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Multiple-input and multiple-output synthetic aperture radar (MIMO SAR) is a very potential technique for high-resolution and wide-swath (HRWS) imaging due to the fact that it can provide more degrees of freedom. In this article, 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, leading to 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

In this paper, a novel Doppler rate (DR) estimation-based two-dimensional (2-D) subaperture motion compensation (MOCO) approach is proposed for high-resolution scanning synthetic aperture radar (ScanSAR) mode. It can estimate space-variant phase errors in different subswaths simultaneously by exploiting data of multiple subswaths. High-resolution ScanSAR images, produced by full-aperture processing algorithm, are often defocused by uncompensated phase errors. As a result of the periodic gapped data in each subswath in ScanSAR mode, the performance of the commonly used stripmap MOCO approaches will be degraded or even not work when using full-aperture processing algorithm. The effectiveness of the proposed approach is demonstrated by an airborne ScanSAR real data set containing different types of terrain, with a high geometric resolution of about 3.5 m (mid or far range) or 5 m (near range) in azimuth working at C band. [ABSTRACT FROM PUBLISHER]

Future spaceborne SAR missions will require complete and frequent coverage of the Earth with a reasonably high geometric resolution. To achieve this imaging capacity, multichannel SAR systems with a planar antenna will be widely adopted in the next generation spaceborne microwave remote sensing missions. However, the high data rate, the increased system complexity, the difficulty of multichannel signal processing, the heavy load, and the wide bandwidth downlink may hamper the demonstration of these imaging modes. With the rapid development of electronic engineering and digital signal processing, these problems will be well resolved. Furthermore, regarding the implementation complexity, multichannel SAR systems with a deployable reflector antenna seem to be a better alternative for HRWS imaging [17, 18]. However, the major drawback of the reflector antenna is the limited beam-steering capacity compared with a planar antenna, which will affect some of the remote sensing applications. Thus, multichannel SAR systems with a planar antenna rather than a reflector are more suitable for the higher geometric resolution, especially for the higher-carrier frequency (such as X-band). [ABSTRACT FROM PUBLISHER]