PARIS, detecting reflected satnav

The Passive Reflectometry and Interferometry System (PARIS) concept involves a dedicated constellation of satellites picking up reflected satnav signals from GPS, Galileo and other navigation satellite constellations to gather data on Earth's sea and land surfaces. Operating on the same basis and in a similar way as current radar altimeters and scatterometers, these returning signals can be used to build up global maps of sea-surface height and wind and wave measurement over the ocean, determine ice extent and thickness of the icecaps and indicate soil moisture and biomass across land.
(Figure courtesy of ADS-SAU).