ESA HydroGNSS delivers first water signals

The European Space Agency said on May 21 that HydroGNSS had delivered its first signals from orbit, giving the agency its first live return from a mission built to track key parts of Earth’s water cycle. The two-satellite mission was launched on November 28, 2025 on a SpaceX Falcon 9 from Vandenberg Space Force Base in California. ESA says the spacecraft are designed to monitor soil moisture, wetlands and inundation, freeze-thaw conditions over permafrost and above-ground biomass. ### What exactly did HydroGNSS just do? ESA said the satellites have begun generating the reflected navigation-signal measurements at the center of the mission, even though HydroGNSS was still described in March as being in its commissioning phase. The agency said the spacecraft use signals from systems such as GPS and Galileo after those signals bounce off Earth’s surface, a method known as Global Navigation Satellite System reflectometry. (esa.int) The measurements are turned into products linked to hydrological climate variables. ESA says the mission is intended to provide global information on soil moisture and other water-related indicators at finer resolution to support climate monitoring, agriculture, meteorology and planning for floods and drought. ### Why use navigation signals instead of a conventional radar payload? (esa.int) HydroGNSS was built as ESA’s first Scout mission, part of a smaller and faster line of Earth-observation missions under the FutureEO program. ESA says Scout missions are meant to deliver value-added science either by miniaturizing existing technologies or by testing new observing techniques on shorter development cycles than its larger Earth Explorer missions. (earth.esa.int) The HydroGNSS approach avoids carrying a large transmitter. ESA says the satellites compare direct GNSS signals with the reflected ones, and the amplitude, delay and polarization of those reflections carry information about land, ice and water. ### Which parts of the water cycle is ESA trying to measure? ESA says HydroGNSS focuses on four parameters identified as Essential Climate Variables or closely related to them: soil moisture, inundation and wetlands, freeze-thaw state over permafrost, and above-ground biomass. (esa.int) Those variables are used to study how water moves through land systems and how climate and human activity affect them. (esa.int) The mission page says the twin satellites orbit 180 degrees apart, a configuration intended to expand revisit coverage. ESA has described HydroGNSS as a global mission that will feed scientific products up to Level 2, meaning processed geophysical data rather than raw signal returns. ### What else did ESA announce this week? ESA said on May 20 that it had selected two new Scout missions, Hibidis and SOVA-S, from four finalists. (esa.int) The agency said Hibidis is designed to study understorey biodiversity and ecosystem functioning, while SOVA-S will investigate atmospheric gravity waves and their effects on the upper atmosphere and thermosphere. (esa.int) That means ESA’s Scout line is expanding beyond HydroGNSS into biodiversity and upper-atmosphere science. ESA said the four candidate missions had been narrowed from nine proposals and that the selected missions will now move into implementation. ### What happens next for HydroGNSS? March 12 was the date ESA last said HydroGNSS was still in commissioning, and the May 21 first-signal milestone points to the mission moving toward routine science operations. (esa.int) ESA’s Earth Online mission page says HydroGNSS will provide global soil-moisture information and other water-system measurements for climate, agriculture and hazard applications. (esa.int) ESA has not, in the material reviewed, given a separate public date for full operational service, but the mission is now part of the agency’s active Scout fleet and the next public milestones are likely to be additional calibrated data products and science releases from the HydroGNSS team. (esa.int)