Air traffic control, abbreviated ATC, refers to any of a number of mechanisms used to keep aircraft from crashing into the ground or each other. It first began to appear in the USA during the Roaring Twenties, when air travel began to become popular: the first modern radio-equipped control tower started operations in 1930 at Cleveland Municipal Airport, and by World War II virtually all major airports in the US were equipped with some form of ATC.

This form of air traffic control, focusing around airports, is known as terminal control. In the mid-1930's, the Bureau of Air Commerce began experimenting with ATC systems for aircraft traveling between airports: these systems are now collectively known as airway traffic control. At the time, aircraft radios were not powerful enough to make regional ATC centers a reality, so the airway controllers relied on telephone links to control towers across the country to keep track of which aircraft were flying where.

Following the war, control towers across the US began procuring surplus radar units from the Navy. During the 1950's, most control towers were using shrimp boats atop radar displays to track aircraft. Starting in the 1960's, IBM began selling mainframes to the new Federal Aviation Administration: these could simultaneously process radar signals, flight plans, and transponder codes, making it possible to track and identify aircraft in flight on one machine.

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Today, there are many different types of air traffic controllers working simultaneously to ensure the safety and reliability of air travel.

"Clearance delivery," in the airport's control tower, is the first controller involved in any flight. At least 30 minutes before takeoff, every pilot is required to send their itinerary to clearance delivery, and are given specific airways to follow, as well as a unique transponder "squawk" code that will allow ATC stations to track the aircraft for the duration of its flight. Clearance delivery relays this information to every other ATC station on the aircraft's route, so that they will be ready when the plane passes through.

"Ground control" at the airport guides the aircraft from the gate through the taxiway network to the threshold of the runway. It is then "local control," also simply known as "tower," that gives clearance for takeoff.

"Departure control" kicks in after the aircraft leaves the ground. These controllers work out of TRACON facilities, which monitor all low-level air traffic within a metropolitan area. They monitor the aircraft until it leaves the vicinity of its origin city and begins to approach crusing altitude.

Once the aircraft is traveling cross-country, the network of Air Route Traffic Control Centers (called "center" over the radio) guide the pilot along his assigned airway, ordering the aircraft to detour if necessary to avoid congestion or weather.

As the aircraft nears its destination, the local TRACON's "approach control" takes over. Approach control guides the aircraft to a standard approach path into the airport, and helps to line the pilot up with the runway. Ten miles from the airport, the pilot calls the tower and follows the Instrument Landing System (or his eyes) to land at the airport.

While the plane is still rolling down the runway, local control gives the pilot a taxiway to exit on, and passes him back to ground control to guide the aircraft on to the terminal.

Sources: is chock full of tidbits has pretty pictures

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