We propose a modified range doppler algorithm to attain as clear as photograph in a Ku-band frequency modulated continuous wave automobile-based synthetic aperture radar (FMCW Auto SAR). The algorithm that is revised to be proper for the radar system installed in an automobile from the conventional range doppler algorithm is explained in this paper. The specification of the FMCW SAR system is produced to verify the remarkable performance. We had gathered the raw data on the bridge moving in constant velocity. The validity of the radar to get SAR images is demonstrated in the practical results.