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Q. What is Differential GPS (DGPS)? What is the GBR 21/GBR 23 Differential Beacon Receiver? A. The U.S. and Canadian governments (among others) have set up Differential GPS (DGPS) stations to transmit correction signals. They are broadcast on existing marine beacon frequencies around 300 kHz. They are operational in coastal areas and on many navigable river systems. This DGPS system is available for use without fee. There are other competing systems, both ground-based and satellite-based, that provide correction signals for a subscription fee. In addition to compensating for errors in the position, these correction signals can also correct for deliberate degradation of the accuracy of GPS by the U.S. Department of Defense under their Selective Availability (SA) program. Please refer to the US Coast Guard's web site: http://www.navcen.uscg.mil/ for locations and status of their differential stations. The GARMIN GBR 21 GBR 23 are optional receivers designed to receive the US Coast Guard's broadcast differential signals and supply these signals to your GARMIN GPS unit. When connected to your GARMIN GPS, the GBR 21 and GBR 23 will increase its accuracy to under 10 meters when within receiving range of Differential Beacon stations.

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