Time synchronization and phase synchronization are indispensable for spaceborne bi-/multi-static SAR systems. The LuTan-1 (LT-1) bistatic SAR system employs a time synchronization scheme combining pulse-per-second (PPS) signals with GNSS-disciplined oscillators (GDOs), along with a novel non-interrupted alternate pulse (NIAP) scheme for phase synchronization. Furthermore, the time synchronization accuracy can be further enhanced by applying the two-way time transfer (TWTT) technique based on synchronization data from the NIAP scheme. To accurately evaluate the synchronization performance of the LT-1 system during in-orbit operation, an assessment method based on the two-dimensional (2D) positional offset of calibration targets is proposed. The method compares the theoretical offsets derived from imaging geometry with the measured offsets extracted from monostatic and bistatic SAR images to determine the system’s time and frequency deviations. Using 16 repeat-pass acquisitions over the calibration field in Hami, Xinjiang, during LT-1’s bistatic operation phase, the in-orbit synchronization performance of LT-1 is demonstrated. The PPS+GDO scheme achieves time synchronization accuracy under 100 ns, while the TWTT algorithm enhances this metric by three orders of magnitude, which is better than 0.10 ns. After NIAP phase synchronization, the LT-1 system exhibits a frequency deviation of approximately −2.17 × 10−3Hz, corresponding to frequency stability of 2 × 10−12.

LuTan-1 (LT-1) is the first civil L-band synthetic aperture radar (SAR) satellite constellation and comprises two identical satellites. LT-1 is designed to fulfill two main requirements, one of which the main tasks is to provide digital surface model (DSM) products covering the areas that are not available using optical satellites. The second task is to provide deformation products to support the geohazard monitoring task. For the first task, LT-1 provides a novel noninterrupted imaging technology to facilitate phase synchronization. For the second task, the orbit maintenance is implemented using a newly proposed in-plane offset semimajor axis and out-of-plane control triggered strategy. Besides, we provide the system performance analysis for the two main tasks. Finally, the first results are produced with root-mean-square-error (RMSE) of the DSM less than 0.7 m in the flat region and 6.7 m in the mountainous region. The RMSE of the first deformation products is less than 2.7 mm in the test region. The results indicate a favorable potential for terrain mapping and deformation monitoring applications.

Scattering properties of lunar surface rocks are important guides for remote sensing observation of the lunar surface and discrimination of geologic features. Previous studies primarily focused on the scattering effects of surface or partially buried rocks in terms of angle or frequency, with limited research addressing SAR simulation images of rocks on the lunar surface. This study proposes a three-dimensional (3D) scattering model that integrates the vector radiative transfer (VRT) method with lunar surface rock scattering mechanisms while effectively combining topographic data, rock abundance data, and radar parameters. To implement this model, an end-to-end computational framework is developed to process input datasets and apply the 3D scattering model for backscattering coefficient simulation, reducing uncertainties in the input parameters. Several case studies are performed on Mini-RF observational data. Simulation results demonstrate that the proposed approach shows good consistency with radar data, particularly in terms of distribution characteristics and average backscattering coefficient errors, with an error margin of less than 3 dB.

LuTan-1 is the first spaceborne L-band bistatic Synthetic Aperture Radar(BiSAR) constellation in China aimed at highly accurate digital elevation model(DEM) and surface deformation monitoring. During the in-orbit commissioning phase, the constellation performed periodic observations over the Hami region in Xinjiang, China, while flying in a controlled helix formation. This paper preliminarily elucidates the capability and the stability of Raw DEM generation utilizing LuTan-1 BiSAR interferometric images. Interferometric processing procedures, including initial baseline estimation and absolute phase estimation, were implemented without the use of external ground control points(GCPs). In comparison with GCPs, the results showed that the mean error and the root mean square error (RMSE) of the eight Raw DEMs exhibit close alignment, accompanied by a small standard deviation, validating the excellent stability and capability of LuTan-1 Bistatic SAR.

Synthetic aperture radar Interferometry (InSAR) has proven to be an effective landslide monitoring technique, especially for very slow landslides without observable morphological features. Integration of multi-source InSAR observations from different satellites/tracks could help reduce omissions and misjudgements of potential landslides, showing a promising trend toward automatic landslide detection and monitoring at regional or national scale. However, existing methods present poor error suppression performance, and are not capable of describing and solving potential information conflicts, due to the neglect of observation uncertainties during the integration. Here we propose a new integrated method based on Dempster-Shafer evidence theory to address these deficiencies. The two key steps are multi-source InSAR integration based on DST and a two-step decision rule. We apply the proposed method to the entire Jinsha River Basin using both ascending and descending Sentinel-1 SAR data from 2014 to 2023. The preliminary result on the Luoshui-Baini section based on two sources (ascending and descending orbits) identified 68 landslides, which is nearly the same between our method and the existing mosaic method.

Azimuth ambiguity in synthetic aperture radar (SAR) images results from the limited azimuth sampling rate and seriously affects image quality and normal applications. Multiple algorithms for azimuth ambiguity elimination have been proposed. However, most algorithms adopt a single-channel model and ignore the effect of additional reconstruction operations on ambiguity in azimuth multichannel systems (MCSs). In this letter, the particularity of azimuth ambiguity for MCSs is analyzed theoretically. A refined ambiguity suppression scheme based on multichannel cancellation (MCC) and ambiguity refocusing is proposed. The ambiguity region is first located through the comparison of multi-look image pairs. Then the interference image for ambiguity extraction is cancellated through MCC. Consequently, the ambiguous image is refocused and extracted. Finally, the ambiguity is reversed to the echo domain to handle the special replicated ambiguity problem for MCSs. With the proposed scheme, ambiguity suppression performance has improved for MCSs. The validity of the proposed algorithm is further verified through LuTan-1 real data.

The echo separation issue of different transmit antennas is the most technical challenge in realizing multiple-input and multiple-output synthetic aperture radar (MIMO SAR) with same frequency band, especially for low-computing echo separation, making it extremely difficult towards the practical application for the spaceborne MIMO SAR. Based on the orthogonal frequency-division multiplexing (OFDM) chirp waveforms, this letter proposes an innovative echo separation scheme with digital beamforming (DBF) and bandpass filtering (BPF) on board and bandpass-null steering on the ground for the spaceborne MIMO SAR. This scheme transfers the complex computing process on board to the ground, thus significantly reduce the computational load and relieve the resource occupation on board. Also, the perfect separation of interested echoes from interference can be achieved by this scheme. Finally, performance comparisons and simulation results show the effectiveness of the proposed scheme. The proposed scheme enables a high-efficiency and great-performance echo separation for the spaceborne MIMO SAR and makes the MIMO SAR a more promising technique for future SAR missions.

The azimuth multichannel synthetic aperture radar (SAR), as one of the mainstream technologies for achieving high-resolution and wide-swath (HRWS) imaging, has been successfully employed in several on-orbit SAR missions. However, the unavoidable phase errors among channels result in azimuth ambiguity, deteriorating the recognizability of targets in SAR images. Additionally, the radio frequency interference (RFI) exacerbates the difficulty of phase error estimation. To address this issue, a phase error calibration method based on least spectrum difference (LSD) is proposed. Firstly, the multichannel signals are reconstructed using the linear mapping form of the reconstruction algorithm. Secondly, the objective function is established based on the continuity of the azimuth spectrum, by which only the signals near the discontinuity points are proposed. Finally, the optimal estimation of the phase differences can be obtained after iteration. In LSD method, the RFI to the objective function is mitigated due to the operation in the range-Doppler domain, and the iteration is proposed only using a few data near the discontinuity points thus saving much computational cost. Experimental results based on the simulated data and real bistatic echoes of the LuTan-1 (LT-1) mission validate the superiority of the proposed LSD method.

Range ambiguity is a technical challenge due to the deterioration of the image quality in the multistatic synthetic aperture radar (SAR) constellations. This article proposes an effective range ambiguity suppression scheme based on multiechoes coherent processing. First, a general signal model impacted by range ambiguity is built up based on the geometry of the multistatic SAR constellation, showing the different phase characteristics of the desired and ambiguous signals between the satellites. Then, an effective processing scheme for range ambiguity suppression is proposed based on the different phase characteristics, and the corresponding ambiguity suppression performance (ASP) is analyzed accordingly. This scheme can achieve a coherent summation of the desired signals by compensating for the corresponding phase difference, while the ambiguous signals are summed incoherently. Therefore, the range ambiguities can be suppressed without increasing the SAR instrument complexity. Finally, the system and imaging simulation results are provided to validate the theoretical analysis of the ASP and demonstrate the effectiveness of the proposed scheme, respectively.

This study proposed an improved Doppler radar imaging model (IDopRIM) to address the overestimation of the original DopRIM and produce better accuracy for investigating the current-wave interaction effect on ocean surface current (OSC) retrieval. The IDopRIM’s performance, with the root-mean-square (rms) errors mostly under 0.3 m/s, is validated against the empirical model and actual measurements. It shows a notable improvement over the original model in which the maximum deviation could be up to about 1.5 m/s at moderate-to-high wind speeds and upwind direction for HH polarization. Furthermore, this study uses both numerical simulations and real synthetic aperture radar (SAR) imagery to assess the impact of current-wave interaction on OSC retrieval, especially in scenarios involving ocean internal waves (IWs), using the Doppler centroid anomaly (DCA) method incorporated with the proposed IDopRIM. The results underscore the significance of incorporating current-wave interactions in OSC retrieval for IW conditions, revealing that neglecting these interactions can result in relative errors of over 30% in certain cases.

With the growing scarcity of spectrum resources, frequency sharing among different systems is becoming quite common, which causes radio frequency interference (RFI) to normal SAR applications. Recently, in the RFI suppression field, algorithms based on the low-rank property (LRP) assumption have become a popular research topic. However, this assumption is not valid in complicated cases, especially when there is a wide variety of RFIs. To address this problem, in this paper, factors that affect the LRP are studied through theoretical analysis and simulation verification, a general RFI model is established to fit complicated cases, and an advanced RFI suppression scheme based on low-rank property restoration (LRPR) is proposed. The LRPR scheme has three steps. The first step is RFI localization, which captures the rough profile of the RFI in the time-frequency domain for RFI spectrum separation and extraction. The second step is RFI clustering, in which a novel similarity evaluation metric and corresponding two-step clustering algorithm are designed. The final step is RFI suppression. Operations for LRP recovery in each RFI group are proposed, consequently, classical low-rank approximation methods are applied for RFI suppression. The simulation results for various types of RFI show the robustness and performance improvement of the proposed scheme. In addition, in LuTan-1 data processing, the proposed scheme outperforms classical algorithms in both intensity image recovery and phase preservation.

Frequency scanning (F-SCAN) synthetic aperture radar (SAR), as an advantageous choice for SAR systems operating at higher carrier frequencies, can achieve high performance in terms of swath width, azimuth resolution, and signal-to-noise ratio (SNR). In this article, a novel nonlinear F-SCAN (NF-SCAN) SAR imaging mode is proposed. Compared to the F-SCAN SAR, NF-SCAN SAR has two main advantages: one is to change the distribution of the transmit bandwidth over the swath by precisely tuning the beam scanning response to frequency. The other is to adapt the SNR distribution by adjusting the beam scanning response to time. First, this article derives the system parameters for NF-SCAN SAR. Second, methods for designing nonlinear beam scanning responses (NBSRs) to frequency and time (NBSR-F and NBSR-T) are proposed, by which a well-balanced ground range resolution and adaptive SNR are acquired. Third, a waveform design method dedicated to NF-SCAN SAR is given and the corresponding signal model is derived in detail. To reduce the data rate of NF-SCAN SAR with an incompletely compressed echo window, an improved data subsampling algorithm (IDSSA) is proposed, where a precise filter is designed, thus avoiding the loss of effective bandwidth. Finally, simulation experiments are conducted to verify the superiority of NF-SCAN SAR imaging mode. The proposed NF-SCAN SAR can be viewed as an important candidate for high-performance spaceborne SAR.

Spatiotemporal data fusion provides an efficacious strategy for addressing data gaps within time series datasets. This approach significantly enhances the feasibility of large-scale remote sensing applications by, for example, enabling the creation of seamless Data Cubes (SDC). Nevertheless, strict data input requirements and low computational efficiency of current methods severely limit the practicality of large-scale SDC production. In this study, we propose an efficient spatiotemporal data fusion method, the Fast Variation-based Spatiotemporal Data Fusion (FastVSDF) method. FastVSDF consists of 3 steps, i.e., unmixing, distributing global residuals, and distributing local residuals. In the unmixing process, FastVSDF introduces the fast abundant variation classification (FAVC) to mitigate sample imbalance and expedite the unsupervised classification. Then, the in-class Gaussian weight function is introduced to accelerate the distribution of local residuals by considering the classification to introduce the information on spectral similarity. Besides, FastVSDF employs Fast Guided Filter to combat the "block artifacts" of global residuals efficiently. Results show that FastVSDF demonstrated superior performance over Fit-FC, STARFM, RASDF, and FSDAF. More importantly, FastVSDF yields a remarkable improvement in computational efficiency, reducing predicting time by 43 to 573 times. As a practical application, we generated the Sentinel-2 SDC for the Yangtze River Basin, China. The fusion process for a single period’s Yangtze River Basin dataset was accomplished within 20 minutes, with an average of 3.85 seconds for each Sentinel-2 scene. Comprehensively considering the efficiency, accuracy, feasibility, and universality, FastVSDF demonstrates the practical potential for constructing large-scale and long-term SDC. Our code will be publicly available at https://github.com/ChenXuAxel/FastVSDF.

Tropospheric delays have been a major limitation on the precision and accuracy of Interferometric Synthetic Aperture Radar (InSAR). InSAR data-based tropospheric delay correction methods, especially the local window methods, could estimate heterogeneous tropospheric delays in the same resolution as InSAR data, thus, are increasingly desired for modern high-resolution InSAR products. However, the phase-elevation relationship estimation at local windows can be severely contaminated by topography-correlated deformation. In this paper, we present a new InSAR phase-based tropospheric delay correction method based on texture correlation, which is shown to be relatively insensitive to topography-correlated deformation. The texture information represents the spatially high-frequency component of a two-dimensional image, which can be obtained using high-pass filtering. The method first produces a low-resolution tropospheric delay estimation using the window-wised texture correlation in the space domain, then refines it to high resolution by fitting the residual with the previously estimated tropospheric phase-elevation slope in the time domain. We apply the proposed method to ALOS-2 data over the Kirishima volcanic complex in Japan, encompassing typical topography-correlated deformation. The estimated phase-elevation slope shows seasonal oscillation in agreement with independent ERA5 prediction. The proposed method reduces the median spatial standard deviation of the residual phase from 1.3 cm to 0.7 cm, showing superior performance compared with other existing methods without compromising the deformation signal.

The Bistatic SAR (BiSAR) employs the Alternating Transmitting Mode (ATM) to capture multi-dimensional scattering information of the target. However, the drawback of the ATM lies in doubling the Pulse Repetition Frequency (PRF) and reducing the echo window (i.e., swath width), consequently limiting the High-Resolution and Wide-Swath (HRWS) imaging capabilities of BiSAR. To achieve HRWS imaging, the paper proposes new insights into the ATM of bistatic multi-channel SAR. First, multi-channel signal models containing time and phase synchronization deviations are established for BiSAR. Then, the imaging mode, clock synchronization scheme, and imaging method to achieve HRWS imaging are detailed. Finally, a group of system parameters for the HRWS imaging of the ATM is designed based on the LuTan-1 (LT-1) BiSAR system, and the synchronization and imaging simulations of the ATM are conducted using the data of the LT-1. Simulation results demonstrate that the ATM can achieve the HRWS imaging mode of3m/60km. In short, these new insights pave the way for the HRWS imaging of distributed SAR.

Benefiting from the wide coverage and high resolution of synthetic aperture radar (SAR) data, interferometric SAR (InSAR) has significant advantages in wide-area deformation detection and monitoring. In order to improve computational efficiency and save computational resources, it is expected to perform the deformation area identification first as accurately as possible, and then perform local time series inversion for the specific deformation area. However, with the interference of tropospheric delay, especially its systematic component, the accurate identification of the deformation area becomes challenging. To address this issue, we propose a two-step tropospheric delay removal method including time domain correction and spatial domain correction. The time domain correction is used to avoid the effect of the systematic component of tropospheric delay so as to derive an accurate deformation rate map. The purpose of the spatial domain correction is to finely remove the effect of tropospheric delay in local area to recover the correct deformation time series. Applying our method, external data based method and existing classical SAR data based method to the reservoir area of the Lianghekou hydropower station with Sentinel-1A ascending data for comparison, the results demonstrate the advantages of our method in deformation area identification and deformation monitoring.

Convolutional neural networks (CNNs) have achieved greate success in a variety of computer vision tasks. However, they are susceptible to elaborate, human-imperceptible adversarial noise patterns, which limit their deployment in safety-critical systems. In this paper, we propose an adversarial training method for synthetic aperture radar (SAR) image segmentation, which can effectively suppress the effects of adversarial perturbations. The proposed method introduces a multiple intermediate domain mechanism to enhance the robustness of the network to adversarial attacks, by dynamically adjusting the distribution of input data during the training process, without modifying the network structure or adding a separate mechanism to detect adversarial images. Experiments validate that our approach not only improves the segmentation accuracy of the network, but also effectively enhances the robustness of the network when facing white-box adversarial attacks.

Phase synchronization plays a crucial role in the LuTan-1 (LT-1) bistatic synthetic aperture radar (BiSAR) system, as it aims to eliminate additional azimuthal phase modulation caused by oscillator differences. However, for the pulse alternating transmission system operating in the L-band, the presence of radio frequency interference (RFI) poses an inevitable challenge. Serious RFIs introduce phase jitters, compromising the accuracy of synchronization. In this letter, an effective method for detecting and removing phase jitters is proposed to enhance synchronization accuracy. In the proposed method, the jitter features are separated by the iteratively reweighted least squares (IRLS) in the instantaneous frequency domain based on the established synchronization phase model. Then the jitter positions are detected by correlated peaks between the designed convolutional kernels and jitters. Finally, a polynomial model is utilized to remove jitters and assist in phase unwrapping. The synchronization phases acquired by the LT-1 mission are used to verify the feasibility of the proposed algorithm. The improved imaging quality demonstrates the effectiveness of the proposed method and confirms its ability to ensure the high-precision generation of the LT-1 BiSAR images.

The distributed multiple-input-multiple-output synthetic aperture radar (MIMO-SAR) system composed of numerous separated transmitters and receivers can realize multi-angle imaging, cross- and along-track interferometry through their high-precision cooperation. However, phase synchronization is a major factor affecting the cooperation of distributed MIMO-SAR. As such, this paper proposes a Link-Free Permanent-C (LFPC) phase synchronization scheme for the first time. The designed system framework, basic principle and error model of the scheme are described, and some simulations are performed to evaluate the synchronization accuracy of the LFPC scheme. The results demonstrate that the LFPC scheme has the frequency stability of 10-15when SNR ≥ 50 dB, and it is a candidate for the future distributed SAR mission.

The innovative bistatic synthetic aperture radar (BiSAR) mission LuTan-1 (LT-1) uses a noninterrupted synchronization scheme to achieve high-precision phase synchronization. While radio frequency interference (RFI) is a major factor in deteriorating phase synchronization performance. To present the effect of RFI on the synchronization phase clearly, the characteristics of RFI in the synchronization link are described in detail, and a precise analytic expression between the amplitude, frequency, and phase of RFI and synchronization phase error is established. Furthermore, a novel pulse-compression-based notch (PCN) method is proposed to eliminate the phase error introduced by RFI. In the proposed method, the saturated distortion signals resulting from strong interferences are detected and discarded by the distribution features of their modes. Then, inspired by contrary thinking, the synchronization signal after pulse compression is notched instead of RFI. A fast missing data iterative adaptive approach (Fast MIAA) is performed to recover the gaped RFI signal and remove it from the compressed signal. Finally, the correct synchronization phases can be extracted from the peak positions of the remaining signals. Experimental results derived from using simulated and real synchronization data of the LT-1 system validate the performance of the proposed RFI mitigation method.

