HydroGNSS mission begins to study Earth’s water inventory from space

On November 28, 2025, the two HydroGNSS twin satellites of the European Space Agency (ESA) were launched into space —along with other satellites— by a SpaceX Falcon 9 rocket, from Vandenberg base, California. Less than 90 minutes after liftoff, the two satellites separated from the rocket. Then, technicians from the company Surrey Satellite Technology Ltd (SSTL), ESA’s main contractor, confirmed the reception of signals, which indicated that both satellites were in Earth orbit correctly. The mission also has partial funding from the UK Space Agency.

What HydroGNSS will do and how

The satellites will use an innovative technique called GNSS reflectometry (GNSS-R), by which they capture L-band signals from satellite navigation systems (such as GPS or Galileo). These signals, when reflected on the Earth’s surface, modify their characteristics. By comparing the reflected signals with the direct GNSS signals, the satellites can extract information about physical properties of the terrain. Each satellite carries a “delay-Doppler mapping” receiver: with two antennas —one zenith (for direct GNSS signals) and another nadir (for reflected signals) — and processes the data to generate delay-Doppler maps. Thanks to this, satellites will be able to measure —on a global scale— variables such as:

• Soil moisture
• Freezing or thawing state of the ground
• Flood level and extent of surface water bodies or wetlands
• Surface biomass (vegetation)

What it will be used for — importance and impacts

• The generated data will allow for the development of a global inventory of the Earth’s water, essential for understanding the global hydrological cycle.
• They will be useful for predicting floods and improving agricultural planning, especially in vulnerable regions.
• They will help identify hidden wetlands under vegetation —key ecosystems for biodiversity and also potential sources of methane.
• In areas with snow or permafrost, freezing/thawing information will provide relevant data on soil energy balance, carbon cycles, and climate change in high latitudes.

Furthermore, this mission represents a technological milestone: HydroGNSS is part of the ESA’s “Scout” program — low-cost, rapid-development projects — designed to test new Earth observation technologies.