Differential GPS (DGPS) uses a GPS receiver at a fixed point whose position is known with submeter accuracy. This is the control unit. The receiver collects data from all visible satellites and computes predicted satellite ranges, which are compared with actual ranges. The difference is the satellite range error, which is then converted to correction signals for use by a roving receiver. The roving receiver would be to one on the system users boat.
It is assumed that this correction will be the same for other GPS receivers that in the same area and are using the same satellites for positioning. If the correction is communicated to other receivers in the area, usually by a beacon on the same site, the range error can be removed from satellite signals and precise fixes calculated by these receivers. It should be noted that not all data errors can be corrected in this way. Errors that are caused by receiver noise (which is inherent in any GPS receiver) and multipath problems cannot be eliminated with differential equipment. Multipath errors occur when the receiver's antenna "sees" the reflections of signals that have bounced off of surrounding objects.
Using DGPS to eliminate the effects of correctable errors requires that the user's GPS receiver be connected to a compatible Differential Beacon Receiver (DBR) and be within range of the broadcasting beacon. The DBR accepts and demodulates the broadcast corrections, which are then relayed to the GPS receiver. The GPS receiver applies the corrections to the navigation data it uses to compute a position solution, and then displays differentially corrected data. Care must be taken to ensure that the DGPS receiver and the GPS receiver are compatible for this procedure to be successful.
Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. Originally initiated by the U.S. Coast Guard, DGPS adds a land based reference receiver, located at an accurately surveyed site. Since this non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location, the station can compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.
The two most common sources of corrective DGPS signals are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive the DGPS correction data from the Coast Guard beacon transmitters, the mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data.