Multi-Mode SAR System Design And Imaging Processing Technology Of Low-Orbit SAR

Synthetic Aperture Radar,as an active microwave detection remote sensing system,is not affected by weather changes and climate changes,and can achieve high-resolution and largearea scene-to-earth imaging capabilities at any time and in any environment.Spaceborne SAR,as an effective means to quickly acquire surface changes,has been widely used in many fields such as land surveying and mapping,resource surveys,military investigations,and environmental monitoring with the continuous development in recent years.For the low-orbit satellite SAR system,its launch cost is low and its application is more extensive.The low-orbit satellite SAR has a long revisit period and a narrow surveying band.It is necessary to design more effective system parameters,including orbit parameters,radar system parameters,and design system working modes.According to different imaging modes,design corresponding imaging algorithms to give full play to low The SAR performance of orbiting satellites,especially in the reconnaissance and imaging of surface ships,better serves my country's national defense modernization.This thesis mainly conducts related research on the integrated design and imaging processing technology of low-orbit spaceborne multi-mode SAR satellite-ground.The main work of the thesis is as follows:1.Systematic study of low-orbit SAR satellite-ground characteristics analysis.First,a geometric model of the LEO SAR orbit was established,and its general motion characteristics were analyzed.The sub-satellite point trajectory,beam coverage,and revisit performance were studied and analyzed.The Doppler parameter expression of the target is mainly deduced,and the Doppler characteristic of the target is analyzed.Secondly,the yaw traction law and the synthetic aperture time of the point target are studied.According to the requirements of the observation task for the resolution and the size of the scene,several imaging modes of the spaceborne SAR system,including strip mode,spotlight mode,and TOPS mode,are studied and designed,and the control strategy of the radar antenna pattern beam pointing under different imaging modes is studied.2.A design method for beam position parameters of low-orbit SAR system is proposed.In view of the high orbital height of the low-orbit satellite and the large scene coverage,the focus is on the selection of pulse repetition frequency(PRF)that meets multiple constraints such as range ambiguity,azimuth ambiguity,and optimal survey zone.The PRF selection strategy and beam position iteration strategy are proposed,and multiple sets of effective radar system parameters are designed,which lays the foundation for the subsequent imaging algorithm research.3.Research on low-orbit SAR imaging algorithm,and propose a frequency domain imaging algorithm for variable PRF spotlight and TOPS working mode.In this paper,CS algorithm is used for imaging in strip mode;In order to deal with the problems of long-distance walking and large echo data redundancy caused by spotlight and TOPS mode,a variable PRF technology is proposed to perform echo memory.Aiming at the problem of azimuth nonperiodic non-uniform sampling in the SAR working mode of variable PRF technology,an imaging algorithm combining extended omega-K algorithm and time-frequency scale transformation is proposed.This method first completes uniform sampling of the azimuthfree signal in the time domain,then uses the omega-K algorithm to complete the range migration correction,and finally uses the azimuth frequency scale transformation method to process the remaining phase.Simulation experiments show that the imaging algorithm proposed in this paper can accurately image multi-mode low-orbit SAR ground observations,which verifies the effectiveness of the algorithm.