Abstract: Time-domain algorithm, such as back projection (BP), can deal with SAR imagery precisely with complicated radar trajectory. However, SAR images reconstructed by time-domain algorithm cannot satisfy the requirements of conventional autofocus method, which constrains the performance of time-domain algorithm in practical situation. To solve the problem, we propose an improved PGA approach which can be embedded into time-domain algorithm for processing circular SAR (CSAR) data. Firstly, based on the imaging geometry of CSAR, the of the motion error encountered in CSAR is presented, and on this basis, the signal formation of back projection is derived. Then, the implementation of improved PGA is presented in detail. Finally, the real data experiment is processed, and the results prove the correctness of the theory analysis and the validity of the proposed approach.