Impact of Atmosphere on Geostationary SAR: Performance Analysis and Optimization

Geosynchronous synthetic aperture radar (GEO-SAR) offers a unique capability for wide-area monitoring with persistent observation, leveraging long integration times to compensate for severe path losses at large sensor-target distances. However, spatial and temporal variations in both the troposphere and ionosphere during these long integrations hinder effective azimuth focusing. This article addresses a geosynchronous synthetic aperture radar (SAR) system whose orbit complies with International Telecommunication Union (ITU) regulations for geostationary platforms. Specifically, it investigates optimizing the satellite orbit under constraints imposed by available orbital slots and regulatory requirements. The upper limit on the velocity is determined by additive noise sources (thermal noise, radio frequency interference, and so on), while the lower limit is constrained by the phase noise due to the atmospheric turbulence. The article first develops a statistical model describing the joint variability of ionospheric and tropospheric delays. It then provides a statistical characterization of atmospheric turbulence, which is quantitatively evaluated through the analysis of large datasets obtained from meteorological measurements and global navigation satellite system (GNSS)-based observations. The combination of the performance model and the statistical characterization of atmospheric turbulence enables the optimization of both the system and the satellite orbit. Results show that while a ±0.1° ITU station-keeping box allows for mission feasibility, expanding the tolerance to ±0.5° - still within experimental ITU limits - yields significantly better performance.