The European Space Agency (ESA) has gained a powerful new tool for monitoring climate change and the global water cycle. On Friday, November 28, 2025, from launch pad SLC-4E at Vandenberg Space Force Base in California, a Falcon 9 on the Transporter-15 mission carried the twin HydroGNSS mission—the first project from ESA's new family of so-called Scout missions—into a sun-synchronous orbit. Two identical small satellites, separated by 180 degrees in orbit at an altitude of about 550 kilometers, will from now on literally "listen" to Earth via navigation system signals to map water and related climate parameters on a global scale.

Less than an hour and a half after liftoff, both satellites successfully separated from the rocket's second stage, and in the evening hours Central European Time, the control center of Surrey Satellite Technology Ltd (SSTL) in the United Kingdom confirmed the receipt of the first signals. This confirmed that both satellites are alive, stable, and ready to begin the complex phase of testing and commissioning, which will precede regular scientific data collection.

A new generation of small satellites for the water cycle

HydroGNSS (Hydrological Global Navigation Satellite System) is conceived as a fast, agile, and relatively inexpensive mission that complements larger research platforms from ESA's FutureEO program. It involves two microsatellites weighing about 75 kilograms and measuring roughly 45 × 45 × 70 centimeters, which offer in a package what required significantly larger and more expensive spacecraft just ten years ago.

The key to the mission is the focus on four hydrologically and climatically crucial parameters: soil moisture, inundation and wet terrain (including wetlands and areas under water), freeze and thaw states in permafrost areas, and above-ground biomass of forests and other vegetation. These are variables that the Global Climate Observing System (GCOS) considers key "essential climate variables" or their close derivatives, as they directly influence the balance of water, energy, and carbon in the climate system.

Along with these primary goals, HydroGNSS will also provide secondary data on wind speed over oceans and on the extent of sea ice. Thereby, the mission directly joins global efforts to monitor changes in polar seas, which play a key role in climate regulation and maritime safety.

GNSS reflectometry: how satellites "hear" water

Unlike classic radar missions that send their own signal towards Earth and then measure the echo, HydroGNSS uses a technique known as GNSS reflectometry. Navigation systems like GPS and the European Galileo continuously emit L-band microwave signals. These signals, after reflecting off the surface of the sea, soil, ice, or vegetation, carry a signature of the physical properties of that surface—for example, how moist the soil is, whether it is frozen or melting, whether there is water or dense vegetation in a certain area.

Each HydroGNSS satellite therefore carries a special receiver called a delay Doppler mapping receiver. It works with two antennas: a zenith antenna facing the sky, which tracks direct GNSS signals, and a nadir antenna directed towards Earth, which captures those same signals after reflection from the surface. By comparing the direct and reflected signal and processing it in the form of so-called delay Doppler maps, the instrument can reconstruct a series of geophysical parameters related to water and vegetation.

The smoother and flatter the surface—like a calm sea—the more concentrated the reflected signals are. Rougher or vegetation-covered surfaces produce more diffuse echoes, while the presence of water in the soil changes dielectric properties and thus the shape of the reflected signal. In permafrost areas, the transition from a frozen to a thawed state leaves a recognizable trace in the way the GNSS signal bounces off the ground. All these nuances, integrated at a spatial level of about 25 kilometers, allow satellites to "see" from space hydric and vegetation processes that are often fragmented and difficult to measure on the ground.

Soil moisture: the foundation for crop and flood forecasting

Soil moisture is one of the variables that directly links meteorology, hydrology, and agriculture. The amount of water in the top few centimeters of soil determines how much precipitation will infiltrate into deeper layers, how quickly surface runoff and potential flash floods will occur, and how much moisture is available to plants in critical growth phases.

Until now, global soil moisture maps have largely depended on a combination of numerical models and measurements from specialized satellites like ESA's SMOS mission or NASA's SMAP mission. HydroGNSS introduces a third approach that uses already existing GNSS infrastructure, thereby reducing costs and increasing the possibility of frequent overflights above the same areas. Two satellites in polar orbit will be able to cover more than 80 percent of the planet's land surface within about two weeks, thus giving an almost continuous picture of soil moisture changes.

Such data are of key importance for the adaptation of agricultural production to increasingly frequent droughts. More precise information on soil moisture will enable farmers, irrigation agencies, and policymakers to better plan water distribution, optimize sowing and harvesting times, and reduce the risk of yield loss in extreme seasons.

