The European Space Agency (ESA) launched the first mission of the new "Scout" family – HydroGNSS – on Friday, November 28, 2025, with the aim of systematically monitoring key variables of the water cycle on Earth. Two identical spacecraft took off at 19:44 CET on the Transporter-15 mission of the Falcon 9 rocket from Vandenberg Space Force Base in California. Less than ninety minutes after liftoff, they separated from the second stage and began independent maneuvers, and at 22:45 CET, the control center in the United Kingdom confirmed the reception of the first signals – a sign that both satellites are safely in orbit and operational.

Why HydroGNSS is important: from "small" satellites to big answers

HydroGNSS is conceived as a "fast" and agile mission that will, at significantly lower costs than classic research projects, deliver valuable scientific data on water in nature and its role in the climate. Within the FutureEO program, ESA's "scout" missions (Scout) are intentionally small, smart, and fast: each must go from the start of development to launch within three years, with a total budget of up to 35 million euros, which includes development, launch, and initial operations in orbit. Such a framework allows for faster testing of new observation techniques and simultaneously complements larger and more expensive fundamental research missions.

HydroGNSS is a dual mission: two satellites are deployed in a Sun-synchronous orbit and fly 180 degrees opposite each other. This achieves uniform and frequent coverage of land on a global scale in a relatively short revisit cycle. The system is designed to cover more than 80% of land surfaces on average in 15 days at a resolution typically of 25 kilometers (depending on reception geometry and surface condition).

Technique that "listens to the echo" of navigation signals

The center of the mission is the technique of GNSS-Reflectometry (GNSS-R). Navigation satellites (such as GPS and Galileo) constantly transmit stable microwave signals in the L-band. When these waves fall on Earth, their reflection changes depending on surface properties: dry or wet soil, calm or rough sea, ice in a melting or freezing state, a river out of its bed, or a forest with a large amount of biomass – all of this leaves a recognizable "fingerprint" on the reflected signal. HydroGNSS "listens" to these echoes and compares them with the direct signals from the same GNSS satellites that it receives simultaneously.

Therefore, on each spacecraft, there is an instrument called a delay-Doppler mapping receiver (DDMR) with two antennas: a zenith one, which captures direct GNSS signals, and a nadir one, directed towards Earth, which receives reflected echoes. In special "maps" of delay and Doppler shift, the instrument analyzes how reflected waves differ from direct ones, thereby extracting information about the physical properties of the surface. The advantage of the GNSS-R technique is that it utilizes the existing global network of navigation transmitters – thus it does not need its own radar with high energy demands – so precise observations can be performed from small, cheaper platforms with very modest energy consumption.

What exactly will it measure: four pillars of the hydrological story

HydroGNSS is focused on a group of variables that scientists call essential for understanding the water cycle and climate processes:

• Soil moisture – changes in soil water content affect evaporation, plant growth, and energy exchange between the soil and the atmosphere. Long-term dry periods and flood events leave a clear trace on GNSS signal reflections.


• Freeze-thaw state – the transition of the surface from an icy to a liquid state and vice versa changes dielectric properties and reflection. Monitoring this cycle at high latitudes is key for energy, moisture, and carbon models in permafrost areas.


• Inundation and wetlands – when rivers overflow their banks or when marshes fill with water, the reflection of the GNSS signal becomes "mirror-like" and stronger. GNSS-R is particularly useful here because, unlike optical sensors, it "sees" well even through clouds and is not sensitive to the day-night cycle.


• Above-ground biomass – forest canopies and vegetation change the way microwave waves scatter; in the long term, statistical changes in "roughness" and absorption provide insight into changes in carbon stocks in forests.

By combining these measurements, a much finer picture of the water cycle is obtained than was enabled by individual missions. The benefits are immediate: from flood forecasting and support for agriculture, through water resource management and monitoring of extreme events, to carbon budget assessments and a better understanding of climate change feedback loops.

Orbital geometry, resolution, and data cadence

Both spacecraft fly in a close, nearly polar Sun-synchronous orbit at an altitude of about 550 kilometers. This geometry was chosen so that the reception angles of reflected GNSS signals would satisfy the widest range of configurations, while simultaneously ensuring regular repeat coverage of the same areas at fixed local times (useful for comparability of measurements). The primary goal is to cover most land every fifteen days on a typical grid of cells about 25 kilometers in size, although the effective resolution will depend on local conditions – a smooth water surface "echoes" differently than a rough forest canopy or a mosaic of agricultural plots.

