Researches On Spaceborne SAR System Working On Spotlight Mode

Spotlight, as one mode of the Synthetic Aperture Radar(SAR), is an effective means of realizing high resolution imaging of small region. It has strong application value in military field. Nowadays developed countries in West have developed airborne and spaceborne Spotlight SAR systems successfully and some of them have been used for military equipments. We have developed airborne SAR experiment system working in Spotlight mode successfully, but little has been done about spaceborne Spotlight SAR. Because of the important status in the military field and increasingly strong requirement of national defence development, this dissertation studies the spaceborne Spotlight SAR detailedly.On the basis of analyzing the characteristic of spaceborne Spotlight SAR, some theory frames and system methods are built including simplified space geometry model, echo model, theory formula of system key parameter selection, real-time control of antenna beam direction, echo simulation, imaging processing and so on. A integral theory system is formed about spaceborne Spotlight SAR.This dissertation is devoted to:1) By introducing local-sphere earth and local-circle orbit in the traditional space geometry model of spaceborne SAR, simplified space geometry model of spaceborne Spotlight is built according to the project fact. This model is strongly representative, as well as universal, which simplies the research of system problems.2) The range equation from SAR to target is deduced in the simplified model . The formula of synthetic aperture central place is deduced according to the requirement of zero centroid and frequency spectrum symmetry. The method to decide the start place and end place of synthetic aperture is provided. It is proved that spaceborne Spotlight SAR is working at squint looking mode usually.3) On the aspect of system parameter selection, the theory formulas of computing the azimuth resolution, synthetic aperture length and antenna sizeare deduced on the simplified model considering the earth rotation with SAR working at squinted looking mode. This dissertation introduces azimuth subsampling theory into Pulse Repetition Frequency(PRF) selection of spaceborne Spotlight SAR. It is proved that the method of PRF selection is consistent with that of spaceborne Stripment SAR.4) In the study of antenna beam direction real-time control, Broad-Sense Spotlight (BSS) mode is put forward as a new conception. Spotlight factor and efficiency function are defined for describing BSS SAR. The formulas of antenna beam real-control are deduced on the basis of analyzing the influence from yaw steering.5) The focus radius of squinted looking Spotlight SAR is studied which proves that it has the same formela as that of side looking Spotlight SAR.6) The detailed flow and experiment result of echo simulation are presented in this dissertation. Imaging algorithms for spaceborne Spotlight SAR are researched.