The GPS (Global Positioning System), present in many land, air, and marine vehicles but also in a majority of smartphones, is now part of our daily life.
Since 1978, the US Army maintains a fleet of satellites whose function is to continuously send a radio signal containing information, namely the position of the transmitting instrument and the time of sending the signal.
On the ground, GPS geolocation devices such as are found in automobiles or smartphones have antennas that pick up these signals. A GPS receiver notes the time at which it receives a given signal, the time at which the signal was transmitted and understands the time taken to complete the route. Since radio waves propagate at the speed of light, we can assume the distance from the satellite.
The GPS system comprises at least 24 satellites operating in the Earth’s orbit at an altitude of approximately 20,000 km. They work by triangulation and make it easy to determine a location.
These satellites carry extremely precise atomic clocks which give the geolocation an accuracy of about 30 meters. The clocks of the European Galileo system which will soon be put into service are three times more precise. The geolocation then approaches in the accuracy of the meter.