This paper derives the motion errors model of small size airborne low frequency ultra-wideband synthetic aperture radar (UWB SAR)

and analyze the difference of the motion errors between the low frequency UWB SAR and the high frequency narrow band SAR as well as the reason of the traditional motion compensation (MoCo) method failure.To resolve this problem

a three-stage MoCo method is proposed.The procedure of the proposed MoCo method is as follows:At the first stage

removing the echo envelope error completely and the phase error partly by the MoCo based on the sensor data.Then

compensating the second-order phase error by the MoCo based on the Doppler chirp rate estimation at the second stage.At last

the residual high-order phase error is compensated by applying the autofocus algorithm in the image domain at the third stage

and the focused UWB SAR image is obtained.Experiment results on real data prove the validity of the proposed MoCo method.