The U.S. Air Traffic Control (ATC) system is one of the most complex structures in the world. The mission of the ATC system is to the safe, orderly, and expeditious flow of air traffic in the U.S. National Airspace System (NAS). The ATC system is composed of a vast network of surveillance systems, automated data processing systems, communications equipment, navigation systems, and ATC facilities (Government Accounting Office [GAO], 1998)
