As soon as satellites are added to the operational constellation, the Master Control Station implements a method of control which limits civilian access to the system's full capabilities. This method of control is referred to as 'Selective Availability'. Using the travel time multiplied by the speed of light provides a calculation of range to each satellite in view. From this and additional information on the satellites orbit and velocity, the internal GPS receiver software calculates its position through a process of triangulation. They are military equipment and require encryption keys to operate at their full level of accuracy.
Receivers are for civilian use.
The SPS receivers provide 100 metre accuracy under good satellite geometry. These method uses a known position, such as surveyed control point, as a reference point to correct the GPS position error. The U.S.A. and Candian Coast Guard are establishing a series of radio beacons to transmit the DGPS corrections for accurate navigation along the Great Lakes, the Mississippi River and tributaries, the Gulf Coast,and the Eastern and Western coasts of North America. There are a variety of different types of GPS receivers on the market for commercial and public use.