GNSS reflectometry (GNSS-R)

GNSS reflectometry (GNSS-R) is a passive bistatic radar technique that exploits navigation signals from GPS, Galileo, and BDS constellations as signals of opportunity. Reflected GNSS waveforms are captured by a nadir-facing antenna and processed into Delay-Doppler maps (DDMs), which encode surface roughness and dielectric properties to retrieve ocean wind speed, soil moisture, sea ice extent, and surface inundation.^[1]

TechDemoSat-1 (launched July 2014) carried the SGR-ReSI instrument developed by Surrey Satellite Technology Ltd and demonstrated the first spaceborne GNSS-R measurements of ocean wind speed and sea ice classification.^[2] CYGNSS (launched December 2016) scaled the approach to an eight-microsatellite constellation in a 520 km, 35-degree-inclination orbit, enabling near-daily tropical coverage. Its DDMI instrument delivers L2 ocean wind speed, L3 soil moisture, and L3 flood inundation products distributed via NASA PO.DAAC.^[3] ESA's HydroGNSS Scout constellation (launched November 2025) extends the application set to soil moisture, freeze-thaw state, wetland inundation, and above-ground biomass.^[4] Spire Global operates a commercial CubeSat constellation with GNSS-R capability, offering Spire Soil Moisture Insights as an operational commercial product.

L-band navigation signals penetrate vegetation canopy more effectively than shorter-wavelength active radar, making GNSS-R an efficient complement to dedicated active microwave sensors for land surface monitoring. The technique requires no onboard transmitter, which substantially reduces instrument mass and power compared to monostatic active radar.