This paper proposes an advanced radio frequency interference (RFI) mitigation scheme for the phase synchronization of the bistatic SAR (BiSAR) system based on the blind source separation (BSS) technique, by which narrowband and wideband RFI can be suppressed significantly. First, the RFI signal model is built up and the impact of the RFI on the phase synchronization is analyzed accordingly. Second, a phase synchronization system with multiple receiving channels and the corresponding blind identification diversity (BID) method are proposed to separate the synchronization signal from the contaminated data with less signal loss. In BID, a novel peak detection algorithm is presented to solve the inherent ambiguity of the BSS. Finally, the simulation and experimental results based on the BiSAR system LuTan-1 are presented to validate the advantages of the proposed scheme.

The channel imbalance and crosstalks are the two major factors for the polarimetric calibration of the circular quad-polarimetric (CQP) synthetic aperture radar (SAR) systems. In existing methods using distributed targets, the latter is usually ignored, which may lead to unbearable errors in target classification, surface parameter inversion, and so on. To address this issue, this article proposes a modified iterative calibration method using distributed targets that satisfy quasi-azimuth symmetry to calibrate both the channel imbalance and crosstalks of the CQP SAR systems. First, a stable distributed target selection strategy is proposed based on the correlation coefficient of LL polarization and RR polarization, the cross- and co-polarization backscatter ratio, and the equivalent number of looks (ENL). These parameters are insensitive to polarization distortion, and their typical ranges are determined via numerical simulations. Their combination helps select the targets that satisfy the quasi-azimuth symmetry, which is critical for calibrating the crosstalks. Then, the calibration can be conducted using the selected target. Finally, the phase ambiguity of the receive channel imbalance ratio, commonly found in distributed-target-based algorithms, is eliminated using a dipole target. Through the calibration of Gaofen-3 data and the statistical analysis of residual distortion, the effectiveness of the proposed method is verified.

An advanced sparse multi-channel system is proposed for spaceborne digital beamforming synthetic aperture radar (DBF-SAR), which can suppress pulse extension loss (PEL) and frequency dispersion loss (FDL) without increasing the computational load and system complexity. First, conventional scan-on-receive (SCORE) technique is reviewed and a matching ratio (MR) is proposed to evaluate the mismatch between the formed beam pattern and the pulse signal amplitude. To mitigate the PEL and FDL, the novel sparse SCORE (S-SCORE) based on the optimization of the sparse channel distribution is proposed. The impact of sparse channel distribution is analyzed and the method to optimize the distribution based on the maximized MR is proposed accordingly. Finally, the results of simulations and experiments are provided to demonstrate the superiority of the proposed S-SCORE technique. The work in this paper can be seen as an important candidate for future spaceborne DBF-SAR.

An accurate baseline of synthetic aperture radar (SAR) interferometry (InSAR) is an important parameter for the geodetic application of the InSAR data. Although some advanced SAR satellites have precise orbit determination, there are still many SAR satellites suffering from baseline inaccuracies, such as GF-3. In this article, an image-based estimator for baseline correction is proposed, which requires only the external digital elevation model (DEM) data. The idea of the method is to project the orbit error phase onto the phase components carrying the baseline error information, which is called orbit error phase bases in this article, and to correct the baseline according to the projection coefficients. Since the pure orbit error phase is unavailable, the residual phase of the interferogram is used to approximate the orbit error phase, and a series of processes are introduced to weaken the effect of this approximation. Both the simulated and real data from GF-3 SAR are used to validate the proposed method, and a comparison with the conventional nonlinear least-square and the latest proposed flat-Earth phase-based baseline refinement methods are made. The results indicated the superior accuracy and robustness of our method, especially in areas with higher relief and wider coverage.

Insipred by the cocktail party effect, an advanced scheme based on blind source separation is put forward to suppress the range ambiguity of spaceborne SAR in this paper. In the scheme, multiple sub-antennas are used to collect multiple echo data sets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During post-processing, a detailed flow with the blind source separation algorithm is proposed to separate the desired echoes from those data sets. To verify the scheme, the point targets simulation is carried out. The results indicate that range ambiguity can be reduced by more than 14 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing system complexity. The scheme has the potential to be applied to the future spaceborne SAR missions, such as LuTan-l (LT-l) misson.

In this paper, we investigate the potential of polarimetric synthetic aperture radar (SAR) in identifying flat areas using fractal dimension and polarimetric scattering similarity. A two-step method is proposed, including rough selection and fine selection. First, rough selection is performed by calculating the fractal dimension of the radar backscattered total power image. Then for each candidate region, the fine selection is conducted using polarimetric scattering similarity parameters. Furthermore, the effectiveness of the method is verified by GF-3 quad-polarimetric SAR data and SRTM1 DEM data in desert areas of China. Results show that for the final selected flat area (320 × 320 m), the maximum elevation deviation is 3.39 m and the elevation standard deviation is 0.72 m. Therefore, without depending on additional DEM data, the proposed method can effectively achieve flat areas identification, which can be helpful for the future application of polarimetric SAR data in the Moon.

Due to the large equivalent transmit crosstalk in hybrid quadrature polarimetric SAR system, traditional calibration methods using azimuthally symmetric distributed targets (AS-Targets) cannot work. In this manuscript, a preprocessing method of AS-Targets' covariance matrix is proposed, which makes equivalent crosstalk after processed become small to achieve system calibration through the AS-Targets. Simulation and airborne SAR data testing verify the reliability of the proposed method. The results show that the method presented in this manuscript can well calibrate the hybrid quadrature polarimetric SAR system using AS-Targets.

Cocktail party effect is a fascinating thing about our amazing brains. Inspired by this, a new synthetic aperture radar (SAR) concept is first proposed for wide-swath imaging. Here, it is termed as IP-SAR, because this idea originally comes from voIce-seParation (IP). IP-SAR is a radar system with an array antenna divided into multi-channels along azimuth and/or el-evation directions, and it can image multiple sub-swaths at the same time. In order to verify the wide-swath imaging ability of IP-SAR, scene simulations are conducted, and the gener-ated echoes are processed according to a novel procedure.

Digital beamforming (DBF) is an effective method for improving the signal-to-noise ratio (SNR) in synthetic aperture radar (SAR) images. However, further enhancing the SNR in DBF-SAR is limited by the antenna size and the expensive system cost. To address this problem, in this letter we implement a hybrid strip-map/spotlight mode for DBF. By utilizing the proposed processing method, DBF is adapted for hybrid strip-map/spotlight mode data. Furthermore, the additional synthetic aperture acquired from the hybrid strip-map/spotlight mode is employed to implement multilook processing for further enhancing the SNR of DBF-SAR images. The proposed processing method was verified by an airborne X-band 16-channel DBF-SAR system. The results confirmed that a remarkable additional SNR improvement in DBF-SAR images can be obtained using the proposed method without increasing system complexity.

In previous publications, it is generally believed that the rotated buildings in built-up areas can induce polarization orientation angles, which cause overestimation of the volume scattering contribution. However, we found that the rotation effect of the double-bounce scattering wall-ground structures cannot be accurately expressed by an orientation angle. In this letter, dihedrals are used to simulate the wall-ground structures, and a novel technique for inversion of rotation angles in built-up areas is proposed. This letter is committed to directly inverting rotation angles rather than orientation angles. Then the rotation angles are compensated into the measured scattering matrix instead of deorienting the coherence matrix. C-band GF-3 and L-band ALOS-2 polarimetric data of San Francisco are used to verify the effectiveness of the proposed inversion technique.

Landslide inventory mapping (LIM) plays an important role in landslide susceptibility analysis. Many LIM approaches based on change detection techniques have been proposed, but with various drawbacks. For example, existing approaches have limited capability to capture the objects of varying shapes/sizes present in an area impacted by landslide. Many existing approaches are supervised and require parameter tuning. Moreover, some methods are prone to salt-and-pepper noise. To overcome these limitations, in this letter, an algorithm based on automatic adaptive region extension using very-high-resolution remote sensing images is developed. First, a simple yet effective k-means clustering method is used to generate training samples for landslide and nonlandslide classes, which refer to changed and unchanged areas, respectively. Second, an automatic adaptive region extension algorithm is developed and applied to each pixel of the postevent image, and the label of an extended region around a pixel is determined by the nearest distance between the central pixel and the changed or unchanged samples. Finally, the labels of a pixel are recorded because a pixel in different adaptive regions may be reassigned dissimilar labels, and the final label of the pixel is consistent with its maximum assigned label. To verify the performance of the proposed approach, we conducted experiments on two different landslide sites with VHR remote sensing images in Lantau Island, Hong Kong, China. Experimental results clearly demonstrate that the proposed approach has several advantages in improving the performance of LIM with VHR remote sensing images.

Synthetic aperture radar (SAR) systems require a favorable waveform for imaging as the radar waveform directly affects the performance of SAR systems, such as image quality and resolution. It is well known that the nonlinear frequency modulation (NLFM) waveform can adjust the time-frequency relation to shape the power spectral density (PSD) and then provide a matched filtering output with lower sidelobes without losing signal-to-noise ratio (SNR). However, it will broaden the main lobe, which means the resolution of the SAR image decreases. Therefore, in this paper, a novel NLFM waveform is proposed. It employs the piecewise linear function (PWL) model to define the instantaneous chirp rate function, and genetic algorithm (GA) is then applied to optimize the waveform. The novel NLFM waveform promises enhanced performance and flexibility due to greater design freedom. Through this method, lower sidelobe is achieved within the same 3-dB main lobe width. Finally, simulation results are presented to verify the practicability of the proposed NLFM waveform.

The nonlinear frequency modulation (NLFM) waveform is a promising candidate for the linear frequency modulation (LFM) signal because its autocorrelation output exhibits low sidelobes without loss of signal-to-noise ratio (SNR), also avoiding the transmitting power loss for spaceborne synthetic aperture radar (SAR). However, the acquisition of the analytical expression of the NLFM waveform is often closely related to the indirect instantaneous frequency function generated by the principle of stationary phase (POSP), thus it is not inconvenient for the real-time generation of the efficient and precise NLFM signal on board. In this letter, based on the harmonic decomposition, closed-form expressions of the NLFM waveform, which are directly calculated by the predefined window, are derived in both time and frequency domains. Therefore, it is very beneficial to the real-time generation and process of the NLFM waveform for the SAR system. All the simulation results and analyses validate the promising potential of the closed-form expressions of the NLFM waveform for SAR application.

Distributed synthetic aperture radar (SAR) has various applications, including multi-angle imaging, SAR tomography, and interferometric SAR. However, these applications are affected by several factors, especially clock synchronization. Recently, an innovative push-to-talk (PTT) scheme was proposed to tackle the clock synchronization problem. As the PTT scheme can directly obtain the time and frequency deviations of the ultra-stable oscillators (USOs), it gives the possibility to achieve the clock synchronization of distributed SAR in real-time. Therefore, this paper builds a real-time framework of the PTT scheme. Compared with the previous clock synchronization module (CSM), an estimation module and a control module are added in this framework to fabricate an improved CSM (ICSM). The time and frequency deviations obtained by the ICSM are used to directly correct the GPS disciplined rubidium clocks. Based on the ICSM, some simulations are performed. The results indicate that the established real-time framework can achieve high precision time and frequency synchronization. In addition, we discuss the system implementation.

In an azimuth multichannel synthetic aperture radar (SAR) system, the unavoidable channel error will result in virtual targets or azimuth ambiguity, which significantly degrades the quality of high-resolution and wide-swath (HRWS) SAR image. Calibration of the imbalances between channels has been an important topic. First, to present the channel error effect on the final image clearly, a precise relation between the amplitudes of virtual targets and amplitude–phase error is established by matrix trace, which applies to the case of a certain amplitude–phase error. In addition, the total amplitude of all targets represented by the${L^{1}}$-norm reaches the minimum when there is no channel error. Based on this principle, an image-domain least${L^{1}}$-norm method is proposed to estimate the phase error between channels. By utilizing the focused high signal-to-noise ratio (SNR) images, the proposed algorithm achieves high estimation accuracy. Moreover, no redundant channel is required in the proposed algorithm compared with the subspace-based method. Finally, the effectiveness of the proposed channel error effect analysis and calibration method is validated by both the simulated and real SAR data.

Due to the minimum antenna area constrain of synthetic aperture radar (SAR), high-resolution and wide-swath (HRWS) imaging is difficult to be achieved using classical modes, such as scan, spotlight, and so on. Range ambiguity is one of the main technical challenges limiting HRWS imaging in spaceborne SAR. Insipred by the cocktail party effect, an advanced scheme based on blind source separation is put forward to suppress the range ambiguity of spaceborne SAR in this paper. In the scheme, multiple subantennas are used to collect multiple echo data sets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During postprocessing, a detailed flow with the blind source separation algorithm is proposed to separate the desired echoes from those data sets. To verify the scheme, point target and scene simulations based on GF3 are carried out. The results indicate that the range ambiguity can be reduced by more than 15 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing the complexity of the system design under the current SAR system conditions. The scheme has the potential to be adopted in future spaceborne SAR missions.

Polarimetric synthetic aperture radar (SAR) has been extensively used in various remote sensing applications. In this article, a unified framework is designed to compare the classification performance of different polarimetric systems, which include quad-polarimetric (QP), compact-polarimetric (CP), and dual-polarimetric (DP). To avoid problems, such as the lack of uniform standards in feature extraction, the classification algorithm is directly based on the statistical characteristics of the coherency/covariance matrix and is implemented by extending the Wishart mixture model (WMM). The GF-3 data set in San Francisco and the AIRSAR agricultural data set in Flevoland are used in the experiment, and the following conclusions are generated. QP can achieve the highest classification accuracy in all classification tasks. When distinguishing three typical classes (water, urban, and vegetation) with very different scattering characteristics, the performance of different polarimetric systems is similar, and QP has only a slight advantage. For classification tasks of different classes with similar scattering characteristics, CP performs better in agricultural scenes, and the overall accuracy (OA) is only reduced by 3%–4% compared with QP. DP performs better in urban scenes, and OA is only reduced by 1%–3% compared with QP. These conclusions can provide guidance for future payloads’ design and the choice of polarimetric operation mode for existing multi-polarimetric SAR systems to achieve the purpose of giving full play to the advantages of different polarimetric systems.

This article deals with the synthesis and analysis of quasi-orthogonal nonlinear frequency modulation (NLFM) waveforms to mitigate the impairments of ambiguous returns in quadrature-polarimetric (quad-pol) synthetic aperture radars (SARs). To this end, focusing on signals with a continuous piecewise linear instantaneous frequency, the design of a waveform pair exhibiting both a low cross correlation energy (CCE) and low peak to sidelobe ratios (PSLRs), is considered. To handle the resulting nondeterministic polynomial (NP) hard problem, a coordinate descent (CD) method is employed, where, at each step, the marginal minimization is tackled via a MATLAB optimization toolbox. Hence, transmission/reception schemes jointly capitalizing quasi-orthogonal NLFM waveforms and azimuth phase coding (APC) techniques are proposed to suppress ambiguity interference. Moreover, a systematic framework for the evaluation of the resulting azimuth ambiguity-to-signal ratio (AASR) and range ambiguity-to-signal ratio (RASR) is provided. Finally, detailed simulation experiments based on the LuTan (LT-1) parameters are carried out to verify the practicability and effectiveness of the newly proposed transceiver schemes.

Range ambiguity is a technical challenge for current spaceborne synthetic aperture radar (SAR) systems. To this end, a novel range-azimuth joint modulation transmission scheme is proposed, and the corresponding imaging processing and performance analysis are detailed. Compared with the azimuth phase coding (APC) technique, this scheme fully exploits the sampling margins of the range and azimuth dimensions, resulting in the range ambiguities experiencing a double suppression effect. Starting from the range-azimuth joint modulation scheme, to obtain the best ambiguity suppression performance, the design of a nonlinear frequency modulation (NLFM) waveform with a continuous piecewise linear instantaneous frequency is formulated and tackled via a MATLAB optimization toolbox. The detailed simulation results based on LuTan-1 (LT-1) parameters illustrate that the proposed methodologies outperform the APC method and provide considerable ambiguity suppression.

An alternate transmitting mode (ATM) is an important synthetic aperture radar (SAR) imaging mode as it can provide rich waveform design degrees of freedom to improve the system performance, especially to mitigate range ambiguities. However, the azimuth ambiguity issue caused by the differences between the autocorrelation functions of transmitted waveforms is ignored in existing studies. In this article, a deep understanding of the ambiguities in the ATM allows a correct evaluation of the ambiguity-to-signal ratio and the design of quasi-orthogonal nonlinear frequency modulation (NLFM) waveforms optimized for ambiguity suppression. Moreover, a novel azimuth compensation method is developed to remove the azimuth ambiguities caused by waveform diversity. Finally, detailed simulation experiments are carried out to verify the theoretical analysis.

Hybrid-polarimetric architecture may become a new option for spaceborne quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) mainly due to the range ambiguity improvement without loss of polarization information. To inherit the analytical methods developed for the conventional quad-pol SAR data, it is necessary to transform the hybrid quad-pol SAR data into a linear polarimetric basis. However, this operation will result in deterioration of azimuth ambiguity performance. The theoretical analysis on range and azimuth ambiguities in quad-pol SAR has been relatively complete, which is also summarized in this article. Yet, there is no real data to verify the equivalence of polarization information and to compare the ambiguity levels. Based on P-band airborne experiment, real hybrid and conventional quad-pol SAR data are used for comparative verification. In addition, considering that polarimetric target decomposition is often used in the practical applications, we suggest a new perspective from target decomposition for the ambiguities in quad-pol SAR. Identical polarimetric processing results for two sets of comparative data verify that hybrid quad-pol SAR data are indistinguishable from conventional quad-pol SAR data. The demonstrated intensity images and decomposed RGB images with azimuth ambiguities, acquired by the two quad-pol modes, completely conform to the theory.

One of the challenges of the low-frequency spaceborne synthetic aperture radar (SAR) is that propagation through the ionosphere will introduce nonnegligible errors in the final SAR product. In the low-frequency bistatic SAR (BiSAR) system, the ionosphere will degrade the imaging performance and cause nonnegligible phase errors in the single-pass SAR interferometry application, which results in undesired errors of digital elevation model (DEM). In this article, a method that embedded into the focusing procedure is proposed, which aims to estimate and remove ionospheric effects on L-band spaceborne BiSAR system. First, the impacts of ionospheric effects on the BiSAR system are demonstrated, including the deterioration of imaging performance and the geometric distortion. Then, a method is proposed to correct ionospheric effects. Afterward, the simulations, including point targets and distributed targets, are carried out to verify the effectiveness of the proposed method. The imaging results and the DEM reconstruction results show that the proposed method can effectively estimate and remove ionospheric effects on spaceborne BiSAR systems.

Digital beamforming (DBF) with scan-on-receive (SCORE) technique in elevation is a powerful technique that enables a spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide swath (HRWS) imaging. In the spaceborne DBF-SAR system, sampling signals from each channel are weighted by weights generated by a digital signal processing system in real-time. However, the contradiction between the shortage of spaceborne hardware resources and resource demand of the multichannel real-time signal processing increases the difficulty of system design. In order to solve this problem, a novel weight generator and an improved intermediate frequency (IF) DBF real-time processing architecture are proposed in this article. By taking advantage of the special properties of the SCORE algorithm, the proposed weight generator calculates weights using a linear polynomial algorithm. The simulation result shows that a low-order approximation can achieve high performance. The proposed generator can correct multichannel amplitude and phase error at a low cost on hardware resources. The effectiveness of the proposed method is verified by experiments with a raw data processing instance of an X-band 16 channels DBF-SAR.

Synthetic aperture radar (SAR) is an advanced ground-observing remote sensing technology, and high-resolution wide-swath (HRWS) imaging has always been the goal of SAR. In general, an improved azimuth resolution requires a large pulse repetition frequency (PRF), resulting in high system requirements. To overcome this difficulty, azimuth multichannel technology has gradually developed, achieving HRWS imaging and decreasing the PRF by increasing the spatial sampling, i.e., increasing the number of antenna subapertures in the azimuth. This article proposes a theoretical architecture that generates multiple virtual receiving apertures in the azimuth rather than real apertures. The virtual receiving apertures are formed by multiplying the azimuth signals by linear phase histories provided by vortex beams carrying different orbital angular momentum (OAM) modes. Vortex beams with different OAM modes have different oblique phase wavefronts, so virtual receiving positions are generated in the along-track direction. This approach aims to reduce the PRF without increasing the azimuth real receiving aperture. Simulation results demonstrate the effectiveness and limitations of the method. Finally, to overcome the inherent limitations of the method, two possible implementation schemes are proposed.

