The synthetic aperture radar (SAR) is a crucial tool for maritime observation, with ships being the main targets at sea. Detecting these ships is foundational for other downstream tasks, making the study of ship detection algorithms highly significant. Currently, ship detection primarily relies on deep learning algorithms, and training neural networks requires a large amount of data. In various maritime observation tasks, identifying defocused ships is particularly important. However, current databases lack defocused ship data, and SAR ship image generation algorithms are not yet mature. Therefore, this letter proposes generating defocused ship data using the existing ship data. First, we introduce the signal model of defocused ships and the causes of defocusing. Next, we describe the generation of defocused ships with nonuniform rotation by introducing motion errors through resampling. We also explain the generation of defocused ships with translational motion using initial phase compensation and envelope shift. Finally, expansion experiments using spaceborne SAR data demonstrate the effectiveness of our method.

Synthetic aperture radar (SAR), capable of ultra-wide swath imaging, serves as a vital tool for ocean surveillance. Ship detection in SAR imagery holds significant importance for maritime security and traffic management. Current ship detection algorithms exhibit excessive computational complexity, and both conventional and deep learning-based approaches struggle to achieve rapid detection in ultra-wide swath SAR imagery acquired by next-generation SAR systems. This paper proposes a rapid adaptive anchor framework for ship detection. First, the detection criterion statistics are decomposed into multiple prefix sum matrices, ensuring minimal computational complexity for anchor boxes of arbitrary sizes. Second, the image is quadrant-divided, with detection criterion statistics computed separately to determine ship presence in each quadrant. Third, quadrants containing ships undergo iterative quadrant division, while ship-free quadrants are annotated using differential matrices. Finally, the initial anchor boxes are systematically shifted to repeat the second and third steps across the entire image, ultimately generating a sea clutter mask. The computational complexity of the proposed ship detection framework remains equivalent to that of loading the full SAR image, introducing negligible additional overhead to existing systems. The framework is validated using empirical SAR datasets, demonstrating enhanced computational efficiency without compromising detection accuracy.

Range ambiguity is a critical factor degrading the high-resolution and wide-swath (HRWS) imaging performance of spaceborne synthetic aperture radar (SAR), arising primarily from the antenna sidelobe characteristics. Recently, blind source separation (BSS) methods have shown promise in mitigating range ambiguity. However, existing studies have mainly focused on the determined scenario. In contrast, underdetermined cases are often more prevalent in practical settings. To address this gap, this article proposes a novel range ambiguity suppression scheme specifically designed for the underdetermined BSS (UBSS) scenario. Point and distributed targets simulation based on Sentinel-1 system is conducted to verify its effectiveness. The results indicate that for the point target imaging performance of two channels, peak sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR) are improved by an average of 6.62 and 9.47 dB, respectively. In the distributed target case, the separation and recovery of the echo signals in the target region achieve an average similarity (pixel, structure, and cosine metrics) exceeding 94.27%, and demonstrate robustness at signal-to-noise ratios above 25 dB. These findings provide insight into the feasibility of UBSS-based strategies for range ambiguity suppression and offer valuable reference points for future investigations involving single-channel implementations.

The Hongtu-1 (HT-1) synthetic aperture radar (SAR) system is the first spaceborne single-pass multibaseline (MB) interferometric SAR (InSAR) system based on four HT-1 SAR satellites flying in a cartwheel formation. This setup includes three secondary satellites that function as receive-only units to create a compact multistatic SAR system. The primary objective of the HT-1 mission is to generate a consistent global digital elevation model (DEM) at 1:50000 scale. In addition, the HT-1 mission will feature novel SAR imaging technology demonstrations. On March 30, 2023, four HT-1 satellites were successfully launched and started to provide spaceborne radar data services to users. This article provides a detailed description of the HT-1 MB InSAR system, including the SAR performance, the cartwheel formation designed for multistatic SAR data collection with desired baselines, and the uninterrupted synchronization link. The interferometric performance is thoroughly analyzed. With the recorded data, imaging and interferometric processing procedures are introduced, and the capabilities of single-pass MB InSAR DEM generation are demonstrated. Compared with those of ICESAT, the height errors are less than 2 m in flat terrain and less than 5 m in mountainous terrain. Moreover, the resolution and swath of multisatellite mosaic imaging are 3 m and 80 km, respectively. The repeat-pass differential InSAR measurement for surface deformation monitoring is also included.

Ground moving target focusing performance and processing efficiency are two important metrics in the synthetic aperture radar (SAR) system. Owing to the complex nonstationary phases caused by target motions, ground maneuvering targets are commonly smeared and distorted in an SAR image. To realize the ground maneuvering target imaging, exhaustive high-order parameter searching and optimization processing are usually needed to achieve the optimal processing performance. However, the large computational resource consumption may limit their real-time processing capabilities. To deal with this issue, an efficient SAR imaging approach for ground maneuvering targets is proposed by adopting the mixed symmetric function (MSF) transform. After performing the multichannel SAR clutter rejection and target initial detection by applying the constant false alarm rate (CFAR) technique, the linear envelop shift of detected ground target is eliminated by using the well-known Keystone transform (KT). Then, an MSF is constructed to rectify the range curvature and relieve the Doppler broadening after carrying out the nonuniform fast Fourier transform (NUFFT) corresponding to the second-order time variable. After removing the second-order Doppler spreading influence, the quadratic chirp parameter is estimated via the NUFFT after the symmetrical correlation processing. Finally, after accomplishing the motion compensation, the ground maneuvering target can be well focused and relocated in the SAR image. Compared with the traditional approaches, the proposed approach not only realizes the high-precision Doppler parameter estimation, but is also computationally efficient without grid searching procedure. Numerical simulation and real-measured SAR experiment results are depicted to verify the correctness and effectiveness of the proposed approach.

Synthetic aperture radar (SAR) is a crucial technology in marine surveillance. Moving ships are always defocused in SAR images, which affects the following identification process. Inverse SAR (ISAR) technology has been employed to focus moving targets in SAR images, typically involving two steps: translation motion compensation and rotation motion compensation. Range envelope alignment is generally considered a well-established method for translation motion compensation. In rotation motion compensation, current approaches, however, rely on signal parameter estimation and optimization algorithms, which lack robust methods. This article presents a rotational motion compensation framework, named “azimuth envelope alignment,” which extends the range envelope alignment to the azimuth direction. First, a novel concept of “azimuth envelope” is introduced, and the process of azimuth envelope alignment is discussed. Then, a specific method is presented: the use of the iterative adaptive approach (IAA) and resampling for implementation of “azimuth envelope alignment.” This method first estimates the alignment parameter (AP) using IAA, followed by achieving alignment through resampling. This method exploits the high-precision estimation capability of the IAA while avoiding its sparse estimation results, enabling the robust, nonsparse refocusing of moving targets. Finally, simulations and experimental data validate the effectiveness of the proposed algorithm. The azimuth envelope alignment framework introduced in this article can serve as a general approach for moving target imaging, similar to range envelope alignment.

