ESA’s HydroGNSS is already showing in its first months why Europe launched a new generation of small satellites to monitor water
The European Space Agency has entered a new phase of Earth observation with the HydroGNSS mission, and the first results suggest that this relatively small and cost-efficient project could have a significantly greater scientific and operational impact than might be expected at first glance. Three months after the launch of the two satellites, the mission is still in the commissioning phase, but it is already collecting data that confirm that the instruments in orbit are operating according to plan and that the approach on which HydroGNSS is based has real potential for monitoring key processes related to the Earth’s water cycle.
HydroGNSS is the first mission from ESA’s Scout line, developed within the FutureEO programme. It is a concept that relies on the so-called New Space approach: faster development, lower costs, more compact platforms, and the testing of new ideas in a shorter timeframe than with large, multi-year research satellite missions. In practice, this means that Europe does not wait a decade to test a new method of observing the planet, but instead tries to bring science and technology into orbit quickly enough for the results to help both researchers and public services while climate change and extreme weather events are already under way.
What HydroGNSS actually does
Unlike conventional satellites that image the Earth using their own radar or optical instruments, HydroGNSS uses reflected signals from navigation systems such as GPS and Galileo. These systems continuously transmit microwave signals in the L-band, and when they are reflected from soil, water, ice, or vegetation, their shape and strength change. HydroGNSS records precisely those changes and compares them with the direct signal, producing a new type of information about surface conditions.
The central tool of this method is the so-called Delay Doppler Maps, that is, maps showing how much the signal was delayed after reflecting from the surface and how its frequency changed due to motion. Although this description sounds highly technical, the meaning is fairly clear: different surfaces leave different “signatures” in the signal. Calm water or a flat sea-ice sheet produce a strong and sharp peak, while rough seas or a rugged land surface generate a more diffuse and weaker pattern. From such patterns, it is possible to read data on soil moisture, floods, wetlands, freeze-thaw cycles, and above-ground biomass, and over the oceans also information on wind and sea ice.
The distinctive feature of HydroGNSS is that it does not collect such data in just one configuration, but uses two frequencies and two polarizations. This increases the amount of information scientists can extract from the reflected signal and improves the ability to separate different influences on the surface, such as soil roughness, the presence of vegetation, and the actual water content. Such an approach is especially important in hydrology, where one signal often carries several mutually intertwined pieces of information.
Why water is at the centre of this mission
Changes in the water cycle are today among the most direct indicators of climate disruption. Droughts affect agriculture and drinking water supply, extreme precipitation increases flood risk, and changes in frozen ground and permafrost affect greenhouse gas emissions, ground stability, and the functioning of entire ecosystems. ESA therefore does not view HydroGNSS as just another technical experiment in orbit, but as a tool for monitoring processes that have direct consequences for the environment, the economy, and human safety.
The mission tracks four key groups of variables. The first is soil moisture, important for drought assessment, agricultural management, and understanding how water is retained in or lost from the landscape. The second is flooded areas and wetlands, which are ecologically extremely valuable but also climate-sensitive because they can store carbon, regulate the water regime, and at the same time be a source of methane. The third is the freeze-thaw state of the ground, especially in permafrost regions, where even small changes can trigger broader changes in energy and carbon fluxes. The fourth is above-ground biomass, that is, the amount of vegetation and woody mass, which is strongly linked to water availability and estimates of carbon stocks in forests.
In that sense, HydroGNSS does not seek to replace larger and more expensive satellite missions, but to complement them. Its advantage is that it uses existing navigation signals, so it does not need to carry a bulky active radar. This reduces mass, cost, and energy requirements, while at the same time providing frequent global coverage. ESA states that two satellites can cover more than 80 percent of land within 15 days at a spatial resolution of 25 kilometres, which is a very valuable compromise between frequency and detail for this type of climate monitoring.
First results from orbit: small satellites, serious signal
Launched on 28 November 2025 by a Falcon 9 rocket from Vandenberg, California, HydroGNSS-1 and HydroGNSS-2 separated from the rocket less than ninety minutes after liftoff, and the first signal from the satellites was confirmed that same evening. This marked the start of the most operationally sensitive part of the mission, the so-called commissioning phase, during which subsystems are gradually switched on, spacecraft behaviour is checked, instruments are calibrated, and it is confirmed that the entire data-processing chain is ready for routine operation.
According to data from SSTL, the mission’s main industrial contractor from the United Kingdom, both satellites began collecting Delay Doppler Maps of reflected GNSS signals within the first few weeks. One of the early examples was recorded over central Africa just seven days after launch, when HydroGNSS-2 simultaneously captured signal reflections from the Galileo and GPS systems. Such an early result is important for several reasons. First, it shows that the instrument’s basic principle works in real orbital conditions. Second, it confirms that the data can be collected in a form that is useful for later scientific processing. Third, it gives teams on the ground concrete material for calibration, algorithm verification, and comparison with other data sources.
SSTL also stresses that the mission is still in the refinement phase and that additional calibration adjustments, validation of processing chains, and detailed characterisation of satellite behaviour in orbit still lie ahead. This is precisely the usual path for every new Earth observation mission: the first signal in itself is not enough, but only opens the work of turning a technical measurement into a stable scientific product. But the fact that the first data sets arrived so early and that they confirm the expected operation of the instrument represents for ESA and its partners a strong sign that the mission is on a good path towards the full operational phase.