This article proposes deep convolutional neural networks to detect and map localized, rapid subsidence caused by mining activities using time-series Sentinel-1 synthetic aperture radar (SAR) images. A deformation detection network (DDNet) is developed to automatically identify rapidly subsiding areas from wrapped interferograms, and a phase unwrapping network (PUNet) is designed to unwrap the cropped interferogram patches centered on the detected subsiding locations. To train the two networks, interferogram simulation strategies are developed to generate various training samples using the distorted 2-D Gaussian surface and fractal Perlin noises. The performance of the DDNet is verified by simulations on a synthetic dataset with 13 large interferograms, while the PUNet is evaluated by simulations using synthetic datasets with different levels of deformation gradients and noises. Compared with the traditional and deep-learning methods, the PUNet exhibits excellent performance and efficiency in unwrapping interferograms with rapid mining-induced deformation. The proposed networks are further verified by applying them to Shanxi province, China, which is characterized by serious ground subsidence hazards caused by long-term coal mining activities. The time-series deformations of 1344 detected subsidence areas are calculated with the vertical velocities ranging from −19.7 to −254.8 cm/year. The results are validated using the ascending and descending Sentinel-1 Interferograms and an L-band ALOS-2 interferogram covering the same area within the acquisition period, showing highly consistent vertical deformation rates. The proposed strategy and methods introduce deep learning to the time-series interferometric SAR (InSAR) processing chain and may have profound implications on the detection and monitoring of localized mining-induced deformation using InSAR.

Phase unwrapping is a critical step of interferometric synthetic aperture radar processing, and its accuracy directly determines the reliability of subsequent applications. Many phase unwrapping methods have been proposed, most of which assume that the phase has spatial continuity, while decorrelation noise and aliasing fringes invalidate the assumptions, resulting in poor performance of these methods. To obtain more reliable unwrapping results, in this article, a deep convolutional neural network, called a discontinuity estimation network (DENet), is proposed for predicting the probabilities of phase discontinuities in interferograms. The main advantages of DENet are: 1) using branching structure to extract detailed and high-level features separately and retain details while making full use of contextual information; 2) using multichannel input, including interferogram, range/azimuthal phase gradients, and residues map, to provide effective guidance for discontinuity prediction; and 3) using a single network to estimate phase discontinuities in both range and azimuth directions simultaneously. To train the network, a dataset simulation strategy is proposed to generate enough training samples. The strategy considers a variety of phase components, such as terrain-related phase, random deformation, atmospheric turbulence, and noise. The phase discontinuity estimated by DENet is then converted to costs in the minimum cost flow (MCF) solver of the statistical-cost, network-flow algorithm for phase unwrapping (SNAPHU) to obtain the final unwrapped phase. Based on validations of simulated and real interferograms, the proposed method exhibits excellent performance compared to traditional and deep learning unwrapping methods. The proposed method can effectively unwrap large-scale, low-quality interferograms, which is expected to significantly improve the accuracy of synthetic aperture radar interferometry (InSAR) applications.

Interferometric synthetic aperture radar (InSAR) is an efficient tool for global large-scale digital elevation model (DEM) generation. However, for steep terrain, the current approaches cannot stably reconstruct valid DEM products from a single-baseline InSAR image pair without an external reference DEM due to the influence of shadow/layover geometries, phase noise, and the Itoh condition limitation in the phase unwrapping (PU) process. In this article, a novel stereo-radargrammetry-assisted PU (SAPU) approach with no need for external auxiliary information is proposed to eliminate the constraint of the Itoh condition by exploiting the internal stereo-radargrammetric shifts. The method reduces the phase gradient in the interferogram and guides the PU process with automatically selected tie points. Notably, the current stereo-radargrammetry approaches will deteriorate to an incoherent state in steep terrain, hampering the reliability and accuracy of SAPU. Accordingly, we also propose an adaptive weighted subwindow-coherent stereo-radargrammetric shift estimation (AWS-CSE) method to improve the accuracy of subpixel shifts by introducing local topographic phase consistency in coherence estimation. We quantitatively validate the performance of the proposed methods based on the L-SAR 01 simulation data and three pairs of repeat-pass single-baseline Advanced Land Observing Satellite (ALOS) phased array type L-band synthetic aperture radar (PALSAR) images from different areas, comparing the results with those of various traditional and deep-learning-based PU methods. The findings suggest that the proposed methods can generate accurate DEMs from single-baseline measurements in steep terrain while avoiding additional data acquisitions.

The innovative spaceborne Earth observation mission LuTan-1 (LT-1) deploys advanced full polarimetric L-band synthetic aperture radar (SAR) to obtain high-precision, multidimensional ground feature information. As the quad-pol SAR system, LT-1 suffers from strong ambiguities that degrade the quality of the observation products. Antenna pattern synthesis algorithm can suppress the range ambiguities without the increase of the azimuth ambiguities and the system complexity and therefore has great potential to improve the overall ambiguity performance of the quad-pol SAR system. However, the previous research on this kind of method is generally limited to specific situations and lacks of the analysis of actual measurement results. Based on the application requirements of LT-1, this article proposes a novel pattern synthesis algorithm that suppresses the range ambiguities via sequential convex optimizations. In simulation and comparison, the proposed algorithm effectively suppresses the strong co-polarized range ambiguities and shows the flexibility, efficiency, and stability that are significantly better than the previous algorithms. What is more, the analysis of practical performance loss is performed based on the measurement result of the LT-1 antenna, and the practicality and validity of the algorithm are strongly verified.

Bistatic/multistatic synthetic aperture radar (Bi-/M-SAR) has extensive applications, including cross- and along-track interferometry, multiangle imaging, and so on. However, these applications are affected by several factors, especially the time and frequency synchronization that this article focuses on. Here, the new insights about the focus are described, based on which an innovative push-to-talk (PTT) scheme is first put forward to solve the time and frequency synchronization problems of distributed SAR. It realizes unidirectional and long-distance synchronization through the two-segment frequency rate (TSFR) waveform composed of the two linear frequency modulation (LFM) signals with different frequency rates and achieves radar imaging and synchronization with a high rate (${f}_{\mathrm{ syn}}$) at the same time by the frequency diversity (FD) waveforms and bandpass filters. To clarify the PTT scheme, its system framework and synchronization error models are detailed. Besides, the channel impulse response, multiple errors, the signal-to-noise ratio (SNR) of synchronous links, and time synchronization accuracy are analyzed. In addition, some simulations are carried out to evaluate its performance. Those results demonstrate that the PTT scheme can perform the long-distance synchronization with high$f_{\mathrm{ syn}}$, its synchronization accuracy increases with the improvement of SNR and$f_{\mathrm{ syn}}$, and it can measure the time and frequency deviations in different ranges accurately. In a word, the PTT scheme has the potential to be used in future distributed SAR missions.

The non-linear frequency modulation (NLFM) waveform can shape the power spectrum such that its autocorrelation output exhibits very low sidelobes without loss of signal-to-noise ratio (SNR), compared with the linear frequency modulation (LFM) waveform. However, the NLFM waveform has attained little acceptance in spaceborne SAR system due to its distinct disadvantages, e.g., greater system complexity and limited development of the NLFM generation devices. In this paper, we report a parametric piecewise linear (PWL) model for the generation of the general NLFM waveform. Through this model, a novel generation approach, which can significantly reduce the signal computing resources on board, is proposed. Nevertheless, the existed advanced NLFM waveforms, which possess a lower sidelobe under fixed main lobe, suffer from severe performance degradation with this parametric model. To this end, an empirically advanced NLFM waveform is further proposed. This proposed waveform not only allows for a low computing complexity generation by a modified parametric PWL model, but also its performance degradation is dramatically reduced. Finally, detailed simulation experiments are performed to verify the excellent performance of the proposed NLFM waveform.

Multiple-input and multiple-output synthetic aperture radar (MIMO SAR) is a very potential technique for high-resolution and wide-width (HRWS) imaging due to the fact that it can provide more degrees of freedom. In this paper, an innovative MIMO-HRWS SAR imaging system is introduced with segmented phase coding (SPC) waveforms. However, it is well known that the echo separation issue is the most technical challenge for MIMO SAR. It has shown that the separation of orthogonal echoes from cross-correlation interference can be achieved by scan-on-receive (SCORE). However, this method often requires large computing resources on board, especially for null-steering process. For this, we propose a two-stage spaceborne-ground echo separation scheme. This scheme is mainly divided into two-steps: the first step is a real-time SCORE-beamforming process on board to reduce the downlink data volume, and the second step is a bandpass-null steering process to suppress the interfering echoes on the ground. Thus, this scheme allows for the low computing resources on board and sufficient suppression of the interference simultaneously. Following this scheme, the MIMO-HRWS SAR system enables its number of equivalent phase centers nearly double that of the azimuth multichannel SAR system, thus higher-resolution and wider-swath imaging. Finally, detailed simulation experiments for the MIMO-HRWS SAR imaging system are performed to verify the practicability and feasibility of the proposed scheme.

Multiple-input–multiple-output synthetic aperture radar (MIMO-SAR) has extensive application prospects, mainly including the acquisition of multidimensional scattering information, high-resolution and wide-width (HRWS) imaging, and moving target indication (MTI). Its echo separation is the most technical challenge, and so far, the confirmation for orthogonal waveform encoding MIMO-SAR by airborne experiments has not been reported in any literature. Here, an echo separation experiment based on the segmented phase code (SPC) waveforms and an airborne digital beamforming SAR (DBF-SAR) system is demonstrated for the first time. In the experiment, the SPC waveforms are cyclically transmitted within the adjacent pulse repetition intervals (PRIs) to simulate multiple transmitters, and the scattered echoes are received by the 16-channel antennas in elevation at the same time. In the postprocessing, the echo signals of continuous PRIs are added to obtain the mixed echoes, and a detailed echo separation method is adopted. In the method, the mixed echo signals from close arrival angles and far arrival angles are separated by the time shift and weighting, and by the bandpass filtering and DBF technique, respectively. Through the presented method, the mixed echoes of dual-transmit and 16-receive (2T16R) SAR imaging mode are separated and imaged successfully. The experimental results not only validate the echo separation scheme but also indicate that it is very promising in future MIMO-SAR missions.

Hybrid and$\pm \pi /4$quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) systems are established to simultaneously obtain all polarimetric components, including co-polarized (co-pol) and cross-polarized (cross-pol) components. However, the cross-pol components always suffer from severe azimuth ambiguities. In this article, the hybrid and$\pm \pi /4$quad-pol SAR systems with double receive channels are widely investigated to suppress the azimuth ambiguities of cross-pol components, in which the systems allow for a half pulse repetition frequency but at the cost of antenna size. We firstly provide a more thorough analysis for the double-channel (DC) hybrid and$\pm \pi /4$quad-pol SAR systems. Then, an improved reconstruction method is proposed to suppress the extremely severe azimuth ambiguity caused by the general reconstruction algorithms. However, the cross-pol signals still exist severe azimuth ambiguity. To this end, the displaced-phase-center antenna (DPCA) condition based on digital beamforming reflector antenna is employed, in which the undesired polarized signal can be greatly suppressed. Furthermore, numerical analysis is developed to demonstrate the excellent performance of the DC hybrid and$\pm \pi /4$quad-pol SAR systems with such DPCA condition. Finally, the distributed scene simulation results are presented to verify the advantage of the proposed approach.

Staggered synthetic aperture radar (SAR) is an innovative concept to obtain an ultrawide continuous swath with fine azimuth resolution using multiple elevation beams and pulse repetition interval (PRI) variation. Conventionally, the interpolation of the nonuniform data with gaps (due to the interruption during transmission, i.e., blockage) requires a high oversampling ratio to avoid image quality degradation, which leads to increased range ambiguities and high downlink data rates. In recent years, there has been a growing interest in improving the image quality for low-oversampled staggered mode. This study suggests a new concept, stepwise staggered SAR, which employs stepwise staggered PRIs to facilitate the use of novel linear Bayesian approaches to achieve high-quality recovery of blockage in low-oversampled data. The so-called step in “stepwise” refers to a period that contains multiple constant PRIs in a PRI staggering cycle, ensuring part of the data uniformly sampled to reduce the error propagation during resampling. In addition to the no-consecutive gap condition, the stepwise PRI staggering strategy meets the requirement of no more than one gap at any location within the slant range of interest in each PRI staggering cycle, which makes it possible to easily extract prior information from the consecutive known samples on both sides of each blockage sample to realize the proposed linear Bayesian approaches. Because the prior information adapts to characteristics of backscatter, the linear Bayesian estimation has an excellent performance in blockage recovery, which has been validated by simulations with different scenarios.

For ultrahigh-resolution spaceborne spotlight synthetic aperture radar (SAR), the effective swath width is limited by the severe range migration. By adapting the pulse repetition interval (PRI) according to the change of the slant range, however, the range migration can be substantially reduced, which makes the varying-PRI spotlight mode an attractive solution for future spaceborne SAR missions. This study provides a complete preprocessing framework for the varying-PRI spotlight SAR data, by which data equivalent to uniform sampling can be obtained, so that traditional processing algorithms designed for data on a regular grid can then be applied. Furthermore, in response to the non-ideal focusing results of the spotlight data, an in-depth analysis of the effects of the SAR acquisition geometry and processing algorithms on the Doppler spectral distribution is presented. Finally, a varying transmitted pulse duration scheme is proposed to enhance the performance of the varying-PRI spotlight SAR system.

Interferometric synthetic aperture radar (InSAR) technique can periodically measure small deformation over a large area, providing many supports for landslide studies. However, landslide detection with InSAR is still a challenge problem, especially in the efficiency over large-area. In this paper, the workflow of InSAR driven landslide detection and monitoring is optimized based on small baseline sets. It adopts a two-step strategy: (1) regional detection by small baseline InSAR-stacking, (2) continuous monitoring of detected landslides by small baseline sets InSAR (SBAS-InSAR), making a trade-off between efficiency and accuracy. This workflow was applied in the Dongchuan Section of Jinsha River Valley. Over more than 5000 km2, 62 active landslides are detected and mapped. Then for one typical landslide, Dapingdi landslide, deformation time-series are estimated to illustrate its growing process.

This paper established a unified framework to compare the capabilities of different polarimetric SAR systems in target detection based on polarimetric covariance matrix. The framework contains two different strategies. The first is based on the original polarimetric matrix and the second is based on the main scattering matrix. An improved algorithm is proposed based on the second strategy. GF-3 quad-polarimetric data is used to compare the performance between different algorithms, the results show that the proposed algorithm can obtain the highest signal-to-clutter ratio (SCR). The performance of different polarimetric systems is further compared, and the results show that the quad-polarimetric (QP) performs best, compact-polarimetric (CP) performance is similar to QP, and dual-polarimetric (DP) performs worst.

The ionosphere is a significant source of the phase dispersion on the low frequency radar signal for spaceborne synthesis aperture radar (SAR). LuTan-1 (LT-1) is an innovative spaceborne bistatic SAR (BiSAR) operated in L-band, which will be launch in 2021. Studying the impacts of ionospheric effects on spaceborne single-pass imaging and interferometry is a necessary work to support the ground processing system of LT-1. In this paper, The geometric distortions caused by the ionosphere, including the offsets of the SAR image and Digital Elevation Model, are analyzed firstly. Then the deterioration of the imaging performance caused by the quadratic phase error and the cubical phase error is analyzed, and simulation results of the point target are given to verify. Finally, a framework of ionospheric effects correction for BiSAR is proposed.

To solve the time and frequency synchronization problems of the distributed synthetic aperture radar (SAR) with longer baselines and more space vehicles, an innovative push-to-talk (PTT) scheme is proposed here. It realizes unidirectional and long -distance synchronization by the two-segment frequency rate (TSFR) waveform composed of two LFM signals with different frequency rates, and also achieves high-rate and noninterference synchronization by the frequency diversity waveforms. To evaluate the performance of PTT scheme, two simulations are executed. Those results indicate that PTT scheme can perform time and frequency synchronization with high accuracy, that its synchronization accuracy improves with the increase of signal-to-noise ratio (SNR), and that it has the potential to be used in future distributed SAR missions.

In recent years, the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) concept has been widely researched because it can provide more degrees of freedom to dramatically improve SAR system performance. However, the echo separation issue of different transmit antennas is the most challenging issue and it stirs up extensive discussions. In this letter, a novel MIMO-SAR imaging scheme based on the improved orthogonal frequency-division multiplexing (OFDM) waveforms is proposed. The main contributions of this work are that:1)Improved generation method for$M$OFDM waveforms is presented. This method can compensate the extra carrier frequency deviation and compared with the other compensation methods, this method is more general and suitable for$M$OFDM waveforms.2)Based on the proposed OFDM waveforms, a novel and low-cost spaceborne MIMO-SAR imaging scheme is proposed.In this scheme, a simple time-shift weighting process and a bandpass filter bank are employed to separate the echoes from the close arrival angles. Then, the digital beamforming (DBF) on receive in elevation is employed to separate the echoes from far arrival angles. Furthermore, the distributed scene simulation results are presented to verify the practicability of the proposed scheme.

Azimuth ambiguity degrades the quality of synthetic aperture radar (SAR) images and leads to the increase of false alarm rate in ground moving target indication (GMTI). Due to the existing azimuth ambiguity, suppression methods do not remove the first-order ambiguity completely and ignore the ambiguity above first order as well, moving target detection is affected by residual ambiguity. Hence, a novel method to suppress first-order and higher order azimuth ambiguities for the dual-channel SAR/GMTI is proposed in this letter. First, the displaced phase center antenna (DPCA) technique is applied to suppress clutter. Then, estimate the local azimuth ambiguity-to-signal ratio (LAASR) to find out the area affected by ambiguity. Finally, an inpainting algorithm is improved to patch the ambiguity area. The proposed method can remove the ambiguity almost completely. Moreover, the method is verified using the GaoFen-3 SAR dual-channel complex image data, and the result shows that the false alarm of moving target detection is degraded without reducing the detection rate.

The high-fidelity deceptive jamming contaminated synthetic aperture radar (SAR) images are very confusing when they are interpreted for target recognition. In this letter, a novel azimuth phase coding (APC)-based method is proposed for deceptive jamming suppression, especially with characteristics of high fidelity and intermittent sampling. The deceptive jamming model is detailedly formulated, and the difference and relation related to the real target are also discussed. Then, the APC-based suppression method is proposed, where the original APC principle and how it is to be introduced into deceptive jamming suppression are deduced and analyzed. Considering the delay feature of deceptive jamming retransmission, azimuth phase modulation, azimuth phase demodulation, and azimuth filtering are successively implemented. Then, parameters discussion is carried out for a deeper understanding. Simulation results demonstrate the superiority of the proposed method.

In this letter, a novel and low-cost echo separation technique for the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is presented, based on the orthogonal frequency-division multiplexing (OFDM) chirp waveforms. The proposed scheme allows the generation of multiple OFDM chirp waveforms on common spectral support. In the new scheme, a series of simple time-domain operations including replica, T-shift, and superposition is applied to eliminate the interference waveform within a limited time. Then, a combination with a bandpass filter instead of a matched filter to focus signal power and digital beamforming (DBF) on receive in elevation enables the suppression of interference signals for a realistic spaceborne SAR scenario, where the swath width exceeds the spatial extension of the transmitted pulse. Furthermore, the distributed scene simulation results are presented to verify the practicability of the proposed scheme.

The minimum antenna principle gives rise to the extremely complex engineering implementation and data processing of spaceborne high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR). To address this problem, a multiswath jump SAR with sound range ambiguity performance is proposed in this letter, which continuously switches multiple radar signals with nonoverlapping spectrum to radiate different areas during the transmitting time. To further obtain high-resolution images, the echo signals are processed by the methods of range spectrum splicing and azimuth multichannel reconstruction. Besides, some simulation experiments are executed to verify the capability of HRWS imaging of multiswath jump SAR and the theoretical analysis.

Space-time waveform-encoding (STWE) synthetic-aperture radars (SARs) can realize waveform diversity in the space-time domain and effectively improve the system performance. However, the benefits of the STWE SAR are based on the accurate separation of the overlapped echoes. Digital beamforming (DBF) in elevation using the null-steering techniques has the potential for echo separation in the STWE SAR. However, the performance of the conventional null-steering beamformer deteriorates when it meets with the extended pulse. Constant time delay of the signal in each channel is usually used to relieve the influence of the extended pulse in the DBF SAR. However, for the STWE SAR in elevation with multiple subswaths, the used delay values are only compatible for one single imaging swath. This letter proposes an innovative echo separation beamformer with digital scalloped beamforming (DSBF) and adaptive multiple null-steering for the STWE SAR. The proposed beamformer can relieve the influence of the pulse-extension loss (PEL) for the echo to be extracted and much better suppress the interfering echoes compared with the conventional methods. Simulation results show the effectiveness of the proposed beamformer. The proposed beamformer enables perfect echo separation for the STWE SAR and makes the STWE SAR a more promising technique for future SAR missions.

