Differential GNSS

Differential GNSS (DGNSS), including differential GPS (DGPS), supplement and enhance the positional data available from global navigation satellite systems (GNSS), such as the American Global Positioning System (GPS) and other similar systems. A DGPS can increase accuracy of positional data by about a thousandfold, from approximately 15 metres (49 ft) to 1–3 centimetres (1⁄2–1+1⁄4 in).

A DGPS system consists of networks of fixed position, ground-based reference stations. Each reference station calculates the difference between its highly accurate known position and its less accurate satellite-derived position. The stations broadcast this data locally—typically using ground-based transmitters of shorter range. Non-fixed (mobile) receivers use it to correct their position by the same amount, thereby improving their accuracy. Direct correction of position estimates is called observation-space correction.

The United States Coast Guard (USCG) previously ran DGPS in the United States on longwave radio frequencies between 285kHz and 325kHz near major waterways and harbors. It was discontinued in March 2022. The USCG's DGPS was known as NDGPS (Nationwide DGPS) and was jointly administered by the Coast Guard and the Army Corps of Engineers. It consisted of broadcast sites located throughout the inland and coastal portions of the United States including Alaska, Hawaii and Puerto Rico. The Canadian Coast Guard (CCG) also ran a separate DGPS system, but discontinued its use on December 15, 2022. Other countries have their own DGPS.

A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a satellite-based augmentation system. Although originally proposed under the name Wide-Area DGPS (WADGPS), it is actually a system that disseminates state-space correction including location-independent (satellite clock, ephemeris, health, etc.) and location-dependent (ionospheric delay) components.