Abstract: The effect of roll angle error on the range dependent gain correction is analyzed for spaceborne Scan Synthetic Aperture Radar（ScanSAR）. The model of the range dependent gain is proposed for the bursts image processing. The basic precision of the roll angle estimation is present to satisfy the image radiometric correction requirement， which provides a theoretical guideline for the design and analysis of spaceborne ScanSAR，as verified by simulations.