As GNSS systems broadcast globally and continuously, HydroGNSS can continuously collect "echoes" from multiple directions, and processing in the DDMR allows for simultaneous mapping of multiple reflections. This opens space for synergies with other missions: GNSS-R, for example, complements SAR radars (which actively illuminate the surface) and high-resolution optical sensors (which offer a detailed image but are limited by cloud cover and light). In combination, these data provide extraordinary context for hydrologists, climatologists, and civil protection services.

Industrial role and ground segment

The prime contractor for the mission is the British company Surrey Satellite Technology Ltd (SSTL), which developed and built the satellites on the SSTL-21 platform, prepared them for flight, and integrated them onto the Falcon 9 multiple payload carrier. SSTL is also in charge of early operations in orbit, as well as the distribution of scientific data according to the agreed "data as a service" model. A network of ground stations is used for telemetry reception and command transmission, with an emphasis on high-latitude locations that offer frequent passes of Sun-synchronous satellites.

Operationally, the early course of the mission includes several clearly defined steps: spacecraft stabilization, verification of the power system and thermal conditions, deployment of solar panels, initial calibration of transceivers and the DDMR, and the so-called commissioning phase in which instrument performance is measured and confirmed. Only after these steps does routine acquisition of scientific products begin, with the gradual expansion of the list of validated geophysical variables.

Applications: from floods and droughts to permafrost monitoring

In the field of risk management, GNSS-R is particularly attractive because it enables global and frequent observations of inundations – river overflows, monsoon floods, coastal torrents – even when optical sensors are "blind" due to clouds or night. In combination with relief models and hydraulic simulations, such data can be incorporated into early warning systems and operational models for population evacuation.

In agriculture, soil moisture is one of the crucial inputs for irrigation models and yield assessments. GNSS-R shows sensitivity precisely to changes in the water content of the surface layer, so it can serve as a cheap and robust source of information for wide-area agronomic support – for example, for more precise determination of optimal irrigation times or for early drought detection.

For high latitudes, where permafrost dominates, information about the freeze and thaw cycle is important both for energy (radiation and heat balance) and for carbon (methane and CO₂ emissions associated with thawing). HydroGNSS will, together with other missions, improve parameters that enter into climate models and assessments of long-term trends.

Forests, carbon, and "hidden" wetlands

GNSS-R will not replace specialized biomass missions, but it gives an additional angle of view: changes in reflections associated with above-ground biomass and vegetation structure can, in a statistical sense and on larger spatial scales, complement estimates of carbon stocks in forests. Particularly interesting is the mapping of wetlands hidden beneath forest canopies, where optical sensors often fail. Since wet soils and shallow water surfaces reflect the L-band more strongly and "smoothly", GNSS-R can serve as an indicator of variable inundation in such ecosystems – important both as methane sources and as natural carbon sinks.

Technical figures and operational framework

The two spacecraft have a mass of around 75 kilograms and dimensions of approximately 45 × 45 × 70 centimeters. They are designed for multi-year operation in orbit, with a nominal lifespan longer than three years, with the possibility of extension if system health permits. An orbital altitude of about 550 kilometers and a Sun-synchronous intersection with local crossing time was chosen to achieve an optimal combination of reflection geometry and pass cadence. Standard scientific products will include soil moisture maps, inundation indicators, freeze-thaw state maps, and indicators related to above-ground biomass, along with metadata on imaging geometry and quality measures.

"New Space" in European service: Scout as a supplement to Earth Explorers

HydroGNSS is not an isolated case – it is the first "scout" of a wider series of small missions that ESA is introducing to accelerate the introduction of innovations into operational Earth observation. The model is simple: faster to first data, cheaper per unit of science, and more flexible regarding risk – because a series of smaller, targeted missions tests new techniques more easily than one large satellite. Scouts thereby complement the portfolio of Earth Explorer missions, and scientific communities get faster access to pioneering datasets.

Launch with a full manifest: IRIDE and Greek radar satellites

Transporter-15 was a classic "sold out" flight: alongside HydroGNSS, the spacecraft carried more than a hundred commercial and national payloads into Sun-synchronous orbit. For European users, two other segments of the manifest are particularly important. The first is the Italian IRIDE – a national constellation under the auspices of the Italian government and in coordination with ESA and the Italian Space Agency (ASI). A new series of Eaglet II satellites took off on this flight, part of the mosaic with which IRIDE builds services for Italian public authorities: from monitoring ground displacement and land cover to surveillance of waters, coasts, and other environmental parameters. The program is financed through the Italian recovery and resilience plan and is conceived as infrastructure for civil protection and environmental management.