Permafrost and freeze state: a signal of melting in the north

Another key task of HydroGNSS is monitoring freeze and thaw states in high latitudes, especially in permafrost areas. Permafrost—permanently frozen ground—stores vast amounts of organic carbon. When the ice in the ground begins to melt, microorganisms decompose organic matter and release greenhouse gases, primarily methane and carbon dioxide.

Changes in the freeze state also affect soil stability, thereby increasing the risk of infrastructure collapse in Arctic communities: buildings, roads, pipelines. The data that HydroGNSS will deliver, in combination with local measurements and other satellite missions, will help scientists better understand how permafrost behaves under conditions of accelerated warming and where the critical zones of future changes are.

Monitoring the transition from a frozen to a thawed state—and vice versa—is also important for calculating the surface radiation balance. Snow and ice reflect more solar energy than darker soil or vegetation. The loss of snow cover and the melting of surface ice change the ratio of reflected and absorbed energy, which further intensifies regional warming. HydroGNSS will supplement existing satellite data on snow and ice with information on what is happening in the thin surface layer of soil immediately beneath the snow.

Inundation, wetlands, and hidden sources of methane

The third set of parameters that the mission will observe relates to inundation and the extent of wetlands. Floodplains, seasonal ponds, and large wetland complexes are often hidden beneath forest canopies, which is why optical satellites have difficulty distinguishing them, especially in cloudy tropical regions. GNSS reflectometry, thanks to the penetration of L-band signals through vegetation, can detect the presence of water even where it is actually invisible to the human eye.

Wetlands are one of the most important, but also most sensitive types of ecosystems. They simultaneously serve as carbon sinks, natural water filters, and areas with high biodiversity. However, certain types of wetlands are at the same time significant natural sources of methane—a potent greenhouse gas. Better maps of seasonal and inter-annual changes in inundation will enable more precise estimates of methane emissions from these ecosystems and improve models of feedback loops between the biosphere and the climate.

For European countries, including Croatia, more precise satellite information on floods and wetlands also has a very practical dimension. Data from HydroGNSS can be integrated into operational early warning systems for floods, planned management of retention areas, and the restoration of degraded wetlands as natural "sponges" that mitigate extreme water surpluses and shortages.

Biomass and carbon in forests

Above-ground biomass—primarily in forests—is directly linked to the amount of carbon stored in vegetation. In a situation where climate policy increasingly counts on the role of forests as carbon sinks, reliable and up-to-date biomass estimates become key for planning measures, from forest management to emissions trading projects.

HydroGNSS will detect differences in vegetation structure and density through changes in the reflected GNSS signal. Although the mission itself will not replace specialized radar or lidar missions dedicated to biomass, its data will serve as a valuable additional layer of information: for example, in detecting areas where major changes have occurred, which then need to be imaged in more detail by other instruments.

In combination with national forest inventories and high-resolution satellite imagery, HydroGNSS can help in discovering illegal logging, monitoring forest recovery after fires or storms, and verifying the effectiveness of reforestation projects and the restoration of degraded forest areas.

Scouts: fast and agile missions in the New Space era

HydroGNSS is the first launched mission from ESA's new Scout series, designed under the influence of New Space philosophy. The basic idea is that relatively small, targeted missions are designed, built, and launched within three years from the start of the project and within a financial framework of about 35 million euros, including development, construction, and operation in orbit. Thereby, ESA strives to introduce start-up dynamics into the traditionally slower space sector, while at the same time not sacrificing the scientific value of the data.

Scouts are intended to complement larger and more expensive Earth Explorer missions, which bring revolutionary technologies and new measurement principles but require a longer development cycle and significantly larger budgets. HydroGNSS relies on the technology of already proven GNSS reflectometry missions, such as NASA's CYGNSS constellation or the TechDemoSat-1 demonstrator, but transfers it into a new, more operational framework and focuses on a series of clearly defined climate variables.

Since the Scout family is conceived as a series of smaller missions that complement each other, HydroGNSS will in the future share an orbit with other satellites from that portfolio, whereby the European Earth observation system will gain additional flexibility. This creates prerequisites for the faster introduction of new technologies, such as more advanced receivers for GNSS reflectometry or combinations with other passive and active sensors.

Industrial partners and the British footprint

The main industrial partner of the mission is the British company Surrey Satellite Technology Ltd (SSTL), a pioneer of commercial small satellites, which has already participated in the development of previous GNSS reflectometry projects. SSTL is responsible for the design and construction of both satellites, the development of the key delay Doppler receiver instrument, but also for operations in orbit and data distribution to end users.

