Pulse Repeat Frequency (PRF) is a very important parameter in the Synthetic Aperture Radar (SAR) system. It affects other parameters and determines the radar performance. For a conventional procedure to select PRF for SAR, the earth is treated as a ball, and the orbit as a circle. At this time, the error from the calculating distance between the satellite and the target on the earth surface must exist. It severely affects the right selection of PRF. To avoid this error, in this paper, the model for the earth is an ellipsoid and the orbit is an ellipse. And it is specified a space coordinate, in which the error for the distance between the satellite and targets is corrected and the PRF selection for SAR is discussed. Finally the approach is used to derive an available PRF plot and the azimuth/range ambiguity to signal ratio. As comparison, the same results in circle orbit and round earth are specified.