what is Differential GPS (DGPS)? Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. 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. This allows the station to 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 currently 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 DGPS correction data from Coast Guard beacon transmitters, a 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.

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