Cost-effective sensors from the University of Bonn are measuring water levels along rivers and coastlines in Africa and the Pacific region.
Using a low-cost sensor and GNSS Interferometric Reflectometry (GNSS-IR), river water levels can be monitored around the clock. The water-level data are automatically transmitted via cellular networks to an analysis center.
Researchers at the University of Bonn developed the method several years ago and tested it on the Lower Rhine. With support from the European Space Agency (ESA), the monitoring system is now also being used in Africa and the Asia-Pacific region.
Researchers at the Institute of Geodesy and Geoinformation at the University of Bonn, led by Makan Karegar, have transferred water -level monitoring technology from the Rhine to Africa, Australia and the Philippines as part of ESA projects. Originally developed in the DFG Collaborative Research Center SFB 1502 (DETECT), the technology enables continuous, freely accessible monitoring of inland and coastal waters in data-poor regions worldwide.
Active on three continents
The technological centerpiece is the Raspberry Pi Reflector (RPR), a compact, solar-powered sensor developed at the University of Bonn. Using GNSS-IR, it measures water levels with centimeter-level accuracy.
Only a portion of the signals emitted by the GNSS satellites is directly captured by the antenna. The rest is reflected by the water surface and reaches the receiver via this detour. When superimposed with the directly received signal, it forms specific patterns known as interference patterns. These can be used to calculate the distance from the antenna to the water surface.
Each unit costs less than 800 euros, is powered by solar energy, and transmits data daily via mobile networks. “Modern gauge stations are prohibitively expensive, and conventional ones are highly vulnerable to flood damage,” said Makan Karegar, project manager. “These two factors together have left many countries in the global south with little to no ground-based water-level monitoring. The low-cost GNSS-IR sensor was developed precisely to address this gap.”
CAMEO-WAGST Project
The CAMEO-WAGST project (“Cameroon Advanced Measurements for Enhanced Observations of Water levels using Affordable GNSS-IR and Sentinel-3 & 6 Technology”) has established the first dedicated GNSS-IR network for monitoring water levels along coasts and rivers in Camroon and was funded by ESA. Between May and June 2025, researchers collaborated with Loudi Yap, director of the Research Laboratory in Geodesy at the National Institute of Cartography to install eight RPR sensors in Cameroon: two on the Sanaga River and six along the coast. “A lack of infrastructure for reliable hydrological and coastal monitoring in Cameroon has so far hindered effective flood risk management and early warning systems,” Yap said.
This collaboration, under the umbrella of the EO Africa Research and Development Facility, is already bearing fruit, said Roelof Rietbroek, research coordinator at ESA’s EO Africa R&D Facility. “We hope this paves the way for more reliable monitoring of flood-prone regions in Africa.”
St3TART-FO Project
Building on this success, the follow-up project St3TART-FO also was launched in collaboration with ESA. A total of 17 RPR sensors will be installed in seven countries, including West Africa, Australia and the Philippines. “The goal is to create a freely accessible reference measurement network for calibrating satellite data,” Karegar said. For the first time, the network will provide continuous water-level data at previously unmonitored locations.
The collaboration is based on years of scientific exchange between Africa and Europe. Partners include:
- International Institute for Water and Environmental Engineering (2iE), Burkina Faso
- National Institute of Cartography, Cameroon
- Environmental Protection Authority (EPA), Ghana
- Nigeria Hydrological Services Agency (NiHSA)
- University of Maiduguri, Nigeria
- Assane Seck University of Ziguinchor, Senegal
- University of Southern Queensland, Australia
- University of the Philippines Diliman.
Technology Transfer and Capacity Building
Both projects promote technology transfer and local capacity building through training, workshops and mentoring, enabling partner institutions to operate RPR networks independently. “We want to leave behind a sustainable monitoring capacity that is operated by local scientists and institutions, openly shared with the world, and maintained well into the future,” Karegar said.
With financial support from the Transdisciplinary Research Area (TRA) “Sustainable Futures” at the University of Bonn, Karegar developed the open-access data platform gnss4surfacewater.com, which provides an independent, ground-based service for monitoring current and historical water levels using GNSS-IR. Also visit CAMEO-WAGST GitHub for code and field photos.