The challenging problem to realize high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) imaging is the ambiguity suppression in the azimuth and range directions. According to the spatial angle difference of each ambiguity component, the current technical approach is to design the spatial filter for achieving the ambiguity suppression based on the 2-D multichannel system. Along with the increasing of HRWS imaging requirements, the number of system channels also gradually increase and further result in the complex structure design of the phased array antenna system. Meanwhile, the traditional phased array antenna cannot effectively control the direction of the transmit beampattern in range. Unlike the traditional phased array, frequency diverse array (FDA) employs a small-frequency increment across the whole array elements and forms the range-angle-dependent S-shaped transmit beampattern, which can be utilized to separate the different range ambiguous region. Considering the above-mentioned characteristics and the range periodicity problem of transmit beampattern, this letter devises a scheme for spaceborne SAR HRWS imaging mode in the view of transmit beampattern utilizing 2-D planar array, i.e., the FDA in azimuth for removing the range nonperiodicity ambiguity and the conventional phased array in elevation for removing the range periodicity ambiguity. Simulation results have been presented to validate the effectiveness of the proposed scheme.

The displaced phase center antennas in azimuth and digital beamforming (DBF) in elevation are two state-of-the-art techniques for achieving high-resolution wide swath (HRWS) imaging in the multichannel synthetic aperture radar (SAR) systems. However, due to the atmospheric turbulence, airborne HRWS-SARs inevitably suffer trajectory disturbances, which will consequently defocus the SAR image. Although various motion compensation (MoCo) methods have been proposed, they are mostly designed for the traditional single-channel SAR and, therefore, ignore the channel-dependent posture error. The posture motion error will introduce residual motion and time-variant channel errors, which not only defocuses the image but also causes azimuth ambiguity and degrades the SNR improvement of the final images of the azimuth multichannel and DBF-SAR, respectively. To solve these problems, a novel MoCo scheme for 2-D multichannel SAR systems is proposed. The posture error is described and calculated in matrix form through the use of quaternions. Then, the posture error is transformed into the translational motion error for each receiving channel and is subsequently compensated precisely. To address the residual aperture-variant motion error, a modified aperture-dependent MoCo is integrated into the proposed scheme. Simulations and airborne experiments, including processing the data of a C-band SAR system with four azimuth channels and an X-band DBF-SAR system with 16 elevation channels, have been implemented to validate the effectiveness of the proposed MoCo scheme.

The echo separation issue associated with different transmit antennas is the most technical challenge in realizing the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) system. In this article, a novel MIMO-SAR imaging scheme based on an advanced radar waveform, namely, segmented-phase-code (SPC) waveform, is proposed. Compared with the state-of-the-art short-term shift-orthogonal (STSO) waveform beamforming schemes, this scheme relieves the short-term shift-orthogonality condition of transmitted waveforms without losing the imaging performance, which extends the optional waveform space. In this scheme, the separation of the echoes from close arrival angles is ensured by a simple time-shift weighting processing. Furthermore, a range bandpass filter bank and the digital beamforming (DBF) technique are employed to ensure that the echoes from far arrival angles are separable. Finally, detailed simulation experiments are performed to verify the feasibility of the proposed scheme, and in-depth discussions of different waveforms and MIMO-SAR imaging schemes are presented.

The bistatic synthetic aperture radar (BiSAR) plays an important role in remote sensing. However, the deviation between the two oscillators in BiSAR systems will cause a residual modulation of the echo signal. Therefore, the phase synchronization is an important issue that must be addressed in the BiSAR system. An advanced noninterrupted phase synchronization scheme is used for LuTan-1. The synchronization pulses are exchanged immediately after the ending time of the radar echo receiving window and before the starting time of the next pulse repetition interval, which will not interrupt the normal SAR operation. In order to evaluate the accuracy of the phase synchronization scheme, the model of phase synchronization is introduced at first. The hardware design and processing flow of LuTan-1 are introduced in detail. An innovative internal calibration strategy is also described. Then, the test data acquired by the ground validation system are analyzed to verify the effectiveness of the phase synchronization scheme. The signal-to-noise ratio (SNR) and the synchronization rate are the two most important factors to influence the accuracy in phase synchronization. The conclusions have guiding significance for the synchronization module design of LuTan-1 and the future BiSAR system.

Variable atmospheric conditions are typically ignored in the retrieval of geophysical parameters of the Earth’s surface when using spaceborne passive microwave observations. However, high frequencies, for example, 91.7 GHz, are sensitive to variable atmospheric absorption, even in winter’s dry conditions. In this article, the influence of variable atmospheric absorption on snow cover extent (SCE) mapping was quantitatively investigated. A physical method was derived to enable atmospheric correction for variable atmospheric conditions. The total column precipitable water vapor (TPWV) from Moderate Resolution Imaging Spectroradiometer (MODIS) was parametrized into transmittances in this correction method. The corrected brightness temperature at 19 and 91.7 GHz from the Special Sensor Microwave Imager Sounder (SSMIS) was applied to the threshold algorithm for snow mapping over China. Compared with the Interactive Multisensor Snow and Ice Mapping System (IMS) data in winter from 2012 to 2013, for Qinghai–Tibet plateau (QTP), a significant improvement after correction was obtained from February to March over ephemeral and shallow snow, where the largest daily improvement of accuracy is up to 20%. The accuracy (incl. precision, recall, and F1 index) improved on average is from 0.77 (0.60, 0.68, and 0.63) to 0.79 (0.69, 0.7, and 0.68) over the full winter time from December to March. Over forest-rich Northeast China, where snow in winter is thicker, small improvement was observed at the onset of the snow season and over snow margin area. It was evidenced that high frequency is a promising way of snow cover mapping with the proposed atmospheric correction method.

The azimuth phase coding (APC) technique is known for its very low implementation complexity and its effectiveness for point and distributed ambiguities in conventional synthetic aperture radar (SAR) systems. In recent years, as an extension, the APC technique has been briefly discussed for multichannel SAR systems. However, the properties of the APC technique are no longer guaranteed in the multichannel SAR systems based on the digital beamforming (DBF) on-receive, and only a slight APC gain in the suppression of the range ambiguity can be obtained. In this article, we first provide a more thorough analysis for an APC-multichannel SAR system with respect to a uniform pulse-repetition frequency (PRF). Then, the APC/multichannel SAR system with nonuniform spatial sampling is briefly discussed, and an improved reconstruction approach based on a quadratically constrained optimization model is proposed to increase greatly the APC gain with respect to existing multichannel reconstruction algorithms. This proposed approach allows the minimization of the range ambiguity with a given azimuth-ambiguity constraint. In particular, for some specific PRFs, the proposed method permits a cancellation of the odd-order range ambiguity. Finally, simulation experiments are performed to verify the advantages and effectiveness of the proposed approach.

Spaceborne multistatic synthetic aperture radar (M-SAR) has extensive applications, including multiangle imaging, digital beamforming (DBF) and cross- and along-track interferometry. However, it is difficult for classical satellite formations to meet the multimission (e.g., cross-track interferometry and along-track interferometry, i.e., XTI and ATI) requirements of M-SAR concurrently. Therefore, superpolyhedron (SP), an innovative satellite formation based on the dual-helix and pendulum formations, is proposed to maximize the coverage ratio of the effective cross- and along-track interferometric baselines at the same time. A method of constructing the SP formation is detailed. The method is a three-step procedure, in which the first two steps aim at obtaining an initial formation via exploiting the geometry of the formation dynamics; the last step solves an optimization problem. Then, the design of a supertetrahedron (ST) formation, a special case of the SP formation, is investigated as a numerical example. The merit of SP formation is represented as the coverage ratios of effective cross- and along-track interferometric baselines. The result is compared with those of the classical satellite formations. It shows that only the coverage ratios of the ST formation are greater than 70% simultaneously. Therefore, the ST formation can realize both effective XTI and ATI. It implies that the SP formation has the potential to be used in future spaceborne M-SAR interferometry.

In the Earth observation mission of the synthetic aperture radar (SAR), wide swath can be used to complete global monitoring in a short time and high resolution can provide rich detailed information about the feature space and prominent structure and texture. However, the traditional single-channel classical SAR system cannot meet high-resolution and wide-swath (HRWS) imaging demand due to the constraint of minimum antenna area. Fortunately, this fundamental limitation can be overcome by using multiple receive subapertures in combination with advanced digital beamforming (DBF) technique. DBF in elevation can provide high gain and better system performance and has recently gained much attention in the field of SAR imaging. This article presents a 16-channel in elevation airborne X-band DBF-SAR system with 500-MHz bandwidth, characterized by high speed data acquisition and storage, as a test bed to provide the technical reserves and support for a future spaceborne DBF-SAR system in China. The hardware configuration of this system is designed according to a realistic flight mission. To verify the feasibility and operability of this advanced 16-channel DBF-SAR system, an outfield airborne flight experiment was successfully conducted in eastern Guangdong Province in November 2019. Meanwhile, considering the inevitable channel mismatch from airborne system, a precise strategy as well as the underlying signal processing is proposed to process the experiment data. In addition to the channel mismatch due to the topographic height, the Scan-On-Receive (SCORE) pattern loss (SPL) is also an inherent factor, which will deteriorate the output SNR in final SAR images. Therefore, this article also implements a quantitative assessment of SPL combined with the practical flight parameters and the real airborne data. Finally, the corresponding processing results are presented and analyzed in detail. The practical SNR improvement of 11.23 dB emphasize that DBF technology can significantly improve the quality of SAR images and will make an essential contribution to next generation of HRWS technology for environment monitoring.

For ultra-high resolution spaceborne spotlight synthetic aperture radar data processing, an imaging algorithm is proposed in this paper, which takes into account the variation of the effective velocity caused by the curved orbit. First, an airborne SAR one-step motion compensation (MOCO) method is performed to compensate for the errors introduced by the curved orbit. Then, an azimuth decompression filter is utilized to separate the rang-focusing and the azimuth-focusing after a modified Stolt interpolation. Finally, a residual range cell migration correction (RCMC) and the azimuth compression are implemented successively, considering the range dependence of the effective velocity. Simulation results confirm the effectiveness of the proposed algorithm.

An airborne P-band SAR experiment was conducted to detect targets in a forest in October 2019 and different results of target detection in the forest under snow-covered and snow-free states were obtained. When there is no snow on the branches, the targets in the forest are not easy to be seen in the image, but when the branches are covered with snow, the targets in the forest are clearly visible in the image. Three quad-pol data sets were used for this paper to compare and analyze the differences of target detection in the forest that Yamaguchi Four Component Decomposition and H/α unsupervised classification methods were used to explain. The results reveal that snow affects the scattering characteristics of the forest, and indirectly affects the penetration depth of the P-band electromagnetic waves into the forest, resulting in a large difference between the two detection results.

Mining induced subsidence seriously damages the ecological environment and may cause casualties. Therefore, the rapid and reliable monitoring is particularly important. However, due to severe noise and dense fringes, traditional InSAR methods often severely underestimate the deformation rate. Here, we propose a new processing flow and develop two deep convolutional neural networks for fast detection and phase unwrapping of local subsidence cones. The proposed method is applied to Datong City, Shanxi Province, which is rich in mining activates. The processing results verify the reliability of the method.

Staggered synthetic aperture radar (SAR) faces the challenge that a high oversampling ratio is required to avoid degradation in image quality at the cost of increased range ambiguities and data rates. This paper suggested that the opportunities for low-oversampled staggered SAR arise from novel pulse repetition interval (PRI) variation and advanced signal processing. We proposed a stepwise PRI variation scheme that spreads the blind ranges more evenly and benefits the estimation of the missing information. We combined deramping, spectral estimation, and linear Bayesian estimator to improve the recovery accuracy of the blockage data. Quasi-stationary signals with slowly changing spectra are obtained after deramping. Then the linear Bayesian estimator with the estimated spectrum as the prior information is applied for blockage recovery. Simulation results have verified that the proposed method has better performance than existing methods due to its adaptation to ground scattering characteristics.

For synthetic aperture radars (SARs), an alternate transmitting mode based on waveform diversity is widely discussed for suppressing range-ambiguity in many letters, because it is easy to implement and there is no need to improve the pulse repetition frequency (PRF). These studies mainly focus on the discussion of pseudo-orthogonal waveform design, such as up-down chirp waveforms and orthogonal-frequency-division-multiplexing (OFDM) waveforms; however, the effect on imaging caused by waveform diversity is ignored. This letter, for the first time, provides a demonstrative derivation of imaging for the alternate transmitting mode, which will deepen the understanding of this mode and be helpful for the future research. In this letter, we point out that transmitting different waveforms will introduce a phase-amplitude periodic modulation in the azimuth domain; furthermore, it will cause the aliasing of the azimuth spectrum. In addition, the simulation experiment is performed for verifying the correctness of the theoretical analysis.

Phase synchronization is one of the key issues that must be addressed for the bistatic synthetic aperture radar (BiSAR) system. LuTan-1 (LT-1) is an innovative spaceborne BiSAR mission based on the use of two radar satellites operating in the L-band to generate the global digital terrain models in the bistatic interferometry mode. An advanced synchronization scheme is used for the LT-1 system. The synchronization pulses are exchanged immediately after the ending time of the radar echo-receiving window and before the starting time of the next pulse-repetition interval. Therefore, it cannot interrupt the normal SAR data acquisition, further improving the synchronization accuracy and avoiding the data missing effect. In this letter, a robust phase-error estimation and compensation method is proposed to improve the accuracy of the synchronization by using the Kalman filter. The test data acquired from the ground validation system of the LT-1 synchronization module are used to demonstrate the feasibility of the proposed scheme. The results validate the effectiveness of the proposed scheme and prove the promise for its future application in LT-1.

It is well known that the nonlinear frequency modulation (NLFM) chirp waveform can shape advantageously the power spectrum density (PSD) such that the autocorrelation function exhibits reduced sidelobes as the window function. However, differences between the PSD and window function due to the Gibbs effects caused by the Fresnel integral would lead to deteriorative performance of the NLFM chirp waveform. In particular, the correlation function of NLFM waveform with Chebyshev PSD is seriously inconsistent with the lowest sidelobe level of the Chebyshev window function possesses. To overcome the inconsistency, therefore, in this letter, a novel NLFM waveform with modified Chebyshev window PSD is proposed, which combines the Chebyshev with edge distortion compensation, allowing, in theory, low sidelobe level as Chebyshev window. Using theoretical analysis also confirmed by simulation, this letter shows that the novel NLFM waveform possesses low sidelobes without high computational complexity in design.

LuTan-1 (LT-1), i.e., TwinSAR-L, mission is an innovative spaceborne bistatic synthetic aperture radar (SAR) mission that is based on two satellites operating at L-band with flexible formation flying, which is planned to launch in 2020. The primary objective of LT-1 is to generate a highly accurate global digital elevation model (DEM). Beyond that, LT-1 will serve for the demonstration of some state-of-the-art technologies in radar field and some applications, such as biomass inversion, disaster forecasting, and climate and environmental monitoring, and phase synchronization is a technical challenge in realizing the highly accurate topography and deformation measurements. The pulsed alternate synchronization scheme, which is proposed to solve the synchronization problem of TerraSAR-X add-on for Digital Elevation Measurements (TanDEM)-X, is an efficient and accurate method. However, it interrupts the normal work of the TanDEM-X, accordingly flowing a series of problems. For LT-1, a novel synchronization scheme, in which the phase synchronization signal is exchanged by virtue of a time slot between radar signals, is applied. Thus, the working efficiency and synchronization accuracy can further be improved. Furthermore, the performance prediction and phase synchronization experiment for this synchronization scheme is presented, which verifies the feasibility of the proposed scheme. Finally, in order to guarantee the transmission quality of the synchronization signal, the illumination combinations and gain variation of the synchronization antenna in an orbital period are detailed.

LuTan-1 [(LT-1), i.e., TwinSAR-L] mission is an innovative spaceborne bistatic synthetic-aperture radar (BiSAR) mission focusing mainly on differential interferometry, which will be launched in 2020. This article introduces some important aspects of the LT-1 mission for the first time, including the formation configuration, imaging mode, application scenarios, and the efficient baselines between the master satellite and the slave satellite. To realize the high accurate topography and deformation measurements, a wide swath BiSAR focusing algorithm with phase reserving ability should be developed. This article proposes a modified bistatic range-Doppler algorithm based on 2-D principle of stationary phase spectrum, which reduces the phase error introduced by the root term expansion and has an excellent focus performance and a good phase reserving ability. Finally, the spaceborne bistatic simulation experiments using orbital parameters and imaging mode of the LT-1 mission, including point targets and scene targets, are utilized to illustrate the validity and accuracy of the proposed algorithm.

This article establishes a multiobjective optimization model to suppress the azimuth ambiguity power and noise simultaneously in signal reconstruction for a multichannel synthetic aperture radar (SAR) system. This multiobjective optimization model extends the theory of multichannel signal processing for reconstructing the SAR signal from the aliased signals. Linear scalarization and a quadratically constrained method for the multiobjective optimization model are applied to obtain l1norm optimization, l2norm optimization, and quadratically constrained optimization, respectively, in signal reconstruction. Azimuth ghosts can intuitively reflect the effects of azimuth ambiguity on SAR images. The l1norm optimization solution leads to a minimum upper bound of azimuth ghosts. A lowest azimuth ambiguity-to-signal ratio (AASR) can be derived by l2norm optimization. By relaxing the constraint of total ambiguity power suppression, one can obtain a minimum noise level in the case of quadratically constrained optimization. The reconstruction performances of the multiobjective optimization model in terms of AASR, signal-to-noise ratio (SNR), and signal-to-ambiguity-plus-noise ratio (SANR) are investigated with respect to the pulse repetition frequency (PRF) and compared with other methods for a multichannel SAR system.

LuTan-1 (LT-1) refers to an innovative mission of spaceborne bistatic synthetic aperture radar (BiSAR) adopting the phase synchronization scheme of pulse exchange, which is scheduled to be launched in 2020. In this mission, the multipath effect caused by the reflection of satellites and Doppler frequency shift resulting from the relative motion between satellites constitute two critically latent factors deteriorating the performance of the phase synchronization scheme. This article details a loss factor to evaluate the fading of synchronization signals under the reflection of smooth and rough surfaces and estimates the influence of Doppler frequency shift on synchronization signals. Furthermore, the multipath channel impulse response is modeled, and the correlation characteristics of multipath channel are described by the coherent bandwidth and coherent time. In addition, two simulation experiments based on the system parameters of LT-1, are implemented to assess the influence of time-invariant and -varying multipath effects on the phase synchronization scheme. A scale experiment is also executed to quantify the influence of multipath effect on phase synchronization, and its result demonstrates that the phase synchronization scheme of LT-1 can not only achieve full coverage of space and backup capability of pulse exchange but also meet the application requirements under multipath effect.

Digital beamforming (DBF) with Scan-On-Receive (SCORE) in elevation is a powerful technique for spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide-swath (HRWS) images. DBF in spaceborne SAR should be implemented in real-time to reduce the volume of the downloaded data. Current digital beamformers in SAR systems usually use phase shift instead of a time delay network to ensure real-time implementation. However, conventional phase shift DBF SAR with narrow array bandwidth is not applicable for broadband signals. The beam steering deviation as a function of frequency is obvious for signals with large fractional bandwidth, which appears as a phenomenon of frequency dispersion. If the signal bandwidth is wider than the array bandwidth, frequency dispersion can significantly deteriorate the signal-to-noise ratio (SNR) and must be well relieved. This article analyzes the frequency dispersion in DBF SAR and proposes an innovative DBF-SCORE scheme with specially designed scalloped beam. The proposed digital scalloped beamforming (DSBF) scheme is effective in relieving the influence of frequency dispersion in DBF SAR. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme. The proposed DSBF scheme gives practical solution to the problem of frequency dispersion in broadband DBF SAR, which makes DBF-SCORE a more practical technique for future spaceborne HRWS SAR.

