The Global positioning system (GPS) is a satellite-based radio navigation system that broadcasts a signal which is picked up by receivers in the aircraft. The receiver tracks multiple satellites and determines a pseudo-range measurement that is then used to determine the user location.

There are three functional elements of the GPS system:

1. The space element consists of 24 Navstar satellites called a “constellation." The satellites are located in six orbital planes (with four in each plane) at about 11,000 miles above the earth. At least five satellites are in view at all times. 

2. The control element consists of a network of ground-based GPS monitoring and control stations that ensure the accuracy of the satellite clocks as well as their positions. The current system has five monitoring stations, three ground antennas, and a master control station. 

3. The user element consists of antennas and receiver-processors onboard the aircraft that provide positioning, velocity, and precise timing to the user.