While you can get more accurate fixes from such a system, it requires a number of additional, time-consuming steps, after data collection. Once you know the source of the differential corrections file, you have to get it, load it onto your computer, and execute software to produce the final corrected file. Wouldn’t it be nice if the GPS personal tracking device unit simply gave accurate positions at the time you took the data? Further, there are some applications in which there simply isn’t time to postprocess GPS fixes–bringing a ship into a narrow harbor, for one example. Well, actually there is a way to provide instantaneous, accurate fixes: Real-Time Differential GPS.
Real-Time Differential GPS (RDGPS) may be approached in several ways:
• The user can set up a base station over a known point and arrange for transmission of a radio signal from the base station to roving receivers.
• The user’s GPS setup can receive correction signals broadcast from an antenna in the area operated by a corporation or a government agency. Such installations operate a base station continuously and broadcast the correction data, sometimes as
part of another signal such as a standard FM broadcast. The U.S. Coast Guard maintains such a service in some coastal areas, and companies such as Accupoint, Inc. and Differential Corrections, Inc. sell a correction service.
• The user’s GPS Tracking SIM Card setup can receive correction signals from a communications satellite parked over the Equator. These signals come from data taken by base stations located in disparate parts of the United States. The data are analyzed, packaged, and sent to a geostationary communication satellite for rebroadcast to Earth. One such system uses this approach with a satellite located on a meridian that passes through Lake Michigan.
With all three of these methods, many of the requirements for base stations remain the same. They must take data from all satellites that the rover might use. They must take data frequently (GPS for vehicle ). They should have a separate channel for each satellite so as to track it continuously. With all the methods of doing real-time differential correction, you might wonder how these pieces of equipment manage to talk to each other. While the frequencies on which broadcasts take place may vary, the content of real-time differential correction data is standardized. The current standard is RTCM SC-104 (version 2), promulgated by the Radio Technical Commission for Maritime Services, in Washington, DC. Look in more detail at the three steps for real-time differential correction.
The user may operate her or his own base station. Those who need really precise coordinates relative to some nearby known point (e.g., land surveyors) use this method frequently. The complications go up with differential correction (real-time or otherwise), of course. Here the user has to have at least two GPS receivers, plus a transmitter associated with one of them (the base station) and receivers for each rover. In addition to the roving Vehicle GPS Tracking Device and all of its settings and conditions for good position finding, the user has to also be concerned about the same factors for the base station, plus making the radio link between the stations function at some distance. Additional complications relate to mounting the base station antenna at a precisely known point.