|GPS is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military.
GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.
Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.
Position and Time from Four GPS Satellite Signals