Study On Imaging Mechanism And Algorithm Of High-resolution Circular Synthetic Aperture Radar

Synthetic Aperture Radar (SAR) is a kind of radar in microwave remote sensing which can obtain high-resolution image of target area with the capability of all-weather and all-time operations. High resolution imaging is one of the important goals during the development of SAR. With the advancement of Stripmap SAR and Spotlight SAR, the resolution of SAR imaging is continuously improved. However, because of the straight observing path of Stripmap SAR and Spotlight SAR, the extent of spatial spectrum is limited, and thus the resolutions of both range and azimuth are restricted. Circular SAR breaks up this restriction on the extent of spatial spectrum with circular observing path and realizes the objective of improving the resolution further.The dissertation focuses on theoretical and technical problems of high-resolution Circular SAR. According to the relationship between spatial resolution and the extent of spatial spectrum, the principle of high resolution imaging of SAR is discussed from the perspective of signal processing. The discussion about resolution in range and azimuth makes a beginning for understanding SAR. From the difference of constructing the synthetic aperture between Stripmap SAR and Spotlight SAR, the dissertation explains the reason why Spotlight SAR gets finer resolution in azimuth than that of Stripmap SAR. For Stripmap SAR and Spotlight SAR, wider azimuth spectrum leads to higher azimuth resolution while range resolution keeps almost invariant. The dissertation summarizes the conclusion that signal bandwidth determines the range resolution and length of synthetic aperture determines the azimuth resolution in linear SAR.Based on the analysis of linear SAR, the principle of Circular SAR getting higher resolution in both directions is examined. Using Computerized Tomography (CT) and its Filtered Backprojection algorithm as references, the imaging mechanism and algorithms of Circular SAR are studied. The procedures and results of algorithms of Time Domain Correlation, Backprojection and Circular path approximated by linear subpaths are demonstrated. The characteristics of Circular SAR imaging are observed through the study of imaging algorithms as follows:under condition of ideal point target in focusing plane with full aspect angle(360o) observation, the resolutions of range and azimuth obtained by Circular SAR are almost equal. The resolutions are determined by the wavelength of signal rather than bandwidth of signal. When the maximum frequency of signal is fixed, the increase of signal bandwidth decreases the resolution and the main effect of signal bandwidth is to reduce the sidelobe.Circular synthetic aperture makes Circular SAR with single frequency signal possess the ability to get high-resolution image of target area. Theoretical analysis of this ability provides a good basis for deriving the formulation of system response of Circular SAR with certain signal bandwidth. Therefore, the characteristics of Circular SAR imaging are verified from the aspect of mathematics. Another ability of Circular SAR is to distinguish targets with different heights. Compared with linear SAR, these particular features of Circular SAR are studied.The problems of sidelobe reduction, motion compensation and moving target imaging are also considered in the dissertation. In the end, the experiment schemes for testing the performances of Circular SAR are presented and the applications on near space platform and space platform are discussed.