Technical data

Tables
Precise Positioning Service (PPS)
This service is only available to authorised users with cryptographic
equipment and special receivers. Access is limited to the U.S. and Allied
military, U.S. Government agencies, and selected civil users specifically
approved by the U.S. Government
Geodetic Datums
Geodetic datums, and the co-ordinate reference systems based upon
them, were developed to describe exact geographic positions for
surveying, mapping, and navigation.
Over a period of time, the "shape of the earth" was refined from flat-
earth models, to spherical models of sufficient accuracy to allow global
exploration, navigation, and mapping. True geodetic datums were
employed only after the late 1700s, when accurate measurement then
determined that the Earth was in reality, ellipsoidal in shape.
The default geodetic datum of the Global Positioning System is WGS 84
(World Geodetic System 1984).
Many countries have developed their own geodetic datums, which may
not match directly with other datums.
NOTE
It is vitally important to understand the danger of discrepancies between geodetic
datums. There may be differences of several hundreds of meters between different
datums. This is probably the largest cause of error and problems to users of precise
GPS equipment.
GPS Timing Receiver Installation and Operation Manual
D-5