GNSS reflectometry

Principles

GNSS reflectometry is passive sensing that takes advantage of and relies on multiple active sources - with the satellites generating the navigation signals. For this, the GNSS receiver measures the signal delay from the satellite (the pseudorange measurement) and the rate of change of the range between satellite and observer (the Doppler measurement). The surface area of the reflected GNSS signal also provides the two parameters time delay and frequency change. As a result, the Delay Doppler Map (DDM) can be obtained as GNSS-R observable. The shape and power distribution of the signal within the DDM is dictated by two reflecting surface conditions: its dielectric properties and its roughness state. Further derivation of geophysical information rely on these measurements.

GNSS reflectometry is a bi-static radar, where transmitter and receiver are separated by a significant distance. Since in GNSS reflectometry one receiver simultaneously can track multiple transmitters (i.e. GNSS satellites), the system also has the nature of multi-static radar. The receiver of the reflected GNSS signal can be of different kinds: Ground stations, ship measurements, airplanes or satellites, like the UK-DMC satellite, part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd. It carried a secondary reflectometry payload that has demonstrated the feasibility of receiving and measuring GPS signals reflected from the surface of the Earth's oceans from its track in low Earth orbit to determine wave motion and windspeed.

Space missions

• CYGNSS, satellite constellation by NASA using GNSS-R for improving hurricane forecasting, launched in 2016
• TechDemoSat-1, technology demonstration small satellite by ESA, launched in 2019
• PRETTY, technology demonstration CubeSat by ESA measuring sea state, sea ice, and ocean currents, launched in 2023
• 'HydroGNSS''''', 2 identical small satellites by ESA for monitoring Essential Climate Variables related to the hydrological cycle, launched in November 2025
• Atlantic Constellation, Earth observation satellite constellation by Spain, Portugal, and UK, coordinated by ESA, first launch expected in 2026

GNSS Interferometric Reflectometry

References

References

1. "A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry". ESA Journal.
2. (2000). "IGARSS 2000. IEEE 2000 International Geoscience and Remote Sensing Symposium. Taking the Pulse of the Planet: The Role of Remote Sensing in Managing the Environment. Proceedings (Cat. No.00CH37120)". IEEE.
3. (2013-07-15). "A zeppelin experiment to study airborne altimetry using specular Global Navigation Satellite System reflections: A ZEPPELIN EXPERIMENT TO STUDY AIRBORNE ALTIMETRY". Radio Science.
4. (2017). "Feasibility of GNSS-R Ice Sheet Altimetry in Greenland Using TDS-1". Remote Sensing.
5. (2009-09-22). "Bistatic Scattering of GPS Signals Off Arctic Sea Ice". IEEE Transactions on Geoscience and Remote Sensing.
6. (2011). "Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique". IEEE Transactions on Geoscience and Remote Sensing.
7. (2005). "Detection and Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing". IEEE Transactions on Geoscience and Remote Sensing.
8. M. P. Clarizia ''et al.'', [http://www.agu.org/pubs/crossref/2009/2008GL036292.shtml Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean] {{Webarchive. link. (2011-06-06 , [[Geophysical Research Letters]], 29 January 2009.)
9. Ruf, Christopher. (2019). "In-Orbit Performance of the Constellation of CYGNSS Hurricane Satellites". Bulletin of the American Meteorological Society.
10. Gutierrez, Peter. (2025-01-20). "GNSS Reflectometry Project HydroGNSS to Launch in 2025".
11. (September 2013). "The Accidental Tide Gauge". IEEE Geoscience and Remote Sensing Letters.
12. (September 2009). "Can we measure snow depth with GPS receivers". Geophysical Research Letters.