The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals. One receiver is generally referred to as the Reference or Base Station, whose coordinates are known in the satellite datum. The second receiver may be stationary or moving. However its coordinates are determined relative to the Base Station. In carrier phase-based positioning this results from the determination of the baseline vector, which when added to the Base Stations coordinates generates the User's coordinates. In pseudo-range-based GPS positioning, the coordinates are derived from the User receiver's observations after they have had the differential corrections applied (either in the real-time or post-processed mode).