ESA HydroGNSS Mission Launches to Monitor Global Water Cycle Using GNSS Reflectometry

The European Space Agency (ESA) successfully launched its inaugural Scout mission, HydroGNSS, on November 28, 2025, an endeavor designed to enhance the monitoring of global water availability and the effects of climate shifts. This rapid, low-cost project operates under the 'New Space' philosophy, intended to complement ESA's larger Earth Explorer programs by demonstrating a streamlined approach to critical Earth science objectives.

Spearheaded by the United Kingdom, the mission involves twin satellites constructed and managed by prime contractor Surrey Satellite Technology Ltd (SSTL) in Guildford. The two identical spacecraft were propelled into orbit from the Vandenberg Space Force Base in California aboard a SpaceX Falcon 9 rocket as part of the Transporter-15 rideshare flight. The launch sequence began at 19:44 CET, with successful orbital insertion confirmed less than 90 minutes later upon spacecraft separation. SSTL later verified the operational status of both units by confirming signal reception.

Orbiting approximately 550 kilometers above Earth in a sun-synchronous configuration, the twin satellites maintain a precise 180-degree separation. Their primary scientific method utilizes Global Navigation Satellite System (GNSS) reflectometry, which captures L-band microwave signals from systems like GPS and Galileo after they reflect off the Earth's surface. Each satellite is equipped with a specialized delay Doppler mapping receiver featuring two antennas: one facing zenith to track direct GNSS signals and one facing nadir to collect reflected signals for processing.

HydroGNSS is tasked with quantifying four primary hydrological parameters over land, aligning with Essential Climate Variables (ECVs) defined by the Global Climate Observing System (GCOS). These variables include soil moisture, the freeze–thaw state of the ground, the extent of inundation or wetlands, and the amount of above-ground biomass. The resulting data streams are essential for improving flood prediction, informing agricultural planning, and advancing the understanding of the global carbon cycle. Secondary data products will also include measurements of ocean wind speed and sea ice extent.

ESA's Director of Earth Observation Programmes, Simonetta Cheli, noted that the launch marks a significant milestone for the quickly developed, cost-effective Scout mission family. The UK Space Agency provided £26 million in funding for the mission, highlighting British scientific and industrial capabilities. The HydroGNSS technology builds upon prior GNSS-R missions, such as NASA's CYGNSS, demonstrating the scalability of this sensing method for geophysical parameters.