This paper presents a new approach to calculate the accurate fourth-order Doppler parameters for Geosynchronous Synthetic Aperture Radar (Geo-SAR). To get exact calculation results, the Earth is modeled as an ellipsoid and the relative motion between the sensor in an elliptical orbit and the rotating Earth is analyzed. The J2, J3 and J4 orbital perturbation items and attitude steering are analyzed. Ignoring the perturbation force would produce errors of the Doppler parameters for spaceborne SAR because it can influence the six orbital elements. Since the Doppler parameters are related to the antenna beam pointing directions and influenced by attitude of SAR platform, the calculation results before and after attitude steering are shown. Furthermore, the Doppler parameter properties during the whole orbital periods of Geo-SAR are compared with those of Low-Earth-Orbital SAR (Leo-SAR). Finally, the effects on Doppler parameters stemmed from the radar beam pointing accuracy are analyzed.
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