Differential GPS
It consisted of broadcast sites located throughout the inland and coastal portions of the United States including Alaska, Hawaii and Puerto Rico.
The Canadian Coast Guard (CCG)[3] also ran a separate DGPS system, but discontinued its use on December 15, 2022.
The advent of a global navigation satellite system (GNSS) could provide greatly improved accuracy and performance at a fraction of the cost.
The military received multiple requests from the Federal Aviation Administration (FAA), United States Coast Guard (USCG) and United States Department of Transportation (DOT) to set SA aside to enable civilian use of GNSS, but remained steadfast in its objection on grounds of security.
Additionally, another major source of errors in a GPS fix is due to transmission delays in the ionosphere, which could also be measured and corrected for in the broadcast.
[1] The US Coast Guard was one of the more aggressive proponents of the DGPS, experimenting with the system on an ever-wider basis throughout the late 1980s and early 1990s.
"Production quality" DGPS signals began to be sent out on a limited basis in 1996, and the network was rapidly expanded to cover most US ports of call, as well as the Saint Lawrence Seaway in partnership with the Canadian Coast Guard.
As of November 2013 the USCG's national DGPS consisted of 85 broadcast sites which provide dual coverage to almost the entire US coastline and inland navigable waterways including Alaska, Hawaii, and Puerto Rico.
Additionally, during the Gulf War of 1990–1991 SA had been temporarily turned off because Allied troops were using commercial GPS receivers.
In addition to continued deployments of the USCG and FAA sponsored systems, a number of vendors have created commercial DGPS services, selling their signal (or receivers for it) to users who require better accuracy than the nominal 15 meters GPS offers.
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 0.67 metres per 100 kilometres (3.5 ft/100 mi) from the broadcast site [11] but measurements of accuracy across the Atlantic, in Portugal, suggest a degradation of just 0.22 m/100 km (1.2 ft/100 mi).
[13] European DGPS network has been developed mainly by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
The system is an expansion of the previous Maritime Differential GPS (MDGPS), which the Coast Guard began in the late 1980s and completed in March 1999.
MDGPS covered only coastal waters, the Great Lakes, and the Mississippi River inland waterways, while NDGPS expands this to include complete coverage of the continental United States.
[20] In response to the comments received, a subsequent 2016 Federal Register notice announced that 46 stations would remain in service and "available to users in the maritime and coastal regions".
[22] As of June 2020, all NDGPS service has been discontinued as it is no longer deemed a necessity owing to the removal of selective availability in 2000[23] and also the introduction of newer generation of GPS satellites.
The marine DGPS service of 16 ground stations covering the Australian coast was discontinued effective July 1, 2020.
The improvement of GPS positioning doesn't require simultaneous measurements of two or more receivers in any case, but can also be done by special use of a single device.
