Study On The Wavefront Curvature Compensation For The Polar Format Algorithm In Synthetic Aperture Radar

Synthetic aperture radar (SAR) can achieve fine-resolution images in two dimensions and providesome specific information for long-distance targets in all day and all weathers making it widely usedin military and civil areas. Image formation algorithm is the core of SAR signal processing. In allkinds of time-domain and frequency-domain algorithms, polar format algorithm (PFA) is popularbecause of its high calculation efficiency and easy combination with motion compensation capability.Unfortunately, the classic PFA, due to wavefront curvature, has limits on the geometric fidelity andscene size. At the same time, a high aperture is required to achieve the high resolution in azimuth,making the wavefront curvature compensation in PFA more difficult.In bistatic SAR, the separation of transmitter and receiver results in a complex form of theDoppler phase term of the received signal, and hence the wavefront curvature compensation forbistatic PFA is complex. The core of this paper is the wavefront curvature compensation for PFA.According to above problems, we study the methods based on spatially variant post processing anddigital spotlighting, and use the principle of chirp scaling (PCS) to realize the two-dimensionresample for high algorithm efficiency.Chapter1is the introduction. The history and latest developments of monostatic and bistatic SARare introduced firstly. And then the key problems during the research are presented and the maincontents of this thesis are outlined.In chapter2, the implementation of the principle of chirp scaling is proposed, which could totallyavoid interpolations. Firstly, the principle of PFA is briefly introduced. Then, the range cell migrationcorrection process in polar format transform is studied, which shows that the polar format transformcan also be interpreted as a combination of a range scaling and an azimuth scaling. Based on thescaling transform interpretation, the processing flow of the PCS based PFA is derived in detail. Finally,the conventional interpolation based PFA and PCS based PFA are compared from the viewpoint ofimage quality and processing speed.In chapter3, the PFA based on spatially variant post processing is studied. Firstly, we analyse thephase error caused by wavefront curvature, which is the foundation of compensating the hypothesis ofplane wave. After that, the space variant filter is designed and applied to compensate for thedefocusing effect, and interpolation operation is used to correct for the geometry distortion. Theseoperations can enlarge the effective focused scene size in PFA image.Chapter4analyzes the PFA based on digital spotlighting. The flow of digital spotlight technology is introduced first, and the method is improved adaptively to reduce the computation complexity. Thenthe algorithm process is deduced and introduced detailed, and an improved method is showed. Thesimulation experiment compares the PFA with digital spotlighting to the PFA with spatially variantpost processing in large maneuvering.Chapter5analyzes and corrects for the wavefront curvature error in bistatic PFA image. Thebistatic spotlight mode SAR data collection geometry and bistatic PFA are introduced first. Then wederive the analytical expression of wavefront curvature and design the space variant filter. At last, thenotes in digital spotlighting use for bistatic PFA are discussing.In the end, the research work of this thesis is concluded and some other aspects to be furtherstudied are pointed out as well.