Study On High-Resolution Wide-Swath SAR Moving Target Processing Method

In recent years, high-resolution wide-swath SAR system has attracted more and more attention for its ability to enhance SAR system performance. A multichannel along the azimuth direction high-resolution wide-swath SAR system has been presented to deal with the conflict between high-resolution and low pulse repetition frequency(PRF) which has encountered in high-resolution and wide-swath SAR imaging. Moreover, a high-resolution wide-swath SAR system can offer great potential for ground moving target indication(GMTI) capability, since the spatial degrees of freedom in azimuth can also be utilized to suppress the background clutter. The key issues and the thorny points for the moving targets detection and imaging based on the HRWS SAR system is studied in this dissertation. Based on the problems encountered with the real data processing, some novel signal processing methods are proposed for moving targets detection and imaging for HRWS stripmap SAR system, moving target artifacts suppression in SAR images for HRWS stripmap SAR system, moving targets detection and imaging for HRWS TOPS SAR system and air moving targets detection and imaging. The main content of the dissertation is summarized as follows.1. In high resolution and wide swath(HRWS) synthetic aperture radar(SAR) systems, three problems will be led by utilizing the conventional PD-STAP technique for SAR-GMTI processing. First, the target Doppler spectrum will be wrapped because of the Doppler shift caused by the cross-track velocity of the moving target. The ambiguities will be appeared if directly using the matched filtering. Second, in the case of the signal undersampling, it will cause a PRF shifting in azimuth for a moving target in the focused image. This will make the moving target detection much more complicated and challenging. Third, the traditional STAP technique also has a high computational complexity. To overcome these problems, a novel space-time adaptive processing method based on Deramp processing is proposed. Simulation results validate the effectiveness of the proposed algorithm.2. In order to overcome the dislocated and smeared moving targets in SAR images, the space-time adaptive processing framework of Chapter II is utilized to remove the moving target artifacts. The main idea is to compress the space-time spectrum of the moving targets so that there are no aliased signal components for moving targets in space-time plane. By using the proposed space-time adaptive processing framework, only one spatial degree of freedom is needed for suppressing one moving target. In addition, for the multiple moving target case, dynamic steering vector concept is further proposed. If the multiple focused moving targets in the imaging scene are not overlapped each other in azimuth, one degree of freedom is enough for suppressing all these moving targets. By using the dynamic steering vector concept, in theory the number of degrees of freedom required for suppressing the moving targets needs to be equal to the maximum number of focused moving targets that overlap each other in azimuth. However, the focused moving targets can be regarded as sparse in the SAR image. They are only located at some angles and Doppler frequency bins in the space-time plane and therefore it is less likely that moving targets will overlap each other in azimuth. As a result, in general, two degrees of freedom are enough for suppressing the moving targets. The processing results from simulation and real data have demonstrated the effectiveness and superiority of the method.3. The azimuth bandwidth of background clutter for HRWS TOPS SAR mode is much larger than the instantaneous signal bandwidth. Therefore, the background clutter will be highly aliased if the pulse repetition frequency(PRF) is limited. Direct application of space-time adaptive processing(STAP) techniques will significantly compromise the clutter suppression performance. Hence, we propose a space-time adaptive processing method based on spectrum compression. The spectrum compression process can reduce the azimuth bandwidth of background clutter to the instantaneous signal bandwidth. Space-time adaptive processing is then performed to effectively suppress the clutter, and the clutter suppression performance is greatly improved. Furthermore, Deramp processing is used to focus the moving target in order to avoid the target ambiguities arising from the Doppler shift. Meanwhile, we can perform the moving targets tracking by utilizing the highly overlapping full-aperture images which are obtained by the TOPS SAR mode. Simulation results validate the effectiveness of the proposed algorithm.4. An air moving target detection and imaging method based on the spaceborne SAR system is proposed. First, the background clutter can be suppressed by using the DPCA technique. Then, the moving target detection can be performed by the adjacent correlation processing. As a result of this step, the envelope migration correction can be implemented and makes use of modified keystone transform to eliminate the effect of range walk. And itis beneficial to moving target detection by using the traditional CFAR test since the moving target is a single point in the two dimensional plane. Moreover, we can reconstruct an accurate azimuth reference function by using the azimuth position of the moving target in the two dimensional plane without the knowledge the along-track velocity of the moving target. Finally, simulation results validate the effectiveness of the proposed algorithm.