This article proposes a generalized compact polarimetry (GCP) mode along with two optimal polarization mode selection parameters to address the challenges of polarization mode selection in classification tasks across diverse scenarios. Theoretically, we conduct an in-depth analysis of the differences between circular and linear transmit polarizations, demonstrating their fundamental equivalence in terms of information content. For the first time, we propose that different classification tasks require different optimal polarization modes, and the optimal transmit polarization mode may lie in the elliptic polarization domain of synthetic aperture radar (SAR) systems rather than traditional circular compact polarimetric (CP) or linear dual-polarization (DP) modes. The proposed approach is validated using full-polarimetric SAR data from San Francisco and Hainan, showing that the optimal elliptical polarization mode achieves classification accuracies that are 2% to 42% higher than those of traditional CP or DP modes for certain categories, and performs comparably to full polarization. This improvement in accuracy stems from the interaction between the transmit polarization and the target scene, rather than advancements in classification algorithms. Using the two proposed parameters, the overall and category-specific classification performance of GCP modes can be effectively evaluated, enabling the identification of the optimal polarization mode for a given task. These findings provide significant insight into the design of future polarimetric SAR systems and offer new perspectives and directions for mission planning and mode selection for on-orbit satellites.

Synthetic aperture radar (SAR) tomography (TomoSAR) technology effectively provides precise three-dimensional (3D) forest vertical structure information. However, conventional TomoSAR methods require abundant acquisitions for accurate 3D reconstruction, which is time-consuming and low-efficiency for forest vertical structure mapping. To address these limitations, this paper proposes a novel micro-stack TomoSAR imaging approach utilizing four images, referred to as the double iterative adaptive residual approach (DIARA). The DIARA innovatively combines iterative inner-outer adaptive spectral estimation and residual optimization to enhance both processing efficiency and accuracy. For validation purposes, both simulated and airborne TomoSAR experiments are analyzed at P- and L-band. The P-band results from the BorTomoSAR campaign indicate that the DIARA acquires higher accuracy for estimating forest height than the conventional methods, i.e., improvingR2from 0.443 to 0.628, mean absolute error (MAE) from 0.779 m to 0.625 m, mean absolute percentage error (MAPE) from 4.629% to 3.680%, and root mean square error (RMSE) from 0.941 m to 0.771 m. Additionally, the performance is further validated by the TropiSAR P-band campaign, which confirms the robustness of the proposed DIARA in high-canopy, complex forest environments. Furthermore, the L-band results from HaiTomoSAR campaign demonstrate that the DIARA method successfully detects the weak ground scatterers beneath dense forest canopies, which validates its super-resolution capability in vertical structure reconstruction.

Airborne Synthetic Aperture Radar (SAR) Tomography (TomoSAR) technology facilitates the extraction of three-dimensional (3D) information of target scatterers. However, phase screens induced by radar platform trajectory errors causes TomoSAR defocusing, which adversely affects the accuracy of the forest vertical structure retrieval. Additionally, the phase screens exhibit space-variant characteristics, which significantly degrade the calibration performance estimated by traditional phase gradient autofocus (PGA). This paper proposes an improved phase calibration method synthesizing unconstrained optimization model and weighted PGA (WPGA), which effectively address the above challenges. Firstly, the SAR data stack is segmented into multiple subareas by assuming that phase screens are space-invariant within each small area, which reduces complexity and improves computational efficiency. Secondly, a WPGA method synthesizing the scatterer heights derived from an unconstrained optimization model is proposed to estimate the phase screens. Simulation experiments are conducted to validate the effectiveness of the proposed phase calibration method. Furthermore, the full-polarization SAR data stacks acquired by the BorTomoSAR campaign are used for tomographic focusing analysis. Experimental results demonstrate that the proposed method accurately estimates the phase screens at the P, L, and S bands, providing a efficient solution for the forest vertical structure retrieval.

The primary technical challenge for multi-input multi-output synthetic aperture radar (MIMO-SAR) systems is separating independent channel responses from aliased echoes while maintaining imaging performance. However, the most promising short-term shift-orthogonal (STSO) and segmented-phase-code (SPC) waveform require the use of elevation digital beamforming (DBF) to achieve echo separation. The cost of using elevation DBF for echo separation is the loss of elevation degrees of freedom and a significant increase in system complexity. To solve this problem, this paper proposes a novel coded waveform that introduces phase characteristics for echo separation through two-dimensional phase encoding of the transmitted waveform in both elevation and inter-pulse (azimuth) direction. In this scheme, azimuth DBF is used in the Doppler frequency domain to suppress interference signals, while elevation phase demodulation is employed to separate the echoes. This scheme eliminates the dependence of MIMO-SAR on waveform orthogonality and allows the direct use of a large number of single-station waveforms, providing flexibility in waveform selection. Additionally, retaining more degrees of freedom enables the multi-modal operation of MIMO-SAR. Finally, detailed simulation experiments are performed to verify the potential of the proposed scheme, and advantages and contributions are systematically analyzed.

Synthetic Aperture Radar (SAR) demonstrates advantages in continuous, all-weather, high-resolution Earth observation. However, barrage jamming in both the time and frequency domains can overwhelm SAR signals, leading to a significant overlap between interference and target signals, thus making separation challenging. In particular, barrage interference from multiple jammers can render SAR images almost indecipherable, substantially diminishing the quality of SAR image products. This paper introduces a barrage jamming suppression method for multiple jammers utilizing elevation multi-channel SAR. By first performing blind source separation on the mixed echoes, signals from different imaging bands are extracted, followed by imaging and other processes. Simulation experiments have confirmed that this approach offers benefits such as low system complexity and effective jamming suppression.

