Synthetic Aperture Radar (SAR) system operating in squint working mode has high flexibility and can be widely used in many fields. This paper mainly focuses on decoupling and imaging processing of squint stripmap SAR data. A modified Chirp Scaling Algorithm (CSA) based on the Linear Range Walk Correction (LRWC) method is proposed for this squint stripmap imaging mode. By introducing the Azimuth Non-Linear Chirp Scaling (ANLCS) filtering, the azimuth variant Doppler Frequency Modulation Rate (DFMR) can then be equalized. Simulation is carried out to validate the effectiveness and efficiency of the proposed algorithm.

Global navigation satellite system (GNSS) has good global coverage, which provides convenience for moving target detection (MTD) using low-orbit satellites. However, range ambiguity and range migration (RM) will occur in long-range target detection. This paper proposes a new algorithm based on range correction to solve the problem of range ambiguity and RM of uniform motion targets under a long-time coherent integration detection system. The algorithm realizes the purpose of resolving the range ambiguity with errors by the Newton iteration (NI) method. The NI algorithm introduces the velocity parameter by establishing a uniform motion target model to reduce the measurement range errors caused by long-time integration. Then there are still range errors. The NI method is used to realize the ambiguous range's fast and accurate solution process. Finally, by simulation to verify this proposed algorithm and by Monte Carlo simulation to compare the algorithm with the Chinese remainder theorem (CRT) and the clustering algorithm (CA) to validate the accuracy and efficiency of the algorithm.

Spaceborne bistatic synthetic aperture radar (SAR) system employed different locations for transmitter and receiver could provide additional interference information compared to monostatic SAR. The spaceborne bistatic multichannel SAR system can lift the restrictions between high-resolution and wide-swath. Multichannel SAR system utilize pulse-repetition frequencies less than Doppler bandwidth at the cost of ambiguous Doppler spectrum. This represents that the radar echoes received by each channel are under-sampling. Unambiguously reconstruct the Doppler spectrum is necessary for SAR image focusing. However, Conventional azimuth reconstruction method in the Doppler spectrum for bistatic multichannel SAR result in failure because of the variety of expression for round-trip slant range of the bistatic SAR echoes. To address this problem, an improved approach to azimuth spectrum reconstruction for bistatic multichannel SAR is proposed in this paper. Application of matrix filter method can realize the recovery for signals of all channels. The effectiveness of the proposed approach in this paper is demonstrated with simulation experimental results.

Squint spaceborne synthetic aperture radar (SAR) can significantly improve the ground observation performance of platform sensor through flexible beam pointing; however, it is still challenging for squint spaceborne sliding spotlight SAR data focusing since the significant range cell migration (RCM) and range azimuth coupling (RAC) caused by the squint angles. Whereas the linear range walk correction (LRWC) can deal with these problems, the traditional chirp scaling (CS) algorithm cannot adapt to the signal model after the LRWC operation, especially in the case of sliding spotlight mode. In this letter, a hybrid CS algorithm (HCSA), integrating the modified CS (MCS) in range time domain with the frequency CS (FCS) and frequency nonlinear CS (FNLCS) in azimuth frequency domain, is proposed for squint sliding spotlight SAR system. After LRWC and deramp operations, the hybrid CS (HCS) based on the updated signal model is employed to fulfill whole scene focusing, which can equalize the Doppler frequency modulation rate (DFMR) and handle the back-folding issue in azimuth time domain simultaneously. The geometric correction is then be applied to obtain the focused SAR image without LRWC geometry distortion. Simulation demonstrates that for the spaceborne SAR with 1.0 m resolution in both range and azimuth directions, and a squint angle of 30°, the proposed HCSA can realize effective focusing within the full scene.

In this study, we introduce a novel self-supervised learning adapter based on permutated image restoration (PIR) for effectively transferring pretrained weights from natural images to remote sensing object detection tasks. The adapter’s unique methodology encompasses a three-phase process: segmenting and permuting image blocks, estimating permutation matrices for sequence reconstruction, and applying specialized loss functions for accurate block positioning. The use of our approach results in the maintenance of fidelity in both absolute and relative block positions as demonstrated by the evaluation of block similarities. The empirical results indicate significant performance enhancements for diverse datasets spanning optical and SAR data types, including HRSC2016, SODA-A, and RSDD, while effectively avoiding overfitting.

High-resolution and high-squint spaceborne synthetic aperture radar (SAR) system has excellent earth observation performance. However, high-squint spaceborne SAR data is more challenging to process than the general broadside counterpart because of the severe range-azimuth coupling (RAC). Whereas the classic linear range walk correction (LRWC) method can handle this problem, it is performed in azimuth time-domain and seriously constrains azimuth swath width. In this article, a decoupled hybrid correlation Algorithm (DHCA), combining the range-azimuth decoupling (RAD) in 2-D frequency domain with the modified hybrid correlation (MHC), is proposed to handle sliding spotlight spaceborne SAR data of high-squint angle case. The decoupled hybrid correlation (DHC) is the main body of the proposed algorithm, and it starts with RAD filtering in 2-D frequency domain, which is designed to eliminate the majority of the range cell migration (RCM) and RAC caused by high-squint angles. A nonlinear chirp scaling (NLCS) in range direction is subsequently performed to equalize the variant range chirp rate caused by residual RCM and RAC. Coarse range compression can then be realized uniformly by the range-matched filtering. The NLCS operation and range coarse compression ensure that the range-Doppler (RD) signals of all targets within the whole scene occupy very narrow range cell scopes. By making full use of the principle of stationary point (POSP) to signals, the range compression position and range frequency mapping relationship after NLCS can be derived. The MHC can therefore be fulfilled by extracting the RD signals along the residual RCM and constructing the reference correlation function after the range NLCS. Thus, both the efficiency and the accuracy of focusing processing are guaranteed. Simulations are carried out to validate the proposed algorithm.

