How GPS Works

When people talk about "GPS," they usually mean a GPS receiver unit, but the Global Positioning System (GPS) is actually a constellation of 27 Earth orbiting satellites. The U.S. military developed and implemented this satellite network as a military navigation system, but opened it up to the private sector. Each of these solar powered satellites circles the globe making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each satellite, and use this information to determine its own location. This operation is based on a simple mathematical principle called trilateration. We will not explain the details of trilateration at this time.

Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. To make the navigation more user-friendly, most receivers will download this raw data into map.

A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. The receiver stays in constant communication with GPS satellites to see how your location is changing. With this information and its built-in clock, the receiver can give you several pieces of valuable information: How far you've traveled (odometer), How long you've been traveling, Your current speed (speedometer), Your average speed and A "bread crumb" trail showing you exactly where you have traveled on the map.