Over the past decade, jamming methods for Synthetic Aperture Radar (SAR) target detection and recognition, such as generating false targets, electromagnetic (EM) deception, and spoofing, have garnered significant attention. Distinguished from conventional SAR jamming methods, the target jamming scenario for SAR based on a new man-made material cloak introduces enhanced possibilities. These technologies exhibit the capability to effectively scatter incident EM waves in arbitrary or desired directions, thereby concealing pertinent information associated with the critical target. In this paper, we propose a broadband target jammer of SAR based on an information metasurface, which integrates intelligent information processing algorithms with space-time-coding digital metasurface, providing the capability to manipulate incident EM waves freely to achieve multi-mode jamming protections for critical targets. In simulation experiments, the results demonstrate adjustable EM deception and the generation of multiple false targets without any cooperation with the SAR system. Our work brings the available protection strategies for SAR closer to a wide range of real-time, broadband, and controllable applications, enhancing the concealment of critical targets in hot conflict zones.

The Hongtu-1 (HT-1) Synthetic Aperture Radar (SAR) constellation is the first in-orbit spaceborne single-pass multi-baseline interferometric SAR (InSAR) system. The system has the ability to conduct high-resolution earth observation and high-precision, high-efficiency terrain surveying. The highest resolution of the system is better than 0.5 m, and it has a 1:50000 scale global digital elevation model (DEM) and digital surface model (DSM) surveying capability. This paper provides a basic introduction to the HT-1 constellation, and demonstrates the advantages of single-pass multi-baseline InSAR results and its advantages over steep area.

Miniaturized and lightweight unmanned aerial vehicles (UAV) provide a flexible platform for synthetic aperture radar (SAR). The application of UAV interferometric SAR (InSAR) is gradually increasing in interferometric measurement fields. Leveraging the P-band signal and the capability to penetrate leaf clusters, these UAV InSAR systems demonstrate significant potential for topographic mapping and deformation monitoring, even in challenging surface conditions. This paper introduces the primary challenges associated with UAV InSAR, with a specific focus on analyzing the coherence of UAV InSAR. Additionally, we present interferometric measurement results obtained by our self-developed P-band UAV InSAR system and the processing flow we proposed. These results effectively address key challenges and validate the feasibility of the system for repeat pass interferometric measurements.

The accuracy of the polarimetric evaluation of low frequency polarimetric synthetic aperture radar (PolSAR) systems based on distributed targets is uncertain when the ionospheric information is unknown. Therefore, in this paper we investigate the potential of polarimetric evaluation of LT-1 images based on a set of calibrators arranged for L-band LT-1 PolSAR images. First, reasonable assumptions are made for common calibration models for LT-1 images. The ideal backscatter matrix of the set of calibrators is subsequently used in conjunction with the polarimetric distortion matrices (PDMs) to construct constraints for solving the PDMs. Finally, for possible ambiguous values of the solution, we use phase consistency as well as the Frobenius norm to remove the ambiguous values of the PDMs and obtain the final evaluated results. In the experimental part, we verify the feasibility of the method based on simulation data. Thus, the method contributes to accurate evaluation of LT-1 images.

Multibaseline (MB) synthetic aperture radar inter-ferometry (InSAR) is capable of reconstructing steep terrain height profiles, and MB phase unwrapping (PU) is one of the most critical and challenging steps in its processing chain. Existing MB PU algorithms usually suffer from poor noise robustness or low computational efficiency. In this work, we propose a fast and robust MB PU algorithm, including three main steps: 1) construct the triangulation network guided by the quality map; 2) obtain robust estimation of ambiguity number gradients based on the intrinsic relationship between interferograms and gradient regularization; 3) solve minimum cost flow problem with modified weights. The mean absolute errors of the height profiles constructed by the proposed method are 1.6m and 1.3m for simulated data and real data experiments respectively, verifying the effectiveness of the proposed method.

Typically, synthetic aperture radar (SAR) utilizes linear frequency modulation (LFM) signal to acquire high-resolution images, requiring spectral windowing to suppress sidelobes while sacrificing signal-to-noise ratio (SNR). In contrast to LFM signal, nonlinear frequency modulation (NLFM) signal can reconstruct the signal power spectral density (PSD) without sacrificing SNR, providing autocorrelation outputs with lower sidelobes. Despite the excellent application potential of NLFM signal, the real-time generation of NLFM faces numerous challenges due to the high complexity of the systems involved and constraints imposed by waveform generator devices. In this letter, a low-complexity, high-precision and high-resolution intermediate-frequency NLFM signal generation device is developed, requiring only eleven parameters to generate real-time NLFM signal of arbitrary time width and bandwidth, with a maximum bandwidth reaching 1.2 GHz. This NLFM signal generator will be employed in the unmanned aerial vehicle (UAV) SAR system. Finally, the performance of the NLFM signal generator has been validated through ground experimental results.

Digital beamforming (DBF) is an effective approach for accessing high-resolution wide-swath (HRWS) imaging in spaceborne synthetic aperture radar (SAR). By utilizing DBF, the performance of imaging, including signal-to-noise ratio (SNR) and system sensitivity, can be greatly improved. In actual missions, DBF for SAR imaging can be combined with various operating modes like squint-strip SAR, spotlight SAR, and TOPSAR. In squint-looking scenarios, the receiving beam of the antenna scans the scene not only along the vertical direction (elevation) but also along the horizontal direction (azimuth). For the first time, this article proposes the application of DBF for squint SAR imaging, establishes a geometric model of beam scanning, and derives the analytic expression of the signal model. The performance of both the conventional DBF scheme and the digital scalloped beamforming (DSBF) scheme is analyzed in squint-looking scenarios. In order to further reduce the loss of gain when processing received signals with broadband widths and long pulse durations in the conventional DBF schemes, as well as to minimize the digital resource consumption in the DSBF scheme, where the resource usage is directly proportional to the number of subbeams, a novel multibeam DBF scheme is proposed. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme, making it as a more realistic technology for future spaceborne SAR.

Synthetic aperture radar (SAR) is important for marine surveillance. The image resolution of SAR is obtained via coherent integration. However, with increasing coherent processing intervals (CPIs), the focusing quality of maneuvering ships in images will worsen, impacting target recognition. Defocusing is caused mainly by the change in the image projection plane (IPP) of the maneuvering ship and the change in the effective rotation vector (ERV). In this paper, an improved ISAR method is used to focus the ships in SAR images. First, the factors influencing the instantaneous imaging position are analyzed. Then, a new method based on data resampling for maneuvering ship imaging with a long CPI is proposed. This method consists of two main steps: ERV estimation and data resampling. The change in the ERV is estimated by using the instantaneous imaging position of all the scatterers, and the ship echo is resampled according to the estimation of the time-varying ERV. After resampling, we can obtain the echo from a constant rotating ship and can ensure a well-focused ship by using the traditional range-Doppler algorithm. This new method overcomes the restriction of ERV variation. Thus, a longer CPI can be used for imaging, while the IPP remains stable. Finally, simulation and measured data are used to verify the effectiveness of the proposed algorithm.

