Abstract: Radar imaging of ocean-going ships is a function of both SAR and ISAR imaging techniques. Both the radar and the target are moving so the range-Doppler image formation is not due to either a SAR or ISAR imaging mechanism acting alone, but to contributions from both. In this paper we analysis the motions of radar and the ship based on target image projection plane, and present a method to estimate the relative motion between the radar and the ship by Doppler extension of the sub-images. Images of good quality are required to be from radar data corresponding to the time intervals of smooth change of the Doppler extension. Simulation results are presented which verify this analysis.