Definition: GPS augmentation system

A system that improves the accuracy of the GPS satellite navigation system. A satellite's positioning accuracy is impeded by errors in its clock and signal delays caused by atmospheric conditions. Also called a "differential GPS" (DGPS), an augmentation system compensates for those discrepancies by transmitting corrections to the GPS receivers either via satellite or terrestrial radio. Instead of the normal GPS accuracy, which is approximately 15 to 70 feet, augmented systems pinpoint a location within a range of two to 10 feet, depending on the system, and as little as four inches in the case of commercial systems.

They Work Because of Known Locations
An augmentation system uses earth stations that have been carefully surveyed, and their locations are known with great precision. As they receive signals from the GPS satellites, they are compared with the values they should be receiving, and the differences are used to calculate corrections. The corrections are transmitted either to the GPS receivers via geostationary satellites or terrestrial radio.

Space Based vs. Ground Based
A space-based augmentation system (SBAS), also called a "wide area augmentation system," transmits corrections to one or more geostationary satellites, which have a wide footprint on earth. Unlike GPS satellites that revolve around the earth, augmentation satellites rotate with the earth and are always in a fixed location. The predominant space-based systems are WAAS in the U.S., CDGPS in Canada, EGNOS in Europe and MSAS in Japan (see WAAS, CDGPS, EGNOS and MSAS).

Cmmercial space-based systems such as OmniSTAR, Fugro and StarFire can pinpoint a location with extreme accuracy in the oil, gas, mining, construction and agriculture industries.

A ground-based augmentation system (GBAS) uses radio towers to transmit corrections to the GPS receiver. There are hundreds of ground-based augmentation systems around the world transmitting in frequencies from 162.5 kHz to 2.95 MHz. In the U.S., the Nationwide Differential GPS system is a major example (see NDGPS). See GPS, LORAN, Galileo and Selective Availability.

GPS Augmentation

Obtaining corrections from space or earth, each system comprises numerous terrestrial stations and processing centers. This illustration highlights the receiver signal paths.

Extreme Precision

Used in agriculture, these ultra-precise GPS systems enable farmers to move their equipment in precise rows as tight as a few inches. Systems like this use specialized receivers, not the in-dash navigation units found in vehicles. (Image courtesy of John Deere.)