air traffic control

Air traffic control (ATC) is a service provided by ground-based air traffic controllers (people) who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organise and expedite the flow of traffic in the air, and provide information and other support for pilots.
Personnel of air traffic control monitor aircraft location in their assigned airspace by radar, and communicate with the pilots by radio. To prevent collisions, ATC enforces traffic separation rules, which ensure each aircraft maintains a minimum amount of 'empty space' around it at all times. It is also common for ATC to provide services to all private, military, and commercial aircraft operating within its airspace; not just civilian aircraft. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to obey, or advisories (known as flight information in some countries) that pilots may, at their discretion, disregard. The pilot in command of their aircraft always retain final authority for its safe operation, and may, in an emergency, deviate from ATC instructions to the extent required to maintain safe operation of their aircraft.

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