With the development of SAR（Synthetic Aperture Radar） remote sensing techniques

SAR images have been widely applied in a lot of fields.But because of its side-looking character

we have to rectify the images into orthoimages before we use them.Building mathematical models is an important step in SAR rectification.In this paper we first recapitulate some mathematical models used for SAR image rectificaion

including Soviet Model

Polynomial Model and G.Konecny Model.The characteristics of these models are analyzed.Then we introduce the F.Leberl Model in detail

which is based on the distant condition and the zero-Doppler condition.In airborne condition the range direction is perpendicular to the flight track so the Doppler center frequency is zero.But in spaceborne condition

the Doppler center frequency does not always equal to zero

so the zero-Doppler condition needs to be corrected.In this article we use a polynomial to substitute the value of the Doppler center.By this way we corrected the zero-Doppler condition.In the experiments we used both the original F.Leberl Model and the corrected model to validate the precision of our model.From the results we come to the conclusion that the new model is acceptable to both the airborne SAR and the spaceborne SAR

at the same time it can get higher precision than the original model in spaceborne condition.