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differential GPS[‚dif·ə¦ren·chəl ¦jē¦pē′es]
A technique for improving the accuracy of the Global Positioning System (GPS) in which error corrections are transmitted to users based on measurements of GPS signals by one or more reference receivers situated at known locations. Abbreviated DGPS.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
differential GPS (global positioning system)
The system of establishing position by getting the fix from a constellation of navigational satellites, in which the errors caused by the atmosphere or the ionosphere and those induced by selective availability are corrected. The system has a much higher accuracy than the conventional GPS. It uses two receivers, a rover at an unknown location (aircraft's GPS receiver), and a base station at a known fixed location. The base station computes corrections based on the differences between its actual and observed ranges to the tracked satellites and sends the correction to the rover receiver.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
GPS augmentation systemA 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.
|A GPS receiver can obtain corrections from space-based or ground-based augmentation systems. The receivers must be specialized for each type of correction service, and many earlier receivers used only the GPS signals. This illustration highlights only the receiver signal paths. Each system comprises numerous earth stations and terrestrial processing centers.|
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