Air Traffic Control

Air Traffic Control (ATC) is a safety system for aircraft, primarily commercial, that maintains positive control of the most critical flights. Positive control requires an aircraft to file a flight plan that documents a planned trip's course via specific airway segments between airfields and arbitrary navigation points, with assigned altitudes and separation within each airway. While typically smaller aircraft can operate by Visual Flight Rules (VFR) based on a "see and be seen" model, usable in good weather, commercial and all-weather aircraft operated by Instrument Flight Rules (IFR).

In 1958-59, the Civil Aeronautics Agency introduced “Positive Control”, the division of the continental airspace into strata, making it a federal offence for a plane to stray into certain airways without ground control confirmation.

Tracking
Originally, aircraft position reporting depended on radio reports to ground controllers. There was a strong movement towards positive tracking by ground radar, but technical limitations of pure radar caused it to migrate to a hybrid system based on active radar transponders aboard aircraft. Transponders cooperate electronically with ground tracking systems, no longer requiring identification of a radar reflection from metal, but, when interrogated by radar, send back a coded signal including the aircraft's flight number and its altitude. The ground radar combines this with information calculated from the time of arrival, and relative angle to, the radar receiving antenna.

Organization
The basic system splits into three levels:
 * Terminal control around airports
 * Terminal (radar) approach control (TRACON)
 * Air route traffic control