Positioning with GPS can be performed in either of two ways: point (absolute) positioning or relative positioning. Classical GPS point positioning employs one GPS receiver that uses the measured code pseudoranges and the broadcast ephemeris to determine the user's position instantaneously, as long as four or more satellites are visible at the receiver. The expected horizontal positioning accuracy from the civilian C/A-code receivers has increased (improved) from about 100m (2 drms) when SA was on to about 22m (2 drms) in the absence of SA [1]. As stated in Chapter 2, an additional 15 percent to 20 percent improvement in the positioning accuracy is expected as a result of the recent upgrade in the ground control segment. Classical GPS point positioning is used mainly when a relatively low accuracy is required. This includes recreation applications and low-accuracy navigation. Recently, PPP was introduced, which uses ionosphere-free linear combinations of carrier-phase and pseudorange measurements along with precise ephemeris and clock products. PPP provides positioning accuracy comparable to that of relative positioning (i.e., centimeter to decimeter accuracy).

GPS relative positioning, on the other hand, employs two GPS receivers simultaneously tracking the same satellites. If both receivers track at least four common satellites, a positioning accuracy level on the order of a few meters to millimeters can be obtained [2]. Carrier-phase and pseudorange measurements can be used in GPS relative positioning, depending on the accuracy requirements. The former provides the highest possible accuracy. GPS relative positioning can be made in either real-time or post-mission...