Global Positioning
    System
 The System
 The Position Solution
 Hardware
 Ionospheric And
    Atmospheric Delays
 The Signals
 Dynamic Positioning
 Static Positioning
 Coordinate Systems
 Conclusions

The Position Solution

If we know the distance from one satellite to our receiver, we know we are somewhere on the surface of a sphere of this radius distant from the satellite. If we can determine another distance from another satellite, we narrow down our possible location to somewhere on the surface where these two spheres meet. If we can determine a third distance then this gives us only two possible locations for our receiver. One of these locations is usually nonsensical (for example inside the planet) so the other location is our position.

If we needed to be certain, we would determine the position from another satellite, so 4 distance determinations gives our location as well as eliminate clock errors in the receivers (a systematic error). As you are aware by now, surveyors would like to have a few redundant measurements so the precision and accuracy can be determined, so we often take readings to as many satellites as are visible in the sky. The use of GPS for high precision positioning involves quite convoluted processing of measurements made to as many satellites as are available.