This object is not a navigation device on its own, but is a radio data link system used to connect multiple GPS devices.

A Differential GPS (D-GPS) Base Station is a GPS device located at a fixed known position and it uses this knowledge to estimate the errors in the GPS signals it sees. These errors can be turned into corrections and sent over the data link system using the RTCM SC-104 protocol to other mobile GPS devices which can then correct its measurements and so produce a more accurate position fix.

Between March 1990 and May 2000, the US government introduced “Selective Availability” (SA) to GPS to limit the accuracy that civilian GPS navigation equipment could achieve. SA worked by dithering the clocks on the GPS satellites to introduce errors of the order of +/- 65 m, which in turn led to Horizontal position errors for navigation users at the +/- 100m level.

Differential-GPS techniques could reduce these errors to as little as +/- 5m for users who were within, say 10-20 km of the DGPS Base Station.
Bill Clinton, as his outgoing president’s parting gift to the nation, decreed in May 1990 that Selective Availability be turned off. These radio links were used with XR5M (see ZBA9326) and XR6-SHARPE (see ZBA9328) systems.