GPS satellites circle the planet twice a day in very precise orbits and transmit signal information to Earth. Each satellite totes an atomic clock, permitting it to send out a signal at regular, exact intervals. GPS receivers take this information and use triangulation to calculate an exact surface location.

The entire system operating at present consists of 28 satellites.

A GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. With distance measurements from multiple satellites, the receiver can determine the user's position.

A receiver must be locked on to the signal of at least three satellites to calculate a two-dimensional position -- latitude and longitude -- and track movement.

With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, a GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, even sunrise and sunset time.

Real-world applications of GPS fall into five broad categories:

• Location - determining a basic position
• Navigation - getting from one location to another
• Tracking - monitoring the movement of people and things
• Mapping - creating maps of the world
• Timing - bringing precise timing to the world