Satellites and mission
The HydroGNSS mission consists of two identical microsatellites orbiting the Earth on opposite sides of the planet, 180 degrees apart, to provide maximum global coverage.
Each satellite weighs around 75 kilograms and measures 45 × 45 × 70 centimetres, built on the compact and reliable SSTL-21 platform.
HydroGNSS carries a Delay Doppler Mapping Receiver that listens to GPS and Galileo satellite signals. It receives the direct signals through a top-facing (zenith) antenna, and the reflected signals – bounced off Earth’s surface – through a bottom-facing (nadir) antenna. By comparing these two, the satellites can create Delay Doppler Maps (DDMs) to study hydrological variables like soil moisture and inundation.
The antennas use advanced dual-frequency, dual-polarised metal patch technology, enabling them to capture detailed signal information. Built-in Low Noise Amplifiers with calibration loads help ensure accurate measurements.
The satellites produce DDMs once per second, precisely mapped to their location using on-board terrain models. They can also collect high-resolution data over areas with strong reflections, and occasionally record raw signal samples to support new research and testing on the ground.
The satellites are operated by Surrey Satellite Technology Ltd (SSTL) in the UK. SSTL also distributes the data to users.
All data are stored on the satellite and transmitted several times a day to ground stations. From there, the Payload Data Ground Segment processes and archives the information. Partner science teams generate data products, which are made available to users through an online portal at SSTL.
Each satellite is designed to naturally re-enter the Earth’s atmosphere within 25 years, in line with space debris regulations in place when the mission began. HydroGNSS also carries spare fuel that could be used either to shorten its orbit and deorbit sooner, or to extend its operational life if needed.
