3. How the Global Positioning System works

Here's how GPS works in six steps:

1. GPS works by using a method called "triangulation".
2. It needs to get a message from at least three, preferably four satellites
3. To "triangulate" a GPS receiver measures distance between itself and each satellite. It can measure distance because it works out exactly how long it took for each satellite message to arrive.
4. To measure travel time, GPS needs very accurate timing which it achieves with atomic clocks on board each satellite.
5. Along with distance, the device needs to know exactly where the satellites are in space at any given time. This information is held inside the GPS receiver itself.
6. Finally, because it knows exactly where the satellites are at that instant, by using some very clever mathematics, it can work out where it is on the ground.

Triangulation, is the key idea behind GPS. It makes use of satellites in space as reference points for locations here on earth. By very, very accurately measuring our distance from at least three satellites we can "triangulate" our position anywhere on earth.

With GPS it is more usual to use four line-of-sight satellites as you can see in the image above.