How does GPS work?
The science behind GPS
The location of the satellites
The GPS satellites float in space over 12,000 miles/19,000km from Earth. Ironically, the fact that they’re so far away makes the job of locating the satellites easier. Because they’re well clear of Earth’s atmosphere, they orbit Earth according to (relatively) straightforward mathematics.
Each GPS satellite has been projected into a very precise orbit, ensuring your navigation device always gets signals from at least 3 satellites.
A fourth satellite signal is necessary to synchronize the receiver clock with the satellite clocks. See also paragraph The 4th satellite.
The GPS chip of every navigation device is programmed with an ‘almanac’, or log book. This almanac contains long-term general information about the position of the constellation and its satellites. This helps the device finding the satellite signals. The satellites also send out almanac information with their signal, so that the almanac of your navigation device is regularly updated.
Meanwhile, the monitoring stations use the GPS observations of the satellites to work out each satellite’s position and hence its exact orbit. This position information is sent back to the satellite, which then broadcasts it back to your navigation device with its signals.