Two UK‑built HydroGNSS satellites reached orbit on 28 November, inaugurating the European Space Agency’s first Scout climate mission. The pair launched on SpaceX’s Transporter‑15 rideshare from Vandenberg Space Force Base, with first signals confirmed the same evening.

Led by Surrey Satellite Technology Ltd (SSTL) in Guildford and backed by £26 million from the UK Space Agency, HydroGNSS is designed as a rapid, low‑cost Earth observation mission focused on water. It also marks SSTL’s 75th and 76th spacecraft, launched in the company’s 40th anniversary year; SSTL will operate the satellites and distribute the data.

Rather than firing its own radar, HydroGNSS listens for faint L‑band signals from navigation satellites such as GPS and Galileo and analyses how those signals change when they reflect off the surface. This technique-GNSS reflectometry-can sense soil moisture, surface water and freeze–thaw in all weather, including through thick cloud and vegetation that hinder optical imagers.

The mission arrives as Europe grapples with worsening water extremes. In 2024 the continent saw its most widespread flooding since 2013 and its warmest year on record; almost one‑third of the river network exceeded at least the ‘high’ flood threshold, with at least 335 deaths across the year, according to C3S/ECMWF and the World Meteorological Organization.

For forecasters, starting‑state soil moisture is crucial. Peer‑reviewed research shows that assimilating satellite soil‑moisture data can improve peak timing, reduce false alerts and cut error bars in large catchments. ECMWF has already tested ESA’s SMOS data within the European Flood Awareness System (EFAS), finding promising skill gains once biases are corrected.

HydroGNSS adds a fresh L‑band view alongside ESA’s SMOS and NASA’s SMAP. SMOS delivers global soil‑moisture maps roughly every three days at tens‑of‑kilometres scale, while SMAP’s radiometer continues to provide 36 km soil‑moisture and freeze–thaw products. Together, these missions offer complementary perspectives on the water cycle.

Because GNSS reflections come from many satellites at once, HydroGNSS can keep gathering measurements during storms-exactly when decision‑makers most need reliable data. That resilience makes the mission well suited to the critical hours that shape flood guidance, reservoir operations and emergency logistics.

There is a clear route to impact in the UK. The Environment Agency already publishes rapidly produced, remotely sensed flood extents during incidents, and the UK Water Resources Portal blends live river flows with soil‑moisture readings. HydroGNSS can be bias‑corrected and integrated into these workflows to strengthen situational awareness.

Farmers and land managers also stand to gain. More reliable soil‑moisture maps support planting and irrigation decisions, while freeze–thaw monitoring helps manage compaction risk and winter crop damage. Globally, WMO reporting points to persistent water‑resource stress in many catchments; HydroGNSS offers a new way to track where deficits are emerging.

What happens next is commissioning, calibration and distribution. ESA confirms that SSTL will run the platforms and deliver data products under the Scout programme’s fast‑turn model. It is a small mission with a practical brief: deliver dependable hydrology data that can flow into models, warnings and planning tools without delay.