Autofocus for Motion Error Compensation in High Range Resolution UAV Synthetic Aperture Radar

Unmanned air vehicle (UAV) sensitivity to atmospheric turbulence leads to deviation from the straight trajectory and results in motion error in the UAV synthetic aperture radar (SAR) images. In this paper, motion error compensation of a UAV SAR, which works in the stripmap mode at low altitude and uses stepped linear frequency modulation (LFM) signaling for high range resolution is investigated and two joint motion compensation and image formation approaches are proposed. For motion compensation, the existing phase gradient autofocus methods are extended to the case of stepped-LFM signaling. The motion error is estimated based on the echoed data of the middle stepped-LFM sub-pulse and is applied for error compensation of all sub-pulses. This idea significantly reduces the autofocus computational complexity in high resolution UAV SAR. Simulation results of the proposed approaches for polynomial and sinusoidal errors show their good motion compensation performance with ISLR and PSLR values close to -20 dB for point targets. Therefore, the proposed approaches are appropriate for motion error compensation and image formation in high resolution UAV SAR systems.