At the highest level of the air traffic control system is the air route traffic control center (ARTCC), usually just referred to as “center”. There are 22 centers, each assigned a particular geographic jurisdiction. Together, they provide air traffic control services to the entire United States.
Centers are not located at an airport, and center controllers to not work in a tower. Instead, radar feeds and communications lines are routed into the center from all over the geographic area under its control. The name of the center denotes the major city near which the center is physically located. A center controller is often separated from the airspace they control by several hundred miles. All Class A and most Class E airspace is controlled by a center.
Class B and C airspace is delegated from the centers to approach control facilities throughout the country. Class D airspace is delegated from the centers to tower facilities. The personnel at these approach and tower facilities specialize in operations for a much smaller geographic area. Most of the time, they are physically located in the center of their airspace, often in a tower.
This specialization, along with the controller’s ability to directly communicate with and sometimes even see all their traffic, allows approach and tower controllers to move traffic much more effectively than a center could. Approach and tower facilities are allowed to use less restrictive rules than center. They also develop and use procedures specific to their airspace, which allow the movement of traffic to safely be expedited.
A center controller is limited by several factors, and so uses more restrictive separation standards. One such limitation is the result of the radar display used at the center. Instead of a single radar antenna, computers at each center process signals from several available radar feeds. The computer then uses these multiple radar feeds to make its best guess as to the actual location of the radar target. This best guess is displayed to the center controller. At first glance, the five miles of lateral separation required between IFR aircraft might seem excessive. But, this is due to the fact that there is some ambiguity as to the actual location of the aircraft. It is not uncommon for radar targets to suddenly jump one or two miles on a center display.
Since airspace is delegated from the center, the center would take over airspace should a facility shut down for some reason. Most large approach control facilities are 24/7, but many smaller ones close at night, when there is little traffic. During these times, the center takes the airspace and the airspace classification changes. Many tower’s also close. They often simply become non-towered airports and the airspace reverts to a lower classification, such as Class E and G.