The GEO atmospheric sounder (GAS) is a potential European future instrument, which utilizes interferometric synthetic aperture radiometry to obtain the desired spatial (30 km) and temporal (30 minutes) resolution for measurement of atmospheric temperature and humidity profiles under all weather conditions. This paper presents a simulation model for the entire imaging process. It enables detailed simulation of the impact of the instrument characteristics on the imaging performance. The model has been used as a tool to predict the performance of an instrument demonstrator under development.