TheReal-time Coarse Compensation ofmotion error BasedonUAVSAR

Keywords Unmanned AerialVehicles(UAV), SAR, Coarse compensation, Real-time imaging *I. INTRODUCTION Themethodofmotion compensation ofSARcanbe divided into twokinds: themotion compensation based on motion sensor andthemotion compensation based onthe echodata. Theformer utilizes theforward velocity to control pulse repetition frequency (PRF)toeliminate the azimuth aberration, whichiscaused byaircraft forward velocity change. Fortheboresight motion error, itutilizes radial acceleration tocompensate thechange ofDoppler frequency ofecho, sothat eliminates distance error between aircraft andtarget; Foraircraft attitude errors, itutilizes antenna servo device tocompensate yaw,pitch androll, so that theantenna canpoint tothenominal imaging area. The latter utilizes phase compensation ofechodata toeliminate thephase error caused bymotion error. Inthesystem of highresolution SAR,these twocompensation methods are usually usedtogether torealize precision-imaging. This paper mainly studies thetechnique ofmotion compensation based onechodata during real-time imaging.(I) Thedifficulty ofSAR motioncompensation has relation totheresolution andtheaircraft. Themotion compensation isveryimportant forhighresolution SAR (for example UAV SAR)onsmall aircraft, especially the motion compensation ofreal-time imaging. Thepre-filter andblockimaging areusually needed intheprocess of real-time imaging, andpre-filter includes low-pass filter and band-pass filter, in orderto improveSNR (Signal-to-noise ratio) andreduce dataratio. Theaircraft motion error mainly causes phaseerrors andsampling rangeshift. Andthephaseerrors maketarget Doppler frequency depart fromthecorrect frequency. The frequency shift errors caused bymotion error will besmall whentheaircraft motion error issmall. Pre-filter will not causethelossofechosignal, andmotion compensation canbecarried outafter pre-filter. Thefrequency shift of target echocaused bymotion error will belarge whenthe aircraft motionerrorislarge, andthismakestarget Doppler frequency outofthebandwidth ofpre-filter. In this case, thecoarse compensation ofmotion error mustbe donebefore pre-filter. Otherwise, pre-filter will lose useful echosignal andretain theuseless signal, sothat theimage oftarget can't beacquired. Theexisting motion compensation algorithms areall based onsmall motion error oroff-line process. Forhigh resolution SAR imaging system basedonsmall aircraft, thecoarse compensation ofmotion error isoneofthekey techniques inreal-time imaging, andaffects theimaging quality ofreal-time process andthegeometrical precision directly.