Answer
The GPS system uses accurately synchronized clocks to find the travel time of a radio signal between a satellite and receiver. This determines the receiver's distance to the satellite, whose position is very well-known. One "very good" satellite, by knowing the distance to the receiver, could only locate the receiver on an imaginary sphere surrounding the satellite.
Three or more GPS satellites need to be "in view" of the receiver to accurately triangulate the receiver's position and to track movement.
This is discussed on page 197, in the "Finding Your Way" essay.
