Advanced Land Observing Satellite-4 (ALOS-4) is a spaceborne high-resolution and wide-swath synthetic aperture radar (HRWS-SAR) that uses a variable pulse repetition interval (VPRI) technique to achieve continuous wide imaging. In some ALOS-4 images, azimuth fractional ambiguity caused by the VPRI is observed, and it differs from the usual integer ambiguity resulting from inter-channel errors in that it occurs at smaller intervals. In this paper, we propose a sequential Doppler offset (SDO) method for locating the original target (OT) that causes azimuth fractional ambiguity. First, the ratio of the interval of integer ambiguity to that of fractional ambiguity is obtained, which is used to generate SAR images with different Doppler center frequencies. Second, the coherence between the sum image of the generated images and the image with a zero Doppler center frequency is calculated. Third, some points with coherence greater than a threshold are selected based on the coherence. Finally, the final OT is obtained by detecting the filtered selected points. Some experiments are conducted based on ALOS-4 L1.2 data, and the results demonstrate that the method locates the OT accurately. In short, the proposed method provides a starting point for fractional ambiguity suppression in HRWS-SAR.

Multiple-input-multiple-output synthetic aperture radar (MIMO-SAR) has potential for high-resolution and wide-swath (HRWS) imaging. Current research mainly focuses on its echo separation. However, how to achieve HRWS imaging based on separated echoes has been rarely discussed and remains as if all echoes are separated and reconstructed ideally so that some conventional imaging algorithms can properly focus. Thus, this paper proposes an HRWS imaging mode for spaceborne MIMO-SAR, named virtual delay-emission (VDE). First, the VDE delays the transmission time of some channels to reduce the effective phase center (EPC) redundancy of MIMO-SAR and increase the effective antenna area. Second, the VDE transmits orthogonal waveforms with the same autocorrelation function. Third, the VDE separates the mixed echoes and reconstructs the separated echoes in the frequency domain to achieve low azimuth ambiguity and high-resolution imaging. Some system simulations based on ALOS-2/-4-like system parameters are conducted, and the results demonstrate that the VDE achieves the 3m/900kmmode if the system hardware is implemented properly.

Multiaperture echo signals collected by distributed array SAR can be reconstructed to realize High-Resolution and Wide-Swath (HRWS) imaging. To achieve the HRWS imaging of distributed SAR, an advanced Azimuth Multi-Aperture Reconstruction (AMAR) method is put forward for the first time. First, to eliminate the trajectory offset of distributed SAR, the first-order baseline deviation, second-order baseline deviation and azimuth space variation error are compensated in turn for the calibrated and undersampled echo signals, and the echoes containing distorted azimuth ambiguity can be obtained. Second, the distortion of ambiguity is corrected by range decompression, anti-migration correction, etc. Finally, the echoes satisfying the Nyquist theorem are derived by the AMAR of Train formation, and the bistatic SAR imaging is performed. In addition, some simulations are conducted to evaluate the performance of the proposed method, and the results demonstrate that the method can achieve AMAR and HRWS imaging of distributed array SAR.

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

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

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

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

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

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.

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

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

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.

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.

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.

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.

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.

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

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.

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.

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.

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.