Ultra wideband (UWB) synthetic aperture radar (SAR) is being used to operate in low frequency for detecting obscured targets beneath the foliage and in the ground from airplanes or satellites. Although it has the obvious military interest, it also possesses civilian applications such as geophysical studies, weather forecasting and so on. Numbers of image processing algorithms have been proposed and also been applied to low frequency UWB SAR. The algorithms are grouped mainly into two categories, frequency domain and time domain. This thesis paper focuses on frequency domain in general and Range Migration Algorithm (RMA) in particular. RMA performs one dimensional interpolation in range. This operation is known as Stolt interpolation. In this thesis work, image processing of an airborne monostatic SAR has been studied. Even though the study has been made for spotlight SAR, due to large antenna beamwidth the SAR operation can be considered somewhere in between of spotlight and stripmap. Main intention has been to take care of the defocused image of the moving target and to refocus it by the method proposed for RMA. This method applies a new wavenumber in azimuth emerging from the Doppler effect between platform and target when both are in motion. This focusing approach also helps out to confirming the presence of a moving target in the image. For simulation, UWB low frequency parameters have been taken from CARABAS II SAR system.