Air Traffic Control (ATC)

Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC
systems worldwide is to separate
aircraft to prevent collisions, to
organize and expedite the flow of
traffic, and to provide information and other support for pilots when able.
Preventing collisions is referred to as separation, which is a term used to prevent aircraft from coming too close
to each other by use of lateral, vertical
and longitudinal separation minima;
many aircraft now have collision avoidance systems installed to act as a backup to ATC observation and
instructions. In addition to its primary
function, the ATC can provide
additional services such as providing
information to pilots, weather and
navigation information and NOTAMs (NOtices To AirMen). In many countries, ATC services are
provided throughout the majority of
airspace, and its services are available
to all users (private, military, and
commercial).
When controllers are
responsible for separating some or all aircraft, such airspace is called
"controlled airspace" in contrast to "uncontrolled airspace" where aircraft may fly without the use of the air traffic
control system. Depending on the type
of flight and the class of airspace, ATC
may issue instructions that pilots are required to follow, or merely flight
information (in some countries known
as advisories) to assist pilots operating
in the airspace. In all cases, however,
the pilot in command has final
responsibility for the safety of the flight, and may deviate from ATC
instructions in an emergency.