Developed in response to market demands for greater GPS accuracy, GARMIN's GBR 21 differential beacon receiver corrects for Selective Availability (SA) and atmospheric error, enabling an accuracy of 5-10 meters. This small, remote-mounted, black box-style receiver links to all GARMIN differential-ready marine receivers and is in use only when turned on. The GBR 21 is waterproof, weighs less than 7 ounces, and measures only 5.2"L x 3.6"W x 1.5"H. All tuning and operating functions are controlled by the GPS unit while the GBR 21 is being used, including beacon receiver setup and beacon logging.

The GBR 21 uses the differential GPS corrections broadcast by the U.S. Coast Guard and the International Association of Lighthouse Authorities' beacons (283.5–325 KHz), which are supplied free of charge to mariners. These signals provide satellite status and pseudo-range information, in addition to the location and status of the beacon transmitter. The GBR 21 then decodes this information for use by a GARMIN GPS receiver, providing the improved repeatability and accuracy required for commercial applications and precision navigation.