What is a GPS?

Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. The correction signal is typically broadcast over UHF radio modem.

The term can refer both to the generalized technique as well as specific implementations using it. It is often used to refer specifically to systems that re-broadcast the corrections from ground-based transmitters of shorter range. For instance, the United States Coast Guard runs one such system in the US and Canada on the longwave radio frequencies between 285 kHz and 325 kHz. These frequencies are commonly used for marine radio, and are broadcast near major waterways and harbors.

Australia runs two DGPS systems: one is mainly for marine navigation, broadcasting its signal on the longwave band;[1] the other is used for land surveys and land navigation, and has corrections broadcast on the Commercial FM radio band.

Two systems for air navigation and precision landing of aircraft, in Australia, will eventually replace the Instrument Landing System. Both utilise DGPS techniques and are called the Ground Based Augmentation System and Ground based Regional Augmentation Systems. Both of these systems broadcast corrections via the aviation VHF band.

A similar system that transmits range corrections from orbiting satellites instead of ground-based transmitters is called a Satellite Based Augmentation System. Different versions of this system include the Wide Area Augmentation System, European Geostationary Navigation Overlay Service, Japan's Multi-Functional Satellite Augmentation System, Canada's CDGPS and the commercial VERIPOS, StarFire and OmniSTAR.