In high-resolution spaceborne Synthetic Aperture Radar (SAR) system design, spotlight and sliding spotlight imaging mode are generally applied. As the increasing resolution and swath length in azimuth, the required scan angle of the antenna pattern become larger. However, due to curved orbits of satellites, the scan speeds and the steering angles are spatial variable. In this condition, the traditional steering method with one dimensional uniform scannning speed is inadequate. Therefore, in this paper, the two dimensional scanning regulation of steering angle of antenna pattern is analyzed in detail. All the analysis and simulation results demonstrate the particularities and show the required two-dimensional nonlinear scanning angles for high-resolution spaceborne SAR system.

Multichannel in azimuth synthetic aperture radar (SAR) operating in sliding spotlight imaging mode serves as an promising tool to achieve very-high resolution and wide-swath imaging of the earth surface. This paper investigates the aspect of phase mismatch calibration for multichannel sliding spotlight SAR imaging, and an extended azimuth cross correlation method is proposed to deel with the influence of beam rotation in azimuth. Processing with experimental data obtained by a C-band dual-channel sliding spotlight airborne SAR shows the effectiveness of channel phase mismatch cancellation.

Landslide, the main geological hazards, needs to be periodically monitored for disaster forecasting and prevention. An effective method to do this is to apply the offset tracking technique on SAR satellite data. Successful applications of this technique depend on the appropriate estimation window. However as a shortage, the selection of estimation window often relies on rich processing experience or multiple attempts. In this paper, we proposed an optimal estimation window offset tracking (OEW offset tracking) technique. As an improvement, this technique provides a theoretical basis and a work-flow to design the optimal estimation window for offset tracking, considering both measurement accuracy and processing efficiency. This technique was applied to the Baige landslide, based on GF-3 data and achieved successful results.

It is well known that the nonlinear frequency modulation (NLFM) waveform with the advantage that it can shape the power spectral density (PSD) to provide a radar matched filter output with lower sidelobe without the loss of signal-to-noise ratio (SNR) when compared with the linear frequency modulation (LFM) waveform. But NLFM waveform would also broaden the main lobe and reduce the range resolution. In this paper, we report a novel waveform optimization framework. Through this framework, an advanced nonlinear frequency modulation (NLFM) waveform with lower sidelobes and a smaller main lobe is constructed. In addition, we apply it in a real synthetic aperture radar (SAR) system with a bandwidth of 100 MHz at 9.6 GHz carrier frequency and the imaging results validate the proposed NLFM waveform.

The phase synchronization is one of the key issues to be addressed for the bistatic synthetic aperture radar system. In this paper, an advanced non-interrupted phase synchronization scheme is proposed. Both satellites are equipped with four synchronization antennas for a mutual exchange of synchronization pulses, which are transmitted rightly after the radar signal transmitting and before the echo receiving. Therefore, it can not interrupt the normal SAR data acquisition, which can further improve synchronization accuracy and avoid the data missing effect. The ground validation system for TwinSAR-L synchronization module is described in detail. The results are also evaluated to demonstrate feasibility of the proposed scheme.

A novel imaging algorithm is presented in this paper for focusing the very-high resolution spaceborne synthetic aperture radar (SAR) data of spotlight mode. First, the first step of two-steps processing approach (TSPA) is used to get a high azimuth sampling rate which is higher than PRF for the raw data. Then, the "fast-time" effect of stop-and-go approximation is corrected in 2-D frequency domain. Finally, the back-projection algorithm (BPA) is used to correct the error introduced by the curved orbit and the "slow-time" effect of stop-and-go approximation. The simulation results confirm that the proposed method has good performance. The spaceborne SAR data acquired by Gaofen-3 SAR systems demonstrate the feasibility of the proposed method.

Synthetic aperture radar (SAR) tomography (TomoSAR) is well established for three-dimensional (3D) reconstruction in urban buildings. There are many methods usually require a large data stacks for a reliable reconstruction and very few methods to handle small data stacks. This paper proposes a new processing method for 3D reconstruction of tall buildings using extremely small data stacks with large baseline interval. In this method, it is necessary to select persistent scatterers (PSs) with high signal-to-noise ratio (SNR) as reference points and estimate the elevation ambiguity issue by using the geometric relationship of SAR imaging. This paper presents the experimental results and shows a well performance of the proposed method in 3D reconstruction of tall buildings with simulated data and SAR data acquired by Chinese spaceborne SAR GF-3 satellite.

TwinSAR-L (Terrain Wide-swath Interferometric L-band SAR mission) is an innovative space-borne bistatic SAR mission for global dynamics, which will be launched in 2020. This paper investigates ionospheric effects on phase and Faraday rotation of interferometry for TwinSAR-L systems. This ionospheric offset arises from different incident angles along each path in inhomogeneous ionosphere. Plus, the inhomogeneity of ionospheric TEC will cause different range delay and defocusing due to dispersion and azimuth shift. The analysis in this paper will be important for TwinSAR-L mission and potential Tandem-L mission in the future.

Intermittent sampling deceptive jamming is widely used in electronic warfare because of its similar characteristics to echo signal and higher gain after two-dimensional matched filtering. In this paper, a novel range intermittent sampling deceptive jamming suppression method is proposed based on azimuth phase coding (APC), which is originally used to suppress range ambiguity. Utilizing the feature that deceptive jamming is generated by intercepting the previous pulse and repeating after at least one or more pulse repetition intervals, an azimuth phase modulation and demodulation are employed to the transmitted signal and received signal respectively. Finally, the interference can be separated in doppler domain, then removed by azimuth filtering rather than azimuth defocusing as conventional suppression methods do. Simulation results show the suppression effectiveness and superiority of the proposed method.

The GaoFen-3(GF-3) satellite is the first full-polarized synthetic aperture radar (SAR) imaging satellite of china, which was launched in August 2016. This paper obtained the geographic information based on the actual data from GF-3 satellite. Range-Doppler Model is used to process GF-3 data to verify the different between the position accuracy after correction and the practical positioning accuracy. The difference provides a certain reference for domestic satellites to promote actual positioning accuracy.

TwinSAR-L (Terrain Wide-swath INterferometric L-band SAR) is an innovative spaceborne bistatic SAR mission, whose primary objectives are achieving a global DTM dataset with high accuracy and observing the land deformation using differential InSAR technology within short revisit time.This paper presents a general methodology for the end-to-end raw data simulation for the spaceborne interferometry SAR systems, which requires correct terrain-mapped phase preserving, including the monostatic and the distributed systems. This paper describes the raw data generation method and its application in TwinSAR-L development phase.

High-resolution wide-swath (HRWS) synthetic aperture radar (SAR) images are valuable for disaster and environment monitoring. Digital beamforming (DBF) in elevation is a powerful technique for future HRWS SAR. However, real-time DBF processing requires massive digital resources, which are precious for spaceborne SAR. The intermediate frequency (IF) process scheme of DBF SAR is advantageous in reducing the required digital resources. Since DBF processing is performed before digital demodulation in IF DBF SAR, the channel imbalance of the system must be compensated with IF real signal. That will be quite different from the conventional scheme and has never been researched. This paper gives solution to this problem based on the IF process scheme without adding to the system complexity. China's next generation airborne DBF SAR (IDBSAR), operated by the Institute of Electronics, Chinese Academy of Sciences (IECAS), will serve as a test bed.

This paper presents a 3.6 GHz X-band wideband airborne SAR system, featured by full-bandwidth transmitting and receiving. After the general introduction, the architecture, inter-connect design and system composition are discussed in detailed. To avoid the main-lobe distortion and asymmetrical side-lobe, the pre-distortion signal is constructed in time domain to compensate the system errors. The results of the experiment were accomplished successfully and validate the effectiveness and applicability of this airborne SAR system.

Range ambiguity suppression is a technical challenge for current synthetic aperture radar systems. A potential solution is to orthogonally modulate the transmitting pulses; however, these orthogonal waveforms (e.g., up-down chirp waveforms) actually cannot reduce the cross correlation energy (CCE). Nonlinear frequency modulation (NLFM) waveform can change the time-frequency relationship to adjust the energy distribution within the bandwidth to reduce the CCE. In this letter, a novel orthogonal NLFM waveform optimization framework is proposed. Through this framework, advanced NLFM waveforms with low sidelobe and CCE are constructed. Furthermore, point and distributed scene simulation results are presented to verify the practicability of the proposed waveforms. In addition, the system scheme, waveform design, and range ambiguity suppression performance are detailed.

Generally, synthetic aperture radar (SAR) system transmits linear frequency modulation (LFM) signal to obtain the high-resolution image and weighted windowing is usually employed to suppress sidelobes. However, it will cause a 1-2-dB signal-to-noise ratio (SNR) loss. Nonlinear frequency modulation (NLFM) signal, which can construct the signal's power spectral density (PSD) to reduce sidelobes without loss of SNR, is a promising candidate. However, the real-time generation of precise NLFM signal is still a technical challenge. In this letter, a high-precision NLFM signal generator with the ability of predistortion compensation is developed, and this signal generator will be employed in LuTan-1 (LT-1, i.e., TwinSAR-L) mission which is an innovative spaceborne bistatic SAR mission and planned to launch in 2020. In addition, a two-step error compensation method is developed to compensate the system error. Finally, the ground experiment is performed to validate the designed signal generator.

In the multichannel synthetic aperture radar (SAR) systems, unavoidable channel errors will significantly degrade the performance of the ambiguity suppression. To address this problem, a channel phase error estimation method is proposed in this letter. First, the multichannel echo model and the weighted backprojection algorithm are introduced. The channel phase errors are then estimated by maximizing the image intensity using the gradient descent method. The simulation results confirm that the proposed method has higher accuracy and robustness than the orthogonal subspace method. The airborne SAR data acquired by a four-channel strip map C-band airborne SAR system demonstrate the feasibility of the proposed method.

Atmospheric artifacts and phase unwrapping errors have unfavorable effects on differential synthetic aperture radar interferometry (DInSAR) deformation monitoring. In this letter, we present an alternative small-baseline DInSAR inversion algorithm, velocity-constraint L1-norm minimization. The proposed algorithm improves the robustness of time series deformation estimation by combining a smoothing constraint and L1-norm minimization. The smoothing constraint can minimize temporal atmospheric artifacts, and the L1-norm minimization outperforms L2-norm minimization in the presence of phase unwrapping errors. The iteratively reweighted least square algorithm is employed to adjust the weights of DInSAR observations and smoothing constraints in L1-norm minimization. The proposed algorithm is validated using simulated data and TerraSAR-X data. The experimental results show that the proposed algorithm is suitable for the inversion of approximately linear deformation processes affected by both atmospheric artifacts and unwrapping errors.

Bistatic and multistatic synthetic aperture radar (SAR) can greatly improve system performance in various aspects, especially in achieving high-resolution wide-swath (HRWS) images. However, the specific system structure may lead to a higher degree of range ambiguities. This letter studies the structure of multistatic multiple elevation beams (MMEB) system which achieves HRWS images with the drawback of more serious range ambiguities compared with the conventional SAR which must be suppressed to get an acceptable system performance. The character that the receivers form, a prospective distributed multichannel system in azimuth, can give a guide in estimating the strong range ambiguities and remove them. An innovative method in estimating and removing (not just smearing) the strong range ambiguities based on this character is proposed. Theoretical analysis and experimental results show the effectiveness of the method. The proposed method considerably improves the range ambiguous performance of the MMEB system without using digital beamforming or pulse coding and makes full use of the system character, and thus improves the applicability of the system.

With the development of high-resolution radar satellite for global comprehensive environmental monitoring, day-and-night and all-weather surveillance has become an active and growing research field. However, in all cases, these applications require radar to have a high-efficiency radar module (e.g., T/R module), and high system transmitting power. These requirements may put an important limitation on the performance of a radar satellite with a high-power configuration. In this paper, we report a novel waveform optimization framework. Through this framework, an advanced nonlinear frequency modulation (NLFM) waveform with lower sidelobes and a smaller main lobe, which can significantly relieve the restriction of very limited satellite power, is constructed. In addition, we apply it in a real synthetic aperture radar (SAR) system with a bandwidth of 100 MHz at 9.6-GHz carrier frequency and the whole process of the NLFM waveform for radar imaging is discussed in detail, including the system architecture and configuration, a system error compensation method, and a modified chirp scaling algorithm (CSA). The imaging results demonstrate the excellent performance of the advanced NLFM waveform. Moreover, we observe that the SAR system with the advanced waveform has a higher signal-to-noise ratio (SNR) of 1.29 dB compared with the conventional linear frequency modulation (LFM) waveform. The improvement of 1.29-dB SNR means that the real radar system can reduce transmitting power with a ratio of 25%. This effect is likely to be a potential feature of NLFM waveform, which can reduce the transmitting power requirement, especially for radar satellite.

Multi-channel synthetic aperture radar (SAR) in azimuth can overcome the minimum-antenna-area constraint of the conventional SAR in high-resolution and wide-swath (HRWS) imaging. However, the SAR system suffers from amplitude and phase mismatch among channels and nonideal antenna pattern, which will result in azimuth ambiguity and ghost targets in the final image. Therefore, taking the nonbandlimited signal and channel errors into account, a practical azimuth ambiguity-to-signal ratio (AASR) model of multi-channel SAR system is established. Meanwhile, the baseband Doppler centroid (DC) frequency related to channel errors also has an influence on image quality. Then, an effective method is proposed to calculate the baseband DC frequency according to the jumping points of the channel phase errors estimate. Subsequently, considering the effect of azimuth antenna pattern (AAP), a corresponding relationship between the ideal steering vectors and the signal subspace from the decomposing covariance matrix is established. After that, based on the uniqueness of the signal subspace and the correct corresponding relationship, an accurate method is proposed to estimate the channel phase errors by minimizing the minimum mean square error (MMSE) of the signal subspace. Finally, an accurate multi-channel SAR imaging diagram is shown to effectively mitigate the azimuth ambiguous energy caused by channel errors. Simulation and real data experiments, including four channel airborne SAR data with a bandwidth of 210 MHz and the Chinese Gaofen-3 dual receiving channel (DRC) spaceborne SAR data, validate the effectiveness of the proposed calibration method, particularly in low signal-to-noise ratio (SNR).

A novel algorithm of strong clutter suppression for spaceborne dual-channel SAR/GMTI is proposed in this paper. First, the adaptive two-dimensional (2D) channel calibration is performed to calibrate difference between channels. Then, the amplitude correction is performed cell by cell to balance channels amplitude. At last, the refined phase correction based on the selected strong clutter is performed to correct phase error between channels. Through the process, channel error is corrected. Clutter, especially strong clutter, is well suppressed. The validity of the proposed algorithm is verified by GaoFen-3 SAR real data.

The next generation of spaceborne synthetic aperture radar (SAR) remote sensing systems will emphasize on high-resolution and wide-coverage imaging. For these design goals, variable pulse repetition interval (PRI), which is also called staggered SAR, is a promising candidate to deal with the inherent problem of blind range effect in conventional SAR with constant PRI. This letter presents an improved work of signal processing to cope with two unavoidable problems in staggered mode: the loss of echo signal and nonuniform sampling in azimuth. Since the existing signal processing methods are mostly based on a high oversampling ratio, which would increase the range ambiguity to signal ratio and the amount of data to be downlinked, this letter focuses on signal processing with low oversampling factors. A two-step processing scheme - recovery and reconstruction - is proposed to recover the missing sampled data using the spectral-estimation techniques first and then reconstruct the in-band energy and minimize the ambiguity energy, which is from band unlimited SAR raw data, to obtain uniform sampling in azimuth. Point target simulations and artificially gapped data generated from China's S-band spaceborne SAR system HJ-1-C are shown to validate the proposed scheme.

In this paper, an advanced X-band airborne SAR is introduced for demonstrating two-dimensional Digital Beamforming (DBF) techniques. The advanced system consists of 64 channels split into 4 column in azimuth and 16 rows in elevation. This airborne experiment system serves as a test bed for the development, implementation and testing of digital beamforming techniques for future High-Resolution Wide-Swath (HRWS) SAR instruments. The set up and functions of this experiment system are presented in the paper. The flight campaign for this airborne SAR system will be undertaken by Institute of Electronics, Chinese Academy of Sciences (IECAS) in 2017.

The multitemporal interferometric synthetic aperture radar (InSAR) technique is a potential tool for measuring digital elevation models and surface deformation. It has the advantage of high precision, competitive spatial resolution, and wide coverage. To improve the accuracy of the final results, some adaptive multilook strategies have been proposed in which the identification of statistically homogeneous pixels (SHPs) is the key task. However, these methods are not always reliable in the case of small data sets. To improve this reliability, SHPs are identified based on the adaptive joint data vector comprising of temporal sample and spatial information in this letter. Additionally, the formulation of adaptive joint data vector is combined with local spatial features of SAR images. The presented adaptive multilook approach can be used in many interferometric applications, such as InSAR data filtering and coherence estimation. Experiments on six TerraSAR-X stripmap images of Tianjin in China validate the feasibility and effectiveness of the proposed approach.

This letter presents a methodology for urban area mapping with density-based spatial clustering of applications with noise (DBSCAN) using the Advanced Synthetic Aperture Radar (ASAR), Sentinel-1A, and HuanJing-1C data. Urban areas have a diversity of shapes, including circles, squares, strips, and other irregular shapes, and the DBSCAN clustering algorithm is suitable for identifying clusters of arbitrary shapes. Exploiting DBSCAN to extract urban areas is a key aspect of this method, and improvements via the incorporation of synthetic aperture radar data preprocessing and postprocessing also play important roles in optimizing the extractions. Different test site sizes were chosen to demonstrate the effectiveness and feasibility of the proposed method, and the validation results showed that the method is efficient and accurately extracts urban areas ranging from small towns to super metropolitan areas.

Due to the light weight and small size, synthetic aperture radar (SAR) sensors with light-small aircraft are very sensitive to atmospheric turbulences, which cause serious trajectory deviations. Limited by the cost or payloads, light-small aircraft sometimes cannot carry high-accuracy inertial navigation systems (INS)/global positioning systems (GPS). For the very high resolution (VHR) case, residual range cell migration (RCM) often exceeds several range resolution cells during the course of the synthetic aperture, thus degrading the image quality substantially. In this paper, a robust scheme for the correction of residual RCM is presented. The scheme consists of three principle steps. First, the range-compressed data in the signal domain are divided into subapertures in azimuth. For each subaperture, a sliding window is used to select the subblock data with the highest signal-to-clutter ratio. Second, for each subaperture, residual RCM is estimated by using the selected subblock data based on a nonparametric entropy minimization technique. Third, the estimated residual RCM of all subapertures are filtered and coherently combined to perform the correction for the full-aperture data. Processing the results of VHR SAR raw data shows that the proposed scheme is effective for highly precise imaging with light-small aircraft equipped with only low-accuracy INS/GPS.

Synthetic aperture radar (SAR) images require a high-resolution system for accurate interpretations. This high range resolution can be achieved by stepped frequency chirp signals. To reconstruct a wideband waveform from each subband signal, amplitude/phase/delay imbalance between the channels should be precisely compensated. In this paper, the system configuration is first presented. Further, a calibration strategy was proposed based on three calibration loops: the reference calibration, transmitting calibration, and receiving calibration loops for coarsely compensating the channel imbalance. Then, two different methods based on the cost functions were proposed to remove the residual channel imbalance. The proposed methods were validated using stepped frequency SAR data acquired by an X-band airborne SAR system with a total bandwidth of 3.6 GHz, yielding (unweighted) a 3-dB range resolution of 4 cm.

In the processing of high-resolution wide-swath spaceborne synthetic aperture radar data, the slant range history is difficult to formulate with a high accuracy over a long data-acquisition interval, and the variation of the imaging parameters over the illuminated scene makes it necessary to implement 2-D space-variant processing. To deal with these issues, first, a slant range expression is derived based on a geometry model using a circular arc to approximate the curved trajectory in this paper, which is composed of a hyperbolic term, a cubic term, and a quartic term. Then, the imaging flow based on the slant range model is proposed: a cubic filtering in range is employed to normalize the range-variant secondary range compression; the nonuniform fast Fourier transform (NUFFT) is used to correct the additional range displacement introduced by the cubic filtering and the nonlinear range cell migration (RCM) caused by the range dependence of the velocity; necessary modifications are made on the matched filter for azimuth compression to accommodate the cubic filtering in range and to deal with the high-order terms of the slant range model. The simulation results show that the proposed slant range model has a relatively high accuracy; the range processing procedure that involves an NUFFT following a cubic filtering can apply to the case where the nonlinear chirp scaling algorithm is adopted, and it has the advantage of being capable of dealing with the nonlinear RCM; the azimuth processing procedure can provide satisfactory focusing qualities over a rather large azimuth extent.

