HydroGNSS satellites for SSTL

This animation shows how the HydroGNSS satellites will measure important climate variables, including soil moisture, permafrost health, wetlands, and above ground biomass, using a technique called GNSS reflectometry. It uses existing signals from GPS satellites as radar signal sources. These signals are reflected off the land, ice and ocean and collected by HydroGNSS to passively measure these data.

More details here.