In a technological environment where climate change is becoming more pronounced, Earth observation missions are gaining increasing importance. In this context, the latest project of the European Space Agency (ESA) – the HydroGNSS mission – is rapidly entering its final stages before launch. They are currently in California, ready for the final procedures before taking off into orbit.

What is HydroGNSS and why is it important

The HydroGNSS (Hydrological GNSS Reflectometry) mission is part of ESA's FutureEO programme, within which the "Scout" category of missions is defined — fast, agile, and relatively inexpensive satellite campaigns aimed at demonstrating new methods and technologies for Earth observation. Scout missions are designed to deliver scientific results within a short period (about three years) on a limited budget.

HydroGNSS is planned to be launched at the end of 2025, with the aim of complementing and linking data from existing missions such as ESA's SMOS or the upcoming Biomass mission.

Technical framework: two microsatellites and GNSS reflectometry

Unlike traditional satellite instruments, HydroGNSS uses a method known as GNSS reflectometry (GNSS-R). This technique uses navigation signals that global satellite navigation systems (such as Galileo and GPS) emit towards the Earth — but it not only receives direct signals, but also those that are reflected off the Earth's surface. By analyzing the differences between the direct and reflected signals, it is possible to draw key conclusions about the characteristics of soil, vegetation, water, ice, and biomass.

HydroGNSS will use two identical microsatellites, stationed in an orbit at an altitude of ~500–600 km, positioned 180° from each other to improve the temporal frequency of data revisits. Each satellite weighs approximately 65 kg and has dimensions of about 50 × 50 × 70 cm.

The operating principle is as follows: GNSS navigation satellites continuously emit L-band microwave signals directed towards the Earth's surface. Part of these signals reaches the satellite's receiver directly, while another part is reflected off the Earth's surface and is later received as a reflected signal. Changes in the reflected signal (in phase, strength, polarization) carry information about the physical properties of the surface — soil moisture, vegetation, water bodies, freeze/thaw state, and above-ground biomass.

Targeted climate variables and scientific application

The main task of the HydroGNSS mission is to measure several key variables that are part of the so-called Essential Climate Variables (ECV), as defined by the Global Climate Observing System (GCOS). These are:

• Soil moisture


• Inundated areas and wetlands


• Freeze/thaw state – especially over permafrost


• Above-ground biomass

In addition, the mission will monitor ocean wind speed and sea ice spatial coverage as secondary products.

Data on soil moisture from space are extremely important for meteorology, flood and drought forecasting, water management, agriculture, climate change modeling, as well as for monitoring permafrost and vegetation status. Biomass measures how much organic matter is above the ground — a key parameter for understanding the global carbon cycle, monitoring forest resources, and assessing the risk of forest fires.

Current status: approval, tests, flight acceptance

By September 2025, the HydroGNSS mission has passed a significant milestone — the Flight Acceptance Review (FAR). This final set of tests confirms that the satellites are ready for transport to the launch site and for launch, and that they meet all technical, safety, and mission requirements. It is currently located at the facilities of Surrey Satellite Technology Ltd (SSTL) in the United Kingdom, where clean rooms and final checks have been carried out.

According to the latest information, it is planned to transport the satellites to California for placement on the launch pad, after which the launch is expected in the fourth quarter of 2025 using a Falcon 9 rocket.

It is important to note that, originally, the mission was planned with only one satellite, but during development it was decided to build two identical satellites to increase the coverage frequency and scientific efficiency. The idea is that a dual constellation will reduce the time between repeated measurements of the same location by almost half.

Concepts of fast (agile) missions and the role of the Scout programme

Scout missions, such as HydroGNSS, are conceived as a complementary segment to traditional research missions (e.g., Earth Explorer). They strive to bring innovation and flexibility through smaller satellites, lower costs (< ~ €35 million), and a shorter development cycle (~3 years).

In the context of ESA's Earth Observation programme, Scout missions serve to test new techniques — such as GNSS reflectometry — and to examine their practical utility in everyday satellite observation. HydroGNSS is the first of three planned Scout missions, thus laying the foundation for future approaches in climate and hydrology observation.

Through its agility and innovation, the mission could pave the way for low-cost, replicable satellite networks that could in the future provide continuous data for monitoring climate, water, and vegetation — without high costs and long development timelines.

Technological challenges and expected impact

Although GNSS reflectometry has great potential, it is not without its challenges. For example, the signals are very weak and challenging to distinguish from background noise. Sophisticated signal processing techniques, polarization corrections, multi-frequency measurements, and inversion models that directly convert reflections into physical parameters (e.g., soil moisture) are required.

Furthermore, for biomass data, the mission will attempt to distinguish vegetation components — leaves, branches, and trunks — based on how the signal is reflected and loses energy through the vegetation layers. This requires models that link the structural features of the forest with measurable signals.

In the final phase, the mission's results could significantly improve Earth system models, as they will offer high spatial and temporal resolution data for key variables of the hydrosphere and biosphere. In areas sensitive to climate extreme events — such as droughts, floods, changes in water regimes — such data can prove to be of very high value for risk reporting and prevention.

In addition, HydroGNSS is a project model of sustainability — developed on a relatively small budget, with low-mass satellites and a modular design, aiming to demonstrate that scientifically valuable missions do not have to rely exclusively on large space programs.

Where it will provide much-needed data – at a time of transition

One of the key reasons for launching the mission right now is the fact that complementary missions that have so far provided data on soil moisture, such as ESA's SMOS or NASA's SMAP, are slowly reaching the end of their operational life. HydroGNSS has the potential to take over their role and ensure continuous data.

Furthermore, HydroGNSS will enhance biomass data relative to the Biomass mission, covering areas outside the operational range of this mission and enabling faster re-measurements and detection of changes in vegetation.

The path to launch and what follows

After the satellites have passed the Flight Acceptance Review (FAR) — which is the final check of their readiness for launch — the next step is transport to the launch site in California, integration onto the rocket, and preparations for launch.

The Falcon 9 rocket, which they plan to use for the launch, has so far carried out numerous missions and is considered a reliable option for putting satellites into orbit. The plan is for the satellites to launch together as a small constellation pair.

Once placed in orbit, a commissioning phase will follow, including instrument calibration and system testing. Then, daily operational measurements and data transmission to Earth will begin, where sophisticated algorithms will process the reflected signals into useful geophysical parameters.

The HydroGNSS mission is — despite its relatively "small" scale — a major step forward in the development of space technology for climate and hydrographic applications. If it is realized as planned, it could become a key element of a global network for monitoring soil moisture, hydrological cycles, and the state of vegetation — at a time when such continuous monitoring is essential.