ESA’s HydroGNSS mission publishes data to deepen our knowledge on Earth’s water cycle

The HydroGNSS Scout mission of the European Space Agency (ESA), in which the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) is involved, has completed the commissioning phase and is now in full scientific operations. It was launched in November 2025 to observe key hydrological variables from space using the Global Navigation Satellite System Reflectometry (GNSS-R) technique.

Following extensive testing and validation of the two satellites, instruments and ground segment, HydroGNSS data are now freely available to the scientific community and users worldwide, opening new opportunities to advance our understanding of water availability and the impacts of climate change on Earth’s water cycle.

The mission captures L-band signals transmitted by navigation satellites such as GPS and Galileo, Europe’s own civilian-controlled GNSS, after they have reflected off Earth’s surface. By comparing these reflected signals with those received directly from the navigation satellites, HydroGNSS can derive information on soil moisture, inundation and wetlands (surface waters), freeze–thaw dynamics, and above-ground biomass, helping scientists better understand the processes shaping our changing planet.

Over the last eight months, mission teams, including the one made up of IEEC researchers at the Institute of Space Sciences (ICE-CSIC), have carefully evaluated the satellites’ performance, verified the instruments, and calibrated the data products to ensure they meet mission requirements. The successful completion of commissioning confirms that the satellites and ground segment are performing as expected and are ready for routine operations.

“After years of intense work preparing the mission, for which we have contributed since the beginning—mission conception, suggestion of a new high-resolution observation mode, scientific and technical assessment as well as development of the surface inundation product, we are now thrilled to see the results being delivered to the scientific community,” says Estel Cardellach, IEEC researcher at the ICE-CSIC, and pioneer in the study and development of the GNSS reflectometry technique. 

“We are convinced these datasets will foster new science relevant to hydrology and climate, while supporting emergency situations due to extreme floods,” she adds. 

IEEC researchers at the ICE-CSIC have developed and implemented—and are now maintaining—the algorithms for surface water detection, what is called the surface inundation and wetlands product. These algorithms run operationally at the payload data ground segment to deliver, in short latency, the locations with presence of water over land, due to permanent water bodies, flooding, inundation or wetlands’ extension. 

The GNSS-R technique has the capacity to detect surface water during day and night, in all weather conditions and even under extremely thick vegetation canopies. The Earth Observation research group at ICE-CSIC, to which Estel Cardellach and Weiqiang Li belong, has over 20 years of experience in this innovative technique that complements traditional approaches, such as optical imaging or synthetic aperture radars, which present limitations or are one to two orders of magnitude more expensive.

“We present two images to show HydroGNSS’ ability to detect surface water at high resolution, including beneath vegetation canopies.

Knowing the presence or absence of water under forest canopies is important to help track methane gas,” says Weiqiang Li, ICE-CSIC and IEEC researcher.

The left image shows measurements from HydroGNSS’ coherent channel, a key feature of the mission, which uses the behaviour of reflected GNSS signals to identify water surfaces.

The data are collected along satellite tracks at 300 m resolution, with observations from both satellites combined over time to build a more complete picture.

The right image shows a global flood map generated using C-band radar data from Sentinel-1 over the same region. Unlike GNSS-R, radar signals can have difficulty detecting water beneath dense vegetation. “This is an example of how much more information can be gathered using GNSS-R that cannot be obtained using other techniques,” adds Weiqiang Li. 

HydroGNSS therefore provides a new way to observe wetlands hidden below forest canopies, helping improve estimates of these important ecosystems and their role in the global carbon cycle.

As ESA’s first Scout mission, developed under the Earth Observation FutureEO programme, HydroGNSS exemplifies the NewSpace approach. The Scout programme enables innovative mission concepts and emerging satellite technologies to be developed and demonstrated rapidly and cost-effectively.

Using this approach, the mission was developed for ESA by Surrey Satellite Technology Ltd (SSTL), which designed, built and now operates the two satellites. SSTL developed an advanced instrument capable of exploiting multiple GNSS constellations, frequencies and polarisations to deliver enhanced environmental observations on soil moisture, flooding and wetlands, seasonal freeze–thaw conditions, and vegetation, supporting research into the global water cycle and its response to climate change.

Research centres and universities have worked along SSTL, such as the IEEC, ICE-CSIC, Sapienza and Tor Vergata University of Rome, Finnish Meteorological Institute, Istituto di Fisica Applicata “Nello Carrara” (IFAC-CNR), National Oceanography Centre (NOC), University of Nottingham and Technical University Vienna.