Research Of Sub-Aperture Motion Compensation In Airbore Ultra-Wideband Synthetic Aperture Radar

Motion compensation is a key technology to acquire high resolution SAR images in airborne Ultra-wideband Synthetic Aperture Radar (UWB SAR).In the first place, the basic problems of motion compensation in UWB SAR are analyzed, then the conclusion is obtained that the space-variant phase errors are of great importance and of great difficulties to be corrected. Having set up the model of the SAR platform motion errors, we study the effects of the motion error both in azimuth and range direction of the SAR platform on the images' focused performance. The acquired compensation precision for the motion errors of the platform is obtained according to the given guide line of imaging in UWB SAR.An algorithm named signal based motion compensation (SBMC) is selected to be deeply researched. The reason why it can not be used in UWB SAR is lie in the two following factors. One is that velocity errors estimated by Doppler tracker as well as displacement errors by Range tracker are added by fake errors introduced by serious Range Cell Migration (RCM). Another is that displacement errors gotten by Range tracker is not actual motion errors of SAR platform, but the "modulated" errors in LOS induced by wide azimuthal angle. An improved SBMC algorithm carried out after correction of RCM is presented to eliminate the negative effects of serious RCM. It is proved to be very powerful and can precisely pick up and efficiently correct the motion errors of the platform.With a wide azimuth angle and a wide width considered in UWB SAR, azimuth-variant and range-variant phase errors are analyzed quantificationally. Then, with the hypothesis of lowly varying motion errors, a new method of 2-D sub-block compensation based on original sub-aperture compensation algorithm is presented. In this method, the data range compressed is divided into sub blocks both in azimuth direction and in range direction. Accurate Compensation factors are calculated and Space-variant phase errors are compensated in every sub block according to the corresponding correlation between Doppler frequency of one target and its azimuth location and range location relative to the platform. For the sake of eliminating the effects of serious RCM on range-dependent errors, we proposed an improved sub-aperture compensation algorithm to achieve more precise compensation.