The mission is partially funded by funds from the United Kingdom, which thereby further solidifies its position in the segment of modern Earth observation technologies. For the European space industry, HydroGNSS is also a demonstration of how a combination of public funding, agile industry, and international cooperation can result in rapid deliveries of sophisticated missions without multi-year delays.

Transporter-15: joint launch of three national initiatives

The Falcon 9 within the framework of the Transporter-15 mission did not carry only two HydroGNSS satellites into orbit. Launched on the same flight were also new satellites for the Italian national program IRIDE and two radar satellites from the company ICEYE for the Greek National Small Satellite Program. Thereby, one commercial rideshare flight became a sort of overview of key European Earth observation initiatives of the next decade.

IRIDE is one of the most ambitious Italian space programs in history. It involves a large constellation of satellites for Earth observation funded by the Italian government through the National Recovery and Resilience Plan (PNRR), with additional funds from the National Complementary Plan. The program is coordinated by ESA in close cooperation with the Italian Space Agency (ASI) and more than 70 Italian companies and institutions.

Ultimately, IRIDE should encompass six separate constellations, each with a different type of sensor—from optical and radar to thermal and hyperspectral instruments. The Eaglet-II satellites that were launched together with HydroGNSS are part of that broader puzzle and are designed to ensure a high frequency of imaging of Italian territory and surrounding areas. Data from IRIDE are intended primarily for Italian public institutions, including Civil Protection, but also for scientists, local authorities, and the private sector.

Main applications include monitoring ground displacement, landslides, and earthquakes, surveillance of land cover and changes in land use, control of air and water quality, surveillance of coastal erosion, and early fire detection. In the context of climate change, IRIDE will give Italian Civil Protection and other institutions a new tool for rapid risk recognition—from flash floods to long-term droughts.

Greece and ICEYE: radar eyes for the Mediterranean

Another national initiative that got its first orbit on the same flight is the Greek National Small Satellite Program, specifically its radar segment under the designation "Axis 1.2". The program is implemented in cooperation with the European Space Agency, and the key industrial partner is the company ICEYE, a global leader in the field of synthetic-aperture radar (SAR) technology.

Greece has signed a contract with ICEYE that includes the development and launch of two SAR satellites, the construction of a production line in the country, and access to the global ICEYE constellation. The program is financed through the national recovery plan "Greece 2.0", with the support of European Union instruments, primarily the Recovery and Resilience Facility. The goal is not only the procurement of satellites but also the building of a sustainable domestic space industry—from high-tech jobs to research infrastructure.

Radar satellites from this program will play a key role in monitoring natural disasters, especially floods, fires, and earthquakes, in the surveillance of maritime traffic and illegal activities at sea, as well as in the control of critical infrastructure. Since SAR technology can "see" through clouds and image at night as well, data from the ICEYE constellation will be a valuable addition to information provided by optical missions like IRIDE or passive missions such as HydroGNSS.

European Earth observation architecture: piecing the mosaic together

HydroGNSS, IRIDE, and the Greek radar program are not isolated projects, but parts of a broader European Earth observation architecture. While the Copernicus program with its fleet of Sentinels ensures global, operational data for numerous applications, new national and thematic satellites fill gaps, bring specialized data, and can react faster to the specific needs of member states.

HydroGNSS has a particularly interesting role here. On one hand, it is a relatively small and financially efficient project, but on the other hand, the data it will collect go directly to the core of the global climate agenda: water, energy, and carbon. If one compares the cost of the mission with the potential benefits—from better flood and drought forecasts to more precise tracking of methane emissions and stored carbon in forests—it is clear why ESA is increasingly pushing the concept of fast, focused missions.

In a practical sense, HydroGNSS data could be integrated in a relatively short time into existing information systems used by national meteorological and hydrometeorological services, water management agencies, and institutions responsible for civil protection and disaster management. Through open data policies and cooperation with international initiatives, such as the World Meteorological Organization (WMO) and GCOS, the mission has the potential to become a global reference source for hydrological variables that have hitherto been difficult to monitor with sufficient spatial and temporal resolution.

Equally, the national programs IRIDE and the Greek SAR segment show how Recovery and Resilience Facility instruments can be used not only for short-term economic recovery but also for strategic investments in space infrastructure that will generate data and knowledge for decades. In that context, Transporter-15 was not just another commercial rideshare flight, but a symbol of a new phase in which European states and ESA use available tools to accelerate their own space ecosystem and simultaneously respond to the greatest challenge of our time—the climate crisis.