![GNSS-R system diagram](https://upload.wikimedia.org/wikipedia/commons/9/98/GNSS-R_system_diagram.svg)
GNSS reflectometry (or GNSS-R) involves making measurements from the reflections from the Earth of navigation signals from
Global Navigation Satellite Systems
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
. 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 mate ...
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 called ...
, 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
The Disaster Monitoring Constellation for International Imaging (DMCii) or just Disaster Monitoring Constellation (DMC) consists of a number of remote sensing satellites constructed by Surrey Satellite Technology Ltd (SSTL) and operated for the A ...
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 mor ...
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 ra ...
, 29 January 2009.
See also
*
Cyclone Global Navigation Satellite System (CYGNSS)
The Cyclone Global Navigation Satellite System (CYGNSS) is a space-based system developed by the University of Michigan and Southwest Research Institute with the aim of improving hurricane forecasting by better understanding the interactions b ...
References
Further reading
*
*
*Cardellach, Estel (2015)
E-GEM – GNSS-R Earth Monitoring; State of the Art Description Document
*Emery, William and Camps, Adriano (2017): Introduction to Satellite Remote Sensing 1st Edition Atmosphere, Ocean, Land and Cryosphere Applications, Chapter 6: Remote Sensing Using Global Navigation Satellite System Signals of Opportunity, Elsevier, 20th September 2017, Paperback , eBook {{ISBN, 9780128092590
* A complete list of references maintained by the GNSS-R Community can be found at: https://www.ice.csic.es/personal/rius/gnss_r_bibliography/index.html
External links
The Engineer Online, 28 November 2006.
Artech House, September 2009.
Satellite navigation
Remote sensing