GNSS signal sensing

GNSS signal sensing is the opportunistic use of L-band signals broadcast by Global Navigation Satellite System (GNSS) constellations - GPS, Galileo, GLONASS, and BeiDou - either reflected off Earth's surface (GNSS Reflectometry, GNSS-R) or refracted through the atmospheric limb (radio occultation), to retrieve geophysical parameters without any onboard transmitter.

In GNSS-R, a downward-looking antenna on a LEO satellite captures the surface-reflected L-band signal and constructs Delay-Doppler Maps (DDMs) from the amplitude, phase, delay, and Doppler shift of the return. The DDM waveform shape encodes ocean surface roughness (a wind speed proxy), land surface dielectric constant and roughness (soil moisture and flood inundation), or sea ice concentration. Reflected signals are approximately 20-40 dB weaker than the direct signal, requiring coherent integration and sensitive receivers. CYGNSS wind speed retrievals achieve uncertainty below 2 m/s RMS meeting NASA Level-1 science requirements.

In radio occultation, a LEO satellite tracks the Doppler-shifted excess phase as a GNSS satellite sets or rises behind the atmospheric limb. Abel transform inversion of the bending angle profile yields atmospheric refractivity, then temperature and water vapour profiles at approximately 200 m vertical resolution and approximately 1 K temperature accuracy. The technique is self-calibrating and requires no vicarious calibration.

Representative missions include CYGNSS (NASA, 8-satellite constellation, tropical ocean wind speed and flood inundation, 2016 onward), COSMIC-2/FORMOSAT-7 (NOAA/NSPO, 6-satellite radio occultation constellation, 2019 onward), Spire Global (commercial, approximately 100-satellite GNSS-R and radio occultation), and HydroGNSS (ESA, GNSS-R for hydrology including soil moisture, freeze-thaw state, inundation, and above-ground biomass, planned).

Applications span ocean surface wind speed (including tropical cyclone inner-core monitoring), soil moisture and flood inundation mapping, sea ice extent and type, inland water level, atmospheric temperature and water vapour profiles for numerical weather prediction assimilation, and ionospheric electron density profiling.

The technology's passive nature - requiring no transmitter - gives each satellite very low power and cost, enabling large constellations. Opportunistic illumination from more than 150 GNSS satellites provides near-continuous global coverage. Core limitations are coarse spatial resolution (approximately 25 km for GNSS-R ocean wind; approximately 300 km horizontal for radio occultation), restricted polar and high-latitude coverage from CYGNSS (inclination approximately 35 degrees), and limited GNSS-R soil moisture sensitivity above moderate vegetation density.