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Sci-Tech Dictionary:

air-traffic control

(′er ¦traf·ik kən′trōl)

(navigation) A service which promotes the safe and fast movement of aircraft operating in the air or on an airport surface by providing rules, procedures, and information and advisory services for pilots. Abbreviated ATC. A system comprising enabling legislation, operating procedures, and navigation and communication equipment which is intended to make for the safe and expeditious movement of aircraft from the time that they leave the departure gates to arrival at the terminal gates.


 
 
Sci-Tech Encyclopedia: Air-traffic control

A service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that aircraft do not operate in airspace where operations are prohibited; and assisting aircraft in distress. Orderly and expeditious flow assures the efficiency of aircraft operations along the routes selected by the operator. It is provided through the equitable allocation of system resources to individual flights.

In the United States, air-traffic control (ATC) is the product of the National Airspace System (NAS), comprising airspace; air navigation facilities and equipment; airports and landing areas; aeronautical charts, information, and publications; rules, regulations, and procedures; technical information; and personnel.

Flight rules

Two principal categories of rules governing air traffic are visual flight rules (VFR) and instrument flight rules (IFR). Visual flight rules govern the procedures for conducting flight where the visibility, the ceiling, and the aircraft distance from clouds are equal to or greater than established minima. Ceiling is the height above the Earth's surface of the lowest layer of clouds or obscuring phenomenon that significantly restricts visibility. The minima for operation under visual flight rules vary by airspace. In controlled airspace, the ceiling must be at least 1000 ft (305 m) and the visibility must be at least 3 statute miles (4830 m). The aircraft must remain clear of clouds, at least 500 ft (150 m) below, 1000 ft (305 m) above, and 2000 ft (610 m) horizontally. Instrument flight rules go into effect when visibility, distance from clouds, and ceiling conditions are less than the minima specified for visual flight rules. To operate under these rules, the pilot must pass an instrument flight examination and have an adequately instrumented aircraft.

Aircraft operating under visual flight rules (VFR aircraft) maintain separation from other aircraft visually. IFR aircraft in controlled airspace operate in accordance with clearances and instructions provided by air-traffic controllers for the purpose of maintaining separation and expediting the flow of traffic. Flight crews operating under instrument flight rules are responsible for seeing and avoiding other aircraft, but the air-traffic control clearances they receive provide substantial added assurance of safe separation. Consequently, flight crews often will operate under instrument flight rules even though the weather satisfies visual meteorological conditions.

Flight plans

A flight plan is filed with the authority providing air-traffic control services [in the United States, the Federal Aviation Administration (FAA)] to convey information about the intended flight of the aircraft. All flight plans contain essentially the same information, that is, aircraft identification number, make and model, and color; planned true airspeed and cruising altitude; origin and destination airports; planned departure time and estimated time en route; planned route of flight, fuel, and number of people on board; pilot's name and address; navigation equipment on board; and the aircraft's radio call sign, if different from the aircraft identification number.

Generally, a flight plan is not required for a flight under visual flight rules. However, if a flight plan is filed and the aircraft is overdue at its destination, search and rescue procedures will be initiated. Hence the flight plan under visual flight rules provides a significant safety benefit. An IFR flight plan is required for operation in controlled airspace when instrument meteorological conditions prevail.

Airspace

The two principal categories of airspace are controlled and uncontrolled airspace. In controlled airspace some or all aircraft are required to operate in accordance with air-traffic control clearances in order to assure safety, to meet user needs for air-traffic control, or to accommodate high volumes of traffic. Air-traffic control services including air-to-ground communications and navigation aids are provided in controlled airspace. Uncontrolled airspace simply is airspace that has not been designated as controlled; air-traffic control services may not be available in such airspace.

Two specific examples of controlled airspace are class A (the positive control area or PCA) and class B (the terminal control area or TCA). The positive control area is, with a few exceptions, the airspace within the conterminous 48 states and Alaska extending from 18,000 to 60,000 ft (5490 to 18,290 m) above mean sea level. Terminal control areas are centered on primary airports and extend from the surface to specified altitudes. An air-traffic control clearance and prescribed equipment are required prior to operating within a terminal control area regardless of weather conditions.

Air-to-ground communications

Two-way air-to-ground voice communications between civil pilots and air-traffic controllers are conducted in the very high frequency (VHF) band. In addition, certain radio navigation aids can provide one-way communications from controllers to aircraft. These channels generally are used to broadcast weather and aeronautical information to pilots. See also Radio spectrum allocations.