According to the characteristics of the Doppler history in ScanSAR mode, a novel ship detector is presented based on the raw echo data. The proposed approach is very efficient, since it does not need the computational imaging process. Its effectiveness is validated by an airborne ScanSAR real data set, collected by a C-band experimental system.

The next generation of space-borne synthetic aperture radar (SAR) systems will emphasize on high-resolution and wide-swath imaging. For these design purposes, multichannel technology in azimuth is a promising candidate. In the multichannel SAR system, the nonuniform azimuth signal needs to be reconstructed when the pulse repetition frequency (PRF) deviates from a specific one. The traditional digital beamforming (DBF) algorithm works well just in the cases of uniform sampling and moderately nonuniform sampling. If there is highly nonuniform sampling or coinciding sampling, this method may fail. In this paper, a new method to handle the cases of highly nonuniform sampling is proposed. The proposed method is based on an equivalent sampling strategy and the minimum mean square error (MMSE) criterion. It is robust over a wide range of PRF. Compared to the conventional algorithms, this method dramatically improves the SNR, which is validated by simulation experiments.

It has been proven that SqueeSAR technique has validated the potential to increase the density of measure points (MP) involving persistent scatter (PS) and distributed scatter (DS) candidate in nonurban areas. In SqueeSAR, DS candidate exhibiting high extended temporal coherence can be processed jointly with PS for deformation estimation after phase estimation by the phase triangulation algorithm (PTA). The PTA extracts phase values of DS by using all possible interferograms under the Gaussian scattering assumption. However, the statistics hypothesis, Gaussian random variable, is no more applicable, with radar resolution increasing. More precisely, it has been shown that clutter in high resolution SAR images can be modeled as a compound Gaussian process. In this letter, a modified approach to phase estimation of DS named MPTA is proposed based on the compound Gaussian model. Experiments on real data are presented to demonstrate the effectiveness of the proposed method.

Statistically Homogeneous Pixels (SHPs) selection is a significant step of multi-temporal interferometric synthetic aperture radar (InSAR) for Distributed Scatterers (DS). A series of studies namely, Anderson-Darling test (AD test) and its variants, have demonstrated their advantages. However, these algorithms have a similar drawback that they put little attention on the spatial amplitude distributions and cost too much time of processing. To solve the problem, this paper proposes a modified statistically homogeneous pixels selection algorithm (MoSHPS). It utilizes the amplitude values to get the prior information of the images through an unsupervised classifier, in order to guide the SHPs selection. It can improve the accuracy for SHPs selection, and promote the computing efficiency. In the end, results on a series of real TerraSAR-X datas, acquired over a Tianjin area, confirm the effectiveness of this algorithm.

Precise range history model and accurate focusing algorithms are challenges for high-resolution spaceborne Synthetic Aperture Radar (SAR) due to the curved orbits of satellites and spatial variations of signal models. The existing range models include HRM, AHRM and DRM. This paper focuses on the range history model for spaceborne SAR. In the paper, an accurate range model based on fourth-order Doppler parameters is proposed to accurately formulate the range history between SAR and targets. Compared with other models, such as DRM4 which is also based on fourth-order Doppler parameters, the proposed range model has much improved accuracy. All the simulation results validate the high precision of the proposed range model.

This manuscript focuses on velocity estimation for synthetic aperture radar (SAR) in high-resolution wide-swath mode. HRWS mode is proposed to overcome the system-inherent limitation, which allows for collecting additional samples under relatively low PRF. Unlike the previous work of SAR ground moving target indication (GMTI) in single channel and multiple channels, the velocity estimation in HRWS GMTI would face two inherent unavoidable problems: the under-sampled data in single channel and non-uniform samples in azimuth. The proposed method is effective, accurate and efficient under these two problems. Some relevant simulations are shown to validate the proposed scheme.

Calibration of the active phased-array antennas for synthetic aperture radar (SAR) is a complicated mission. The phase-encoding method is used to monitor and characterize individual transmit/receive modules of the active phased-array antenna, which has been demonstrated to be an effective method by TerraSAR-X in a spaceborne environment. The error source due to finite phase-encoding setting accuracy is modeled and analyzed, based on which a modification of the phase-encoding matrix is done to improve the measurement accuracy. Furthermore, simulation confirms a significant improvement in the measurement accuracy with the modification, showing an agreement with the model-based theoretical analysis.

Coastline extraction with high accuracy from wide-swath synthetic aperture radar (SAR) imagery is still a challenging problem due to speckle, sea state condition, and land type. This letter presents a novel approach for coastline extraction from SAR images with complex scenarios and large geograp hical coverage, using the combination of modified K-means method and adaptive object-based region-merging mechanism (MKAORM). First, a modified K-means method is used to produce initial oversegmentation for the following region-merging stage. Second, an adaptive and coarse-fine object-based region-merging scheme using subregion classification is exploited to extend the automatically selected “sea” seed and “land” seed, respectively, to extract the final coastline. MKAORM can reduce the high computation cost of coastline extraction from wide-swath SAR imagery while keeping high accuracy. More than 93% of the detected coastlines lie within 2-pixel distance in comparison with the manually traced coastlines in experiments taken on Sentinel-1A (S1A) IW (Interferometric Wide-swath) mode SAR imagery.

A backprojection algorithm (BPA), a time domain synthetic aperture radar (SAR) imaging algorithm, can be widely used without modification in various imaging modes, like stripmap, spotlight, sliding spotlight, etc. However, BPA also suffers when used in azimuth multichannel SAR due to the nonuniform sampling problem caused by a nonideal pulse repetition frequency. In this letter, we propose a weighted BPA (WBPA) for azimuth multichannel SAR imaging. Derived from the filter-bank-based reconstruction method, WBPA adds a weighting procedure to BPA. Simulations and an airborne SAR data experiment demonstrate that WBPA can perfectly suppress the azimuth ambiguities under band-limited circumstances. WBPA extends the applicable scope of BPA.

Statistically homogeneous pixels (SHPs) are considerably significant in many interferometric applications, such as interferometric filtering, distributed scatterer selection, small baseline subset, and SqueeSAR processing. It is very important to achieve SHPs efficiently and accurately. Previous studies on SHPs' selection are based on spatial restrictions and likelihood ratio test, such as Lee filtering, Kolmogorov-Smirnov test, and Anderson-Darling test. However, these algorithms do not stand up to test for the spatial similarity hypothesis for complex terrains with a few images. To solve the problems, this letter proposes a modified SHPs' selection algorithm. It utilizes geometric distance and target features for reaching a priori information to help the similarity hypothesis tests. The proposed algorithm has been tested on simulated and real data to prove the improvements in terms of accuracy and computational efficiency.

For the next generation of spaceborne synthetic aperture radar remote sensing satellites, high resolution and wide coverage are important goals. Digital beam-forming (DBF) with multichannels in elevation is a great and promising candidate to cover wide swaths. In this letter, we focus on onboard digital processing for DBF in elevation. It is generally believed that the onboard processing for DBF is challenging due to the limitations in flight-hardware availability, the heavy computational load, and the high resource occupation. In order to reduce the computational load of DBF, one novel processing scheme is proposed. This proposed scheme performs a modified time-variant weighting on a real intermediate frequency signal of each subchannel, and the weighted signals of all channels are summed to two real data streams. To obtain a correct DBF output, a modified quadrature demodulation process is presented. Then, the scheme is extended to apply FIR filters for overcoming pulse extension loss. Furthermore, improved time-variant weighting coefficients are derived to compensate the phase errors brought by the FIR filtering process. Compared with the present processing flow, the proposed scheme could significantly reduce the computational load and resource occupation.

Range sidelobe suppression in synthetic aperture radars (SARs) is conventionally realized using amplitude weighting with windowing functions in either time or frequency domains, which would result in a reduced signal-to-noise ratio (SNR) of the output. To counterbalance the loss, a system with more complexity and cost is needed, especially for spaceborne SAR. Fortunately, nonlinear frequency modulation (NLFM) chirp waveforms, which can shape the signal's power spectral density and offer radar matched filter output with lower sidelobes at no cost of reduced SNR, is a promising candidate. In this letter, the proof-of-principle experiment is presented to construct NLFM transmitting pulses using a real SAR system platform on the ground and analyze the characteristics of the pulses. Two approaches are developed to perform system-specific predistortion and the advantage of NLFM waveforms with better SNR is demonstrated using the theoretical derivation and experimental results. For spaceborne NLFM SAR, the effect of Doppler shift on NLFM SAR imaging cannot be neglected. Therefore, for further promoting the application of NLFM in SAR, one effective compensation approach is developed. All the experimental results and analysis validate the promising potential of NLFM for SAR application.

For synthetic aperture radar (SAR), a system impulse response with low sidelobes is very important because sidelobes may interfere with the nearby scatterers and contribute to multiplicative noise. It is well known that a nonlinear frequency-modulation (NLFM) chirp waveform can shape the signal's power spectral density and provide a radar matched filter output with lower sidelobes without loss of the signal-to-noise ratio when compared with the linear frequency-modulation chirp. These advantages make the NLFM waveform to be a promising candidate to improve the imaging quality for SAR. However, so far, to our knowledge, there is no real application of NLFM waveforms for SAR. This letter, for the first time, demonstrates the airborne SAR experiment using an NLFM waveform. In the underlying experiment, the construction of the NLFM signal is investigated and a modified range migration algorithm (RMA) is developed to adapt it for focusing the NLFM SAR data. Both simulation and experimental results exhibit the promising power of the NLFM chirp and show the accuracy of the proposed modified RMA.

For spaceborne synthetic aperture radar systems, minimizing the Doppler centroid variation over the range swath may bring many benefits, including improving the accuracy of the Doppler centroid estimation, alleviating the complexity of the data processing, and so on. In the current literature, it is well known that the Doppler centroid for the broadside-looking mode can be reduced to 0 Hz by employing 2-D attitude-steering law which includes a yaw and a pitch steering. However, for the squint mode, 2-D steering is not enough. In this letter, the iso-Doppler surface is first derived using the motion state vectors of the Earth and the satellite, based on which the 3-D attitude-steering strategy including an additional roll steering is proposed to minimize the Doppler centroid variation. Finally, the effectiveness of the proposed strategy is validated by the simulation results when the antenna pointing error is either absent or present.

The next generation of spaceborne synthetic aperture radar (SAR) remote sensing systems will emphasize on high-resolution and wide-coverage imaging. For these design goals, digital beamforming (DBF) in elevation is a promising candidate. DBF-SAR can provide global monitoring capacity for the continuous observation of a highly dynamic and rapidly changing world with high spatial resolution and short repeat intervals. A spaceborne experiment regarding a real complex scene and real spaceborne wave propagation channel effects remains a necessary step to complete the experimental verification of this advanced technique. Fortunately, the spaceborne-stationary bistatic configuration offers a potential chance to validate the advanced technique. The aforementioned experiment can be considered as a test bed for the development and implementation of DBF radar techniques applicable to Earth observation science and planetary measurements. The DBF experiment based on spaceborne-stationary bistatic configuration with TerraSAR-X as an illuminator has been successfully conducted in June 2013 by the Department of Space Microwave Remote Sensing System, Institute of Electronics, Chinese Academy of Sciences.

This paper addresses some important aspects for the spaceborne/stationary bistatic synthetic aperture radar (SAR) (SS-BiSAR) imaging with the transmitter, TerraSAR-X, operated in staring spotlight (ST) mode. With the large integration time reaching 7.5 s and the azimuth steering span reaching ± 2.2°, several significant effects occur, including troposphere delay, precision phase and time synchronization, the curved orbit effect, azimuth spectrum aliasing problem, and efficient frequency domain focusing algorithm. To circumvent the main effects, corresponding solutions are proposed, including a precise synchronization strategy with troposphere delay correction based on the direct signal from the transmitter and a modified and integrative bistatic polar format algorithm (PFA). This paper covers the theoretical development, implementation, and analysis of the SS-BiSAR PFA based on 2-D fast Gaussian gridding nonuniform fast Fourier transform with wavefront curvature correction. Furthermore, the high-resolution ST-mode SS-BiSAR image processed by the proposed algorithm is acquired, and the differences of scattering behaviors between monostatic and bistatic SAR images are analyzed in detail.

This paper proposes a synchronization algorithm for bistatic synthetic aperture radar (BiSAR) imaging in a spaceborne/stationary configuration. In real bistatic systems, synchronization errors are generally introduced into the received data. Additionally, the lack of precise imaging parameters, such as the position of the transmitter and the accurate sampling time, could affect the imaging quality greatly. Fortunately, the image could be well focused by the proposed algorithm in the case of lack of the accurate position of a transmitter and the sampling time. First, a preprocessing step is employed to remove synchronization errors through matching an echo signal with a direct signal. Then, a modified chirp scaling factor containing an error phase term is constructed, and the accurate position of the transmitter and the sampling time can be acquired by the phase extraction of the direct signal and the searching method based on contrast optimization. After that, the corresponding imaging process can be implemented. Finally, the proposed algorithm is validated by the simulation and experimental results, where TerraSAR-X is used as the illuminator.

Wide area surveillance (WAS) radar can monitor a large area repeatedly and acquires information of moving targets appeared in interesting area. In this paper, a novel algorithm of moving targets detection and parameters estimation is proposed for dual-channel WAS radar. The paper analyzes parameter ambiguous problem, and presents the corresponding solution. The effectiveness of the proposed algorithm is demonstrated by both the simulated data and real data.

In this paper, a stepped‐frequency spaceborne/airborne bistatic synthetic aperture radar (SFSA‐BiSAR) configuration is proposed to show to improve the range resolution of BiSAR. First, the geometry and the signal model of SFSA‐BiSAR are formulated, and then an analytical bistatic point target reference spectrum (BPTRS) is derived. Second, based on the developed BPTRS, two imaging algorithms called combination‐before‐focusing algorithm (CBFA) and combination‐after‐focusing algorithm (CAFA) are proposed to obtain BiSAR images with improved range resolution. CBFA uses sub‐band combination to obtain synthetic BPTRS, and then uses a new bistatic frequency‐domain algorithm (BFDA) to obtain the improved BiSAR image, whereas CAFA first uses the BFDA to focus all the sub‐pulses, and then uses sub‐image combination to obtain the improved BiSAR image. Both of the two algorithms can focus the SFSA‐BiSAR signal well and comparison simulations are performed to verify the authors' conclusion. Computation loads are also calculated to compare the two algorithms and show that CAFA uses less floating‐point operations than CBFA. Besides, some important parameters of SFSA‐BiSAR are analysed both theoretically and numerically to provide a reference for system design.

This letter presents a new filtering technique for interferometric synthetic aperture radar (InSAR) phase images. Traditional local denoising algorithms all suffer from the drawback of removing texture detail information. In contrast, several nonlocal methods have attained good performance in InSAR applications. However, these methods only take radiometric similarity. The patch-based locally optimal Wiener filter (PLOW) utilizes both geometrically and radiometrically similar patch information by clustering analysis and nonlocal filtering; thus, it can better balance between detail preservation and denoising. Nevertheless, PLOW itself is not fitted for InSAR. In this letter, we modify and improve the original algorithm while considering the coherence coefficient and the characteristics of InSAR. This new method provides better filtering results, but with a higher computational cost. Moreover, we introduce the box dimension in fractal geometry as a new index. Experiments on simulated and real data demonstrate that this algorithm outperforms traditional denoising methods.

This letter focuses on a modified complex-valued Markov random field (CMRF) modeling filter to improve the performance in the filtering of the synthetic aperture radar interferometric phase. In the reference CMRF modeling phase map filter, the CMRF model was employed to update residues and their neighbors. By this, the residues were reduced, and phase jumps were preserved simultaneously. However, residue reduction was still insufficient. Furthermore, incorrect model parameter estimation leads to wrong filtering in some blocks. To solve the two aforementioned problems, we propose a modified CMRF modeling filter, where the original interferogram is divided into overlapped blocks. In addition, the adaptive weighted neighbor values are used to estimate the CMRF model parameters. Both simulated and real data experiments are performed to validate this method.

In this letter, an improved full-aperture imaging algorithm for scanning synthetic aperture radar (ScanSAR) mode is proposed, which fills the data gaps between bursts through a linear-prediction-model-based aperture interpolation technique in a subaperture manner before azimuth compression. It can significantly suppress the spikes induced by periodical data gaps and, at the same time, enhance the signal-to-noise ratio of the obtained ScanSAR imagery. This approach has a great potential in the interferometric context. The effectiveness of the proposed approach is demonstrated by both simulated and real ScanSAR data with different types of terrain. All the experimental data were acquired by the C-band SAR system with a bandwidth of 200 MHz, which was developed by the Department of Space Microwave Remote Sensing System, Institute of Electronics, Chinese Academy of Sciences.

In order to enable a testbed for spaceborne scanning synthetic aperture radar (ScanSAR) mode, in this letter, a ScanSAR autofocusing approach for airborne platforms has been developed. Autofocusing algorithms, such as the phase gradient autofocus (PGA) algorithm, prove to be a useful postprocessing technique to get refocused synthetic aperture radar images. However, conventional stripmap PGA does not work in ScanSAR mode when the full-aperture approach is used, due to the periodic data gaps in each subswath. To solve this problem, we extend the stripmap PGA to ScanSAR with some modifications, mainly in the subaperture segmentation and phase error estimation steps. The performance of extended stripmap PGA is evaluated by an airborne ScanSAR data set containing different types of terrain, with a high spatial resolution up to 3.5 m in azimuth.

The traditional spectrum reconstruction algorithm for nonuniform sampling of a displaced phase center antenna (DPCA) synthetic aperture radar (SAR) system can provide an unambiguous recovery of an original signal through the inverse computation of a joint filter matrix. However, when the sampling positions of different receivers coincide spatially, the joint filter matrix will be non-invertible, thus causing the ineffectiveness of the algorithm. This letter proposes a regularized spectrum reconstruction scheme when the pulse repetition frequency of a multichannel system operates on these critical frequencies, as well as in their neighborhoods. First, a low-order regularization of the joint filter matrix is suggested so that the original spectrum of the multichannel system can be recovered more accurately with a decreased number of nonredundant channels' measurements. Furthermore, in order to improve the output signal-to-noise ratio (SNR) of the reconstruction network, a combinational low-order spectrum recovery scheme is additionally presented, which takes all redundant channels' samples into account. The optimizations bring significant benefits to both the suppression of aliased ambiguities and the advance of the output SNR of the reconstruction network.

In this letter, an approach based on spectral diversity and conventional Interferometric Synthetic Aperture Radar (InSAR) for Digital Elevation Model (DEM) reconstruction is proposed. Conventional InSAR technique often fails to correctly reconstruct the DEM due to the presence of discontinuities and/or interferometric noise. With the assistance of the spectral diversity method, the ill-posed phase-unwrapping can be regularized. Thus the capabilities of the conventional InSAR technique for DEM reconstruction can be notably improved. Comparing to the multi-channel InSAR technique, the proposed method requires less SAR images. Comparing to the multifrequency technique, the performance of the proposed method depends on the perpendicular or effective baseline and the ratio of the radar's carry frequency to the bandwidth between the central frequencies of the two sub-looks; while the multifrequency technique depends more on the range bandwidth. Thus, the proposed method is more flexible than the multifrequency technique. The effectiveness of the proposed algorithm is validated by experimental results using real data.

In this paper, an adaptive patch-based filter for Synthetic aperture Radar (SAR) Interferometric phase in the Fourier transform complex domain is proposed. The proposed filter fully utilizes the properties of locally planar terrain phase noise model so that the noise can be suppressed according to the local level of noise. Moreover, the denoising performance of the proposed filter is further improved by an adaptive selection strategy for patch size. The experimental results show that the proposed method delivers state-of-the-art denoising performance in terms of objective criteria.

A novel autofocusing technique is developed for image from stripmap-mode synthetic aperture radar (SAR) data. The approach is based on maximizing the image sharpness function that induces the solution to maximum-posterior estimation. In this letter, closed-form expressions are derived for the gradients of the sharpness function with respect to the coefficients of the polynomial expansion, which makes the use of conjugate gradient algorithm available. Additionally, we also design a modified adaptive-order searching strategy, and it helps to remarkably reduce the computational load while maintaining the accuracy. Real airborne SAR data experiments and comparisons demonstrate the validity and effectiveness of the proposed algorithm.

