GNSS reflectometry (or GNSS-R) 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 Center The Langley Research Center (LaRC or NASA Langley), located in Hampton, Virginia, United States of America, is the oldest of NASA's field centers. It directly borders Langley Air Force Base and the Back River on the Chesapeake Bay. LaRC has fo ...

and is also known as ''GPS reflectometry''. Research applications of GNSS-R are found in * Altimetry * Oceanography (Wave Height and Wind Speed) * Cryosphere monitoring * Soil moisture monitoring 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 (the

pseudorange The pseudorange (from pseudo- and range) is the ''pseudo'' distance between a satellite and a navigation satellite receiver (see GNSS positioning calculation), for instance Global Positioning System (GPS) receivers. To determine its position ...

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 Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

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 In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mat ...

and its roughness state. Further derivation of geophysical information rely on these measurements. GNSS reflectometry works as a

bi-static radar Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is calle ...

, 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 Surrey Satellite Technology Ltd, or SSTL, is a company involved in the manufacture and operation of small satellites. A spin-off company of the University of Surrey, it is presently wholly owned by Airbus Defence and Space. The company began ou ...

. 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 A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...

to determine wave motion and windspeed.M. P. Clarizia ''et al.''
Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean

Geophysical Research Letters ''Geophysical Research Letters'' is a biweekly peer-reviewed scientific journal of geoscience published by the American Geophysical Union that was established in 1974. The editor-in-chief is Harihar Rajaram. Aims and scope The journal aims for rap ...

, 29 January 2009.

References

External links

The Engineer Online, 28 November 2006.

Artech House, September 2009. Satellite navigation Remote sensing

See also

References

Further reading

External links