Interferometric synthetic aperture radar (InSAR) has been a valuable tool for mapping topography and subtle deformations. However, dealing with a wide imaging integral angle (IIA), especially for low-frequency band unmanned aerial vehicle (UAV) InSAR systems, introduces challenges. The conventional interferometric phase model depends on the difference in two slant ranges between the synthetic aperture centers and the target in two observations. Accuracy limitations emerge when variations are encountered in differences of slant range history across the entire wide IIA. This article explores the impact of IIA on interferometric measurements and proposes a modified interferometric phase model to address these limitations. For a wide IIA, the proposed model focuses on the integral of differences in slant range history throughout IIA by considering the nonlinear trajectory of the UAV platform. Additionally, measurement models for the IIA are deduced, in which an additional scale factor expanded by the Bessel function is introduced. Simulated and experimental datasets are utilized to demonstrate improvements in the accuracy of topography and deformation measurements. These results validate the effectiveness of the modified model in overcoming the challenges posed by wide IIAs in UAV InSAR systems.

Radar echo signals may experience the significant 2-D space dependence when it comes to the nonlinear trajectory of the synthetic aperture radar (SAR). The backprojection (BP) imaging algorithm is generally effective for achieving satisfactory focused SAR images under this condition. However, the conventional BP algorithm usually selects a uniform reference imaging plane, regardless of the actual topography of the observation scene. In undulating topographies, it has been proved that the range migration of the actual target and its projection point on the reference imaging plane may not remain consistent, leading to residual uncompensated phase errors and resulting in imaging defocusing during nonlinear trajectories. To address this problem, this article proposes an improved imaging method that involves the optimal topographic imaging plane reconstruction based on the image quality evaluation. The coarse plane and subsequent partitioned subplanes are sequentially constructed to create a topographic imaging plane that closely resembles the digital elevation model (DEM). The BP imaging algorithm is then applied to the reconstructed topographic imaging plane to overcome the defocusing problem. Both simulated and actual experiment datasets validate the effectiveness of the proposed method. Moreover, the proposed method significantly alleviates registration difficulties.

Polarimetry synthetic aperture radar (SAR) interferometry (PolInSAR) has been well-established for forest height estimation. However, employing mono-frequency SAR data for PolInSAR tree height inversion presents inherent limitations, posing challenges to ensure inversion accuracy. This article presents a novel method for the inversion of vegetation parameters using dual-frequency (DF) four-stage PolInSAR, aiming to address the limitations observed in mono-frequency inversion. By leveraging the differential penetration of vegetation across distinct frequency bands, this method facilitates the derivation of more precise volume-only coherence and ground phase information. Applying the DF four-stage PolInSAR method to a substantial dataset of simulation results identifies the optimal band combination as P- and L-band. Moreover, the band combination that yields the most significant enhancement in accuracy is determined to be L- and S-band. These simulation results inform the design of the DF full-polarization SAR system. Subsequently, airborne SAR data are acquired using this L- and S-band full-polarization airborne SAR system over the Saihanba Forest Farm in Hebei, China. Ground-based LiDAR measurements serve as reference values for the comparison of PolInSAR inversion results. The DF four-stage PolInSAR method has a 5.46% improvement in the inversion accuracy of airborne SAR data. Both simulation and airborne SAR data inversion outcomes demonstrate a significant enhancement in forest height inversion accuracy achieved through the DF four-stage PolInSAR method compared to the mono-frequency approach.

The dual-frequency (DF) synthetic aperture radar (SAR) in lunar orbit, using L and S bands, is the only full-polarization (FP) SAR. It explores different layers of the moon, relying on the unusual rotational symmetry for polarimetric calibration. Addressing prior issues in polarimetric evaluation, this study tackles the challenges of an expanding dataset (now exceeding 900 scenes) and the necessity to evaluate polarimetric distortion (PD) values in the range direction. A novel polarimetric evaluation framework is introduced, enhancing existing methods. First, a scheme is proposed to eliminate low-quality areas in the range direction based on antenna isolation specifications, validated theoretically with simulated and real data. Second, more than 900 DFSAR scenes, downloaded prior to April 25, 2023, are utilized for evaluation, surpassing previous limitations and enhancing accessibility for users. By evaluating the L-band data, the image calibration results acquired by DFSAR between September 19, 2019, and December 4, 2020, have high accuracy and can be prioritized for lunar applications. In addition, we recommend that researchers pay attention to the issue of changing data quality for releases around March 2021.The amount of data in the S-band with the L-S-joint mode is small, and we provide the availability results directly. Third, the study applies the evaluated results to lunar polarization research, contributing to lunar exploration through polarimetric parameters. This comprehensive framework ensures accurate data utilization, addressing the evolving needs of lunar exploration with improved methodologies and expanded datasets.

In this letter, a high-resolution SAR imaging algorithm based on a modified omega-K algorithm is proposed. The proposed algorithm first multiplies the reference signal. Then, the azimuth non-stationary phase error (ANSPE) is considered after performing the interpolation process based on the generalized frequency scale transformation. Finally, a well-focused SAR image can be obtained after compensating the ANSPE and correcting the residual spatial variant range migration by utilizing the range segmentation technique. The simulation results of point targets and the real-measured airborne high-resolution SAR data simultaneously verify the effectiveness and feasibility of the proposed algorithm.

Range ambiguity suppression is a key technical challenge for synthetic aperture radar (SAR) systems. Waveform diversity technology is a potential solution due to its low system complexity. In this letter, a 2-D method based on orthogonal nonlinear frequency modulation (NLFM) waveform and azimuth phase coding (APC) for range ambiguity suppression is proposed. This approach not only suppresses range ambiguity energies instead of dispersing them, but also works for multiple consecutive orders of range ambiguity energies. The imaging processing and range ambiguity suppression performance are described in detail. In addition, the range-ambiguity-to-signal radio (RASR) is analyzed, and the simulation results for the point target and distribution scenarios are given to verify the effectiveness and practicality of the proposed scheme.