Air-to-ground data communications (that is, data link) increasingly are used to transfer information to and from the cockpit. Many of the communications errors associated with humans incorrectly reading, speaking, and hearing text are eliminated by communications protocols that detect errors in data transmissions, by electronically displaying the information received, and by storing the received information so that it can be reviewed. Data link also permits large quantities of data to be exchanged between ground-based and airborne computers. Civil aviation is exploiting three data-link media: some VHF voice channels, Mode S, and communications satellites.

Radio navigation aids

Radio navigation aids are used to determine the plan position of the aircraft (that is, the position in the horizontal plane) in coordinates referenced either to the navigation aid or to the Earth (that is, latitude and longitude). For most operations, the aircraft vertical position is determined by sensing atmospheric pressure on board and converting this pressure to altitude, based on a standard model of the atmosphere. For the landing phase of flight, precision landing aids provide horizontal and vertical position referenced to the runway. See also Altimeter.

VOR is a principal system used for determining plan position, with approximately 1000 ground stations nationwide. The system provides the magnetic azimuth from the VOR station to the receiving aircraft accurate to ±1°. Position determinations can be obtained from the intersection of radials from VORs with overlapping coverage volumes. With the addition of distance-measuring equipment at a VOR station, it is possible to obtain a position determination from a single station. See also Distance-measuring equipment; Rho-theta system; VOR (VHF omnidirectional range).

Nondirectional radio beacon is an older technology, with few installations remaining. The system radiates a continuous signal from which direction-finding receivers can determine the azimuth to the ground station. See also Direction-finding equipment.

Loran C is a pulsed system, with chains of ground stations each consisting of one master station and at least two secondary stations organized to transmit their signals in synchronism. Loran C coverage in the United States includes the conterminous 48 states and southern Alaska. See also Loran.

In order to conduct approaches and landings in low-visibility conditions, it is necessary that an electronic glideslope (or glidepath) be provided as a reference for controlling the descent of the aircraft. In addition, a stable guidance signal is required to align the aircraft with the runway centerline. The instrument landing system (ILS) has been the standard means for providing precision landing guidance to the runway, and is installed on approximately 1000 runways in the United States. The localizer antenna transmits the lateral (left and right) guidance signal over a 20° sector, 10° on both sides of the extended runway centerline. The glideslope antenna transmits the elevation guidance signal over a 1.4° sector, 0.7° on both sides of the glidepath, which is normally 3.0° above the horizontal. See also Instrument landing system (ILS).

A new standard system for providing precision approach guidance, the microwave landing system (MLS) has been designed to eliminate limitations of the instrument landing system. It utilizes scanning-beam technology to provide proportional landing guidance over 80° in azimuth (40° on both sides of the extended runway centerline) and 15° in elevation. The system can provide three-dimensional landing guidance within the scanned volume, thereby permitting curved approaches and approaches at higher glideslope angles than those available from the instrument landing system. See also Microwave landing system (MLS).

The constellation of Global Positioning System (GPS) satellites provides a highly accurate worldwide position determination and time transfer capability. In the horizontal plane, the position determined by a GPS receiver is within 330 ft (100 m) of the true receiver position at least 95% of the time. The vertical position is accurate to within 459 ft (140 m) on the same 95% probability basis. In addition, the receiver provides Coordinated Universal Time (UTC) with an accuracy of 310 ns (95% probability). Coordinated Universal Time is an internationally accepted time standard that never differs from Greenwich Mean Time by more than 1 s. The principal advantages of GPS are its accuracy and worldwide coverage. See also Air navigation; Electronic navigation systems; Satellite navigation systems.

Surveillance systems

Air-traffic controllers use radar to monitor the positions of aircraft and to monitor areas of heavy precipitation. The radar information is used to develop clearances and instructions for separating aircraft operating under instrument flight rules, and to provide traffic advisories to IFR aircraft and to VFR aircraft receiving the traffic advisory service. Traffic advisories provide the ranges, bearings, and altitudes of aircraft in the pilot's immediate vicinity. The pilot is responsible for visually acquiring and avoiding any traffic that may be a collision threat. Two principal types of radar are used in civil air-traffic control: secondary, or beacon, radar and primary radar. See also Radar.

Secondary radar is an interrogate-respond system. The rotating directional antenna of the ground station transmits a pulse pair to the transponder in the aircraft. The pulse spacing encodes one of two messages, “transmit your altitude” (the Mode C interrogation) or “transmit your identity” (the Mode A interrogation). The aircraft transponder transmits an encoded pressure-altitude reply in response to the first interrogation and a four-digit identity code, assigned by air-traffic control and entered into the transponder by the pilot, in response to the second.

