The GPS System
A squadron of 24 satellites orbiting 12,000 miles above Earth comprises the heart of the GPS. Each satellite, powered mainly by solar energy, completes its orbit every 12 hours.
Each GPS satellite broadcasts radio signals that code the exact time of the pulse and information about the satellite's course and status.
If the handheld GPS receives signals from at least four satellites, it can attempt to calculate its position, based on factors like the time differential between the radio pulse's emission and reception, the satellite's orbit and the speed of light. Four satellites are required to calculate position in three dimensions: latitude, longitude, and elevation.
Potential errors in the communication between satellite and receiver include satellite/handheld unit clock discrepancies, signal bounce off buildings and topography (the multipath effect) and atmospheric signal delay.
The first GPS satellite achieved orbit in 1978, and the entire arsenal of 24 was in place by 1994.