Successful launch of the European Space Agency’s mission that will study Earth’s hydrological properties from space

The European Space Agency‘s (ESA) Scout missions are agile, low-cost Earth observation projects designed to test new technologies. Researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC) participate in the first of these missions, called HydroGNSS, which was launched yesterday, 28 November 2025. Composed of two identical satellites launched from California aboard a SpaceX Falcon 9 rocket, its objective is to advance the global understanding of water availability and the effects of climate change on Earth’s hydrological cycle.

Surrey Satellite Technology Ltd (SSTL) in the UK confirmed that signals had been received by the KSAT ground station in Svalbard (Norway), 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 is a remote sensing technique that uses reflections of navigation satellite signals off the Earth’s surface to measure physical parameters of the Earth. Specifically, HydroGNSS satellites capture L-band signals (radio waves used in satellite communications) from navigation systems such as GPS (Global Positioning System, from the USA) and Galileo (satellite navigation system of the European Union), which transmit L-band microwave signals that vary as they reflect off the Earth’s surface. This allows the satellites to compare the reflected signals with direct GNSS signals, received from navigation systems without touching the Earth’s surface, to obtain relevant information about properties related to the water cycle, among other aspects.

To do this, each satellite carries a delay Doppler mapping receiver, a system that generates an energy map, with a function similar to a photograph, which allows scientists to understand what the surface is like where the emitted signal bounces. This receiver 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. This 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 often hidden beneath forest canopies–ecosystems that can act as significant sources of methane. 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.

“This mission will demonstrate a cost-effective way to monitor key variables of hydrology, the climate and the oceans, with strong impact on our everyday life,” says Estel Cardellach, IEEC researcher at the ICE-CSIC, and pioneer in the study and development of the GNSS reflectometry technique.

The mission will provide measurements of climatic hydrological variables, such as humidity, flooded areas, frost line, and forest biomass. These will be key to helping scientists understand climate change and will contribute to the development of weather models, ecological mapping, agricultural planning, and flood preparedness.

“The HydroGNSS satellites will demonstrate new features of the GNSS reflectometry technique, which will improve spatial resolution of the measurements and their accuracy,” says Weiqiang Li, IEEC researcher at the ICE-CSIC who participates in the mission.

The IEEC team at the ICE-CSIC is responsible for the development and implementation of 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. This makes it a unique technique that complements traditional approaches such as optical imaging or synthetic aperture radars, techniques that present limitations or are one to two orders of magnitude more expensive.

Furthermore, for the first time, a GNSS reflectometry mission works at two different polarizations, two different electromagnetic frequency bands and captures complex signals continuously at high sampling rates.

The Earth Observation research group at ICE-CSIC, to which Estel Cardellach and Weiqiang Li belong, has over 20 years of experience in the innovative GNSS technique. The researchers are part of the mission consortium, which also includes teams from La Sapienza University, Tor Vergata University and Istituto di Fisica Applicata “Nello Carrara” (IFAC – CNR) in Italy, Finnish Meteorological Institute (FMI), National Oceanography Centre (NOC) and University of Nottingham in the UK.

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.” “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,” she concluded.