The velocity error of the airborne SAR will cause the deviation of the Doppler spectrum centroid. The deviation can be corrected by the clutter-lock system. In this system

the error signal of the deviation may be detected by the phase comparator. This paper briefly relates the principle to detecte the error signal using the phase comparator

and derive the analytic formula of the deviation error signal of the single point target.An example and the estimation method about the maximum correct range of the deviation error are given.