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GPS F.A.Q.

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. To view existing coverage plots, please visit the U.S. Coast Guard's Navigation Center web page. The DGPS system is available for use without a fee. There are other competing systems, both ground based and satellite based, that provide correction signals for a subscription fee.

DGPS signals were originally developed by the U.S. Department of Defense (DOD) under their Selective Availability (SA) program (no longer in effect). These signals were meant to correct GPS in case of deliberate degradation of GPS signals by the DOD. DGPS signals are still used today to enhance the accuracy of GPS units. Please refer to the U.S. Coast Guard's website for locations and status of their differential stations.

The Garmin GBR 21 and GBR 23 (both are no longer in production or available) were optional receivers designed to receive the U.S. Coast Guard's differential signals and supply these signals to your Garmin GPS unit. When connected to a compatible Garmin GPS, the GBR 21 and GBR 23 will increase its accuracy to under 10 meters when within receiving range of differential beacon stations.

The development of WAAS has provided another useful method for accurate navigation, and many customers have turned to WAAS-enabled GPS receivers. To read more about WAAS, please visit our What is WAAS? web page.

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