4.1 - SATELLITES POSITION

That 20,000 Km altitude is actually a benefit in this case, because something that high is well clear of the atmosphere. And that means it will orbit according to very simple mathematics. Each GPS satellite has been injected into a very precise orbit, according to the GPS master plan. On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment. The basic orbits are quite exact but just to make things perfect the GPS satellites are constantly monitored by the ground station (GPS Control Segment), using very precise radar to check each satellite's exact altitude, position and speed.