The Earth observation Scout mission, HydroGNSS, will provide measurements of key hydrological climate variables, including soil moisture, freeze–thaw state over permafrost, inundation and wetlands, and above-ground biomass, using a technique called Global Navigation Satellite System (GNSS) reflectometry. In doing so it will complement missions such as ESA’s SMOS and Biomass, Copernicus Sentinel-1 and NASA’s SMAP.