The European Space Agency (ESA) has long been at the forefront of Earth observation, and its Scout missions represent a bold step into the era of “New Space” – emphasizing rapid development, cost-effectiveness, and innovative technologies. These missions aim to validate new scientific concepts using small satellites, paving the way for scalable, larger constellations in the future. As part of ESA’s Earth Observation FutureEO programme, Scout missions are designed to move from concept to launch in just three years, with budgets capped at around €35 million. This agile approach allows for quicker deployment and testing of cutting-edge ideas, ultimately enhancing our understanding of climate change and environmental dynamics.
HydroGNSS: Leading the Charge in Hydrological Monitoring
At the heart of the Scout initiative is HydroGNSS, the first operational Scout mission, led by the United Kingdom and built by Surrey Satellite Technology Ltd (SSTL). Scheduled for launch in November 2025 aboard a SpaceX rocket from Vandenberg Space Force Base in California, HydroGNSS consists of two identical 65kg satellites – each roughly the size of a household washing machine. These satellites will employ Global Navigation Satellite System Reflectometry (GNSS-R), a technique that analyzes reflected signals from GNSS constellations like GPS to measure key hydrological climate variables.
The mission targets Essential Climate Variables (ECVs), which are critical indicators of Earth’s changing climate. HydroGNSS provides data on soil moisture, permafrost freeze-thaw states, inundation (flooding), above-ground forest biomass, ocean wind speeds, and sea ice extent. This information is vital for climate modeling, agricultural planning, flood preparedness, and environmental resilience. With a spatial resolution of 2–7 km and sensitivity ranging from 0.7–3.8 dB per 10% change in geophysical parameters, the satellites promise high-accuracy measurements that can capture dynamic phenomena, such as daily soil moisture fluctuations or permafrost cycles – something traditional remote sensing systems struggle with.
Funded by the UK Space Agency with £26 million alongside ESA contributions, HydroGNSS highlights strong UK involvement. SSTL is responsible for designing, building, and operating the satellites, collaborating with international partners including Sapienza University and Tor Vergata University in Italy, the Finnish Meteorological Institute (FMI), the Institute of Space Sciences (ICE/IEEC) in Spain, the National Oceanography Centre (NOC) in the UK, and the University of Nottingham. This collaboration builds on early UK investments through the Centre for Earth Observation Instrumentation, accelerating key technologies.
As of November 7, 2025, the HydroGNSS satellites have completed final testing and integration onto their launch plate, marking them as fully ready for liftoff. After arriving at the California launch site in late September, the twin satellites underwent meticulous preparations, including environmental testing and deployment simulations. The mission’s planned operational life is three years, with potential extensions to five or six, ensuring long-term contributions to climate science.
CubeMAP: A Promising Concept That Faced Challenges
Originally slated as the first Scout mission, CubeMAP (also known as ESP-MACCS or Earth Scout 1) was envisioned as a constellation of three 12U CubeSats to study processes in the tropical upper troposphere and stratosphere (UTS). The mission aimed to quantify atmospheric composition, variability, and its interactions with climate change, focusing on key greenhouse gases like water vapor, carbon dioxide, methane, and nitrous oxide. These observations would target tropical and sub-tropical latitudes, using novel limb-sounding techniques where the satellites align with the Sun and atmosphere for precise profiling.
CubeMAP promised innovation through miniaturization, employing thermal infrared spectrometers and a visible near-infrared hyperspectral solar disk imager. Led scientifically by the UK, with the Rutherford Appleton Laboratory (RAL) playing a key role, the mission involved an international consortium including GomSpace in Denmark (leading industrial development), and partners in Luxembourg, Sweden, Austria, the Netherlands, and Norway. A contract was signed with GomSpace in December 2021, with an initial launch target of 2025.
However, despite its potential, CubeMAP’s development was terminated by ESA’s Earth Observation Programme Board in June 2023 after careful evaluation. The decision reflected challenges in aligning the mission’s innovative ambitions with the strict Scout timelines and budgets. While the concept remains scientifically valuable for understanding UTS dynamics and greenhouse effects, no revival has been announced.
The Broader Impact of Scout Missions
Scout missions like HydroGNSS exemplify how small satellites can democratize Earth observation, offering faster, more affordable alternatives to traditional large-scale missions. By proving concepts that can scale into constellations, they enable responsive monitoring of rapidly changing environmental phenomena. The UK’s leadership in HydroGNSS not only boosts national expertise in small satellite engineering but also contributes to global efforts in climate resilience and sustainability.
As HydroGNSS prepares for its imminent launch, the Scout programme continues to inspire, demonstrating that innovation in space can yield tangible benefits for our planet. With climate change accelerating, these missions are more important than ever, providing the data needed to inform policy, predict disasters, and protect ecosystems worldwide.
