Embodying the New Space approach, HydroGNSS is ESA’s first Scout mission, developed under the Earth Observation FutureEO programme. Scout missions prioritise speed and innovation, enabling new ideas and satellite technologies to be developed rapidly and at low cost.
At the heart of HydroGNSS lies an innovative method known as Global Navigation Satellite System (GNSS) reflectometry. Navigation satellites such as GPS and Galileo continuously transmit L-band microwave signals that subtly change after reflecting off Earth’s surface. HydroGNSS compares these reflected signals with the direct GNSS signals to extract valuable information on geophysical parameters linked to the water cycle.
Key to this process involves producing Delay Doppler Maps (DDMs), which map out how a GNSS signal changes after bouncing off Earth’s surface. One axis represents delay (how long the signal takes to return), and the other shows Doppler frequency (how relative motion affects the signal).
When the signal reflects off a smooth, mirror-like surface – such as calm water or flat sea ice – it produces a strong, sharp peak. But over a rough ocean, the reflection spreads out into a weaker, curved parabolic shape. A helpful comparison is sunlight reflecting off the sea when viewed from an aeroplane: a perfectly smooth surface gives a bright point, while waves stretch the reflection into a wide glistening area.
The strength and shape of this reflection depends on surface conditions. Roughness matters, but so do factors like soil moisture, whether the ground is frozen, and the presence of vegetation. By accounting for these different parameters, scientists can use Delay Doppler Maps to measure soil moisture, floods, forest biomass and freeze–thaw cycles. Over oceans, these maps can also reveal wind speed and sea-ice extent.
HydroGNSS enhances this approach by generating maps of reflections in two polarisations and also using a second, wider bandwidth, frequency signal available from GPS and Galileo. As well as the DDMs, HydroGNSS introduces a new higher rate measurement called the “coherent channel” that offers the potential of 25 times finer spatial resolution in some regions of the globe. These innovations add extra layers of information that improve the breadth, quality and resolution of hydrological measurements worldwide.
The image below shows four reflections being tracked as HydroGNSS-2 crosses from Wisconsin, US to Ontario, Canada. The crossings of rivers and lakes are clearly visible in the reflections at both frequencies. The amplitude of reflections is affected by the roughness of the surface over water, giving an indicator for wind speed. The amplitude over land also depends on soil moisture and the presence of vegetation. As the satellites head further northwards, the reflections indicate the presence of frozen soil and ice.
The two HydroGNSS satellites are still in their planned commissioning phase. The team at Surrey Satellite Technology Ltd (SSTL) in the UK is busy refining calibration, validating processor chains, and characterising the satellites’ in-orbit behaviour. SSTL is consulting closely regarding results with its science team partners at Sapienza and Tor Vergata Universities in Rome, CSIC/IEEC in Spain, IFAC in Italy, FMI in Finland, TUW in Austria, National Oceanography Centre and University of Nottingham in the UK.
Nevertheless, even at this early stage the satellites are already generating promising measurements, showing that the mission is well on track and moving confidently toward its full operational phase.
Martin Unwin, HydroGNSS Principal Investigator at SSTL, said, “Both of the HydroGNSS Scouts are collecting Delay Doppler Maps of reflected GNSS signals at dual polarisation and dual frequency. In particular, the strength of the GPS L5 and Galileo E5a reflections have been a surprise as these offer ten times the bandwidth of the GPS L1 signals and could be a future enabler for ocean altimetry applications.”
Jonathan Rawlinson, designer of the HydroGNSS reflectometry instrument, explained “the addition of Galileo and dual frequency capability to the instrument presented new technical challenges, but the instrument is already yielding surprising results that widen the applications and justify the approach taken. The high-resolution coherent channel could take GNSS reflectometry to another level.”
“The team is very pleased that the hard work in all the different aspects of the payloads, antennas, amplifiers, software and digital data handling, all came together to enable this spectacular new in-orbit capability.” said Reynolt de Vos van Steenwijk, manager of the HydroGNSS payload.
Pete Garner, HydroGNSS Project Manager at SSTL, added, “Seeing the first datasets from this exciting mission is a fantastic reward for the SSTL–ESA–scientific partners collaborative team, which has worked so hard to overcome the many challenges you would expect from a complex satellite project like HydroGNSS. The whole team is looking forward to what else the mission can show us about our planet.”
ESA’s Project Manager for the Scout missions, Jean-Pascal Lejault, said, “We are extremely pleased with these initial results. The first show that the satellites are in good health and working as they should. This is a great achievement and my thanks go to everyone who has been involved.
“We look forward to finalising the mission’s commissioning phase and moving to operations, and seeing how this first ESA Scout mission will indeed ‘scout’ for water, yielding new information about the properties related to Earth’s water cycle, and more.”