Synthetic aperture radar (SAR) is an advanced remote sensing technique, capable of observing Earth’s surface independent of weather conditions and sunlight illumination. Restricted by the minimum antenna area, however, conventional spaceborne SAR systems cannot achieve high azimuth resolution in a wide swath. In addition, blind ranges are present as the constant pulse repetition interval (PRI) is used. To solve these problems, a PRI-staggered elevation multichannel SAR (EMC-SAR) system is employed in this article. By transmitting the continuously PRI-varied sequence, the blind ranges are located at different regions in different receive instants, effectively avoiding the loss of coverage in elevation. In this system, three issues are required to be addressed: 1) recovering the missed data located at blind ranges; 2) suppressing range ambiguous components; and 3) restoring the PRI-varied signal into a regular grid. To deal with these problems, we propose a novel SAR imaging method for a PRI-staggered EMC-SAR system. To be applied on-ground, assume downlinking of the individual elevation channels. First, the modified$\varepsilon $-insensitive loss tube regression with the L2 regularization method is applied to recover the missed data. Then, the range ambiguous components are suppressed by performing digital beamforming (DBF) based on the elevation multichannel technique, where the covariance matrix is constructed by using an iterative adaptive algorithm. After that, a generalized scaling transform is employed to restore the PRI-varied signal into a uniform sampled grid. Finally, a well-focused SAR image can be obtained by performing the conventional SAR imaging techniques. The effectiveness of the proposed method is validated by both simulated and real SAR data processing results.

For synthetic aperture radar (SAR), multipolarization and multifrequency modes greatly enrich the acquired earth resource information and have been widely applied in remote-sensing fields. In this article, we compare the classification capabilities of multipolarization and dual-frequency SAR. To meet the objective of selecting consistent and complete polarimetric information, a unified classification framework is proposed. In the framework, covariance matrices are used directly as inputs instead of polarimetric indicators. Additionally, the Wishart mixture model (WMM) is utilized to characterize the statistical distribution of polarimetric SAR data. Besides, the data log-likelihood function is utilized to mitigate the influence of the initial values on the expectation-maximization (EM) algorithm. Then, among the combinations of four sample-to-subclass distances and two schemes for obtaining sample-to-class distances, the one with the best classification performance is selected as the default for this framework. In the experiments, the classification capabilities of full polarization (FP), compact polarization (CP), and dual-polarization (DP) modes are first compared through the proposed classification framework. Then, we compare the classification capabilities of dual-frequency SAR in FP, CP, and DP modes. For polarimetric SAR (PolSAR) system design, it is necessary to strike a balance between demand indexes (classification performance, coverage width, and so on) and cost (such as budget, weight, and so on). The comparison results provide a reference for the optimization of polarization modes and frequency bands of the existing multipolarization and dual-frequency SAR payloads and the design of future SAR systems.

Digital beamforming (DBF) can provide high-gain narrow-beam scanning reception while transmitting wide-beam signals, which greatly improves the signal-to-noise ratio (SNR) of the corresponding systems. It is an effective technique for synthetic aperture radar (SAR) to obtain high-resolution wide-swath (HRWS) imaging capability. However, elevation changes in mountain area will lead to beam-pointing mismatch problems when using the ideal sphere model to calculate the beamforming weighting vector. As a result, the loss of receive gain and the deterioration of the SNR will occur. To solve this problem, adaptive DBF (ADBF) methods based on spectral estimation are typically used, such as Capon and MUSIC. However, the computational complexity of spectral estimation method is high, which is not conducive to on-satellite real-time processing. Therefore, a low complexity ADBF method based on beam-space phase-center cross correlation is proposed. In this method, the whole array is divided into several subarrays, and multiple phase centers are formed by beamforming so that the angle of arrival (AOA) of the signal source can be accurately estimated. Then, the weighted vector of the received beam is updated to mitigate the loss of receiver gain. The simulation results and airborne measured data validate the effectiveness of the proposed method. Compared with methods based on Capon and MUSIC, the proposed method can decrease the computational complexity without reducing the processing accuracy, thus providing a basis for the real-time processing of spaceborne DBF-SAR signals in the future.

The multiple elevation beam (MEB) mode is an effective technique for enhancing imaging width in spaceborne synthetic aperture radar (SAR) systems. This mode combines intra-pulse beam-steering during transmitting and digital beamforming (DBF) during receiving. By sequentially illuminating the far sub-swath followed by the near sub-swath, echoes from different sub-swaths can reach the antenna at the same time and overlap each other in the receiving window. To separate the overlapping echoes, the linear constrained minimum variance (LCMV) beamformer has been used, which is a single-null constrained beamformer. Additionally, a multi-null constraint beamformer has also been proposed on this basis. However, these two methods are insufficient for effectively separating the overlapping echoes when there is a significant energy difference between different sub-beams signals. To solve this problem, an advanced multi-null constrained beamformer with deepened nulls is proposed. Compared with other methods, the proposed method can flexibly adjust the width and depth of the nulls. The simulation results demonstrate that the proposed method can enhance echo separation quality. And the experimental results verify the effectiveness of the proposed method. All the results indicate that the proposed method is helpful to improve high-resolution and wide-swath imaging performance of future spaceborne SAR systems.

As an essential component of interferometric synthetic aperture radar (InSAR) measurements, phase unwrapping (PU) is a critical process that significantly impacts the accuracy and reliability of topography and deformation measurements. As an effective approach that can overcome the limitations of the Itoh condition, the multi baseline (MB) PU method is widely used in PU; however, when it comes to unmanned aerial vehicles (UAVs) InSAR systems that use a single antenna and repeat-pass mode for interferometric measurements, the time-varying baseline can pose a challenge since the airflow disturbance can prevent the repeat flight paths from remaining parallel. This makes the combination of baselines more complicated, which can impede the feasibility of using the MB PU for the UAV InSAR. As a result, using the relationship between different frequencies for PU is a promising alternative method. This article demonstrates that the frequency ratio design is a crucial factor in the success of the multifrequency (MF) PU method. An optimal frequency ratio design method for MF PU that maximizes the minimum Euclidean distance between the intersection vectors determined by the integer ambiguity vectors is proposed. The proposed method is validated using the simulated and experimental datasets, which provide evidence that PU accuracy can be greatly enhanced with the optimal frequency ratio.

Curvilinear spotlight SAR (CSSAR) has a high degree of freedom and can be used for 3-D imaging and video SAR (ViSAR) persistent imaging. The direct and effective processing of CSSAR data is an important part of CSSAR applications. However, CSSAR has higher requirements for motion compensation, especially when the motion measurement is not accurate enough. In this paper, an extended polar format algorithm (EPFA) is proposed based on the non-uniform fast Fourier transform for CSSAR. First, a theoretical derivation and analysis of the 2-D space-varying phase error in CSSAR are carried out. Then, a 2-D autofocus algorithm is proposed, which takes into account the spatial variability of the phase error. The efficiency of EPFA embedded in 2-D autofocus processing is significantly higher than that of back projection. In addition, based on the principle of small-angle approximation and spatial frequency domain sub-aperture technology, a novel ViSAR image generation scheme for CSSAR is proposed, which can significantly reduce the number of redundant calculations. The proposed algorithm is verified experimentally using data with a bandwidth of 2.4 GHz.

