GPS (global positioning system)
GPS (Global Positioning System) is a positioning system that makes it possible to determine one’s position anywhere in the world. The system consists of 24 satellites, each of which is in a fixed orbit around the earth. Each of these satellites has its own signal, which is received by the GPS system, the receiver, currently the size of a postage stamp.
This receiver has a kind of logbook of the orbits that each of the 24 satellites follow. By comparing the received satellite signals with the time at the moment of reception, the receiver can determine its exact position. Indeed, by taking the intersection of at least three satellites, you get a certain point on the globe: your position.
A fourth satellite is needed to calculate the position accurately, but the more satellites the better.