Abstract: Synthetic aperture sonar imaging is affected by random motion and medium instability. Large motion errors can be corrected by means of motion compensation, while the effects of uncompensated errors and medium instability should be compensated for with aotofocus method. SAS error model is derived in range-compressed domain under the assumption of small sway error. Moreover, patch-PGA used in SAR is studied in using SAS and the simulation and underwater experiments in a lake show the validity of the method.