High Resolution Imaging Of GEO SAR Under The Influence Of Terrain Elevation

Geosynchronous Synthetic Aperture Radar(GEO SAR)is a spaceborne SAR with an orbital altitude of about 36,000 Km,which operates in a geosynchronous orbit with a certain orbital inclination.Compared with low earth orbit SAR(LEO SAR),GEO SAR has many unique advantages.Due to its ultralong synthetic aperture time,wide swath,long dwell time and short revisit cycles,GEO SAR has bright application prospects in natural disaster monitoring and earth mapping.At present,the research on GEO SAR focuses on the imaging problem,therefore,this paper addresses the difficult problem of GEO SAR imaging affected by topographic elevation.The main work of this paper are listed as follows:Chapter 2 analyzes the accurate calculation of important Doppler parameters from the GEO SAR system design level.In order to keep the Doppler center frequency zero,yaw steering is needed for control,however,it is impractical to achieve a yaw angle rotation of tens of degrees on the colossal GEO SAR platform,which can be substituted by the two-dimensional(2D)phase scanning and can be adequately described in terms of squint and look-down angles.Within the synthetic aperture,the variation of the squint and look-down angles is considerable and must be taken into account when performing calculations of Doppler parameters(synthetic aperture time,Doppler bandwidth,and azimuthal resolution),otherwise it has a significant impact on subsequent processing such as imaging.Therefore,this section gives an accurate calculation of the Doppler parameters,taking into account the Radar Line of Sight(RLS)variation,and lays the foundation for the analysis in subsequent chapters.Chapter 3 presents a fast distributed target echo simulation method.Compared with the current echo simulation which is mostly about point targets or dot-matrix targets,this chapter focuses on finding a fast and accurate algorithm for distributed target echo simulation.Firstly,the three-dimensional(3D)coordinates of the target in the Earth-centered fixed Coordinate System(ECF CS)can be derived from the geometric relations,in which the position coordinates of each scattering point can be realized by interpolation to improve the calculation speed;then the elevation information can be added to the plane scene according to the constructed geometric mathematical relations or the existing Digital Elevation Model(DEM)to obtain different terrain elevation position coordinates.Finally,the theoretical scattering distribution model or the existing complex SAR image amplitude can be used as the scattering intensity input of the scene,and the accurate echo simulation can be performed according to the geometric model of the satellite platform and the scene.Subsequent imaging and compensation processing are performed on the basis of the echoes obtained by the fast simulation algorithm.Chapter 4 investigates the effect of elevation spatial variation on GEO SAR imaging,and provides the theoretical basis for the proposed improved imaging algorithm and elevation spatial variation error compensation algorithm in Chapter 5.From the basic echo signal model,the echo signal model considering the target elevation under different orders of slant range model is given;then the influence degree of elevation cavitation on LEO SAR and GEO SAR is compared,and the reason why elevation spatial variation must be considered in GEO SAR is explained;finally,the influence of various system parameters on elevation spatial variation is fully analyzed in the GEO SAR system,which provides the theoretical support for the system design of GEO SAR.In Chapter 5,the compensation of the elevation spatial variant error is achieved through a two-step processing process to obtain an accurately focused image and to inverse the elevation of the target based on the phase error obtained in the autofocus compensation process.The first step is to divide the complex scene of the region of interest into a slowly variant slope and a fast variant part based on the existing coarse DEM information;then the proposed improved RD-ACS(Range-Dopper Azimuth Chirp Scaling,RD-ACS)algorithm is used for the first step of compensation,and the relationship between the elevation of the slowly variant slope and its location is used.The elevation spatial variation can be decomposed into range and azimuth spatial variation,and then the range and azimuth spatial variation can be compensated to obtain a coarse-focused image;on this basis,the elevation spatial variation error caused by the remaining fast-variant part is compensated by the second step of processing.By combining the Map-drift Algorithm(MDA)and the Phase Gradient Autofocus(PGA)algorithm,we propose the Block MD-PGA algorithm,which can compensate for the residual elevation spatial variation error and obtain an accurately focused image.Finally,based on the phase error obtained in the autofocus compensation process,the relationship between this error and the target elevation is used to inverse the elevation of the target.