Adaptive Secondary Range Compression Algorithm in Geosynchronous SAR

In geosynchronous synthetic aperture radar (GEO SAR), due to larger equivalent squint angle and extremely complicated geometrical relationship between satellite motion and earth rotation at equator, the Doppler parameter space-variance becomes further strained, i.e., its value gets larger and its direction becomes uncertain. Meanwhile, the large imaging area brings in additional difficulties to compensate the Doppler parameter. In addition, since the synthetic aperture time is up to hundred seconds, the assumptions of the linear trajectory model and the Fresnel approximation appear to be decreasingly effective in GEO SAR, and the range cell migration and two-dimensional coupling also become larger. In allusion to the problems mentioned above, this paper proposes an improved secondary range compression (SRC) algorithm. First, special issues of GEO SAR imaging are analyzed, such as the Doppler parameter space-variance, the error of the linear trajectory model and the Fresnel approximation. Then, the effects of Doppler parameter space-variance on GEO SAR imaging are analyzed. Finally, the core issue of GEO SAR imaging at equator, i.e., adaptive phase compensation, is discussed in detail. The direction of Doppler parameter space-variance is determined, and the effects of Doppler parameter space-variance are compensated by sub-block processing, the processing is operated along the direction of Doppler parameter space-variance at an interval of calculated scale. Simulations of point array targets and area targets at equator are performed, and the correctness of this algorithm is validated.