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.

Space-Time Waveform-Encoding (STWE)-synthetic aperture radar (SAR) enables waveform diversity in the space-time domain to meet the requirements of future high-resolution and wide-swath (HRWS) missions. The STWE-SAR receives echoes from multiple sub-swaths simultaneously with a single receive window. The overlapping echoes are usually separated based on the linear constrained minimum variance (LCMV) beamformer. However, the energy of the echoes from different sub-swaths can have a huge difference in the time domain. The conventional LCMV beamformer cannot effectively separate the overlapped echoes because the echo energy difference is not considered. Based on airborne STWE-SAR data, this paper performs energy equalization pre-processing before echo overlapping. Moreover, this paper confirms that the echo discrepancy is worth considering in the STWE system by comparing the separation results of the LCMV beamformer before and after energy equalization. This paper recommends that the design of future echo separation schemes needs to focus not only on the echo arrival of angle but also on the echo energy based on the actual situation.

Nonlinear frequency modulation (NLFM) signal can construct the signal’s power spectral density to reduce sidelobes without loss of signal-to-noise ratio. LuTan-1 (LT-1) is an L-band spaceborne synthetic aperture radar mission which is launched in the beginning of 2022, and a high-precision NLFM signal generator is developed in LT-1. However, the existing model, i.e., the traditional frozen ionosphere model, can not accurately describe ionospheric dispersion effects faced by the NLFM signal due to the non-linear characteristic of the instantaneous frequency. Thus, an extended model is established in this paper to describe ionospheric dispersion effects of the NLFM signal. Then, the differences of ionospheric dispersion effects on the NLFM and linear frequency modulation signals are compared. Afterwards, a method that embedded into the focusing procedure is proposed, which aims to eliminate ionospheric dispersion effects for the NLFM signal. Finally, the hardware-in-the-loop simulations of point targets and distributed targets are performed to verify the proposed method. The method proposed in this paper is used in the ground processing system of LT-1.

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.

Ground-based multiple-input multiple-output (GB-MIMO) radar is capable of performing structural health monitoring through vibration monitoring. However, when a target’s vibration frequency is comparable to the image acquisition frequency of MIMO radar, its vibration can affect the vibration parameters estimation based on the pixel’s phase series. In this letter, the impact of target vibration on the pixel series is analyzed. A robust method for vibration parameter estimation is proposed by operating iteration between the series compensation and estimation. The correctness and effectiveness of proposed method is verified by both simulation and calibrator experiments.

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.

Time-series interferograms acquired on a deep pit with a Ground-Based Synthetic Aperture Radar (GB-SAR) system showed that the atmospheric phase (AP) could be complexly space variant due to rapid changes of the weather conditions and steep topography. Conventional compensation methods that simulate the AP with typical parametrical models are no longer applicable. Based on the theoretical path integral model of the AP, a grid partition (GP) method is proposed. By dividing one interferogram into a certain number of small grids, the refraction variation inside each grid is assumed to be a constant. A system of linear equations is first built based on sufficient permanent scatterers (PSs). Then, bounds and inequality constraints are set to limit the refraction variation of each grid. A constrained linear least-square problem is solved with two-step process to estimate and compensate the AP. To fully validate the feasibility of the GP method, simulated phase interferograms based on four conventional AP models and with the consideration of deformation areas and noise phase are first processed. Then, four experimental interferograms with different types of AP components are processed and made comparisons with the conventional parametrical methods. The quantitative comparisons of the simulated and experimental data sets both proved that the GP method can well reduce the AP errors.

Currently, in the spaceborne synthetic aperture radar (SAR) community, the sliding spotlight mode has been widely applied to achieve high-resolution imaging. However, limited by the system pulse repetition frequency (PRF), it is usually impossible to realize a large-scale coverage at the same time. The technique of azimuth multichannel alleviates the PRF restriction by receiving additional spatial samples proportional to the number of Rx channels. A system combining the technique of azimuth multichannel with sliding spotlight SAR is able to achieve ultrahigh-resolution and wide-swath imaging simultaneously. In the advanced multichannel sliding spotlight mode, some problems may occur. First, azimuth beam progressive steering leads to an enlarged Doppler bandwidth over$N\cdot \text {PRF}$, and conventional azimuth multichannel reconstruction algorithms developed for the stripmap mode are, thus, not directly suitable for the sliding spotlight mode. Second, the phase characteristics of multichannel sliding spotlight SAR is different from that of multichannel stripmap SAR due to the time variation of the system Doppler centroid. Third, processing of spaceborne multichannel sliding spotlight SAR experimental data shows that the convolution function used in the full-aperture reconstruction method is closely related to the antenna phase configuration strategy of the phased array antenna system. Therefore, an improved processing frame is required for the multichannel sliding spotlight SAR, reexamining the validity of assumptions, such as the stop-go-stop approximation and that of a noncurved trajectory during the relatively long aperture time. Recently, as an exploratory study, a dual-channel sliding spotlight experiment was conducted with the Gaofen-3 (GF-3) SAR. Two experimental data sets were acquired with different PRFs. In this article, the GF-3 payload and the experiment design are introduced, and a complete multichannel sliding spotlight SAR data processing chain is provided. The first imaging results of spaceborne multichannel sliding spotlight SAR are demonstrated.

Multichannel SAR system operating in the sliding spotlight mode has the capacity of imaging with very high resolution as requested by future spaceborne synthetic aperture radar (SAR) mission; hence, it has been attracted more and more attention in the SAR community. In a multichannel sliding spotlight SAR system with$N$channels, the overall Doppler bandwidth of the data acquired by each channel is greater than$N \cdot \text {PRF}$; thus, the preprocessing algorithms (channel mismatch calibration and signal reconstruction) are no more available. To address this issue, a full-aperture imaging method for azimuth multichannel sliding spotlight mode is proposed in this article. First, the raw data of each channel need to be preprocessed based on a deramping operation, which can obtain an unambiguous Doppler spectrum. Afterward, the algorithm proposed for single-channel sliding spotlight mode can be performed. Finally, to avoid possible back-folded SAR images, postprocessing after focusing is demanded in some cases. The imaging results of the data acquired by GaoFen-3 (GF-3) are given for the first time in this article, which proves the effectiveness of the proposed imaging method and demonstrates the capacity of high-resolution wide-swath imaging of the multichannel sliding spotlight mode.

