Differential global positioning systems (DGPS) improve the positioning accuracy of GPS users who are equipped to receive correction messages broadcast over radio links from a GPS reference receiver or a network of receivers at a fixed, known location or locations. The method exploits the fact that the leading error sources for civil GPS receivers will be highly correlated at the reference receiver and at nearby user receivers. These errors include the propagation delay of radio signals through the ionosphere and the troposphere as well as errors that are characteristic of the satellite broadcast signals. The latter include satellite timing (clock) errors and errors in the broadcast satellite ephemerides (which provide precise satellite positions at any given time). A reference station estimates the combined effect of these errors as the difference between the measured value of the line-of-sight range to each satellite and the value of that range predicted from the known locations of a satellite and the reference station.

Public-use DGPS transmit the current value of these ranging errors for all satellites that are in view. Some systems separate out the individual error contributions attributable to satellite clock offsets, ephemeris errors, and propagation errors, whereas others transmit only the composite line-of-sight errors. These two approaches are generally characteristic of wide- and local-area DGPS respectively. An intermediate approach whereby each station transmits local, scalar corrections but a weighted average of corrections from multiple transmitters is used in the user receiver, is called regional DGPS. This concept is used in the Northwest European LORAN (NELS) Eurofix system, where DGPS corrections are transmitted by pulse-position modulation of the LORAN signal.