Système mondial de détermination de positions NAVSTAR

OBS: The System uses a constellation of satellites placed in orbit at an altitude of 20,178 km. They complete their circular orbits in half a sidereal day, that is 11 hr 57 min. These satellites emit two sets of signals on 1,227MHz [megahertz] and 1,575 MHz in the L-band. These signals carry the transmitting satellites' identification and position, as well as the time and other information. The airborne equipment is entirely passive. It consists of a receiver whose position is determined by measuring its distance from four of the system's satellites.