Primary radar operates by transmitting high-power, radio-frequency pulses from a rotating directional antenna. The energy is reflected from any aircraft in the directional beam and received by the antenna. The aircraft is displayed at the azimuth corresponding to the pointing direction of the antenna and the range corresponding to the round-trip time between pulse transmission and receipt of the reflected signal. Primary radar has the advantage that aircraft without air-traffic control transponders can be detected, and energy reflected from heavy precipitation indicates to the controller areas of potentially hazardous weather. However, extraneous returns (clutter) from surrounding buildings and terrain can reduce the effectiveness of primary radar in detecting aircraft. At most air-traffic control radar sites, the secondary radar antenna is mounted on the primary radar antenna, and they are turned by a common drive system.

The secondary radar system has been improved through the addition of Mode S, which employs more sophisticated signaling formats than Modes A and C. Each aircraft transponder is permanently assigned a unique address and interrogations therefore can be addressed to individual aircraft.

In the oceanic environment, the ground-based surveillance systems described above obviously cannot be used. Oceanic operations are now based on rigid procedures and high-frequency (HF) communications that sometimes are unreliable. With the advent of commercially available mobile satellite communication systems, the development of a technique called automatic dependent surveillance (ADS) has been undertaken to provide real-time position information from aircraft over the ocean. In the operation of this system, the position of the aircraft, as determined from on-board navigation sensors, is communicated to air-traffic control facilities when requested by satellite relay. This position information can be displayed to controllers as though it had been determined by a radar system.

Automation

The principal elements of the controller's workstation are the plan view display, a track ball or mouse, the data-entry keyboard, printed flight strips showing the flight plans of aircraft for which the controller is responsible, and interfaces with communications facilities linking the controller with aircraft and with other controllers and facilities. The plan view display shows two principal types of data, map data and radar data. Map data include the locations of airports and their runways, navigation aids, airways, obstructions, and the geographical limits of the facility's airspace. Radar data comprise the positions of aircraft, including their altitudes, ground speeds, and radio call signs, as well as areas of precipitation. The data-entry keyboard allows the controller to modify data stored in the automation system, including flight plans. Extensive automation (computer) equipment is used in maintaining the flight-plan databases and processing radar data. A number of automation aids have been developed to assist controllers in separating aircraft as well as in sequencing and metering aircraft into and out of busy terminal areas.

Flight management computer systems are installed in aircraft for the purpose of guiding the aircraft along its planned route of flight while minimizing operating costs by selecting optimum speeds and altitudes. Extensive databases are stored in the flight management computer system (FMCS), including the current flight plan, wind velocities and air temperatures along the planned route of flight, and the positions and operating frequencies of the radionavigation aids to be used. Interfaces with the FMCS for air-to-ground data communications permit changes to be made to the databases in flight and allow information to be extracted, such as automatic dependent surveillance position reports and estimated times of arrival at specific points along the planned route of flight. See also Aircraft instrumentation.

Airborne collision avoidance systems are installed in aircraft to provide ground-independent protection from midair collisions, as a backup to the conventional air-traffic control system. Within the United States, the system is known as the Traffic Alert and Collision Avoidance System (TCAS). The TCAS equipment in the aircraft interrogates the secondary surveillance radar transponders in proximate aircraft and processes the replies to determine if any aircraft is on a collision course. Traffic advisories are displayed to the pilot to portray the range, bearing, and relative altitude of any aircraft that penetrates a protection volume around the TCAS-equipped aircraft. A resolution advisory will be displayed to tell the pilot how to maneuver to avoid a collision, if necessary.

Airways and procedures

Two fixed-route systems have been established for air navigation. From 1200 ft (360 m) above the surface up to but not including 18,000 ft (5490 m) above mean sea level, there are designated airways based on VORs and nondirectional beacons. The most prevalent are the so-called victor (V) airways defined by VORs. Jet (J) routes are defined from 18,000 to 45,000 ft (5490 to 13,710 m) above mean sea level, based solely on VORs.

There are three principal categories of procedures: departure procedures for leaving terminal areas, arrival procedures for entering terminal areas, and en route procedures. Departure procedures prescribe the process for route clearance delivery to an aircraft, for providing takeoff runway and taxi instructions, and for defining or placing limitations on the climb-out route of the aircraft. Generally, pilots of IFR aircraft call the clearance delivery controller for their route clearance prior to taxiing. The route in the clearance may differ from the filed route because of system restrictions such as excess traffic, facility outages, and weather.

En route procedures deal principally with reporting aircraft flight progress to air-traffic control (position reporting) when the aircraft is outside radar coverage or is operating in holding patterns.

