As mentioned earlier the correlation between most of the GPS biases becomes weaker as the rover gets farther from the base. The term Local Area Differential GPS (LADGPS), is used when the baselines from a single base station to the roving receivers using the service are less than a couple of hundred kilometers. The term Wide Area Differential GPS (WADGPS) is used when the service uses a network of base stations and distributes correction over a larger area, an area that may even be continental in scope. Many bases operating together provide a means by which the information from several of them can be combined to send a normalized or averaged correction tailored to the rover’s geographical position within the system. Some use satellites to provide the data link between the service provider and the subscribers. Such a system depends on the network of base stations receiving signals from the GPS satellites and then streaming that data to a central computer at a control center. There the corrections are calculated and uploaded to a geo-stationary communication satellite. Then the communication satellite broadcasts the corrections to the service’s subscribers.
In all cases, the base stations are at known locations and their. Both the U.S. Coast Guard (USCG) and the Canadian Coast Guard (CCG) instituted DGPS services to facilitate harbor entrances, ocean mapping, and marine traffic control as well as navigation in inland waterways. Their system base stations beacons broadcast GPS corrections along major rivers, major lakes, the east coast, and the west coast. The sites use marine beacon frequencies of 255 to 325 kHz, which has the advantage of long range propagation that can be several hundreds of kilometers. Access to the broadcasts is free and over recent years the service has become very popular outside of its maritime applications, particularly among farmers engaged in GPS aided precision agriculture. Therefore, the system has been extended beyond waterways across the continental U.S. and is now known as the Nationwide DGPS (NDGPS). There are currently 86 base stations. Of these 39 are USCG sites, 38 are Department of Transportation (DOT) sites, and 9 are Corps of Engineers sites.corrections are broadcast to all rovers that are equipped to receive their particular radio message carrying real-time corrections in the RTCM format.
Service originated as an augmentation for marine navigation. Another U.S. DGPS service initiated in 1994 cooperatively by the Department of Transportation and the Federal Aviation Administration (FAA) is known as WAAS. It is available to users with GPS receivers equipped to receive it. The signal is free and its reliability is excellent. While the official horizontal accuracy is 7.6 m, its capability to actually deliver approximately 1 m horizontally makes it a possible alternative to Wide Area DGPS. It utilizes both satellite-based, also known as SBAS, and ground-based augmentations and was initially designed to assist aerial navigation from takeoff, en route through landing. Reference stations on the ground send their data via processing sites to two Ground Earth Stations that upload differential corrections and time to geostationary satellites, Inmarsat III’s, devoted to transmission of GPS differential corrections to users.
More information at http://www.jimilab.com/ .