Differential GPS refers to using at least two receivers, a reference (or base) receiver and a rover receiver, to further improve GPS accuracy. By comparing data collected simultaneously at each receiver, common error sources can be eliminated from the solution. These error sources include satellite orbit error, satellite clock error, and atmospheric signal delays. In addition, carrier phase data are used to further increase precision and reduce error caused by the signal delay in the ionosphere. Differential GPS yields precise baselines (distance between reference and rover antennas). If the base location coordinates are accurate within a particular reference frame, then the rover coordinates will be accurate within that reference frame as well. But baseline precision does not imply coordinate accuracy unless the initial starting coordinates are of a known quality.

1. The Differential GPS on-line tutorial by Trimble Navigation provides an excellent explanation of the basics of differential GPS.