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This paper presents an improved polar format algorithm (PFA) for geosynchronous synthetic aperture radar which undergoes a near-circular track (GeoCSAR). GeoCSAR imaging geometry and signal formulation considering orbit perturbations were derived to ensure accurate slant range between antenna and targets. The illuminated area is more than one million square kilometers due to the long slant distance, resulting in large amount of data to be processed and that the scene is a spherical crown rather than a plane. By assuming spherical wavefronts instead of planar wavefronts, improved polar format algorithm (PFA) was proposed to focus GeoCSAR raw data on a spherical reference surface (ground surface), so that the size of focused scene is no longer limited by the range curvature phase error. Thus, this method could deal with large area imaging for GeoCSAR precisely and effciently. The implementation procedure, computational complexity, phase error and achievable resolution were presented to show the focusing capabilities of this imaging algorithm. Numerical simulation was further performed to validate the feasibility of this imaging algorithm and the correctness of analysis.

AN IMPROVED POLAR FORMAT ALGORITHM WITH PERFORMANCE ANALYSIS FOR GEOSYNCHRONOUS CIRCULAR SAR 2D IMAGING