Digital beamforming (DBF) is a fundamental technique for synthetic aperture radar (SAR) to get high-resolution wide-swath (HRWS) images, which significantly increases the signal-to-noise ratio (SNR) of the system and improves range ambiguity performance. Moreover, the performance of the DBF-SAR system can be improved by using the nonlinear frequency modulation (NLFM) waveform, which can provide a matched filtering output with lower sidelobes without the loss of SNR compared to the linear frequency modulation (LFM) waveform. Combining the DBF technique and the NLFM waveform will enhance the system performance of DBF-SAR from an additional degree of freedom, which has essential engineering significance for reducing the transmit power of the system. However, the previous system architecture and processing method of DBF-SAR are generally based on the LFM waveform and are not practicable in DBF-SAR adopting NLFM waveform. This manuscript demonstrates the potential of adopting the NLFM waveform in DBF-SAR and analyzes the problems of compensating pulse extension loss (PEL) and frequency dispersion loss (FDL) in the new system. Then, an optimized DBF framework that combines sub-digital beamforming and a bank of unequal-width bandpass filters to suppress PEL and FDL in DBF-SAR adopting NLFM waveform is proposed. Simulations demonstrate that the proposed framework shows greater efficiency and stability in suppressing the severe PEL and FDL in the NLFM and LFM systems than previous methods. This manuscript brings an additional degree of freedom to the next generation spaceborne DBF-SAR and provides sufficient technical support for high-performance DBF-SAR when the LFM waveform is not adopted.

For a spaceborne high-resolution and wide-swath synthetic aperture radar (HRWS-SAR) system, it usually uses the digital beamforming technology. However, in practice, because of the influences of temperature, antenna pattern, receiving antenna, and other error factors, there may exist the range synchronization time errors, amplitude errors, and phase errors among different spatial channels. These nonideal factors will significantly degrade the multichannel data reconstruction performance, resulting in a smeared SAR image. To address this issue, in this article we propose a novel channel error correction algorithm based on the orthogonal projection theory. First, the optimal weight of each Doppler ambiguity component is calculated by the orthogonal projection. Then, the cost function is constructed based on the power maximization criterion, from which the channel phase errors can be obtained. Finally, the HRWS-SAR imaging can be finely realized after performing the channel balancing. Compared with the conventional phase error estimation method, the proposed algorithm does not require to perform the matrix eigenvalue decomposition, avoiding the signal leakage phenomenon under low SNR case. The effectiveness of the proposed algorithm is validated by both airborne and space-borne real SAR data.

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.

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.

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.

Dynamic deformation measurement is an important approach to monitor the structure stability. Due to its rapid imaging capabilities, ground-based multi-input multi-output (GB-MIMO) radar has shown great application potential in bridge structure monitoring. This paper proposes a dynamic deformation estimation method based on radar image series. Firstly, an improved clutter suppression method is utilized to overcome the estimation error in complex working status. Secondly, a two-step pixel extraction method is adopted to ensure both quantity and quality of sample pixels. Finally, frequency- and time-domain environmental stimulating methods are jointly considered for stable mode shape estimation. This paper systematically measured the dynamic deformations of bridge structures, including two suspension bridges and a cable-stayed bridge. The time-series deformation, deflection, vibration frequency and amplitude on the bridge structure are obtained by image domain measurement. And for the first time, the mode shapes of bridges are obtained through GB-MIMO radar. The correctness of structure parameter measurement is verified with the finite element model (FEM). Experimental results prove that with the proposed method, stable results could be acquired against weak and complex stimulation conditions.

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.

The azimuth multichannel spaceborne SAR system can achieve the high-resolution and wide-swath simultaneously. In a high-resolution and wide swath (HRWS) SAR system, the Doppler centroid (DC) frequency estimation is an important step for Doppler ambiguity signal reconstruction, which should be precisely estimated. In this paper, we propose a novel Doppler centroid frequency estimation method for HRWS-SAR system based on modified shuffled frog leafing algorithm (MSFLA). Firstly, the cost function is designed based on the average power difference between ambiguities and useful signal. Then, the MLSFA algorithm is proposed to accomplish the DC estimation. Finally, a good HRWS-SAR image is obtained based on the estimated DC. Compared with the traditional time-domain based DC estimation methods, the proposed method is not affected by the troubled cross-terms induced by high-order moments. The effectiveness of the proposed method is verified by the simulation experiments and real-measured SAR data.

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.

An experiment which involves the simultaneous deployment of three ground-based multiple-input multiple-output (GB-MIMO) radar systems to measure 3-D deformation of a displaceable corner reflector (DCR) is outlined in this letter. The DCR successively displaces in three mutually orthogonal directions and each radar measures 1-D deformation independently. Since the displacement directions of the DCR cannot be measured, they are estimated by solving a nonlinear equation set based on the rotation relationship between two 3-D coordinate systems, whose effectiveness is verified by simulation. Considering that the measurement accuracies of the DCR’s displacements along three axial directions of a 3-D coordinate system, that is, 3-D deformation, are related with how the coordinate system is built, the geometric dilution of precision (GDOP) is then utilized to take accuracy analysis. The measured and theoretical GDOP are rather close, which validates the feasibility of 3-D deformation measurement with three radar systems.

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.

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 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.

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.

This paper demonstrates the first experiment of Gaofen-3 (GF-3) satellite operated in dual-channel sliding spotlight mode. First, the raw data of two channels need to be preprocessed, including channel error correction and signal reconstruction, so that it can be focused by the subsequent conventional single-channel sliding spotlight mode imaging methods. After imaging processing, to avoid the possible back-folded SAR image, it is necessary to carry out postprocessing step. The imaging result of the data acquired by GF-3 is first given in this paper, it proves the capacity of achieving high-resolution wide-swath of multi-channel sliding spotlight mode, and the effectiveness of the processing method.

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.

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.

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.

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.

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).

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.

