Investigation On Imaging Algorithm And Scattering Property Of Spaceborned-Ground Bistatic SAR

Recently bistatic radar with a fixed receiver has been a hot topic for its advantageof flexible configuration, abundant information, anti-interception and anti-interferenceetc. However, as for the real bistatic synthetic aperture radar (BiSAR) system, theintroduced time and phase synchronization errors could not be ignored and willinfluence imaging seriously. Meanwhile, it turns harder under the specialconfiguration of BiSAR with a stationary receiver, for the spatial variation of echo intwo dimensional is serious, which make traditional imaging algorithm invalid.This paper firstly deals with the impacts on BiSAR imaging caused bysynchronization errors and spatial variation of bistatic echo, and proposes the imagingalgorithms of synchronous echo(without synchronizatio errors) and non-synchronousecho (uncompensated synchronizatio errors) respectively. Then, considering theadvantange of BiSAR in obtaining abundant target information, this paper mainlyconcerns about the possibility of improving target’s detection and resolution based onthe multi-angle illumination of BiSAR with a fixed receiver, and derives the coherentsynthesis method of small bistatic angle and incoherent synthesis method of largebistatic angle. The detailed contents are as follows:1. The extended non-linear chirp scalling (NLCS) imaging algorithm towardsthe synchronous echo is proposed, which could focus the bistatic echo underlarge scale and squint angle. Firstly, under the special configuration ofSpace-ground hybrid BiSAR, this paper analyzes the impacts of high orderphase and spatial variation of frequency modulation (FM) rate on rangefocusing. Then, considering the invalidity of assumed quadratic phase modelin traditional imaging algorithm, the cubic phase model is proposed andcould compesate the high order phase in the range direction. After that, thespatial variation of range cell migration and FM rate are all compensated,according to the multiply of quadratic and cubic perturbation terms inrange-Doppler domain, as well as the expanded FM rate in the referencepoint. Finally, both the simulation and experimental results are all used tovalidate the proposed algorithm, based on the result comparison of traditionaland extended NLCS imaging algorithm.2. The imaging formation algorithm based on direct signal towards the non-synchronous echo is proposed. As for the real BiSAR system, for thesynchronization errors could not be eliminated in the absence of accurateorbit data and some important system parameters information, thus thetraditional imaging algorithms could not be directly implemented tonon-synchronous echo. Firstly, the quadratic phase model of direct signal isderived to estimate the Doppler chirp-rate, then the focusing in the rangedirection could be finished by searching for the optimal Doppler centroid andsquint angle based on the rule of maximum image contrast and optimazition.After that, the range cell migration correction (RCMC) and the azimuthfocusing are all implemented by the extracted range cell migration and peakphase in the direct signal. Finally, the proposed algorithm is validated byimaging of the BiSAR experiment configured with YAOGAN satellite as thetransmitter.3. Based on the multi-angle illumination of BiSAR with a fixed receiver, thispaper analyzes the coherent synthesis method of small bistatic angle andincoherent synthesis method, which could improve target’s detection andresolution by using all the obtained radar images. At first, under the specialconfiguration of BiSAR, the scattering property of certain targets under themulti-angle illumination of BiSAR is studied in detail, and the correspondingcalculation equations of target’s radar cross section (RCS) are also given.Then, as for the BiSAR system with small bistatic angle, spectrum offset willbe introduced in the received echo under the multi-angle illumination oftransmitter, so the coherent synthesis method thus can be applied tosynthesize the wide-bandwidth signal and realize high resolution in twodimensions. When the bistatic angle turns larger, considering the scatteringproperty of target varies seriously with the look angle, althrough the coherentsynthesis method of small bistatic angle could not be directly used, theobtained abundant target scattering information can be synthesized toimprove target’s detetion and enrich the information of radar images.