[2] Every commercial aircraft flying in U.S. airspace and every private aircraft that has filed U.S. flight plans becomes part of the country's air traffic control system. The planes start their flight with a clearance from one of more than 400 airport towers. At about two miles away from the runway, one of 185 Tracon (terminal radar approach control) facilities takes over, tracking the planes in lower altitudes to, typically, 40 miles to and from an airport. (Twenty-eight Tracons are separate sites, the rest are part of an airport tower.)

At high altitudes, control is transferred to one of 20 en route centers, each of which handles a different section of airspace. At these facilities, some 3400 of the 17 000 Federal Aviation Administration controllers may be working at one time. Each controller is responsible for one sector of airspace, which can be as much as several thousand feet high and from 200 to 20 miles wide. Controllers are responsible for keeping planes separated by 1000 feet vertically and 3 miles horizontally around airports, 5 miles horizontally elsewhere.

Pilots nowadays navigate using a system of very high-frequency omnidirectional range (VOR) transmitters that grid the United States. Tracons monitor traffic with airport surveillance radar (ASR) and the automated radar terminal system (ARTS), which runs on linked Sperry-Univac 1140 computers. ARTS information is displayed on data entry and display subsystems (DEDSs) or on full digital ARTS displays (FDADs).

At the en route centers, planes are tracked by air route surveillance radars (ARSRs), which send their signals to the Hosts, IBM 3083 computer systems, which in turn pass the information on to a mainframe system—either the display channel complex or the display channel complex replacement or the computer display channel—which processes the data for display on plan view displays (PVDs).