Global Positioning System (GPS) navigators use the mathematical technique of trilateration to determine user position, speed, and elevation. GPS navigators constantly receive and analyze radio signals from GPS satellites, calculating precise distance (range) to each satellite being tracked.
Data from a single satellite narrows position down to a large area of the earth's surface. Adding data from a second satellite narrows position down to the region where two spheres overlap. Adding data from a third satellite (see illustration) provides relatively accurate position. Data from a fourth satellite (or more) enhances precision and also the ability to determine accurate elevation or altitude (in the case of aircraft).
GPS receivers routinely track 4 to 7 or more satellites simultaneously.
If a GPS navigator is receiving insufficient satellite data (not able to track enough satellites) it will notify the user, rather than providing incorrect position information.