ESA’s HydroGNSS mission launched to ‘scout’ for water – small twin satellites to track Earth’s water cycle and climate impacts

Mission overview

The European Space Agency’s HydroGNSS mission, the first in its Scout series, was launched on 28 November to improve global knowledge of water availability and how climate change affects Earth’s water cycle. Two identical small satellites were placed into orbit as part of SpaceX’s Transporter-15 rideshare flight on a Falcon 9 rocket from Vandenberg Space Force Base in California.

Launch and early operations

The HydroGNSS pair separated from the rocket less than 90 minutes after liftoff and soon afterward ground controllers confirmed receipt of signals from both spacecraft, confirming they were safely in orbit. Surrey Satellite Technology Ltd (SSTL) in the UK, ESA’s prime contractor for HydroGNSS, is responsible for operating the satellites and distributing mission data.

Science objectives

HydroGNSS is designed to measure key hydrological variables such as soil moisture, surface inundation, freeze–thaw state and above‑ground biomass across most of Earth’s land surface. These measurements will feed applications in flood prediction, agricultural planning, water‑risk management and assessment of forest carbon stocks in the global carbon cycle.

Measurement technique

GNSS reflectometry

Both satellites use Global Navigation Satellite System (GNSS) reflectometry, an approach that analyses navigation signals after they bounce off Earth’s surface. HydroGNSS receives L‑band microwave signals from systems such as GPS and Galileo, comparing the direct and reflected signals to infer surface properties linked to water and vegetation.

Payload and configuration

Each HydroGNSS spacecraft carries a delay Doppler mapping receiver with two antennas: a zenith antenna to track direct GNSS signals and a nadir antenna to collect reflected signals. The receiver converts these measurements into delay Doppler maps that can be processed into geophysical variables like moisture, inundation extent and biomass indicators.

Orbit and coverage

The twin satellites fly in a near‑polar Sun‑synchronous orbit at roughly 550 km altitude, arranged about 180 degrees apart. This configuration aims to revisit most land areas approximately every 15 days on a grid of cells around 25 km, with effective resolution varying depending on surface conditions such as calm water, forests or agricultural mosaics.

Environmental and climate relevance

By mapping inundation and wetlands, including those hidden beneath forest canopies, HydroGNSS will help identify ecosystems that can be substantial sources of methane. Monitoring freeze–thaw transitions and permafrost behaviour will support studies of energy exchange, surface radiation balance and carbon fluxes in high‑latitude regions.

Programme context

HydroGNSS inaugurates ESA’s Scout missions, a line of rapid, low‑cost Earth observation projects developed on lean budgets of around €35 million from design to in‑orbit operation. The mission contributes to ESA’s FutureEO programme, complementing larger Earth Explorer missions with a faster, more agile approach to testing and deploying innovative observation techniques.

Industrial and national contributions

SSTL leads spacecraft development and operations, with the mission partially funded by the UK Space Agency as part of the UK’s role in ESA’s climate and Earth observation activities. The Transporter‑15 launch also carried other European national payloads, including IRIDE satellites and ICEYE spacecraft for Italy and Greece, developed with ESA’s participation.

“As the first of ESA’s Scout missions to launch, HydroGNSS marks an important milestone for this new family of rapid, low‑cost Earth observation missions, and we extend our thanks to the mission’s prime contractor, SSTL.”

“We now look forward to seeing how HydroGNSS will employ GNSS reflectometry to deliver valuable insights into key hydrological variables that shape Earth’s water cycle.”

Author’s summary

HydroGNSS sends two compact Scout satellites into Sun‑synchronous orbit, using GNSS reflectometry to track soil moisture, flooding, freeze–thaw and biomass, sharpening climate and water‑risk monitoring on a modest budget.

European Space Agency — 2025-11-29