The Global Navigation Satellite System (GNSS)-based passive bistatic radar (PBR) which uses the GNSS signal as the illuminators of opportunity is studied for moving target detection (MTD). GNSS-based PBR has many advantages due to the removal of the transmitting device; however, its fundamental limitation is the low power density of the GNSS signal. Therefore, the integration time should be sufficiently long to obtain a promising maximum detectable range. On the other hand, the integration time is limited by the range migration and Doppler migration of the echo caused by target motion. In this letter, a novel MTD algorithm is proposed for the GNSS-based PBR, by employing a modified radon Fourier transform (MRFT) to achieve the required long-time integration for moving targets. The MRFT integrates the echo energy via joint searching of range, Doppler, and Doppler rate of the target, which can handle not only the range migration but also the Doppler migration problems, and significantly improves the signal-to-noise ratio (SNR) of the echo signal. An experiment using the GPS L5 signal as the illumination source is conducted and a moving car is successfully detected by the proposed algorithm, although significant range migration and Doppler migration are present due to variation of its speed.

Scanning synthetic aperture radar (ScanSAR) mode is widely used in Earth observation because of its capability of acquiring wide-swath images with moderate resolution. However, due to the operation mechanism of ScanSAR mode, the acquired images often suffer from the scalloping problem, resulting in significant deterioration of image quality. In this article, a novel scalloping suppression method is proposed for ScanSAR images based on the modified Kalman filter with preprocessing. First, an image model is built to analyze the effect caused by scalloping. Then, a modified Kalman filter is proposed to estimate the intensity of scalloping. However, if the scene is complicated or the scalloping effect is strong, the Kalman filter works with poor performance. Therefore, an innovative preprocessing operation is introduced, involving image segmentation and pixel value filling. Finally, the proposed method is verified by the GaoFen-3 (GF-3) and TerraSAR-X satellite images with different scenes. The results demonstrate that the proposed method can accommodate the complex scene well and achieve effective scalloping suppression.

Based on the multiple azimuth squint angles mode, a novel moving target velocity estimation method is proposed, including both along range and along azimuth directions. The acquisition geometry of multi-azimuth angle mode is given first with variation of antenna azimuth squint angles. Then, range curvature is corrected in range-frequency domain after range compression and the residual range curvature can be neglected which is smaller than a range bin. Furthermore, azimuth velocity is estimated by the slope variation of the range walk trajectory from two certain observations and then is the range velocity estimation based on the slope from one observation. Velocity estimation accuracy is also analyzed, considering the errors caused by azimuth squint angle and trajectory slope. The effectiveness of the proposed strategy is demonstrated by experimental results.

In recent years, electromagnetic vortex waves carrying orbital angular momentum (OAM) arouse extensive attention in remote sensing imaging fields. However, most of the researches concentrate on the staring imaging mode, where the radar is motionless relative to the targets. We establish a novel system operation mode based on the principle of synthetic aperture radar (SAR). The echo model upon this novel operation mode is established and the new imaging method modified from the back-projection (BP) algorithm is deduced. Simulation and image evaluation results demonstrate the validation of the proposed method and OAM-based SAR system can be utilized in the SAR data acquisition and signal processing fields.

Recently, electromagnetic (EM) vortex with orbital angular momentum (OAM) attracts more attention in radar imaging fields, which needs a variety of OAM modes. However, most of them are based on motion relative static. In this paper, we propose a novel SAR imaging method based on electromagnetic vortex with OAM. The geometry model of synthetic aperture radar (SAR) is utilized for EM vortex with OAM and the echo signal model is deduced correspondingly, including the additional phase items produced by EM vortex waves. Then the imaging algorithm aiming at OAM based on Chirp-Scaling algorithm is proposed. Simulation results validate the effectiveness of the proposed method and that OAM beams can be applied in SAR signal processing.

In this paper, a novel algorithm of geometric correction for spotlight and sliding spotlight SAR is presented. Synthetic Aperture Radar (SAR), which is independently of weather and light conditions, is a well-established imaging technique for remote sensing [1]. With the increasing demands for higher resolution, spotlight and sliding spotlight modes have been presented. The key requirement for understanding SAR image is to map the location of pixels accurately, and this requires a more accurate geometric correction algorithm to assure the quality of spotlight and sliding spotlight SAR image products [2]. In 1982, a Range-Doppler location method, which is effective for stripmap SAR, has been presented by John C. Curlander [3], but it's not effective for spotlight and sliding spotlight SAR. Considering the imaging process model [4], the space-time mapping relationship and the space-frequency mapping relationship, we have modified the location equations to improve the geometric correction for spotlight and sliding spotlight SAR. The simulation result shows the validity of the algorithm.