A global Geodetic Datum defined and maintained by the US Department of Defense. As the Control Segment coordinates and the Broadcast Ephemerides are expressed in this Datum, the GPS positioning results are said to be in the WGS84 Datum. In the case of Point Positioning this is largely true, although the level of accuracy achievable under the policy of Selective Availability is so poor that the link to the WGS84 Datum is very approximate. In the case of Relative Positioning, the baseline vector may be determined to quite high accuracy (at the sub-centimetre level using precise GPS Surveying techniques), however the coordinate (and therefore the Datum) of the unknown point is almost completely defined by the Datum of the Base Station. This may not be coincident with the WGS84 Datum at better than a few tens of metres! If GPS Geodesy techniques are used, with known station coordinates expressed in the ITRS and precise ephemerides obtained from the IGS, it is more correct to state that the subsequent set of coordinates are expressed in one of the ITRS frames (e.g. ITRF92, ITRF94, etc.). The WGS84 and the ITRS are compatible at the one metre level. However, the ITRS is a more precise realisation of an earth-fixed, earth-centred terrestrial reference system.

This definition is in context to GPS. See more contextual defintions for World Geodetic System 1984 (WGS84).