Study On Image Formation For Geosynchronous And Low Earth Orbit Collaborative Imaging System

Geosynchronous earth orbit SAR(GEO SAR)has huge potential for application in environment and disaster monitoring with large coverage and short revisit cycle.Compared with monostatic GEO SAR,geosynchronous and low earth orbit collaborative imaging system with a GEO satellite as an illuminator,low earth orbit(LEO)satellites as receivers can provide finer spatial resolution and higher signal-to-noise ratio(SNR)with less system complexity.The research on the system design and imaging algorithm of GEO monostatic SAR has been rich in development so far.However,the research on Geosynchronous and low earth orbit collaborative imaging system is relatively scarce,and it is still in its infancy.That is reason this paper of study on Geosynchronous and low earth orbit collaborative imaging system has been carried out.The main research works of this thesis are listed following:1.Geosynchronous and low earth orbit collaborative imaging system can be modeled as a general bistatic SAR system with different orbit height,velocity,and velocity directions of transmitter and receiver.Under consideration of the influence brought by the rotation of the earth,Doppler parameter characteristics of the system is analyzed.And the patial resolution characteristics of the system are analyzed by the vector gradient method.The design process is given for the parameters of the imaging scene such as the synthetic aperture time,mapping bandwidth and pulse repetition frequency and so on.The feasibility of the system analysis is demonstrated at the end of chapter.2.The design method of multi-base receiver platform for improving the resolution is studied.The influence of the formation configuration between receivers on the improvement of resolution using imaging geometry and signal spectrum is then analyzed.The baseline vector is introduced as an intermediate variable to establish the relationship of relative orbital root number difference and resolution improvement factor,resolution improvement is used as the optimization objective function to acquire the optimal baseline vector using simulated annealing algorithm.Based on the principle of orbital dynamics,the formation configuration of receivers for the resolution improvement is obtained.3.As the imaging quality of traditional bistatic algorithm depends on the the accuracy of spectrum calculated by approximate method,and the calculation process of approximate spectrum is complicated,NUFFT(Non-uniform Fourier transform)based generalized keystone algorithm applied for large-scale,high-resolution imaging of Geosynchronous and low earth orbit collaborative imaging system is studied.The keystone algorithm avoids solving two-dimensional spectrum of the echo.The formula derivation analyzes the principle of using the general keystone method to eliminate the linear distance walk and nonlinear walk in the echo,the applicability range of the algorithm according to the phase error introduced by the keystone algorithm is then analyzed.And the approach of using the two-dimensional non-uniform Fourier for interpolation processing on non-uniform sampling points obtained by keystone transformation is purposed.Compared with the traditional keystone method,the theoretical analysis and simulation results verify that the generalized keystone algorithm has a good imaging effect for large-scale scenes.