Concepts for future spaceborne radar systems are being developed which rely on the transmitter and receiver(s) being carried on separate spacecraft. The potential advantages include lower cost than current spaceborne radars and improved measurement capability. This paper reviews two currently proposed systems: GNSS reflectometry (GNSS-R) and a geosynchronous synthetic aperture radar constellation (GeoSAR). GNSS-R uses reflections of signals from GPS (and Galileo when available) to measure the height and state of the ocean surface. The receiver is typically in a low Earth obit (LEO) and provides global coverage. GeoSAR uses a radar receiver in geosynchronous orbit (slightly displaced from geostationary but still with a period of 1 day). The radar sees a fixed region of the Earth and is able to integrate signals over long periods to obtain a satisfactory signal-to-noise ratio. If several receiver spacecraft are used simultaneously the time to obtain an image can be reduced in proportion to the number of spacecraft used. The principles of these two systems are described and then requirements applying to the system dynamics and control are derived. For GNSS-R the requirements are relatively easy to achieve (coarse pointing and only basic orbit control). GeoSAR’s requirements are more demanding although the environmental disturbances at geosynchronous orbit height are significantly smaller than in LEO. For GeoSAR the most demanding requirement is the need for centimetre-level orbit measurements to allow aperture synthesis to be implement
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