Study On Imaging Algorithms For Circular Trace Scanning Synthetic Aperture Radar

Synthetic aperture radar(SAR)has been dramatically developed and widely used since the concept was first raised several decades ago due to its advantages of high resolution imaging which is independent on the weather and great day and night imaging capability.To further improve the resolution,a variety of operation modes of SAR have been developed.In particular,the curvilinear SAR has the advantages of wide field and real-time imaging compared with linear SAR.Over several kinds of curvilinear SAR,Circular Trace Scanning SAR(CTSSAR)is of special interest thanks to its capability of repetitive imaging and wide angle of view,enabling fast imaging over a large area.For CTSSAR system,the antenna is mounted at a turntable,which is under uniform circular motion at a certain altitude,and its beam is pointing backward to the center,leading to a ring-shaped swath.This thesis mainly focuses on the study of high resolution imaging algorithm of CTSSAR.CTSSAR has different Doppler frequency shift compared with the traditional SAR for its uniform circular motion.This thesis analyzes the range spectrum and the two-dimension(2-D)spectrum of CTSSAR based on its signal model.Then the Doppler frequency shift caused by the relative motion between the antenna and the target is investigated,demonstrating that under certain conditions the echo signal of CTSSAR in azimuth domain can be approximated as linear frequency modulation signal.Moreover,the principle of imaging is described thoroughly from the perspective of high resolution and the governing factors of the azimuthal resolution are examined.It's been found that the obtained azimuth of point target moving along radial direction from CTSSAR using the same algorithm as that of a static target deviates significantly from the actual azimuth,and the deviation has a liner relationship with the radial velocity.To overcome this problem,we proposed to obtain the accurate azimuth by correcting the azimuth matching filter function with the radial velocity of point target taken into consideration,of which the feasibility was verified in various scenarios with single and multiple objects.Finally two methods for the estimation of the radial velocity of moving target were proposed,that is,speed traversal and 2-D FFT algorithm,for which the advantages and constraints were discussed.To test the performance of CTSSAR,the experiments working in different modes are presented at last.To satisfy the requirement of different detection ranges and resolutions,the CTSSAR can work in 14 modes including 10 FMCW modes and 4 pulse modes,respectively.In addition,this thesis presented a new antenna calibration method and the effecltiveness is validated with the field experiment.