Research Of Airborne Circular Synthetic Aperture Radar Imaging Technique

Circular synthetic aperture radar(CSAR)is a special SAR system whose antenna moves along a circular trajectory while illuminating the interior ground region of the scanning path.By acquiring multi-angular measurements over 360°,CSAR can provide higher image resolution,multi-dimension scattering information,and three-dimensional(3-D)imaging in comparison to the linear SAR(LSAR)system.However,the related motion compensation methods as well as the 3-D image reconstruction of CSAR are underdeveloped.The imaging technique of airborne CSAR still needs further research.Combining with the engineering application in practice,this paper focuses on the high-resolution CSAR imaging processing and 3-D image reconstruction.The main contributions of this dissertation can be summarized as follows:1.The characteristics of CSAR system are analyzed,mainly including the imaging geometry,the echo signal model and spatial resolution.Based on general impulse response function,the spatial resolution estimation methods of CSAR images,which are generated by the coherent processing and the noncoherent processing respectively,are derived.Compared with the traditional method,the proposed methods have higher accuracy and better practicality.2.The fast time domain imaging method is studied for the CSAR imaging.The main work includes:(1)By means of subaperture processing,the fast factorized backprojection algorithm for linear SAR is spread to the imaging of airborne experimental CSAR data.(2)The effect due to the error of topographic relief is given in detail,and the compensation method is proposed combining with the fast time domain imaging method.3.The autofocus imaging method for airborne experimental CSAR data is studied.The main work includes:(1)By choosing regions and balancing the energy distribution of the data,the extended autofocus backprojection(EABP)is developed based on the characteristics of CSAR data.(2)A CSAR autofocus imaging method based on subaperture processing is proposed to deal with the low accuracy of trajectory measurement and the long synthetic aperture of small airborne CSAR system.The autofocus imaging method makes an improvement of imaging accuracy,by using the EABP algorithm to process the echo data of subapertures and developing a chain model of image matching to decrease the geometric deformation in the accumulation of subimages.4.The 3D reconstruction strategy based on CSAR data.The main work includes:(1)Based on image fusion of CSAR images generated with different heights imaging plane,an automatic vehicle detection using CSAR image is proposed to enhance detection ability under complex environment,such as intensive parking.(2)A 3D reconstruction strategy of vehicle outline based on single-pass single-polarization CSAR data is proposed.First,an analysis of the distribution of attributed scattering centers from vehicle facet model is presented.And the analysis results show that a smooth and continuous basic outline of vehicle could be extracted from the peak curve of a noncoherent processing image.Second,the 3D location of vehicle roofline is inferred from layover with empirical insets of the basic outline.At last,the basic line and roofline of the vehicle are used to estimate vehicle's 3D information and constitute vehicle's 3D outline.All the research results are verified by the experimental data(obtained by low frequency CSAR system),the Gotcha data(provided by the American Airforce research laboratory),and the simulated data.The achieved researches in this dissertation have great theory signification and potential application for the high accuracy CSAR imaging.