How GPS Works

The principle behind GPS is the measurement of distance (or “range”) between the satellites and the receiver. The satellites tell us exactly where they are in their orbits by broadcasting data the receiver uses to compute their positions. It works something like this: If we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with a radius equal to the distance to the satellite radius. If we know our exact distance from two satellites, we know that we are located somewhere on the line where the two spheres intersect. And, if we take a third and a fourth measurement from two more satellites, we can find our location. The GPS receiver processes the satellite range measurements and produces its position.

GPS uses a system of coordinates called WGS 84, which stands for World Geodetic System 1984. It allows surveyors all around the world to produce maps like the ones you see in school, all with a common reference frame for the lines of latitude and longitude that locate places and things. Likewise, GPS uses time from the United States Naval Observatory in Washington, D.C., to synchronize all the timing elements of the GPS system, much like Harrison's chronometer was synchronized to the time at Greenwich.

Now you should have a fairly clear picture of the GPS system. You know that it consists of satellites whose paths are monitored by ground stations. Each satellite generates radio signals that allow a receiver to estimate the satellite location and distance between the satellite and the receiver. The receiver uses the measurements to calculate where on or above Earth the user is located.