DGPS Survey: A Differential Global Positioning System (DGPS) is an enhancement to the Global Positioning System(GPS) which provides improved location accuracy than GPS. generally, GPS accuracy is 15 Meter. while DGPS accuracy is 10 cm nominal in the best implementations. each DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the measured satellite pseudo ranges and actual internally computed pseudo ranges and receiver stations may correct their pseudo ranges by the same amount. the digital correction signal is typically broadcast locally over ground-based transmitters of short-range every country that has its own DGPS. A similar system that transmits corrections from the arbitrary satellites instead of ground-based transmitters is called a Wide Area DGPS(WADGPS) or Satellite-based Augmentation System. It plays an important role in identifying plot boundaries. The DGPS survey is very useful.
Operation:
Every DGPS has a reference station that calculates differential corrections for its own location and time. users may be up to 200 nautical miles (370 km) from the station, however, some of the compensated errors vary with space. specifically, satellite ephemeris errors and those introduced by ionospheric and tropospheric distortions. this is a major reason the accuracy of DGPS decreases with distance from the reference station. the problem can be aggravated if the user and the station lack "intervisibility" when they are unable to see the same satellites.
Accuracy
The United States Federal Radionavigation Plan and the IALA Recommendation on the performance and monitoring of DGNSS Services in the band 283.5-325 kHz cite the United States Department of Transportation's 1993 estimated error growth of 0.67m per 100km from the broadcast site but measurements of accuracy across the Atlantic, in Portugal, suggest a degradation of jus 0.22m per 100km.
Variations
DGPS can refer to any type of Ground-Based Augmentation System(GBAS) there are many Operational systems in use throughout the world. according to the US coast guard. 47 countries operate a system similar to the US NDGPS(Nationwide Differential Global Positioning System) a list can be found at World DGPS Database for Dxers.
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