The European Space Agency's two HydroGNSS satellites have finished their test phase and are now beaming open data to researchers everywhere. The mission, which uses reflected GPS and Galileo signals to track water on Earth, moved into full scientific operations after eight months of calibration. Anyone can now download measurements of soil moisture, flooding, and freeze-thaw cycles, offering a fresh look at how climate change is reshaping the planet's water cycle.
A clever trick with navigation satellites
The HydroGNSS satellites, launched in November 2025, do not carry traditional sensors. Instead, they catch L-band radio pulses from navigation satellites like GPS and Galileo after those signals bounce off Earth's surface. By comparing the reflected signals with the direct ones, the satellites can calculate how wet the ground is, where water is pooling, and whether soil is frozen or thawed. They also measure above-ground biomass, which helps scientists estimate vegetation density and carbon storage.
This technique, called GNSS reflectometry, was first tested on older missions like TechDemoSat-1 and CYGNSS. Surrey Satellite Technology Ltd in the United Kingdom built the new satellites for ESA and developed an advanced receiver that uses multiple frequencies and polarisations. That makes the data richer and more detailed than earlier efforts.
A fast, low-cost path to space science
HydroGNSS is ESA's first Scout mission, a program designed to prove that small, agile satellites can deliver serious science without the usual price tag. The entire project, from design to orbit, took just a few years, a fraction of the time needed for conventional Earth observation missions. The two satellites were launched together in November 2025 and have been circling Earth ever since, collecting data over every continent.
During the commissioning phase, teams at ESA and SSTL checked every part of the system, from the satellite hardware to the data processing pipeline. They verified that the measurements meet strict quality standards and that the ground segment can handle the flow of information. With that work complete, the mission is now in routine operations, and the data is available through ESA's open access portal.
Why this matters for water watchers
For hydrologists and climate scientists, the appeal is simple: HydroGNSS offers a global, frequent, and free way to monitor water in places where ground sensors are sparse. Floods, droughts, and frozen soils are hard to track from the ground, especially in remote regions. The satellite data can fill those gaps, helping researchers build better models of how water moves through landscapes and how that changes as the climate warms.
The mission also supports disaster response. When a flood hits, the satellites can show where water has spread within days, information that can guide relief efforts. Over time, the data will reveal long-term trends in soil moisture and wetland extent, which are key indicators of climate stress.
ESA's Scout project manager, Jean-Pascal Lejault, called the milestone a success for the entire program. The approach, he said, proves that low-cost missions can deliver real scientific value. With HydroGNSS now fully operational, the next step is for the research community to dive into the data and see what new insights emerge about Earth's changing water systems.
