ESA’s HydroGNSS Scout satellites ready for launch

After arriving at the California launch site at the end of September, the two HydroGNSS satellites have been carefully prepared for liftoff, scheduled this month.

HydroGNSS—a twin-satellite mission—marks the European Space Agency’s first “Scout” venture. By harnessing signals from navigation satellites, HydroGNSS will help scientists gain new insights into key climate variables linked to water.

The identical microsatellites will launch aboard a SpaceX Falcon 9 rocket from Vandenberg Space Force Base, sharing their Transporter-15 ride into orbit with several other small satellites.

Since their arrival, the HydroGNSS satellites have undergone a series of final checks—including testing, propellant loading, battery charging and integration with their launch adapter.

Both satellites are now ready for the next milestone: closing of the rocket fairing.

ESA’s HydroGNSS Project Manager, Jean-Pascal Lejault, said, “Reaching this milestone is a tremendous achievement, and I am incredibly proud of the entire team behind HydroGNSS. Developing ESA’s first Scout mission within such a tight timeframe and budget has required exceptional dedication and ingenuity. My special thanks go to our prime contractor Surrey Satellite Technology Ltd in the UK and the ESA project team—their commitment and creativity have made HydroGNSS possible.”

Once in orbit, the two satellites will travel 180 degrees apart around Earth to maximize coverage. They will use an innovative technique called Global Navigation Satellite System (GNSS) reflectometry to quite literally ‘scout for water.”

Navigation satellites such as GPS and Galileo transmit L-band microwave signals that change when reflected off Earth’s surface. HydroGNSS will compare these reflected signals with the direct GNSS signals to extract valuable information about geophysical parameters related to the water cycle.

The mission focuses on four crucial variables, all recognized as Essential Climate Variables or closely related to them: soil moisture, freeze–thaw state over permafrost, inundation, and above-ground biomass.

These data are vital for improving our understanding of Earth’s water cycle, agricultural planning, flood prediction, and our understanding of wetlands, permafrost dynamics and forest carbon storage.

As ESA’s first Scout mission, HydroGNSS is part of the agency’s Earth Observation FutureEO program. Scout missions are designed to deliver cutting-edge science quickly and affordably, complementing ESA’s larger Earth Explorer missions.

Inspired by the principles of New Space, the Scout program emphasizes agility and innovation—using small satellites to miniaturize proven technologies or demonstrate entirely new ways of observing Earth.

Each Scout mission must move from kick-off to launch within just three years and under a budget of €35 million, covering everything from satellite development to in-orbit commissioning. The resulting data are provided as a service to the scientific community and ESA, helping accelerate our understanding of the planet.