The second segment is Greek: two new ICEYE radar satellites took off as the first pair within the Greek National Small Satellite Programme. The program is led by the Hellenic Space Center and the Ministry of Digital Governance, and ESA provides the framework and technical support. ICEYE has, in addition to delivering data services from the existing constellation, developed sovereign SAR spacecraft with Greek partners intended for the development of domestic capacities for monitoring natural disasters, the environment, and security. Alongside the launch of the first two radars by the end of 2025, a continuation has been announced – including other types of satellites – with the aim of covering daily observation of Greek territory with a range of optical and thermal sensors in the next phase.

Mission schedule and confirmation of first steps

Liftoff was, after several schedule shifts, performed on November 28, 2025, at 19:44 CET (18:44 UTC). The satellites separated from the rocket less than an hour and a half later and established nominal telecommunication links. At 22:45 CET, the reception of the first signals was recorded and confirmed – a key turning point enabling the transition from the "ballistic" phase to conducting initial checks and calibrations. In the first days after launch, teams in control centers monitor temperature, power, orientation, and telemetry, and the instrument is gradually switched on through a plan of test scenarios. Only after successful verification of full functionality will routine collection and publication of scientific products begin.

How data will be available and where it fits in the EO data ecosystem

As foreseen by the Scout framework, the industrial consortium building the mission – in this case SSTL – is also in charge of data distribution. This reduces the burden on agency resources and accelerates the publication of products, and users from the academic community and the public sector receive standardized formats and metadata. HydroGNSS data fits naturally into the European EO ecosystem where Copernicus Sentinels, commercial SAR providers, and a range of national constellations already operate; GNSS-R will fill the "niche" of sensitivity to moisture, wetlands, and freeze status at acceptable costs and very good global reach.

Technical and scientific limits: what GNSS-R can and cannot do

Although attractive in terms of price and robustness, the GNSS-R technique has limitations that users must understand. Product resolution is typically more modest than what users of high-resolution optical and SAR missions are used to; signals are sensitive to imaging geometry, local topography, and the "speckle" phenomenon of forests in microwave scattering. Therefore, smart processing methods, calibration with in-situ measurements, and fusion with other data sources are necessary. But precisely therein lies the strength of the Scout approach: through rapid testing and joint development with users, the technique is honed from cycle to cycle, and cost efficiency allows for future constellations with a larger number of small satellites.

Wider European context: faster innovations under the auspices of FutureEO

HydroGNSS is an important symbol in a programmatic sense as well. With the Scout family, ESA shows that Europe can simultaneously nurture large flagship missions – for example, dedicated to precise biomass tomography or flexible spectroscopy – and on the other hand introduce disruptive concepts within timeframes comparable to industrial "New Space" trends. In that spirit, the next Scout missions targeting greenhouse gases and the geomagnetic field have already been announced, with the same cost and time constraints. The combination of speed and scientific relevance is key for timely support for climate policies, risk management, and an economy that increasingly relies on data from orbit.

What follows after the first signals

In the weeks following November 29, 2025, teams will verify the performance of all subsystems, from navigation and communication to platform stabilization and the instrument's working environment. An in-orbit calibration phase follows, in which the DDMR is "tuned" against known terrestrial and ocean targets, and algorithms for converting reflections into geophysical quantities are harmonized with field measurement campaigns. When these steps are completed, the publication of the first products intended for the wider community is expected. It is already clear that HydroGNSS, as the first European GNSS-R mission of this scope, will fill an important gap between expensive active radars and time-limited optical imaging and create a platform for future, denser constellations.

One manifest, three stories about water

Although having different goals and technologies, HydroGNSS, IRIDE, and Greek ICEYE radars merge around the same theme: water. GNSS-R measures water in soil, inundation, and changes in ice; IRIDE builds capacities for systematic environmental monitoring and support for Italian civil protection, including water resources and coastal zones; ICEYE's radars, meanwhile, enable imaging through clouds and at night – exactly as is most needed during floods or storms. These three segments, launched on the same day, vividly seem like different "cameras" of one and the same phenomenon: how water moves through the landscape and how we should respond to these changes.

Concluding on the beginning (without a conclusion): what we will monitor in the coming months

Key measurable steps in the near future will include stable orbit configuration, publication of the first validated products, and a plan for synergistic campaigns with field measurements and other satellite missions (including radar and optical sensors). In parallel, IRIDE will expand services to Italian users, and the Greek national program will deploy additional elements of SAR and optical capacities. For European services and researchers, this means a growing "stack" of complementary data, faster cadence, and increasingly reliable, multi-source indicators for water risks, agriculture, and climate reporting.