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 corrections are broadcast to all rovers that are equipped to receive their particular radio message carrying real-time corrections in the RTCM format. An example of such a DGPS service originated as an augmentation for marine navigation.
And there are many kinds of networks that can provide this sort of real-time signal. One of them is WADGPS. This actually is used primarily for aircraft guidance. But as you see here, the concept is that there is a upload—you see that on the left with the yellow arrow going up—there's an upload to a satellite, Inmarsat, that then is used to broadcast these correction signals to aircraft. And then, you see GPS satellites that are also broadcasting the signals to the aircraft so that they get the GPS position and then correct it from the signal they receive from WADGPS.