10. Navigation satellites - how they work

There is a constellation of 24 low earth orbit satellites covering the entire earth called NAVSTAR. This gives enough coverage to have 3 or 4 satellites visible from anywhere. Accuracy is good to about a foot or even better with more sophisticated equipment.

trilateration

Here's how GPS works in six steps:

  1. GPS works by using a method called "triangulation" or "trilateration".
  2. It needs to get a message from at least three, preferably four satellites
  3. To "triangulate", a GPS receiver measures the distance between itself and each satellite. It can measure distance because it works out exactly how long it took for each satellite's message to arrive. (distance = time of arrival * speed of light)
  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.

 

challenge see if you can find out one extra fact on this topic that we haven't already told you

Click on this link: GPS system