Radar Parameter Design for Geosynchronous SAR in Squint Mode and Elliptical Orbit

Low-inclined elliptical orbits are recommended for geosynchronous synthetic aperture radar (GEO SAR) to map the regions of interest at middle latitudes. However, the radius variation of such orbit results in a time-variant nadir interference and slant range of illuminated regions. These factors shorten the available imaging time for a specific pulse repetition interval (PRI), which may be even shorter than the required integration time. Besides, the squint mode widely used in GEO SAR brings in the nonorthogonal and nonuniform ground resolution, which cannot be completely described by the traditional range and azimuth resolutions. To solve these problems, first, the ground resolution area is proved to be an ellipse, and the axis lengths of the resolution ellipse are used to present the worst and the best resolutions along various directions. Conditions for orthogonal and uniform ground resolutions are investigated, and analytical expressions of the resolution-related parameters are derived. Second, effects of the orbit radius variation on data acquisitions are analyzed, and a continuous PRI variation method is proposed to lengthen the available imaging time. By varying the PRI with a constant time interval, the acquisition window and transmit interference are changed with the time-variant illuminated region, which loosens the constraint of the transmit interference. Finally, the proposed ground resolution expressions and continuous PRI variation method are verified by computer simulations.