Arrival procedures prescribe the process for making the transition from the en route structure to the terminal area, for approaching the landing runway, and for executing a missed approach when a landing cannot be accomplished. An instrument approach procedure is a series of predetermined maneuvers by reference to flight instruments for the orderly transfer of an aircraft from an initial approach fix to a landing or to a point from which a landing can be made visually. Several procedures, using different navigation and approach aids, may be established for an airport.

Facilities

Air-traffic control facilities include flight service stations, air-route traffic control centers (ARTCCs), and terminal facilities. Flight service stations provide preflight briefings for pilots, accept flight plans, broadcast aviation weather information, assist lost aircraft and aircraft in distress, and monitor the operation of radio navigation aids. Air-route traffic control centers monitor all IFR aircraft not under the control of military or terminal facilities. They assure separation of IFR aircraft by issuing clearances and instructions as necessary and issuing traffic advisories, provide weather advisories, accept amendments to flight plans from flight crews, and assist aircraft in distress. Flight plans submitted to flight service stations usually are transmitted to the parent air-route traffic control center, where they are processed and the route clearance is generated.

At terminal facilities, the ground controller position is responsible for all ground traffic not on active runways. The local controller has jurisdiction over the active runways and the airspace close to the airport. Controllers generally visually acquire and track aircraft and direct their movements by using radio or, when an aircraft has no operating radio, signal lights. In some locations, however, radar indicator equipment is installed in the tower to electronically display traffic that is being tracked by the local air-traffic control radar. See also Air transportation.


 
Columbia Encyclopedia: air traffic control,
the system by which airplanes are safely routed into and out of major airports. Air traffic control in the United States is centered in a number of regional control centers that route airplanes along established airways to airport traffic control centers. There Instrument Landing Systems and Microwave Landing Systems enable planes to land safely in almost any weather conditions. Air traffic controllers, who are responsible for maintaining safe distances between planes, are employees of the U.S. Dept. of Transportation. Air traffic control is made possible by special transponders installed in every commercial and many private aircraft, which automatically transmit information on a plane's altitude and speed to the ground controller. The distortion that often affects voice transmissions can be eliminated by the use of cockpit datalinks; collision avoidance systems provide further safety margins. Knowing the course, speed, and altitude of every plane in the sector, the controller can contact each in turn to give landing or course instructions. Modern air traffic control has contributed to making air travel far safer than highway travel, and on a passenger-mile basis safer even than rail travel.


 
Wikipedia: air traffic control


Air Traffic Control Towers (ATCTs) at Amsterdam's Schiphol Airport
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Air Traffic Control Towers (ATCTs) at Amsterdam's Schiphol Airport

Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and in the air. A controller's primary task is to separate certain aircraft — to prevent them from coming too close to each other by use of lateral, vertical and longitudinal separation. Secondary tasks include ensuring safe, orderly and expeditious flow of traffic and providing information to pilots, such as weather, 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. To ensure communication, all pilots and all controllers everywhere are required to be able to speak and understand English, although they may use any compatible language.


Airport control

Inside the Sao Paulo/Guarulhos International Airport's tower, Latin America's busiest airport.
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Inside the Sao Paulo/Guarulhos International Airport's tower, Latin America's busiest airport.

The primary method of controlling the immediate airport environment is visual observation from the control tower. The tower is a tall, windowed structure located on the airport grounds. Aerodrome or Tower 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 2 to 5 nautical miles (4 to 9 km) depending on the airport procedures.

Radar displays are also available to controllers at some airports. Controllers may use a radar system called Secondary Surveillance Radar also known as Airport Surveillance Radar for airborne traffic approaching and departing. These displays include a map of the area, the position of various aircraft, and data tags that include aircraft identification, speed, heading, and other information described in local procedures.

The areas of responsibility for tower controllers fall into three general operational disciplines; Ground Control, Local or Air Control, and Clearance Delivery -- other categories, such as Apron Control or Ground Movement Planner, may exist at extremely busy airports. While each tower's procedures will vary and while there may be multiple teams in larger towers that control multiple runways, the following provides a general concept of the delegation of responsibilities within the tower environment.

Ground Control

New Control Tower (right) at Chicago's O'hare.
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New Control Tower (right) at Chicago's O'hare.

Ground Control (sometimes known as Ground Movement Control abbreviated to GMC or Surface Movement Control abbreviated to SMC) is responsible for the airport "maneuvering" areas, or areas not released to the airlines or other users. This generally includes all taxiways, holding areas, and some transitional aprons or intersections where aircraft arrive having vacated the runway and departure gates. Exact areas and control responsibilities are clearly defined in local documents and agreements at each airport. Any aircraft, vehicle, or person walking or working in these areas is required to have clearance from the ground controller. This is normally done via VHF radio, but there may be special cases where other processes are used. Most aircraft and airside vehicles have radios. Aircraft or vehicles without radios will communicate with the tower via aviation light signals or will be led by vehicles with radios. People working on the airport surface normally have a communications link through which they can reach or be reached by ground control, commonly either by handheld radio or even cell phone. Ground control is vital to the smooth operation of the airport because this position might constrain the order in which the aircraft will be sequenced to depart, which can affect the safety and efficiency of the airport's operation.

