At 20 000 km in the air there are 30 American satellites that are orbiting the Earth with aprox 14 000km/h ( the orbit period is aprox 12 hours ). The system was initially developed by the US military but now anyone with a GPSreceiver can use them to get their location info.
Wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive.
Your GPS receiver gets information that is transmitted by the GPS satellites and calculates the distance between the device and the satellite by measuring the time is needed by the information to arrive. In order for the calculation to be possible there are needed at least three satellites, the more satellites the preciser is the calculation.
The method that your GPS device calculates the distance to the satellite is called “triateration”.
Trilateration
Really, all that satellites do is broadcast a signal for your GPS receiver to pick up with a specific time and distance.
For example, the first satellite broadcasts a signal that eventually hits your GPS receiver. We don’t know the angle, but we do know the distance. That’s why this distance forms a circle equal in all directions.
This means that your GPS position could be anywhere on this circle at this specific radius.
What happens when your GPS receives a second signal?
Again, this distance is equally broadcasted in all directions until it hits your GPS receiver. This means that the distance could be anywhere on that circle.
But this time, we have two known distances from two satellites. With two signals, the precise position could be any of the two points where the circles intersect.
Because we have a third satellite, it reveals your true location where all three circles intersect.
GPS and Relativity
The GPS sows a real example that the Relativity is real and has an impact. How? Each satellite has an atomic watch on board. Because of their altitude the gravitational pull is a lot more smaller and therefore the atomic watch inside each satellite is running slower then the main atomic watch in Earth.
