Differential GPS

Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). 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).^[1]

DGPSs consist 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.

The United States Coast Guard (USCG) previously ran DGPS in the United States on longwave radio frequencies between 285 kHz and 325 kHz near major waterways and harbors. It was discontinued in March 2022.^[2] 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)^[3] also ran a separate DGPS system, but discontinued its use on December 15, 2022. Other countries have their own DGPS.^{[citation needed]}

A similar system which transmits corrections from orbiting satellites instead of ground-based transmitters is called a Wide-Area DGPS (WADGPS)^[4] satellite-based augmentation system.