A fully polarimetric response of landslide areas at X-band was studied by a Chinese high-resolution airborne synthetic aperture radar system. Polarimetric decompositions, including the Yamaguchi four-component decomposition and the Cloude decomposition, are used to analyze the scattering mechanisms of several typical landslides caused by the 2008 Wenchuan Earthquake in southwestern China. The experimental results indicate that areas affected by large-scale landslides show complicated scattering mechanisms at X-band, which are a mixture of surface, double bounce, and volume scattering. Simple classification results based on supervised Wishart classifier and polarimetric scattering similarity parameters are also presented, which can distinguish landslide areas from others, such as forest and water, very well. However, it does not perform well for urban areas. Additional information, such as prelandslide imagery, is needed to distinguish landslide areas from urban areas or bare soil. From these results, we can conclude that landslide mapping using fully polarimetric data has great potential for rapid response and management of landslide disasters.

Interferometric inverse synthetic aperture radar (InISAR) can achieve 3-D imageries of maneuvering target. However, the quality of 3-D imageries suffers from the noise and sidelobes seriously. In addition, the defocusing effect due to the nonlinear moving of the target can also bring into a bad effect for 3-D imaging. To tackle these issues, a 3-D InISAR imaging approach via compressed sensing (CS)-based super-resolution (SR) ISAR imageries is proposed. First, we establish the target sparsity-constraint optimization model containing both amplitude and phase information of scatterers. Second, a Bayesian CS approach is adopted to get high-quality SR ISAR image. The quasi-Newton solver guarantees both high amplitude and phase information recovery precision. At last, a high-quality 3-D view of the target is achieved via the conventional ISAR imagery pair interferometry technique. Computer simulation and real data experimental results verify the effectiveness of the proposed method.

Both in quality-guide phase unwrapping algorithms and weighted minimum-norm phase unwrapping algorithms, quality maps play a crucial role in obtaining the absolute phase from the wrapped ones. In this letter, a new technique for generating quality maps based on the gray level co-occurrence matrix (GLCM) is proposed. GLCM is a classical second-order statistics method for analyzing the texture features of images. Through exploring the second-order statistics of GLCM, much useful information in the image can be exploited. According to the characteristics of the interferogram, the second-order statistic of GLCM called “difference of entropy” is used to generate the quality maps. Besides, we modified the definition of “difference of entropy” to make the statistic more suitable for the problem. Finally, the new algorithm is compared with other conventional algorithms both in the simulated and real data experiments and the results show its better performance.

This letter extends the “two-step” focusing approach to sliding spotlight (SS) synthetic aperture radar (SAR) with stepped frequency chirps, where two major points should be looked out. One is the slant-range variation among one group of subpulses introduced by the radar platform position changing. In the stripmap mode, an effective approach to compensate this variation is to multiply each subband echo with a phase ramp in the Doppler domain. Herein, we apply this approach to the SS SAR. The other is the residual Doppler bandwidth after azimuth deramping with a fixed reference function. To remove this residual bandwidth, a range-frequency-dependent reference function (FDRF) combined with azimuth zero-padding was developed in a spotlight SAR with a single subband. In this letter, we extend the FDRF to the multiple-subband SS SAR, where the FDRF also depends on the subband carrier frequency. The main contribution of this letter is that the chirp z transform is employed to acquire an identical azimuth output pixel interval for different range frequencies and different subbands. All the aforementioned processing and the bandwidth synthesis are imbedded into the “two-step” focusing approach, which is validated by simulation results.

Conventional frequency-domain frequency-modulation continuous-wave (FMCW) synthetic aperture radar (SAR) raw data simulators (RDSs) can only handle the case of static targets while the scene with moving targets cannot be accommodated. In FMCW SAR, the motion of the radar and moving targets during the pulse duration must be considered, thus posing a challenge for the FMCW SAR RDS. An accurate point target reference spectrum that considers this motion is first derived in the literature. Based on the spectrum, we design an accurate and efficient FMCW SAR RDS. Compared with its counterparts, it can precisely simulate the raw data of moving targets even in the case of high resolution and high squint. Both point target and extended scene simulations are performed to validate the simulator, and its efficiency is also analyzed. Results show its outstanding performance for generating the FMCW SAR raw data of extended scenes with static and moving targets.

In this letter, we propose an approach for multichannel spaceborne/stationary synthetic aperture radar (SAR) interferometry based on maximum a posteriori (MAP). Spaceborne/stationary SAR is a typical bistatic SAR configuration. In order to solve the phase disconnection problem while working with a low signal-to-noise ratio and a limited number of baselines as well as having large look angle variations, we use the height estimation results derived from iterative multibaseline unwrapping as the initial heights for the MAP estimation. The method presented here is highly experimental. An experiment is carried out to verify the effectiveness of the proposed approach. In this experiment, TerraSAR-X, working in the high-resolution spotlight mode with a 300-MHz bandwidth, acts as the transmitter. The receivers with three echo channels are placed on the ground to receive the reflected waveform. As proof of concept, we demonstrate the height estimation of several buildings.

This letter presents an accurate range model, named as Doppler parameter range model (DRM), to perform signal processing for geosynchronous synthetic aperture radar (Geo-SAR). Besides that a credible approach, defined as accurate coordinate numeric model (ACNM), is proposed to simulate the raw echo signals. To improve the accuracy of DRM and ACNM, the rotating Earth is modeled as an ellipsoid and the eccentricity of the satellite's orbit is analyzed. The superiority of DRM that it is more precise than hyperbolic range equation (HRE) and advanced hyperbolic range equation (AHRE), presented before, is illuminated. The point target 2-D frequency spectrum of DRM is derived. Finally, the simulation results validate that DRM and ACNM has a good effect on an L-Band Geo-SAR system with an azimuth resolution around 5 m.

High-resolution and wide-swath synthetic aperture radar (SAR) images can be obtained by multichannel sampling. However, a nonoptimum pulse-repetition frequency is associated with a nonuniform spatial sampling in azimuth. A spectrum reconstruction algorithm and a filter group algorithm can be used to reconstruct the spectrum in azimuth with a nonuniform sampling. In this letter, a comparison of spectrum reconstruction algorithm and filter group algorithm is given. Through a transformation matrix, the relationship between out-of-band signals and the out-of-band signals after reconstruction is built. Furthermore, an equivalent spectrum after reconstruction is obtained, and simulation results proved that it is precise. From the equivalent spectrum, it shows that multichannel SAR can be seen as uniform sampling system but with a different original signal. Finally, channel imbalance is discussed. Simulation results have proved that the imbalance targets have the same location with corresponding ambiguous targets.

As a novel mode of ground-based synthetic aperture radar (GB-SAR), Arc frequency-modulated continuous wave (FMCW) SAR is rarely discussed in the literature up to now. Compared with the conventional rail GB-SAR system, the synthetic aperture is formed by the rotation of antennas, which can scan a wide azimuth extent. Due to its convenience and potential in SAR applications, the Institute of Electronics, Chinese Academy Sciences, carried out a series of Arc FMCW SAR experiments in March 2013, and a lot of results were obtained. In this paper, we discuss the signal processing of Arc FMCW SAR and its first results in change detection and interferometric SAR. The components of the Arc FMCW SAR system are first described, and then, we focus on its signal processing, where the signal model and its properties are investigated; two focusing algorithms are developed for different purposes. The effectiveness of the algorithms is validated by both simulation and real data experiments. We also exhibit the applications of Arc FMCW SAR in landslide detection and digital-elevation-model extraction for the first time. Results show its huge potential in ground-based applications.

High-resolution spaceborne synthetic aperture radar (SAR) mainly poses two challenges to signal processing. The first challenge involves the signal model, where a precise range equation of spaceborne SAR should be considered as the conventional hyperbolic range equation fails to precisely describe the range history in the high-resolution case. The second challenge is an efficient focusing algorithm since the existing SAR processors are inaccurate or inefficient for high-resolution spaceborne SAR. Therefore, in this paper, a novel fourth-order polynomial range equation based on Doppler parameters is proposed, and the method for parameter determination is also addressed. Compared with conventional range equations, the presented one is more accurate and concise for low-earth-orbit SAR so that a higher azimuth resolution can be achieved. Based on the range model, a 2-D spectrum is derived, and an extended range Doppler domain algorithm for SAR image formation in the sliding spotlight mode is also developed. Additionally, we carried out several simulations to validate the presented approach. Results demonstrate high performances of the focusing algorithm as well as the range equation.

Nonnegative eigenvalue decomposition (NNED), which insists and guarantees that each decomposed scattering component corresponds to a physically realizable scatterer, is powerful for polarimetric synthetic aperture radar (SAR) images analysis. Previous NNED is mainly illustrated under the reflection symmetric condition. In this paper, the coherency matrix approach is derived to implement the NNED for the nonreflection symmetry scattering case. We explicitly show the diversifications of the decomposition results between NNED with and without reflection symmetry assumptions, and quantitatively analyze the differences between them using the E-SAR polarimetric data acquired over the Oberpfaffenhofen area in Germany.

This paper analyzes the signal properties of spaceborne squinted sliding-spotlight synthetic aperture radar (SAR). Both the squint angle and the azimuth beam steering during the whole acquisition interval will lead to the Doppler spectrum back-folding. According to the special signal properties of this mode, a new full-aperture focusing approach, which includes three major processing steps, is proposed. In this approach, the first processing step introduces an azimuth convolution and azimuth data mosaic to resolve the azimuth spectral folding phenomenon by generalizing the existing two-step focusing technique for conventional sliding-spotlight SAR data focusing. Afterward, the modified range migration algorithm is adopted to process the resulting raw data. As the azimuth time duration of the raw data is obviously reduced in the first processing step, the obtained SAR image may be back-folded in azimuth. The final azimuth postfiltering step is to resolve the possible aliased SAR image without any azimuth data extension. The proposed full-aperture focusing processor is efficient since only a limited azimuth data extension is required to resolve the back-folded Doppler spectrum and SAR image. Imaging results on simulated raw data validate the proposed imaging approach.

Besides the evident superiority, the Geosynchronous Earth orbit synthetic aperture radar (Geo-SAR) system meets great more challenges than the current Low-Earth-Orbital SAR (Leo-SAR) and the Medium-Earth-Orbit SAR (Meo-SAR). The signal processing approach for spotlight mode Geo-SAR is discussed in this paper. A new range model called the sixth-order Doppler parameter range model (DRM-6) is derived based on the accurate sixth-order Doppler parameter calculation algorithm. The accuracy of Hyperbolic Range equation (HRE) and the DRM-6 are compared. According to the non-linear property of the synthetic aperture, the Back projection algorithm is chosen for image formation. At last the simulation results validate that DRM-6 has a good effect on the L-Band spotlight Geo-SAR system with a azimuth resolution around 2m.

This letter introduces the innovative multichannel sliding spotlight synthetic aperture radar (SAR) system for the acquisition of ultrahigh resolution radar images with wide swath coverage. The instructive system design guideline and the signal processing scheme for the proposed operational mode are exhibited. When deriving the azimuth reconstruction algorithm for the multichannel sliding spotlight mode, a novel squinted geometric model is employed, in which the variable antenna steering direction is taken into account. In addition, an in-depth quantitative analysis of system performance for the presented mode is carried on. Finally, a typical exemplary spaceborne SAR is designed based on the presented strategy, from which numerical simulation results are obtained to justify our derivations.

Interferometric phase denoising is a crucial step of interferometric synthetic aperture radar processing flow because it has significant influence on the following steps. Traditional interferometric phase denoising algorithms have a similar drawback that they will wipe off some texture details in phase images while denoising. Nonlocal (NL) means filter, in contrast, can reach a balance between denoising and texture preserving because it utilizes the feature of recursive structures in the whole image. Taking the characteristics of interferometric phase image into consideration, this letter proposes a modified NL means filter algorithm for phase denoising. Moreover, in order to preserve texture to the biggest extent, an iterative algorithm is invented. In the end, experiments on synthetic and real data validate that this algorithm outperforms other traditional denoising methods.

Multiple-azimuth-channel (MAC) synthetic aperture radar (SAR) systems can overcome the minimum-antenna-area constraint. However, channel phase imbalance and range sampling time error can degrade the quality of the final image. To deal with these two problems, a calibration method based on the azimuth cross correlation is presented in this letter. The proposed method applies the azimuth cross-correlation operation after transforming the received signals of each channel into range frequency domain, where the phase of the cross-correlation function is linear with the range frequency. We obtain the range sampling time error by testing this line's slope, and then the phase mismatch can be derived. Simulation experiments and real data processing results validate the proposed method.

In this letter, we suggest using multiple azimuth channels (MACs) for synthetic aperture radar in the sliding spotlight mode. First, we analyze the scene size constraints of conventional sliding spotlight mode, showing that a large scene size requires long scene illumination time. However, while using multiple channels in azimuth, this could be changed. Dividing a long antenna into small antennas, we can increase the azimuth resolution improvement fact, where the range size will not be reduced. Therefore, with the same illumination time, the azimuth imaging width can be enlarged for a given azimuth resolution. However, using the MAC sliding spotlight mode, the nonuniform sampling problem occurs. Unlike the processing in the MAC stripmap mode, in the MAC sliding spotlight mode, the unambigous reconstruction should be performed after the deramping in azimuth. Finally, an example is given to validate the proposed new configuration and the corresponding algorithms.

In order to enhance the target recognition probability in the inverse synthetic aperture radar (ISAR) imaging domain, the interferometric ISAR (InISAR) mode is presented to achieve the 3-D information of a target. However, the real data results of a maneuvering target are not enough, due to the difficult signal processing. In this letter, a combined processing approach is proposed to realize high-quality 3-D imagery of a maneuvering target. In the approach, the range alignment and phase adjustment are implemented together on echoes to avoid destroying the coherence of the echoes. Then, the high-quality 3-D InISAR imagery of the maneuvering target can be reached. Real data results are provided to confirm the effectiveness of the proposal.

In this letter, a fast backprojection algorithm (FBPA) is proposed for bistatic synthetic aperture radar imaging. For range compression, the signal recorded by the synchronization channel of the receiver on the ground is used as a matched filter to compress the echo signal. This method can reduce the requirements of satellite-orbit measurement and synchronization precision. For azimuth compression, a secondary phase correction is implemented to reduce the effect caused by the approximation involved in the FBPA. The computational complexity of the proposed algorithm is O(N2.5). The proposed algorithm is applied in a graphics processing unit and verified by simulation and real raw data.

This letter presents a motion compensation solution for high-resolution automobile frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR). The motion error model is proposed, and the characteristics of the motion error in automobile FMCW SAR are analyzed by comparing with that of airborne SAR. Furthermore, an effective compensation approach is presented, which can handle motion error while keeping the data well focused in the case of high resolution. Finally, we validate the formulated motion error model and compensation method by using simulated FMCW SAR data as well as the real data that were acquired by the Institute of Electronics, Chinese Academy of Sciences. The results show that the proposed method can work well for automobile FMCW SAR.

This letter presents a novel approach for extracting moving targets in a multichannel wide-area surveillance radar system. In the algorithm, after proper preprocessing and matrix combination, the combined matrix of radar echoes can be regarded as the superposition of three matrices, namely, a low-rank matrix of ground clutter, a sparse matrix of moving targets, and an entry-wise matrix of noise component. Then, the recently proposed relaxed version of principal component pursuit is used to realize ground clutter (low-rank matrix) and moving target (sparse matrix) separation under the influence of entry-wise noise. Both simulation and real data processing results are provided to demonstrate the effectiveness of the proposed method. In addition, the results show the advantage of the proposed method in a nonhomogeneous environment when compared with a reduced-dimension space–time adaptive processing method.

Interferometric synthetic aperture radar (InSAR) multichannel system, which produces more than one interferogram in a multifrequency or multibaseline configuration, allows us to reconstruct highly sloped and discontinuous terrain height profiles. In this letter, a novel method based on a closed-form robust Chinese remainder theorem (CRT) is presented to solve the height reconstruction problem. Proper reference remainder selection and remainder differential process, which do not exist in the traditional CRT, are adopted to improve the robustness of the height reconstruction. We also modify the method by exploiting statistical characteristics of interferometric phase and the combined information of InSAR interferograms. Moreover, a special weighted mean filtering method is adopted to get a better result. Experimental results prove the effectiveness of the method.

In the traditional processing flow of interferometric synthetic aperture radar (SAR) technique, the processing of phase is conducted via two separated and successive steps, i.e., phase denoising and phase unwrapping. That is to say, first, wrapped phases without noise are generated, and then, the true phases without 2π-ambiguities are reconstructed (here and in the rest of this paper, true phase refers to the information-induced unwrapped phase without noise). Such separated steps will inevitably bring in extra estimation error because each step has necessary approximations and presumptions which do not always hold. On the contrary, in this paper, we treat phase denoising and unwrapping as a single problem of true phase recovery from observed ones. Following this methodology, an integrated phase denoising and unwrapping algorithm based upon Markov random fields (MRFs) is proposed. Taking a priori knowledge of interferometric phases into account, MRF is used to model the relationship between the elements in the random variable set including both true phases and their observations. After the model is built up, the energy function of this MRF is defined according to the local-independence property inferred from the MRF structure and then minimized to obtain the estimate of the true phase value. In the end of this paper, experiments on simulated and true phase data are conducted, and the comparison with several commonly used unwrapping methods is proposed to verify the efficiency of the proposed MRF algorithm.

The flexible geometry configuration of the bistatic synthetic aperture radar (SAR) has many advantages. However, it causes serious measurement error in the bistatic SAR system, which degrades the quality of the SAR images and the precision of the digital elevation model (DEM) obtained using stereoscopy bistatic SAR. In this paper, the influence of the scene height estimation error, trigger delay, transmitter position measurement error, receiver position measurement error, and transmission line length measurement error are analyzed. These analyses are very useful in bistatic SAR system design. The scene height estimation error, trigger delay, transmitter position measurement error, and synchronization receiver position measurement error affect both the quality of the images and the precision of the DEM obtained by stereoscopy bistatic SAR slightly. The echo receiver position measurement error and transmission line length measurement error affect the quality of the imaging only slightly, but seriously affect the precision of the DEM obtained by stereoscopy bistatic SAR. Luckily, their measurement precision can be quite satisfactory. Simulations and real bistatic experimental results verify the proposed theoretical analysis.

In the above titled paper (ibid., vol. 51, no. 8, pp. 4518-4543, Aug. 2013), some important details concerning the used data, experimental area, and cooperation partner are missing by accident. The details are presented here.

This paper presents a double-channel hybrid bistatic synthetic aperture radar (SAR) system. It can be implemented with the available illuminator (e.g., spaceborne and airborne SAR systems) to acquire the 2-D and 3-D microwave images for remote-sensing applications. This system can be used as a test system for the validation of complex bistatic acquisitions, novel synchronization algorithms, and advanced imaging techniques. In this paper, we will demonstrate the time and phase synchronization strategies, fast time-domain imaging algorithm, 2-D bistatic SAR, and 3-D bistatic stereo radargrammetry in SAR images. Based on the proposed bistatic system, the acquired bistatic image has a much better sensitivity than its monostatic counterpart, which will facilitate the detection and recognition of targets based on their characteristic and bistatic scattering behaviors. Finally, the experiment results highlight the differences between monotatic and bistatic SAR images.

The inherent limitation between azimuth resolution and range swath width in conventional spaceborne synthetic aperture radar (SAR) systems can be overcome by introducing the displaced phase center antenna technique. In these SAR systems, echoes from all subapertures should be reconstructed and combined together before single-channel SAR processors. However, in the multichannel sliding spotlight and Terrain Observation by Progressive Scans (TOPS) modes, azimuth beam progressive sweeping during the whole acquisition interval leads to that the total Doppler bandwidth spans over several N ·PRF intervals, where N is the number of azimuth channels and PRF is the pulse repetition frequency. As a result, conventional azimuth multichannel reconstruction algorithms for the stripmap mode are not directly suitable for sliding spotlight and TOPS modes. This paper proposes a novel imaging processor for both modes according to their azimuth echo properties. The key point of the proposed focusing processor is the first processing step of multichannel azimuth data reconstruction, which extends the two-step focusing technique to process raw data of the azimuth multichannel case. In addition to azimuth data preprocessing and accurate range cell migration correction steps, the final postprocessing step is added to correct the possible back-folded SAR images. Imaging results on simulated raw data validate the proposed imaging approach.

Interferometric phase denoising is a crucial step of interferometric SAR processing flow. Its performance has great effect on the following steps. Traditional interferometric phase denoising algorithms have a similar drawback that they will wipe off some texture details in the phase image at the same time of filtering noise out. Nonlocal mean filter, in contrast, can find a balance between denoising and texture preserving because it utilizes the recursive structures from the whole image. Taking the features of interferometric phase image into consideration this paper proposes a modified Non-local mean filter algorithm to solve this problem. Moreover, in order to preserve texture to the biggest extend, an iterative algorithm is invented. In the end, experiments on synthetic and real data validate that this algorithm outperforms other traditional denoising methods.

