N° 80–2025: ESA’s HydroGNSS mission launched to ‘scout’ for water
28 November 2025
The European Space Agency’s first Scout mission, HydroGNSS, was launched today, 28 November, marking a significant step in advancing global understanding of water availability and the effects of climate change on Earth’s water cycle. The two twin HydroGNSS satellites were carried into orbit at 19:44 CET aboard a SpaceX Falcon 9 rocket, as part of the Transporter-15 rideshare flight from the Vandenberg Space Force Base in California.
Less than 90 minutes after liftoff, the two satellites separated from the rocket. Then, at 22:45 CET, Surrey Satellite Technology Ltd (SSTL) in the UK confirmed that they had received signals, indicating that both satellites were safely in orbit around Earth.
Both satellites will use an innovative technique called Global Navigation Satellite System (GNSS) reflectometry to quite literally ‘scout for water’.
This process involves the HydroGNSS satellites capturing L-band signals from navigation systems such as GPS and Galileo. These navigation satellites transmit L-band microwave signals that change when they are reflected off Earth’s surface.
The HydroGNSS satellites will compare these reflected signals with the signals the satellites receive directly from the GNSS satellites to reveal valuable information about the properties related to the water cycle, and more.
To do this, each HydroGNSS satellite carries a delay doppler mapping receiver. This consists of two antennas: a zenith antenna, which tracks direct GNSS signals and a nadir antenna, which collects reflected signals and processes them into delay Doppler maps.
Using this technique, these two small satellites, which orbit Earth 180 degrees apart, will measure soil moisture, freeze–thaw state, inundation and above-ground biomass.
These data will not only be vital for advancing our understanding of Earth’s water cycle, but also for supporting applications such as flood prediction and agricultural planning.
Also, by observing the extent of inundation and areas of wetland, HydroGNSS will help reveal wetlands – important ecosystems that can act as significant sources of methane – often hidden beneath forest canopies.
Information on freeze–thaw states will provide insight into the surface radiation balance, energy and carbon exchanges with the atmosphere, and the behaviour of subsurface permafrost in high latitudes.
Meanwhile, data on above-ground biomass will contribute to estimates of forest carbon stocks and their role in the global carbon cycle.
Drawing inspiration from the spirit of New Space, the Scouts champion agility and innovation –harnessing small, smart satellites to shrink proven technologies or test bold new ways of observing our planet.
Each mission races from concept to launch in just three years, on a lean budget of €35 million that covers everything from design and construction to in-orbit operations.
ESA’s Director of Earth Observation Programmes, Simonetta Cheli, said, “As the first of ESA’s Scout missions to launch, HydroGNSS marks an important milestone for this new family of rapid, low-cost Earth observation missions, and we extend our thanks to the mission’s prime contractor, SSTL.
“The launch also represents a key step in the evolution of our FutureEO programme, where the Scouts embody a fast, agile, innovative and cost-efficient approach – complementing our larger Earth Explorer research missions.
“We now look forward to seeing how HydroGNSS will employ GNSS reflectometry to deliver valuable insights into key hydrological variables that shape Earth’s water cycle.”
ESA’s prime contractor for the HydroGNSS mission is SSTL in the UK. SSTL is also responsible for operating the satellites in orbit and for distributing the data. The mission is also thanks to partial funding from the UK Space Agency.
National missions also launched
The SpaceX’s Transporter-15 rideshare flight also carried the next batch of IRIDE satellites, Eaglet-II, for Italy and two ICEYE satellites for Greece. Both of these national missions have been developed with the involvement of ESA.
IRIDE is a national initiative led by the Italian government and coordinated by ESA, with support of the Italian Space Agency (ASI). The mission comprises Earth observation constellations that deliver satellite data to support the monitoring of ground motion, land cover, water resources, coastal zones and other environmental factors. IRIDE provides services for Italy’s public authorities and contributes directly to the country’s Civil Protection Department.
The two new high-resolution ICEYE radar satellites are the first to launch under the Greek National Small Satellite Programme. This programme, which will also include other types of satellite next year, aims to enhance Greece’s satellite technology and application capabilities, foster innovation and job creation, drive economic growth, and strengthen national capacities in disaster management, environmental monitoring, and security. The project is led by the Hellenic Space Center and the Greek Ministry of Digital Governance, with ESA providing the overarching framework and supporting ICEYE in developing both satellites.
Both missions are funded through the EU Recovery and Resilience Facility.
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The European Space Agency (ESA) provides Europe’s gateway to space.
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