DGPS

Defined in the WikiPedia entry http://en.wikipedia.org/wiki/Differential_GPS as follows:

"Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions."

NASA Global DGPS system http://www.gdgps.net/

For hams

DXing http://www.beaconworld.org.uk/dgps.htm