Ground-based synthetic aperture radar (GB-SAR) technique has been widely applied for the deformation monitoring and measurement of the natural and engineered slopes. To extend the 2-D deformation measurement from the conventional 1-D measurement along the radar-target line of sight (LOS), multiple aperture interferometry (MAI) techniques based on phase differences between interferograms of the forward-looking and backward-looking subapertures are tackled in this letter. The optimal subaperture selection is analyzed considering the typical signal-to-noise ratios and correlations in GB-SAR applications. Simulations prove that the coherent integration (CIM) can be utilized to improve the measurement accuracy of the MAI method. Besides, GB-SAR experiments are carried out to validate the feasibility and effectiveness of the 2-D deformation measurement method based on MAI. Accuracy comparison of deformation measurement with the MAI and cross correlation methods is also taken. Experimental results show that the accuracy of deformation measurement along the perpendicular direction to LOS based on MAI and CIM can reach millimeter level for displaceable corner reflector.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Narrow-band interference (NBI) suppression is an important technique in the low-frequency synthetic aperture radar (SAR). NBI sources either intentional or unintentional can mask the SAR signals and cause image degradation. According to the different characteristics of the spectra between the NBI and the SAR signals, a new method using subband spectral cancellation is applied to suppress the NBI. In contrast to the conventional suppression method performed on the raw data, this letter deals with the focused SAR image. By subtracting different range subband spectra of the SAR image, the NBIs are obtained and cancelled. The proposed method does not need to detect and estimate the parameters of the NBI and is easy to put in practice. Its performance is tested on the real data acquired by an experimental SAR system at P-band.

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.

A synthetic aperture radar (SAR) raw data simulator of distributed target scenes is an important tool for testing the system parameters, imaging algorithms, and so on, particularly for these recently proposed novel imaging modes. The Terrain Observation by Progressive Scans (TOPS) mode achieves the wide-swath imaging coverage and overcomes major drawbacks in ScanSAR by electronically steering the azimuth beam along track during the whole acquisition time. In this letter, a novel TOPS mode raw data generation approach from conventional wide-beam SAR imaging modes is proposed, by invoking the fact that the TOPS raw data can be considered as a part of the stripmap/spotlight raw data with the exploited wide azimuth beam interval. In this approach, an azimuth-varying filter is introduced to obtain the desired TOPS raw data and set others to zero. Moreover, the conventional stripmap and spotlight mode raw data can be efficiently generated in the 2-D Fourier transformed domain. Therefore, a fast raw data simulator can be easily achieved by taking this approach. Simulation results validate the proposed simulation approach.

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.

A novel ship wake constant false alarm rate (CFAR) detection algorithm is proposed. The algorithm first detects all the ships and replaces the pixels' gray value of the detected ship with the gray mean value. Then, with the ship target's geometric center as the center, a square image with a certain length is got, and the image is subdivided into four subimages, where the gray intensity contrast of the wake to clutter in the subimage is enhanced. Normalized Hough transform is applied on every subimage, and the probability distribution function in the Hough domain of each subimage is modeled, which can be used for CFAR detection. Finally, the detection results of the subimages are fused to get the final detection. Using our algorithm, the signal-to-clutter ratio of the wake to clutter is enhanced, the ship's navigation direction can be extracted easily, and most importantly, CFAR detection is realized.

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.

The purpose of this letter is to show that the azimuth scanning mode of a synthetic aperture radar can be applied to deriving ocean wave spectra and determining the wave propagation direction for the first time, which reveals its enormous potential and great future in ocean observation. The improved Doppler beam sharpening imaging algorithm is used to produce a sequence of individual subimages of ocean waves in the same scan region from different aspect angles with a high revisit rate. These subimages have an inherent property that they are formed at different discretely delayed times. Therefore, wave propagation direction can be determined from a pair of wave images in different scans. Several different methods are applied to the real airborne radar wave data, including the methods of scan sum (taking the standard Fourier spectrum of the scan-summed image), spectral sum, spectral phase shift, and cross-correlation function of subimages. The processing results demonstrate the effectiveness of the algorithms.

The Terrain Observation by Progressive Scans (TOPS) mode is a novel spaceborne imaging mode which can be used to obtain wide-swath coverage and overcome major drawbacks in conventional ScanSAR. An efficient full-aperture imaging approach, which takes advantage of the two-step focusing technique and the azimuth baseband scaling operation, is presented for processing the TOPS-mode synthetic aperture radar (SAR) data. First, the proposed two-step focusing technique for spotlight and sliding spotlight SAR data focusing is adopted to resolve the aliased Doppler spectrum. Afterward, the following extended chirp scaling processing procedure with azimuth scaling factors is used to implement the residual TOPS raw-data focusing. Since the use of subapertures is avoided and only a limited azimuth-data extension is required, this algorithm is highly efficient. Simulation results validate the proposed imaging approach.

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.

The characteristic difference between targets and clutter is analyzed. Considering the ship target's gray intensity distribution and its shape difference compared to the clutter, in this letter, a new algorithm is presented based on correlation. The algorithm utilizes the strong gray intensity correlation in the ship target; also, the joint gray intensity distribution using 2-D joint log-normal distribution of a pixel with neighboring pixels in the clutter is modeled, which can be used for correlation-based joint constant false alarm rate detection. Using this algorithm, the false alarms caused by speckle and local background nonhomogeneity can be greatly reduced. The detection performance is much better.

Ground moving target indication (GMTI) is one of the most important applications in a general bistatic synthetic aperture radar (SAR) system, where the transmitter and receiver move along nonparallel trajectories with different velocities. In order to improve the capability of clutter cancellation in bistatic SAR/GMTI processing, the coherence between two echoes collected by two receivers is investigated, and the full-coherence conditions are derived. A new coherence-improving algorithm for general bistatic SAR complex image pairs is proposed, which can be realized in the following steps: 2-D range azimuth prefiltering processing, relative geometric deformation correction, and image registration. An approximate implementation of 2-D prefiltering and the corresponding prefilter parameter analysis are also given. Last, two numerical experiment results are given to demonstrate the effectiveness of the proposed algorithm.

Synthetic aperture radar (SAR) distributed target scene raw signal simulation is an important tool for the study and test of SAR systems and processing algorithms, mission planning, and inversion algorithm design. In this paper, the squint spotlight SAR scene model is evaluated to achieve spotlight SAR raw signals in the 2-D frequency domain and the precision of the model is analyzed. It is known that the range migration phenomenon in the time domain is explained as the coupling between the range and azimuth in the 2-D frequency domain. To realize the coupling relation in the 2-D frequency, interpolation may be needed. However, interpolation is a time-consuming manipulation. For efficiency, some signal processing methods are employed to couple the range and azimuth frequencies. Those tricks are derived from some optical principles, which give us some novel thoughts. Therefore, the efficiency of the simulator is highly improved, which facilitates its application to the verification and test of the real-time processor.