With the developing of the computing technology

the technology used in the development of the real time SAR processor is mature.Not only the larger and larger data needs processed

but also the better and better image quality is demanded.The auto-focus algorithm must be used in the real-time SAR processor to improve the SAR image quality and adjust the parameters in the image reconstruction.However due to the special architecture and the real time demand of the real-time processor

the computing load cannot be a random quantity and too large.So that it is not all algorithm fit for real-time processor.In this article

the Map Drift Algorithm（MDA） is used for SAR auto-focus processing.The theory and method of the Map Drift Algorithm are introduced

and its application in real-time SAR processor is discussed.A linear relationship between the secondary phase error in the spectrum of the SAR signal and the position of correlation peak can be confirmed.The iteration times of MDA and also the computing quantity can be reduced effectively using this relationship.An interpolation was introduces to calculate the position of correlation peak in our paper

therefore the errors brought by discrete processing is greatly improved.It also make the estimation of the Doppler parameters more precise.The computing quantity and the image quality of our algorithm are shown as a test result with the Radarsat’s data.