Characteristics of Geosynchronous Synthetic Aperture Radar (GEO SAR), including ground velocity, integration time, azimuth resolution and other factors, are investigated. And the relationship between them and orbital elements is analyzed. Then a subaperture mode based on yawing steering which is more suitable for GEO SAR is proposed. By applying this mode reduction of range migration is obvious, and system parameters of L-band GEO SAR are designed, including the antenna size, the pulse width, the peak transmitted power, and pulse repetition frequency. Finally a modified Range Doppler (RD) algorithm based on quintic polynomial and a Chirp Scaling (CS) algorithm based on spectrum mosaic are advanced. Point target simulation is implemented and the azimuth resolution is assessed according to the rule of being able to distinguish adjacent targets. It is demonstrated that the proposed image formation algorithms can provide a spatial resolution better than 15m.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.