ATC (Air traffic control) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through controlled airspace. And can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of air traffic, and provide information and other support for pilots. In some countries, ATC plays a security or defensive role or is operated by the military. Air traffic controllers monitor the location of aircraft in their assigned airspace by radar and communicate with the pilots by radio. To prevent collisions, Air traffic control enforces traffic separation rules. Which ensure each aircraft maintains a minimum amount of empty space around it at all times. In many countries, ATC provides services to all private, military, and commercial aircraft operating within its airspace. Depending on the type of flight and the class of airspace. Air traffic control may issue instructions that pilots are required to obey or advisories that pilots may, at their discretion, disregard. The pilot in command is the final authority for the safe operation of the aircraft. In an emergency, deviate from ATC instructions to the extent required to maintain safe operation of their aircraft. ATC (Airport traffic control) tower The primary method of controlling the immediate airport environment is visual observation from the airport control tower. The tower is a tall, windowed structure located on the airport grounds. Air traffic controllers are responsible for the separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport itself, and aircraft in the air near the airport. Generally 5 to 10 nautical miles (9 to 18 km) depending on the airport procedures. Surveillance displays are also available to controllers at larger airports to assist with controlling air traffic.