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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]

Digital beamforming (DBF) with scan-on-receive (SCORE) technique in elevation is a powerful technique that enables a spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide swath (HRWS) imaging. In the spaceborne DBF-SAR system, sampling signals from each channel are weighted by weights generated by a digital signal processing system in real-time. However, the contradiction between the shortage of spaceborne hardware resources and resource demand of the multichannel real-time signal processing increases the difficulty of system design. In order to solve this problem, a novel weight generator and an improved intermediate frequency (IF) DBF real-time processing architecture are proposed in this article. By taking advantage of the special properties of the SCORE algorithm, the proposed weight generator calculates weights using a linear polynomial algorithm. The simulation result shows that a low-order approximation can achieve high performance. The proposed generator can correct multichannel amplitude and phase error at a low cost on hardware resources. The effectiveness of the proposed method is verified by experiments with a raw data processing instance of an X-band 16 channels DBF-SAR. [ABSTRACT FROM AUTHOR]

The displaced phase center antennas in azimuth and digital beamforming (DBF) in elevation are two state-of-the-art techniques for achieving high-resolution wide swath (HRWS) imaging in the multichannel synthetic aperture radar (SAR) systems. However, due to the atmospheric turbulence, airborne HRWS-SARs inevitably suffer trajectory disturbances, which will consequently defocus the SAR image. Although various motion compensation (MoCo) methods have been proposed, they are mostly designed for the traditional single-channel SAR and, therefore, ignore the channel-dependent posture error. The posture motion error will introduce residual motion and time-variant channel errors, which not only defocuses the image but also causes azimuth ambiguity and degrades the SNR improvement of the final images of the azimuth multichannel and DBF-SAR, respectively. To solve these problems, a novel MoCo scheme for 2-D multichannel SAR systems is proposed. The posture error is described and calculated in matrix form through the use of quaternions. Then, the posture error is transformed into the translational motion error for each receiving channel and is subsequently compensated precisely. To address the residual aperture-variant motion error, a modified aperture-dependent MoCo is integrated into the proposed scheme. Simulations and airborne experiments, including processing the data of a C-band SAR system with four azimuth channels and an X-band DBF-SAR system with 16 elevation channels, have been implemented to validate the effectiveness of the proposed MoCo 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 azimuth phase coding (APC) technique is known for its very low implementation complexity and its effectiveness for point and distributed ambiguities in conventional synthetic aperture radar (SAR) systems. In recent years, as an extension, the APC technique has been briefly discussed for multichannel SAR systems. However, the properties of the APC technique are no longer guaranteed in the multichannel SAR systems based on the digital beamforming (DBF) on-receive, and only a slight APC gain in the suppression of the range ambiguity can be obtained. In this article, we first provide a more thorough analysis for an APC-multichannel SAR system with respect to a uniform pulse-repetition frequency (PRF). Then, the APC/multichannel SAR system with nonuniform spatial sampling is briefly discussed, and an improved reconstruction approach based on a quadratically constrained optimization model is proposed to increase greatly the APC gain with respect to existing multichannel reconstruction algorithms. This proposed approach allows the minimization of the range ambiguity with a given azimuth-ambiguity constraint. In particular, for some specific PRFs, the proposed method permits a cancellation of the odd-order range ambiguity. Finally, simulation experiments are performed to verify the advantages and effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Due to the light weight and small size, synthetic aperture radar (SAR) sensors with light-small aircraft are very sensitive to atmospheric turbulences, which cause serious trajectory deviations. Limited by the cost or payloads, light-small aircraft sometimes cannot carry high-accuracy inertial navigation systems (INS)/global positioning systems (GPS). For the very high resolution (VHR) case, residual range cell migration (RCM) often exceeds several range resolution cells during the course of the synthetic aperture, thus degrading the image quality substantially. In this paper, a robust scheme for the correction of residual RCM is presented. The scheme consists of three principle steps. First, the range-compressed data in the signal domain are divided into subapertures in azimuth. For each subaperture, a sliding window is used to select the subblock data with the highest signal-to-clutter ratio. Second, for each subaperture, residual RCM is estimated by using the selected subblock data based on a nonparametric entropy minimization technique. Third, the estimated residual RCM of all subapertures are filtered and coherently combined to perform the correction for the full-aperture data. Processing the results of VHR SAR raw data shows that the proposed scheme is effective for highly precise imaging with light-small aircraft equipped with only low-accuracy INS/GPS. [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]

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]

This paper presents a double-channel hybrid bistatic synthetic aperture radar (SAR) system. It can be implemented with the available illuminator (e.g., spaceborne and airborne SAR systems) to acquire the 2-D and 3-D microwave images for remote-sensing applications. This system can be used as a test system for the validation of complex bistatic acquisitions, novel synchronization algorithms, and advanced imaging techniques. In this paper, we will demonstrate the time and phase synchronization strategies, fast time-domain imaging algorithm, 2-D bistatic SAR, and 3-D bistatic stereo radargrammetry in SAR images. Based on the proposed bistatic system, the acquired bistatic image has a much better sensitivity than its monostatic counterpart, which will facilitate the detection and recognition of targets based on their characteristic and bistatic scattering behaviors. Finally, the experiment results highlight the differences between monotatic and bistatic SAR images. [ABSTRACT FROM AUTHOR]