Study On Wide Swath Spaceborne Synthetic Aperture Radar(SAR) Signal Processing

Wide swath imaging is one of the most important trends of the modern spaceborne Synthetic Aperture Radar (SAR) development, and it attracts a wider interest in the spacebome microwave remote sensing community. Conventional burst imaging modes, such as ScanSAR and Terrain Observation by Progressive Scans (TOPS) SAR, obtain the wide swath imaging capacity at the cost of the impaired azimuth resolution.The multi-beams SAR and MIMO-SAR can obtain high resolution wide swath (HRWS) capacity by introducing additional spational samples and overcoming the inherent contradiction between azimuth resolution and range swath width. However, there are still some technical problems with wide swath imaging, such as moving targets imaging, range ambiguity suppression, and multiple pulse echoes separation.The topic of this thesis comes from the National Natural Science Fund Project "HRWS SAR signal processing technology with PRF variation" (project number: 61401431) and "Research of power amplifier energy efficiency based on cognitive and polarization signal processing" (project number: 61271177). In this dissertation, some signal processing problems in the wide swath spacebome SAR imaging modes are deeply studied. Major contributions and innovations of this thesis are listed as follows:1. Echo properties of moving targets in the wide swath spaceborne single beam TOPS SAR are deeply studied, and reasons of moving targets defocusing handled by the conventional TOPS SAR imaging processor are analyzed. According to the echo signal model of moving targets in TOPS SAR, a new imaging approach of moving targets in single beam TOPS SAR is proposed, and it is validated by simulation experiments.2. Signal processing properties of wide swath spaceborne azimuth multi-beams SAR are deeply studied. The signal model of azimuth multi-beams ScanSAR with up and down chirp signals is analyzed, and a new imaging approach is proposed and validated by simulation experiments. simulation experiments on point and distributed targets are carried out to analyze the effect on range ambiguities suppression in wide swath azimuth multi-beams ScanSAR with alternately sending up and down chirp signals. Furthermore, wide swath spaceborne zimuth multi-beams sliding spotlight and TOPS SAR mode are deeply studied.According to Doppler properties of the two modes, two imaging approaches based on sub-aperture data division and full-aperture multichannel reconstruction are proposed, and simulation experiments are carried out to validate the proposed imaging approaches.3. The implementation of range digital beamforming (DBF) in wide swath spaceborne SAR systems is discussed. Furthermore, range DBF processing in spaceborne SAR modes with multiple transmitted sub-pulses in a single pulse repetition interval (PRI) is analyazed. A new processing approach based on the null steering to separate echoes corresponding to different sub-pulses is proposed, and simulation results validate the proposed approach.4. The beamspace multiple input multiple output (MIMO) SAR is deeply studied, and a new imaging approach is proposed according to its echo signal properties. The keypoint of the proposed imaging approach includes two parts: echoes separation onboard and azimuth multichannel reconstruction to resolve the non-uniform sampling. Echoes separation in beamspace MIMO-SAR is implemented by the null steering DBF processing, and MIMO-SAR multichannel reconstruction is implemented based on the MIMO-SAR signal model and the conventional azimuth multichannel reconstruction. Simulation results validate the proposed imaging approach.