A Telecom Compatible Geosynchronous SAR System

We consider a geosynchronous SAR system, to be operated from a small telecommunications satellite, in time or frequency division. We compare the telecom compatible design with more complex systems, that exploits much larger antennas and higher power. We evaluate the effects of the clutter due to moving targets and we show that, besides cost, the telecom compatible design has clear advantages, in that most of the Doppler frequency components generated by target motion fall at azimuths outside the antenna pattern and are thus rejected. Then, we evaluate the defocusing effects due to the temporal and spatial variability of the atmospheric water vapour. Due to the very slow formation of the spatial chirp (minutes or even hours), the temporal change of the local water vapor content could be significant, thus preventing a correct image focusing and the formation of a coherent image. We show that there are no risks for that. In fact, using GPS and ground based radar data, we estimate and model the spatial temporal correlation function of the water vapour delay. Then, for a given spatial scale, we estimate the maximum time within which the temporal evolution of the water vapor should be carried out, to be able to track its temporal evolution. This requisite constrains both the EIRP and the apparent velocity of the satellite, that however is compatible with the telecom operations. Finally, we evaluate the possibilities of real time ground motion analyses, as a function of the spatial and temporal resolution and the EIRP.