Imaging Algorithms For Typical Operation Modes Of Geosynchronous Orbit SAR

Geosynchronous orbit synthetic aperture radar(GEO SAR)runs on the geosynchronous orbit with a certain orbit inclination.Compared with the conventional low earth orbit SAR(LEO SAR),the GEO SAR has the advantages of higher temporal resolution,larger imaging swath,longer time observation and so on.Therefore,GEO SAR is widely used in military or civilian applications,such as military reconnaissance,resource exploration,disaster detection,terrain mapping,marine environment,and other fields.At present,it has been concerned by the aerospace powers such as the United States,Russia,the European Space Agency.However,because of the characteristics of long echo time delay,long synthetic aperture time,slow speed,complex motion,wide scene swath and the earth's rotation,the system parameter design and imaging processing are more complex.For example,the long echo time delay leads to the assumed “stop go stop” in the echo signal model failure.Both of long synthetic aperture time and the complex trajectory led to the conventional secondary slant distance expressions are not suitable for GEO SAR slant distance model description.The squint angle is introduced by the rotation of the earth which causes serious coupling between range direction and azimuth direction.The wide imaging swath and time variant Doppler parameters result in the existence variant in the imaging algorithm.Therefore,the existing imaging algorithm in LEOSAR can not directly used to GEO SAR.In this dissertation,combined with the characteristics of geosynchronous orbit,the echo signal models of GEO SAR in different imaging modes are established,with which the imaging processing methods of three typical imaging modes,such as stripmap mode,-spotlight mode and mosaic mode,are studied.The main and contributions are as follows:(1)Combined with theoretical derivation and simulation example,the GEO SAR imaging characteristics is analyzed in-depth which include sub-satellite points,satellite speed,synthetic aperture time,Doppler frequency characteristics.The imaging characteristics supports a theoretical foundation for imaging algorithm research.Due to the conventional low earth orbit SAR parameters design can not be used to GEO SAR parameters design,the parameters design methods of geosynchronous SAR are proposed based on the principle of the radar,including the resolution,ambiguity and timing design.The system parameters design which supports for parameters design in imaging algorithm is proposed for GEO SAR.(2)The imaging algorithm of GEO SAR in stripmap mode is studied.The echo signal model is established where considering the impact of the “stop and go” assumption.To overcome the problem of that conventional two-order slant range model cannot describe the slant range of GEO SAR accurately,the expression of the slant distance in GEO SAR based on the fourth-order expansion is proposed.An imaging algorithm based on Chirp Z Transform(CZT)and azimuth chip-scaling is proposed.The CZT transform is used to correct the range walk,and to mitigate the Two-Dimensional Spatial-Variant Properties of GEO SAR.The linear time variation of three term expansion coefficients of azimuth signal is eliminated by improved scaling factor.The effectiveness of the algorithm is verified using simulation and the measured data in ground rail vehicle experiment.Simulation and real data processing results show that the proposed algorithm can achieve the strip mode focusing and imaging.(3)The imaging algorithm of GEO SAR spotlight mode is studied.GEO SAR staring observation has ultra-long integration time that conventional frequency domain algorithms cannot handle it because of the used inaccuracy slant model and existing azimuth aliasing.To overcome this problem,this thesis proposes an improved chirp-scaling algorithm that uses a fifth-order slant range model where considering the impact of the “stop and go” assumption to overcome the inaccuracy of conventional slant model and a two-step processing method to remove azimuth aliasing.Furthermore,the expression of two-dimensional spectrum is deduced based on a series of reversion method,leading to the improved chirp-scaling algorithm including a high-order-phase coupling function compensation,range and azimuth compression.The imaging performance evaluation shows that the imaging algorithm can realize the focusing imaging in spotlight mode.(4)The imaging algorithm of GEO SAR in mosaic mode is studied.According to the characteristics of Mosaic mode of GEO SAR,the Doppler frequency is analyzed,and the echo signal model is presented.Then,an improved two dimensional Nonlinear Chirp Scaling(NLCS)imaging processing algorithm is proposed.The space variant in range is effectively solved by considering the characteristics of first-order linear and quadratic nonlinear in equivalent frequency modulation with time delay.Similarly,the space variant in azimuth is effectively solved by the nonlinear chirp scaling algorithm which considers the first-order linear and quadratic nonlinear characteristics in Doppler frequency modulation within time delay.The high order phase error leading by the nonlinear frequency modulation is compensated.The effectiveness of the algorithm is verified by point target simulation.