More GPS tech background
The system consists of a "constellation" of at least 24 satellites in six orbital planes.
The GPS satellites were initially manufactured by Rockwell; the first was launched in February, 1978, and the most recent was launched November 6, 2004.
Each satellite circles the Earth twice every day at an altitude of 20,200 kilometres (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.