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 very carefully surveyed, and their exact 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. The augmentation satellites rotate with the earth and are always in a fixed location above the earth, unlike the GPS satellites, which revolve around the earth. 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). There are also commercial space-based systems such as OmniSTAR (www.omnistar.com), Fugro (www.fugro.com) and StarFire (www.navcomtech.com/StarFire), which can pinpoint a location with extreme accuracy. Used in the oil, gas, mining and construction industries as well as agriculture, such systems require specialized receivers, not the in-dash navigation systems found in automobiles. 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 a wide variety of frequencies, from 162.5 kHz to 2.95 MHz. In the U.S., the Nationwide Differential GPS (NDGPS) system is a major example. See GPS, LORAN, Galileo and Selective Availability.