The spaceborne multistatic synthetic aperture radar (SAR) system offers a flexible baseline that provides more observation angles and higher interferometry accuracy. However, any phase deviation among the independent oscillators in the spaceborne multistatic SAR system can cause a residual modulation of the echoes. Therefore, accurate phase synchronization is crucial for the system. The pulsed alternate synchronization scheme accurately extracts phase errors between different platforms, as verified in the TanDEM-X mission. Furthermore, an advanced noninterrupted pulsed alternate scheme uses the time interval of transmitting sequence to realize phase synchronization without interrupting the normal operation of the radar, which is verified in the LuTan-1 mission. However, with the increasing number of spaceborne multistatic SAR platforms, the time interval of the system may not be sufficient to support noninterrupted phase synchronization. To this end, this article proposes a novel phase synchronization method to improve the efficiency of phase synchronization for spaceborne multistatic SAR systems. First, a model for phase synchronization is built based on the pulsed alternate synchronization scheme, and the constraint between phase synchronization accuracy and waveform properties is analyzed in detail. Second, a quasi-orthogonal waveform optimization method, which can realize the rapid generation of phase synchronization waveform with better correlation properties, is introduced to improve the accuracy of phase synchronization. Third, to further reduce cross correlation energy between waveforms, we introduce generalized short-term shift-orthogonal (STSO) waveforms for phase synchronization. This waveform can achieve local orthogonality with a known baseline, improving the accuracy of phase synchronization greatly. Finally, the proposed method is verified through detailed simulations and ground experiments.

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

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

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

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

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.

In an azimuth multichannel synthetic aperture radar (SAR) system operating in sliding spotlight mode or Terrain Observation by Progressive Scans (TOPS) mode, due to the varying Doppler centroid and the greater processed Doppler bandwidth, the conventional channel phase error estimation algorithms that tailored to stripmap mode will fail and unable to be directly employed. To address these issues, a deramping-based method is proposed in this letter, which extends the conventional methods to sliding spotlight and TOPS cases. By multiplying the signals of each channel with the corresponding deramp phase, the problem of varying Doppler centroid is solved, and its resulting greater Doppler bandwidth is reduced. Therefore, the spectral properties of the signals are the same as those in the stripmap case, and the conventional channel phase error estimation methods can be applied subsequently. Estimation and imaging results of the simulation data and the real data of GaoFen-3 (GF-3) operating in dual-channel sliding spotlight mode demonstrate the effectiveness of the proposed method.

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.

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.

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.

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 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.

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.

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

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.

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.

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) 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.

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

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

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.

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.

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).