This paper concern the issue of signal characteristic and imaging processing of missile-borne side-looking SAR during its descending flight.According to the flight speciality of missile

the geometric model and target range expression are built.The features of range variation with time in single aperture and Doppler parameter variation with operation time are analysed.Because of the altitude decreasing during SAR operation

Doppler parameters change notably and constant Doppler parameters are not suitable for the imaging processing of entire operation period.Analysis and simulation show that focused image can be obtained by employing sub-aperture processing.