Research On Monostatic/Bistatic Imaging Technology For Medium Orbit SAR

Synthetic Aperture Radar(SAR)has the advantages of all-weather,all-weather,long range and strong penetration,and is widely used in military reconnaissance and civil surveying and mapping.With the development of aerospace technology,remote sensing systems that carry SAR payloads on satellite platforms have become a research hotspot.The satellite-borne SAR currently in orbit is mostly Low Earth Orbit SAR(LEO SAR).Its azimuth resolution and the width of the survey strip are mutually restricted,the revisit period is long,and the anti-attack ability is weak.Increasing the satellite orbit height is the solution to these problems.Effective method of problem.Medium Earth Orbit SAR(MEO SAR)has an orbital altitude of2000 to 20000 kilometers.Compared with LEO SAR,its beam coverage is wider,with wide-area surveillance,wide survey band and high-resolution imaging,short revisit period,and resistance.The advantages of strong destruction ability are of great significance to the development of remote sensing systems in the future.Due to the increase in orbit,MEO SAR consumes higher power than LEO SAR.The MEO satellite-aircraft dual-base SAR system with MEO SAR satellite as the transmitter platform and low-altitude aircraft as the receiver can solve this problem and realize the sensing High-resolution imaging of the region of interest.The MEO SAR satellite runs in a higher orbit,and the synthetic aperture time is relatively long,which leads to the bending of the trajectory.The traditional SAR slant range model is no longer valid,which makes the imaging algorithm suitable for LEO SAR ineffective and unable to achieve high-resolution imaging of MEO SAR.Starting from the system characteristics,this paper analyzes the system parameters of MEO SAR,and conducts in-depth research on Doppler characteristics and beam control,single/dual base slant range model and echo signal model,two-dimensional frequency spectrum,imaging algorithm,etc.The main research work is as follows:(1)The space geometric model of MEO SAR is established,the influence of orbit parameters on the trajectory of sub-satellite points is explored,and the motion characteristics of MEO SAR are analyzed.Combined with the system characteristics,the precise synthetic aperture time and two-dimensional resolution of MEO SAR are derived,and the choice of pulse repetition frequency is studied.(2)According to the motion parameters of the satellite,the change rule of the Doppler parameters of the MEO SAR during the satellite operation period is studied,and the changes of the Doppler parameters caused by the satellite attitude disturbance are analyzed.Aiming at the change of Doppler center,the beam yaw control methods in circular orbit and elliptical orbit are respectively introduced.(3)Aiming at the curved trajectory characteristics of MEO SAR,the applicability of the linear slant range model and the nonlinear slant range model is discussed,and a more accurate non-"go-stop" slant range model is given.The echo model of the point target is established,the two-dimensional frequency spectrum is derived,and the imaging algorithm for small scenes is given.Further analysis of the two-dimensional spatial variability of the MEO SAR large scene echo,two compensation algorithms for azimuth spatial variability are proposed,one is the ENLCS algorithm that achieves azimuth equilibrium through azimuth nonlinear scaling,and the other is based on two-step azimuth.Improved CS algorithm for space change compensation.(4)On the basis of the geometric model,the echo model of the MEO satellite-machine bistatic SAR is established,the rationality of the two single-base equivalent slant range models is compared,and the imaging algorithm is given.In order to achieve more accurate imaging,an imaging algorithm based on series inversion was studied.Aiming at the severe spatial-variant of the echo azimuth under the squint situation of the receiver,an accurate spatial variability model is proposed and verified by the NLCS algorithm.