A MIMO-SAR framework based on frequency diversity is presented. The variety of carrier frequency causes the phase modulation and the change of FM rate in azimuth. They will bring spurious responses in the final radar imaging. The influence of phase modulation is overcome by selecting an appropriate sampling rate to the range signal. When the range cell migration can be neglected, we can assume the azimuth signal as the result of a multi-channel sampling system, and then the influence of FM rate changing is eliminated by Doppler spectrum reconstruction. The feasibility of the imaging algorithm is validated by simulation experiments.

The quality of Synthetic Aperture Radar image is affected by the deviation of radar platforms from the nominal straight line which is usually measured based on hardware such as Inertial Measurement Unit and Global Position System with high accuracy. Because of limited budget, many SAR systems cannot carry those expensive equipments. Based on the 3-D Geometry model of the motion error analysis, a new method of aircraft motion error extraction from the SAR raw data is proposed in this paper. To apply this estimation method, we first extract the range cell migration trajectories of strong targets from range compressed data before range cell migration correction procedure. Second, with the help of pseudo-inverse method, the motion errors in line of sight direction will be calculated using the extracted trajectories. Finally, experiments on simulated data validate this proposed method.

A new operating mode of synthetic aperture radar (SAR), referred to as Mosaic mode, can simultaneously obtain high azimuth resolution and wide swath. In Mosaic mode, the radar beam scans in the range direction while the strip line is advanced by steering antenna beam in the azimuth direction. The imaging bursts constituting Mosaic mode have the same acquisition geometry with squint spotlight mode. When the steering angle is large, the effect of the skew of two-dimensional (2-D) spectrum cannot be neglected. In this paper, a modified two-step approach is proposed to focus the data of Mosaic mode. It can remove the azimuth spectrum folding effect caused by the skew of the 2-D spectrum. Simulation results confirm the validity of the proposed algorithm.

Frequency-Modulated Continuous-Wave (FMCW) Synthetic Aperture Radar (SAR) combines FMCW and SAR techniques which makes a lightweight, high-resolution and cost-effective imaging sensor which can work all weather, day and night. However, the failure of stop-and-go approximation leads to additional range-azimuth coupling terms in the Point Target Reference Spectrum (PTRS) of FMCW SAR. To exploit the influence of these terms in FMCW SAR circumstances, this paper presents the remarkable differences between the spectrum formulations of FMCW SAR and pulsed SAR, and validates the significance of the additional range-azimuth coupling terms introduced by the variation of slant range during the long pulse durations. Simulation experiments proved that ignoring these terms will cause serious reduction of image quality.

In this paper, we present a modified frequency scaling processing method for FMCW SAR data. Compared to the conventional frequency scaling algorithm for FMCW SAR, an analytical slant model is employed so that it can work well in all conditions for FMCW SAR. Firstly, the algorithm model is deduced in detail and its implementation is given, and then a simulation experiment is carried on to evaluate its efficiency. Finally, real data of FMCW SAR validate the proposed algorithm.

In this paper, a modified fast back projection (FBP) algorithm is proposed for Synthetic Aperture Radar (SAR) imaging. The azimuth compression of the proposed FBP is divided into sub-image process and finial image process. The secondary phase correction is used to reduce the error cause by approximation before the finial image process. The interlace sub-image grid method makes image quality uniform for all the pixel in the finial image. The applicability limitations and the sub-image two dimensions over sample method are presented. The computational complexity of the proposed FBP is O(N2.5). Simulation on GPU is given to verify the proposed method efficacious.

Compared with SAR interferometry(InSAR), polarimetric SAR interfermotry(PolInSAR) acquires plenty information to estimate parameters, and on the other side, the penetrability of low frequency electromagnetic wave is better than high frequency. So an accurate retrieval of the topography can be implemented from low frequency quad polarimetric SAR interfermotry on the basis of the Random Volume over Ground (RVoG) scattering model abstractly. But there is no paper validation about this theory with experiment data from quantitative point of view. In this paper, analysis and comparison of the quantitative topography accuracy between InSAR and PolInSAR, between X-band PolInSAR and L-band PolInSAR are presented firstly with data validation. Simulated data are used to prove the validity.

Interferometric synthetic aperture radar (InSAR) multichannel system, based on the collection of more interferograms acquired in a multifrequency or in a multibaseline configuration, allows us to reconstruct highly sloped and discontinuous terrain height profiles. In this paper, we present a new method to solve the digital elevation model (DEM) reconstruction problem using the closed-form robust Chinese remainder theorem (CRT). This method uses the remainder differential process, which does not exist in the traditional CRT, to improve the robustness of the DEM reconstruction. The simulation results demonstrate that the performance of the proposed method is better than statistical methods such as maximum likelihood (ML) estimation techniques, when the number of independent interferograms does not large enough.

The range equation employed in the current low earth orbital Synthetic Aperture Radar (Leo-SAR) systems is the hyperbolic range equation (HRE). The key parameters of it are effective velocity and effective squint angle. Traditionally the calculation method of the two values may lead to large errors, and it is only fit for Leo-SAR with moderate space resolution. To get the azimuth resolution on the order of one meter or even higher, an improved method based on the accurate second-order Doppler parameters is used in this paper to decrease the errors of the two parameters above. Then we can get the improved hyperbolic range equation (IHRE). The theory is validated by the parameters of TerraSAR-X, and the image with the space resolution around 0.35m is focused perfectly.

The paper proposes a nonparametric method for ground moving-target indication (GMTI) using along-track interferometric synthetic aperture radar (SAR), which does not require the theoretical derivations of the joint probability density function of the interferometric magnitude and phase for both homogeneous and heterogeneous clutter. In the proposed approach, a best-fit curve is set as the detection threshold to separate moving targets from clutter and noise. A special technique, termed sliding-window technique, is then employed to reduce residual clutter and noise. The validity of the method has been demonstrated by the experiments performed on simulated spaceborne SAR data.

Frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) systems offer smaller size and lower cost compared with pulsed-mode systems. They are therefore widely used for Earth observation where frequent revisits at low costs or small sizes are desirable. By accurately formulating the instantaneous slant range during the transmitting and receiving operations, an analytical point target reference spectrum has been developed for the FMCW SAR system, where an additional range-azimuth coupling term is found. Based on previous work, this letter presents a modified inverse chirp-Z transform (ICZT) algorithm to deal with the range-azimuth coupling term, which appears to be a scaling factor in range. The new scaling factor is formulated in this letter for the first time. An ICZT is well suited for FMCW SAR since it can handle a dechirped signal without interpolations, thus reducing the computing load. Simulation experiment and real data processing result validate the proposed focusing algorithm.

Unambiguous wide swath and high azimuth resolution pose contradicting requirements on the design of SAR systems. The requirements can be overcome by using concurrent subbands technology. This paper reviews the concurrent subbands technology for the imaging of wide swath with high range-azimuth resolutions. In the designed system, the concurrent subbands technology is used to integrate a wider range signal bandwidth, and thus achieve a higher range resolution; multichannel allows a lower Pulse Repetition Frequency (PRF) and enables unambiguous wide swath. The synthetic bandwidth method is used to obtain a wide-bandwidth signal, which is described in detail. With the application of the classical Displaced Phase Centre (DPC) technique, more spatial sampling along the synthetic aperture can be used and thus reduce the PRF.

For synthetic aperture radar imaging, stepped-frequency chirp signals are widely used to obtain high range resolution. One advantage of the approach is the reduction of the instantaneous bandwidth and sampling rate requirements of the radar system. To reconstruct the spectrum of a wideband signal, the amplitude and phase discontinuity between contiguous pulses should be removed. Otherwise, the discontinuity will defocus the range profile and degrade the range resolution. In this letter, a detailed internal calibration technique combined with stepped-frequency mode is proposed to eliminate the discontinuity and thus improve the focusing quality of the final image. The simulation and real raw data experiments are performed to validate the proposed method.

This paper presents a new bistatic point target reference spectrum. It is derived by using the 2-D principle of stationary phase which is first applied in the synthetic aperture radar (SAR) community. The spectrum contains two hyperbolic range-azimuth coupling terms and thus is very similar to the monostatic spectrum. It shows the characteristic of the conventional monostatic SAR besides an additional azimuth scaling term. Therefore, it makes the common Doppler-based monostatic processing algorithms readily suitable to handle the Bistatic SAR (BiSAR) data in the moderate-squint azimuth-variant configurations with two moving platforms. Based on the spectrum, two Doppler-based monostatic imaging algorithms [i.e., range-Doppler algorithm (RDA) and chirp-scaling algorithm (CSA)] are readily implemented to deal with the moderate-squint azimuth-variant BiSAR data. Compared to the processing procedure for the monostatic SAR, the RDA and CSA for the BiSAR need only the adjustment of Doppler parameters. Finally, the potential and limitation of the spectrum are analyzed, and the real raw data in the spaceborne/airborne configurations are used to validate the proposed spectrum and processing methods.

A lot of bistatic experiments have recently been performed by different researchers to show the differences between monostatic and bistatic SAR. The acquired data was needed to verify the developed bistatic processing algorithms. To extend the conventional bistatic SAR experiments, we investigated whether bistatic systems can be used in the field of bistatic interferometry and polarimetry, also shown by [3]. For this purpose we have used our stationary receiver system together with TerraSAR-X as transmitting satellite. This paper describes the experiments and shows some results we obtained using single and repeat pass SAR configurations.

The conventional monostatic SAR shows a limitation of achieving a high azimuth (angular) resolution if a forward-looking geometry is used. Bistatic SAR offers a possibility of the forward-looking formation image, e.g. one platform operates as a separated illuminator while another works in a forward-looking mode. In this special bistatic configuration, the forward-looking beam will introduce two problems. The first one is the significant Doppler shift, which will produce the range-azimuth coupling. The second one is that the targets located at symmetrically about the flight path of the forward-looking platform have the same antenna-to-target range which would cause the range migration ambiguity of the receiver. This paper will develop a mathematical model to describe the Doppler characteristic of bistatic forward-looking SAR (BFLSAR). Based on this model, the previous bistatic point reference spectrum (BPTRS) can be applied. Using the BPTRS, a modified range-Doppler algorithm is proposed to handle the two problems and focus BFLSAR data in the azimuth-invariant configuration. Finally, simulation experiment is used to validate the proposed model and processing approach.

In this paper, we present a motion error model for a Frequency-Modulated Continuous-Wave (FMCW) SAR system, and analyze the characteristics of the intrapulse motion error. Furthermore, we develop an effective compensation approach to handle motion errors based on our previous result. Finally, we validate the formulated signal model and compensation method by using simulated FMCW SAR data.

The spatial separation of the transmitter and the receiver in bistatic synthetic aperture radar (SAR) enables a variety of data acquisition geometries to achieve benefits like the increased information content of bistatic SAR data. In the case of hybrid bistatic SAR constellations where the transmitter is spaceborne and the receiver is onboard an aircraft, one has to deal with a huge discrepancy between platform velocities. This paper presents bistatic spaceborne/airborne SAR experiments, where the radar satellite TerraSAR-X is used as a transmitter and the airborne SAR sensor Phased Array Multifunctional Imaging Radar (PAMIR) of the Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR) is used as a receiver. Both sensors are equipped with phased-array antennas, which offer the possibility of beam steering and could be used for the first time for the “double sliding spotlight mode.” In this mode, the space- and airborne sensors operate with different sliding factors (ratio between footprint and platform velocity). The performance of two different experiments is analyzed, and the novel double sliding spotlight mode is presented. This paper describes the experimental setups, the synchronization system, and the data acquisition. The image results were processed by a modified backprojection algorithm and a frequency-domain algorithm. The analysis of the final bistatic images comprises the spatial resolution and the scattering behavior of selected objects. Parts of the bistatic SAR images are compared with the corresponding monostatic images of PAMIR and TerraSAR-X. It will be shown that hybrid bistatic SAR is a worthwhile and helpful addition to current monostatic SAR.

The combination of frequency-modulation continuous-wave (FMCW) technology and synthetic aperture radar (SAR) promises a lightweight, cost-effective, and high-quality imaging sensor for remote sensing. However, the long signal duration time leads to the failure of the conventional start/stop approximation of the pulsed SAR. In this paper, a signal model is proposed to address the effects of the continuous motion during the transmit time on the echoed signal. Based on the model, an analytical point target reference spectrum is derived. From the spectrum, it will be seen that the continuous motion introduces an additional range-azimuth coupling term and a range walk term compared with the conventional pulsed SAR. The range walk term is well known, whereas the foregoing range-azimuth coupling term is formulated for the first time in the FMCW SAR community. For the squint and spotlight modes, these range walk and range-azimuth coupling terms might significantly degrade the image quality. In this paper, based on the proposed analytical signal model, we further discuss the application of the wavenumber domain algorithm for the FMCW SAR data. In addition, different approximations of the Stolt mapping are made to highlight the effect of the range-dependent higher-order range-azimuth coupling terms on the 2-D impulse responses. Finally, X-band simulated experiments and Ka-band real FMCW SAR data are used to validate the signal model and the processing method.

This paper presents a frequency-domain-based focusing algorithm for the bistatic synthetic aperture radar (BiSAR) data in airborne/stationary configuration. In this bistatic configuration, only the moving platform contributes to the azimuth modulation, whereas the stationary platform introduces a range offset (RO) to the range migration trajectories of targets at the same range. The offset is determined by the azimuth position of different targets with respect to the stationary platform. Since the RO is position dependent, monostatic SAR imaging algorithms are not able to focus the bistatic data collected in this configuration. In this paper, an analytical bistatic point-target reference spectrum is derived, and then, a frequency-domain-based algorithm is developed to focus the bistatic data. It uses an interpolation-free wavenumber-domain algorithm as a basis and performs a range-variant interpolation to correct the position-dependent RO in the image domain after coarse focusing. The proposed algorithm is validated by the simulated data and the real BiSAR data acquired by the Forschungsgesellschaft fu¿r Angewandte Naturwissenschaften's airborne SAR system, PAMIR, in December 2007. In this BiSAR experiment, an X-band transmitter was stationary operated on a hill with PAMIR as the receiver mounted on a Transall C-160.

This paper focuses on the bistatic synthetic aperture radar (SAR) data processing in a spaceborne/airborne hybrid bistatic configuration. Due to the extreme differences in platform velocities and slant ranges, the airborne system operates in the inverse sliding-spotlight mode, while the spaceborne system works in the sliding-spotlight mode to achieve a tradeoff between azimuth scene size and azimuth resolution. In this extreme bistatic configuration, our original bistatic formula shows a limitation of accurately describing the bistatic point-target reference spectrum, owing to the assumption of equal contributions of transmitter and receiver to the total Doppler spectrum. We extend our previous formula using the weighting operation where the weighting factor is the ratio of the azimuth time-bandwidth product (TBP) of the platform to the total azimuth TBP. In this paper, the bistatic-deformation and azimuth-dependent range-cell-migration terms were removed with phase multiplications performed blockwise in range-azimuth subsections. The remaining quasi-monostatic term shows the characteristic of the conventional monostatic SAR besides an additional azimuth-scaling term. For the monostatic characteristic, any precision monostatic SAR processing algorithms can handle. In this paper, we prefer the wavenumber-domain algorithm (also known as Omega-K), since it can accurately correct the range dependence of the range-azimuth coupling, as well as the azimuth-frequency dependence. For the azimuth-scaling term, an inverse scaled Fourier transformation is performed to correct it. Finally, a hybrid spaceborne/airborne simulation experiment is conducted to validate the proposed processing procedure.

This paper discusses the processing method for bistatic SAR data in the constant-offset configuration. The constant-offset configuration is also known as the azimuth stationary or invariant configuration where transmitter and receiver follow each other, moving on identical velocity vector. In this paper, the proposed processing method for bistatic SAR data is based on Loffeld's bistatic formula that consists of two terms, i.e., the quasi-monostatic (QM) term and bistatic-deformation (BD) term. Our basic idea is to linearize the aforementioned two terms and then incorporate the BD term into the QM term to obtain an analogous monostatic spectrum. Based on the new spectrum, any efficient 2-D frequency or range-Doppler domain processor can easily be employed to process the bistatic data, where the Doppler phase parameters of the processor need to be adjusted. In this paper, we concentrate on the application of chirp-scaling-algorithm (CSA) processor. In addition, a bistatic-motion error model is developed where the position deviations of the two platforms are simplified as the bistatic slant-range displacement in the zero Doppler plane. Using this model, the monostatic motion-compensation technique is applied and integrated into CSA to compensate the trajectory deviations of transmitter and receiver. Finally, real and simulated data are used to validate the proposed processing method.

Image registration is known as an important part of generating Digital Elevation Models (DEM) with Interferometric SAR (InSAR) processing and is one of the critical preprocessing steps in remote sensing. It is used in the formation of 3-D models based on 2-D images taken at different view points as well as for mosaicking applications. The paper gives an overview of the information measures which can be used for automatic generation of reference points und finding the correspondences in the second image. The approach is validated with simulated and real image pairs originate from the TerraSAR-X satellite. Additionally to the classic correlation based methods, we will employ, compare and combine theoretical methods, based on information theory, such as mutual information, alignability in conjunction with automated bin size determination to achieve optimally. Considering the co-registration of single look complex SAR data, where correlation approaches are standard, the exploitation of similarity measures and transformation, based on information theory, is rather novel. Comparing these novel approaches with standard references with respect to quality and quantitative measures, will broaden the scientific understanding, and, furthermore, provide new insights concerning co-registration techniques for interferometric applications.

In the SAR community there are many methods offered for solving the problem of unwrapping the phase obtained from a noisy interferometric SAR (InSAR) image [2-5]. Generally a Kalman filter is a powerful tool to obtain accurate model based estimates out of different sources of information. All the given information is fused in a efficient way and also the noise is cancelled optimally. Because of this a Kalman filter based data fusion approach to unwrap and simultaneously filter the phases of interferometric SAR images is developed. The data fusion concept exploits phase information, extracted from the complex interferogram rather than from the phase image and fuses that information with phase slope information extracted from the power spectral density of the interferogram. The paper explains a Kalman filter method for phase unwrapping purposes and discusses the pros and cons of this approach.

A crucial problem in most airborne Synthetic Aperture Radar (SAR) systems is the compensation of motion errors to prevent the image degradation. If these errors are not compensated, some undesired effects will appear, such as loss of geometric resolution and radiometric accuracy, reduction of image contrast, azimuth ambiguities and strong phase distortions. For the bistatic airborne SAR systems, the motion errors become more complex since two separate trajectory deviations contribute to these errors. This paper proposes an approach that will compensate the bistatic motion errors in the frequency domain.

The individual azimuth contribution of transmitter and receiver phase terms has been optimized by weighting them unequally based on a weighting factor mu, in Loffeld's bistatic formula (LBF). Analytical results are verified with the simulations.

This paper concentrates on the bistatic SAR (BiSAR) signal processing for the spaceborne/airborne hybrid bistatic configuration. A bistatic SAR experiment based on this hybrid configuration is being planned in cooperation with FGAN/FHR and DLR. Due to the extreme differences of the platform's velocities and altitudes, the spaceborne system works in the sliding spotlight mode, while the airborne system operates in the inverse sliding spotlight mode. In this paper, our previous work (i.e. ISFT) is applied to focus the hybrid bistatic SAR data based on the extended Loffeld's Bistatic Formula (ELBF).

A bistatic point target reference spectrum (BPTRS) based on Loffeld's bistatic formula (LBF) is derived in this letter. For LBF, the same contributions of the transmitter and receiver to the total azimuth modulation are assumed. This assumption results in the failure of LBF in the extreme configuration (i.e., spaceborne/airborne configuration). For general bistatic configurations, the azimuth modulations are unequal for the transmitter and receiver due to the different slant ranges and velocities. Therefore, the azimuth time-bandwidth products (TBPs) from the transmitter and receiver are different; in some cases (e.g., spaceborne/airborne case), one of them might be very small, which might even result in a serious error of the principle of stationary phase. This letter uses TBP to weight the azimuth phase modulation contributions of the transmitter and receiver to the common azimuth spectrum to approximately obtain the point of stationary phase of the total azimuth phase history. Simulations show that the proposed BPTRS can work well for spaceborne/airborne configurations.