What the “scout” philosophy looks like in practice
HydroGNSS is also important because it serves as a credibility test for ESA’s entire Scout concept. These missions are conceived as faster, more agile, and cheaper than major programmes, but without giving up scientific ambition. ESA states that Scout missions target a path from initial idea to launch in approximately three years, with a budget on the order of 35 million euros for development, construction, launch, and initial operations. At a time when the need for climate data is growing faster than the cycles of major space programmes, such a model is becoming both a technological and a political issue.
If HydroGNSS proves itself in operational work, that will be an argument that an entire family of smaller, focused missions can be used to fill important gaps in observing the planet. This is especially important for variables such as soil moisture and surface flooding, where frequent global coverage is useful, even if the spatial resolution is not at the level of detailed local imagery. In other words, HydroGNSS is not intended as a satellite that will show an individual street under water, but as a system that can regularly monitor large-scale patterns of change, support models, and warn of zones where the situation is changing rapidly.
There is also a broader industrial logic in such an approach. Compact platforms of about 75 kilograms, with instruments that use already existing signals from navigation systems, open the way for faster production, lower launch costs, and the possibility of future constellations. If one such dual mission can deliver relevant results, the next step could be a larger number of similar satellites with shorter revisit times and even more useful data series.
Who is behind the mission
Behind HydroGNSS there is not only ESA and one industrial contractor, but a broader European scientific network. SSTL leads the mission and operates the satellites in orbit, but the processing and interpretation of the data involve partners specialised in individual scientific products. Among them are Sapienza and Tor Vergata in Rome, Spain’s ICE-CSIC/IEEC, Italy’s IFAC-CNR, the Finnish Meteorological Institute, the Vienna University of Technology, the UK’s National Oceanography Centre, and the University of Nottingham.
Such a distribution is not a bureaucratic formality, but a key part of how the mission works. One team develops soil-moisture estimates, another deals with flooded areas, a third with freeze-thaw conditions, a fourth with biomass, while additional partners work on ocean calibration, signal processing, and combining data with other sources. That is why HydroGNSS should be viewed as a complete system: the satellite in orbit is only the first link, and the real value arises when the raw signal is turned into a product that researchers, meteorological services, climatologists, and other users can interpret and apply.
ESA points out that the data will be distributed by SSTL, while products and user access are being developed through the mission’s web portal. This is an important message both for the scientific community and for institutions dealing with risk management, because modern satellite missions no longer live only on the symbolism of launch. Their value today is measured by the speed at which data become usable and by the ability to integrate them into existing models, assessments, and early-warning systems.
What early data could mean for science and public policy
Although it is too early to speak of full scientific results, the direction in which HydroGNSS is heading is already clear enough. Soil moisture is one of the most important input data sets for agricultural assessments, drought forecasting, and hydrological models. Wetlands and flooded areas have long been problematic for satellite monitoring, especially when they are obscured by vegetation or clouds, so any method that can improve their mapping has both climate and conservation value. In permafrost zones, timely recording of transitions between frozen and thawed ground helps in understanding flows of energy, water, and carbon. Biomass, finally, remains one of the key unknowns in estimates of carbon stocks and changes in terrestrial ecosystems.
For public policymakers, this means that behind the technical story of reflected GNSS signals lies something very concrete: better data for understanding drought, floods, land degradation, changes in wetlands, and the behaviour of forest ecosystems. In an era in which climate adaptation is increasingly turning from declarative policy into a financial and infrastructural problem, reliable and regular observation of these processes is becoming an integral part of planning.
HydroGNSS is therefore an example of how space technology is no longer detached from everyday life. When a satellite better estimates how dry the soil is, that is information useful to agriculture. When it tracks flooded areas more precisely, it can help in risk assessment and disaster response. When it provides better insight into permafrost and biomass, it contributes to the models on the basis of which climate trends and future emissions are assessed. It is precisely this connection between orbital technology and very terrestrial problems that is why HydroGNSS is attracting so much attention already in the first months of the mission.
For now, the most important thing is that the satellites are healthy, that the instruments are producing the expected types of measurements, and that the mission is moving towards regular operations. If the coming months confirm what the first data suggest, Europe could gain in HydroGNSS not only a successful first Scout mission, but also proof that key parts of climate observation can be built faster, cheaper, and smarter than before.
Sources:- European Space Agency (ESA) – official announcement of the launch of the HydroGNSS mission on 28 November 2025 (link)- European Space Agency (ESA) – overview of the HydroGNSS mission, its goals, and its place within the FutureEO/Scout programme (link)- European Space Agency (ESA) – explanation of how HydroGNSS monitors soil moisture, wetlands, floods, permafrost, and biomass (link)- European Space Agency (ESA) – technical data on orbit, coverage, satellite mass, and the data distribution approach (link)- European Space Agency (ESA) – description of the satellites, instruments, and Delay Doppler Map processing (link)- Surrey Satellite Technology Ltd (SSTL) – official update on the commissioning phase and the first collected data sets in December 2025 (link)- HydroGNSS project – overview of the scientific team and partners involved in data processing and validation (link)
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