ESA-SSTL Twin HydroGNSS Satellites Return Water Cycle Data During Commissioning

by Sophie Jenkins
London, UK (SPX) ) May 01, 2026
Just three months after launch, the twin HydroGNSS satellites built by Surrey Satellite Technology Ltd for the European Space Agency are producing early science measurements, detecting water-linked surface conditions across Earth even while still in their commissioning phase.

HydroGNSS is ESA's first Scout mission, developed under the Earth Observation FutureEO programme. Scout missions are designed to move quickly and cheaply, translating new satellite technologies into demonstrated capabilities rather than following the longer timelines typical of conventional Earth observation programmes.

The satellites operate using a technique called GNSS reflectometry. Navigation satellites such as GPS and Galileo continuously broadcast L-band microwave signals. When those signals reflect off Earth's surface, their strength and shape change in ways that encode information about surface conditions. HydroGNSS captures both the direct signal and the reflected version, then compares them to extract geophysical data tied to the water cycle.

Central to the processing method are Delay Doppler Maps, which plot how a reflected GNSS signal changes along two axes: signal delay, representing how long the signal takes to return, and Doppler frequency shift, representing how relative motion between the satellite and the surface affects the signal. A smooth surface such as calm water or flat sea ice produces a sharp, strong peak in the map. A rougher surface such as wave-disturbed ocean spreads the return into a broader, weaker parabolic shape - comparable to how sunlight glinting off a calm sea forms a tight bright point from altitude, while choppy water spreads the glint into a wide shimmering band.

Surface roughness is not the only factor shaping the reflection. Soil moisture content, frozen ground conditions, and vegetation cover all influence how reflected signals appear. By accounting for these variables, scientists can use Delay Doppler Maps to measure soil moisture, flood extent, forest biomass, and freeze-thaw cycles. Over ocean surfaces, the same maps yield wind speed estimates and sea-ice extent data.

HydroGNSS extends the standard approach in several ways. The satellites generate reflected maps in two polarisations simultaneously and also exploit a second, wider-bandwidth frequency available from both GPS and Galileo. An additional measurement mode called the "coherent channel" offers the potential for spatial resolution up to 25 times finer than standard GNSS reflectometry methods, applicable to some regions of the globe. Together, these innovations broaden both the range of measurable parameters and the quality of the resulting data.

Early datasets from HydroGNSS-2 crossing from Wisconsin in the United States to Ontario in Canada show four simultaneous reflections being tracked. River and lake crossings are clearly resolved at both frequencies. Reflection amplitude variations over water track surface roughness and give a wind speed indicator, while over land the amplitude responds to soil moisture and vegetation presence. Further north, the data show signatures consistent with frozen soil and ice cover.

The SSTL team at Guildford in the United Kingdom is currently working through calibration refinement, processor chain validation, and in-orbit behaviour characterisation. Science partners supporting the mission include Sapienza University and Tor Vergata University in Rome, CSIC/IEEC in Spain, CNR-IFAC in Italy, FMI in Finland, TUW in Austria, the National Oceanography Centre, and the University of Nottingham.

Martin Unwin, HydroGNSS Principal Investigator at SSTL, said both satellites are collecting Delay Doppler Maps of reflected GNSS signals at dual polarisation and dual frequency. He noted that the strength of the GPS L5 and Galileo E5a reflections "have been a surprise," adding that those signals offer ten times the bandwidth of GPS L1 and "could be a future enabler for ocean altimetry applications."

Jonathan Rawlinson, designer of the HydroGNSS reflectometry instrument, said incorporating Galileo and dual-frequency capability introduced new engineering challenges but the instrument "is already yielding surprising results that widen the applications and justify the approach taken." He said the high-resolution coherent channel "could take GNSS reflectometry to another level."

Reynolt de Vos van Steenwijk, HydroGNSS payload manager, said the team is pleased that work across payloads, antennas, amplifiers, software, and digital data handling "all came together to enable this spectacular new in-orbit capability."

Pete Garner, HydroGNSS Project Manager at SSTL, described the first datasets as "a fantastic reward" for the collaborative SSTL-ESA-scientific partners team, and said the mission team is looking forward to further results.

ESA Scout missions Project Manager Jean-Pascal Lejault said the initial results show the satellites are in good health and operating as intended. He said the agency looks forward to completing commissioning and moving to operations, and to seeing what the mission "will indeed 'scout' for water, yielding new information about the properties related to Earth's water cycle."