Airborne synthetic aperture radar (SAR) is an essential tool for modern remote sensing applications. The motion compensation (MOCO) in SAR processing is usually carried out by assuming a reference level to compute the range displacements and phase corrections to apply to each received echo. This means that phase histories of targets at heights different from the reference level cannot be matched accurately, which might yield several effects in the final compressed image. Topography correction for airborne SAR accommodates topography variations during SAR data processing, using an external digital elevation model (DEM). The aperture-dependent MOCO is compensation the phase error of all targets before azimuth compression, resulting in an enhanced image quality. In this paper, analysis the effect of topography variations in focused image (after range and azimuth compression). Then presented an efficient way to use the information given by an external DEM to take into account the motion of the aircraft along the whole synthetic aperture is presented. Finally, real simulated-data experiments show that the proposed approach is appropriate for highly precise imaging of airborne SAR.