Optimization Of Corner Reflector And Its Application In Spaceborne SAR Image

Synthetic Aperture Radar is a high resolution microwave imaging radar and can work independent of all time,all weather and over long distance.SAR plays a key role in military and civil applications.SAR geometric calibration and radiometric calibration are the premise of quantitative application.But some issues concerning SAR geometric calibration and radiometric calibration are not well resolved,which hinder this advanced technology from being applied on a large scale and being industrialized.The satellite needs to debug,verify and calibrate after the launch,and this stage is short,especially for commercial satellites.As reference target for radiometric calibration and ground control point for geometric calibration,in order to acquire much more test data in the limited time and calibration field,the 3 dB width of corner reflector(CR)should cover all the incident angles of SAR.Then the RCS of corner reflector should be calculated.First,this article shows the importance of corner reflector's RCS calculation,because it is used widely in the SAR geometric calibration and radiometric calibration.The theoretical basis of RCS is introduced including the definition of RCS and calculation method of cavity structure,for example finite difference time domain,method of moment,method of waveguide mode,method of shooting and bouncing rays,method of Gaussian beam;method of complex ray.Because of the SAR CR for electrically large target,the methods of RCS calculation mentioned above are all inefficient and consume a large number of computer resources.Aiming at CR in plate construction,a kind of fast calculation method is proposed based on the combination of geometrical optics and Gordan bin integral method.The method does not need to subdivide the plate,just apply geometrical optics to tracing the light for the lighten area and do the contour integral on them respectively,at last accumulate all the integral results.The fast calculation method of RCS above is verified.First the RCS of dihedral corner reflector is compared.Regardless of the dihedral angle is acute angle,rectangular or obtuse,in the high frequency region,the results in this paper differ with FEKO simulation and literature under 0.3 dBsm.To compare the trihedral corner reflector used in the SAR radiometric calibration,the difference of this paper and FEKO-MLFMM simulation is less than 0.45 dBsm.Especially in the range of 20°?80°(cp)and 20°?80°(?)the difference are both less than 0.1 dBsm.So it is used widely for the minimum edge diffraction in the above angles.In addition,suggested method with FEKO MLFMM and PO algorithm in computation time is shorten nearly 413 times,4696 times(dihedral CR)and 383 times,9174 times(trihedral CR).Moreover the suggested method consumes very few computer resource and the results by FEKO are all calculated under 4 cores 8 threads CPU and 32GB memory.Besides the simulation,the RCS frequency sweep test system of microwave chamber is employed to verify simulation results.Its time domain function can reduce the requirement of test environment and realize high precision measurement greatly.The differencebetween test and suggested results is less than 0.5 dBsm.As seen,the suggested method is accurate,saves calculation time greatly,reduces memory consumption and improves the efficiency of simulation significantly.Compared with the high test cost,the suggested method above is much more economical.By this method,laws of corner reflector' s 3 dB widths are derived through plenty of simulation calculations.The frequency of SAR antenna's incident wave and overall dimension of CR only change the size of RCS and do not change the width and location of 3 dB.But corner reflector whose edges are always equal conventionally can be made optimization design to improve its 3 dB width by the laws.In addition,the 3 dB width will change with the CR placement angles.Base on the above laws,this paper adopts the length ratio and placement angle for the optimization design of TTCR,to obtain 3dB width as much as possible.First,3dB width of different length ratios is obtained through the above method(fixed length a=lm,length b is from 0.2m to 5.5m in the interval 0.1m).Then according to the incident angle of SAR antenna,the placement angle of TTCR is adjusted,so as to reach the wider 3dB.The experimental area is located in Wuhan suburb and shot by the satellite TerraSAR-X.When the length raio is 1:1:1.1 and the placement angle is 4.7°,the placement angles of CR have nothing to do with the incidence of SAR antenna.The corner reflectors after the length ratio and placement angle optimization design are used in the experimental area.Based on the optimization design of CR and radiometric calibration of TerraSAR-X,the validity of CR design is confirmed.Integral method is adopted in the radiometric calibration used in the test area.The precision of relative and absolute radiometric calibration are 0.29 dB and 0.32 dB respectively which are better than 0.3dB and 0.6dB mentioned in TerraSAR-X technical documentation.The Range-Doppler geolocation model and affine transformation model are established to accomplish the geometrical calibration of TerraSAR-X.By comparing different mapping correction model,affine transformation model is more accurate than polynomial model in the test area(hilly land).Higher precision is achieved by less ground control points.Furthermore the precision can be improved by interpolation and centroid extraction.The distance accuracy is 0.3 m,the azimuth accuracy is 0.4 m and corresponding 0.3m,0.5m in TerraSAR-X technical documentation respectively.By comparison of RCS between CRs with azimuth deviation and normal incidence,the 3dB width of CR is more than 36° within incident angles 14°?59°which is bigger than the local azimuth span of SAR antenna(27.350).In other words all transit situation of TerraSAR-X can be overrode.The design is accurate and reduces the complexity of adjusting the CR in the radiometric calibration greatly,increasing the service efficiency of CR.