Roll Steering of Yaw–Pitch Steered SAR for Reducing Ground–Target Pointing Error

While attitude steering in the yaw and pitch axes effectively minimizes a residual Doppler centroid in spaceborne synthetic aperture radar systems, it can induce a ground-target pointing error due to the steered radar boresight. In other words, the beams after steering may no longer illuminate the desired target. In this letter, a novel method is proposed to compensate for the target error. The roll angle and image start time, the two degrees of freedom that may be modified while maintaining the zero Doppler centroid condition, are utilized as control parameters. By correcting the roll and time, the associated target error can be significantly reduced. Two solutions are proposed in this letter. The first is a numerical accurate solution, and the second is an analytical approximate solution. One of the solutions can be implemented on-board after a tradeoff study is performed between the remaining target error and computational load. Simulation results verify that the target error is considerably reduced while the Doppler centroid is minimized with the proposed methods.