Partners in the European Space Agency (ESA)-funded HydroGNSS project, led by Surrey Satellite Technology Ltd (SSTL), will use GNSS reflectometry to provide measurements of key hydrological climate variables, including soil moisture, freeze–thaw state over permafrost, inundation and wetlands, and above-ground biomass.
HydroGNSS is one of a series of ESA missions, the so-called Scout missions, part of the agency’s FutureEO program, designed to quickly and cheaply demonstrate new Earth observation techniques using small satellites.
GNSS signals are differentially reflected or scattered by the Earth’s surface, as affected by water content, specifically permittivity, surface roughness and overlying vegetation. Once analyzed, these reflected signals can provide information about various geophysical properties. Special innovations introduced by HydroGNSS are to include dual-polarization and dual-frequency (L1/E1 and L5/E5) reception, and collection of high-rate coherent reflections.
Compact but powerful Earth observation platform
HydroGNSS uses the SSTL-21 platform, measuring 45 cm x 45 cm x 70 cm and weighing around 65 kg total per satellite. The payload will be operated at near 100% duty, and can support high data download rates using an X-Band transmitter. Star cameras provide precise attitude measurements, and a xenon propulsion system permits orbit phasing, collision avoidance and supports satellite disposal at the end of the mission. The two HydroGNSS satellites will take a ride-share launch into a 550 km sun-synchronous orbit, phased apart by 180 degrees to maximize coverage.
The SGR-ReSI-Z payload is a delay Doppler mapping receiver, tracking the direct GPS and Galileo signals through a zenith antenna and processing the reflected signals from a nadir antenna to create delay Doppler maps (DDMs). The zenith and nadir antennas employ all-metal patch technology, enabling the reception of dual-frequency and dual-polarized signals. Low noise amplifiers include blackbody loads to provide calibration for the amplitude measurement. Generated measurement datasets can be stored in the satellite’s data recorder and downloaded to ground stations at allocated passes several times per day.
Speaking at his annual press briefing in Paris earlier this month (January 2025), ESA Director General Joseph Aschbacher said, “We now expect to launch HydroGNSS in the fourth quarter of 2025, as one of the three so-far-identified Scout missions, which is a series based on smaller satellites, lasting three years of development work and with a relatively limited budget of roughly 30 million for industrial contracts. We see the Scout missions as something very important for our space science work. The scientific community is evaluating them and these are the ones selected and endorsed by them.”