Trilateration is a mathematical technique used by a Global Positioning System (GPS) device to determine the user's position, velocity, and altitude. By continuously receiving and analyzing radio signals from multiple GPS satellites and applying the geometry of circles, spheres, and triangles, a GPS device can calculate the precise distance, or range, to each tracked satellite.

Trilateration is an advanced version of triangulation, although it does not use angle measurements in its calculations. Data from a single satellite provides a general location of a point within a large circular area on the Earth's surface. By adding data from a second satellite, GPS can narrow down the specific location of that point to an area where the two areas of satellite data overlap. Adding data from a third satellite provides a precise position of the point on the Earth's surface.

All GPS devices require three satellites for accurate position calculation. Data from a fourth satellite, or even more than four satellites, further improves the precision of the point's location and also allows factors such as altitude or, in the case of aircraft, elevation to be calculated. GPS receivers routinely track four to seven satellites simultaneously and use trilateration to analyze the information.

The U.S. Department of Defense maintains 24 satellites that relay data around the world. Your GPS device can stay in contact with at least four satellites no matter where you are on Earth, even in wooded areas or large metropolises with tall buildings. Each satellite orbits the Earth twice a day and regularly sends signals to Earth from an altitude of about 12,500 miles. Satellites are solar powered and have backup batteries.