Positioning with GPS can be performed by either of two ways: point positioning or relative positioning. GPS point positioning employs one GPS receiver that measures the code pseudoranges 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 gone down from about 100m (2 drms) when selective availability was on, to about 22m (2 drms) in the absence of selective availability [1]. GPS point positioning is used mainly when a relatively low accuracy is required. This includes recreation applications and low-accuracy navigation.

GPS relative positioning, however, employs two GPS receivers simultaneously tracking the same satellites. If both receivers track at least four common satellites, a positioning accuracy level of the order of a subcentimeter to a few meters can be obtained [2]. Carrier-phase or/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 postmission modes. GPS relative positioning is used for high-accuracy applications such as surveying and mapping, GIS, and precise navigation.