GNSS reflectometry


GNSS reflectometry involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as GPS. The idea of using reflected GNSS signal for earth observation became more and more popular in the mid-1990s at NASA Langley research centre and is also known as GPS reflectometry. Research applications of GNSS-R are found in
GNSS reflectometry is passive sensing that takes advantage of and relies on separate active sources - the satellites generating the navigation signals. For this, the GNSS receiver measures the signal delay from the satellite and the rate of change of the range between satellite and observer. 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 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 works as a bi-static radar, where transmitter and receiver and separated by a significant distance. Since in GNSS-Reflectometry one receiver simultaneously can track multiple transmitters, the system also has nature of multi-static radar. The receiver of the reflected GNSS signal can be of different kind: Stationary stations, ship measurements, air planes 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.