Published on 12 March 2026
Just three months after launch, the European Space Agency’s twin HydroGNSS satellites are already proving their capabilities in orbit. By exploiting reflected signals from navigation satellites – the sophisticated technique they use to generate Delay Doppler Maps in order to ‘scout’ for water across Earth’s surface – these compact satellites are beginning to reveal the scientific potential they were built to unlock, even while still in their commissioning phase.
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.
HydroGNSS uses GNSS reflectometry to ‘scout’ for water
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Key to this process involves producing Delay Doppler Maps, which show 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 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 ‘horseshoe’ shape. A helpful comparison is sunlight reflecting off the sea when viewed from an airplane: 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 understanding these patterns, 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.
Source:
European Space Agency. (2026, March 12). ESA’s HydroGNSS on track to scout for water. ESA Observing the Earth. https://www.esa.int/Applications/Observing_the_Earth/FutureEO/HydroGNSS…