Air route traffic control centers (ARTCCs) have the responsibility of controlling the movement of en route aircraft along the airways and jet routes, and in other parts of the airspace. Each of the 21 air traffic control centers within the United States has control of a defined geographical area which may be greater than 100,000 mi2 in size. At the boundary point, which marks the limits of the control area of the center, control of aircraft may be transferred to an adjacent center or an approach control facility, or radar service may be terminated and VFR aircraft are free to contact the next center. Air traffic control centers are normally not located at airports. Air traffic control centers can also provide approach control service to nontowered airports and to nonterminal radar approach control airports.
Each ARTCC geographical area is divided into sectors. The configuration of each sector is based on equalizing the workload of the controllers. Control of aircraft is passed from one sector to another.
The geographical area is sectored not only in the horizontal but also in the vertical plane. Thus there can be a high-altitude sector above one or more low-altitude sectors. Each sector is manned by one or more controllers, depending on the volume and complexity of traffic. The average number of aircraft that each sector can handle depends on the number of people assigned to the sector, the complexity of traffic, and the degree of automation provided.
Each sector is normally provided with one or more air route surveillance radar (ARSR) units which cover the entire sector and allow for monitoring of separation between aircraft in the sector. In addition, each sector has information on the identification of the aircraft, destination, flight plan route, estimated speed, and flight altitude, which is posted on pieces of paper called flight progress strips, and are superimposed on the radarscope adjacent to the blips which identify the position and identity of aircraft. The strips are continuously updated as the need arises.
At present, communication between the pilot and controller is by voice. Therefore each ARTCC is assigned a number of VHF and UHF radio communication frequencies. The controller in turn assigns a specific frequency to the pilot. However, modernization of air traffic control is planned to include further proliferation of digital communications, known as controller pilot data link communications
(CPDLC) between controllers and pilots.