Some busier airports have Surface Movement Radar (SMR), such as, ASDE-3, AMASS or ASDE-X, designed to display aircraft and vehicles on the ground. These are used by the ground controller as an additional tool to control ground traffic, particularly at night or in poor visibility. There are a wide range of capabilities on these systems as they are being modernized. Older systems will display a map of the airport and the target. Newer systems include the capability to display higher quality mapping, radar target, data blocks, and safety alerts.

Local or Air Control

Local or Air Control (most often referred to as the generic "Tower" control, although Tower control can also refer to a combination of the local, ground and clearance delivery positions) is responsible for the active runway surfaces. The Air Traffic Control Tower clears aircraft for take off or landing and ensures the runway is clear for these aircraft. If the tower controller detects any unsafe condition, a landing aircraft may be told to "go-around" and be re-sequenced into the landing pattern by the approach or terminal area controller.

Within the tower, a highly disciplined communications process between tower and ground control is an absolute necessity. Ground control must request and gain approval from tower control to cross any runway with any aircraft or vehicle. Likewise, tower control must ensure ground control is aware of any operations that impact the taxiways and must work with the approach radar controllers to ensure "holes" or "gaps" in the arrival traffic are created (where necessary) to allow taxiing traffic to cross runways and to allow departing aircraft to take off. Crew Resource Management (CRM) procedures are often used to ensure this communication process is efficient and clear, although this is not as prevalent as CRM for pilots.

Clearance delivery

Clearance delivery is the position that issues route clearances to aircraft before they commence taxiing. These contain details of the route that the aircraft is expected to fly after departure. This position will, if necessary, coordinate with the en-route center and national command center or flow control to obtain releases for aircraft. Often however such releases are given automatically or are controlled by local agreements allowing "free-flow" departures. When weather or extremely high demand for a certain airport or airspace becomes a factor, there may be ground "stops" (or "slot delays") or re-routes may be necessary to ensure the system does not get overloaded. The primary responsibility of the clearance delivery position is to ensure that the aircraft have the proper route and slot time. This information is also coordinated with the en-route center and the ground controller in order to ensure the aircraft reaches the runway in time to meet the slot time provided by the command center. At some airports the clearance delivery controller also plans aircraft pushbacks and engine starts and is known as Ground Movement Planner (GMP): this position is particularly important at heavily congested airports to prevent taxiway and apron gridlock.

Approach and terminal control

Inside the Potomac TRACON
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Inside the Potomac TRACON

Many airports have a radar control facility that is associated with the airport. In most countries, this is referred to as Approach or Terminal Control; in the U.S., it is often still referred to as a TRACON (Terminal Radar Approach CONtrol) facility. While every airport varies, terminal controllers usually handle traffic in a 30 to 50 nautical mile (56 to 93 km) radius from the airport. Where there are many busy airports in close proximity, one single terminal control may service all the airports. The actual airspace boundaries and altitudes assigned to a terminal control are based on factors such as traffic flows, neighboring airports and terrain, and vary widely from airport to airport: a large and complex example is the London Terminal Control Centre which controls traffic for five main London airports up to 20,000 feet and out to 100+ nautical miles.

Terminal controllers are responsible for providing all ATC services within their airspace. Traffic flow is broadly divided into departures, arrivals, and overflights. As aircraft move in and out of the terminal airspace, they are handed off to the next appropriate control facility (a control tower, an en-route control facility, or a bordering terminal or approach control). Terminal control is responsible for ensuring that aircraft are at an appropriate altitude when they are handed off, and that aircraft arrive at a suitable rate for landing.

Not all airports have a radar approach or terminal control available. In this case, the en-route center or a neighboring terminal or approach control may co-ordinate directly with the tower on the airport and vector inbound aircraft to a position from where they can land visually. At some of these airports, the tower may provide a non-radar procedural approach service to arriving aircraft handed over from a radar unit before they are visual to land. Some units also have a dedicated approach unit which can provide the procedural approach service either all the time or for any periods of radar outage for any reason.

En-route, center, or area control

Controllers at work at the Washington Air Route Traffic Control Center.
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Controllers at work at the Washington Air Route Traffic Control Center.
Main article: Area Control Center

ATC provides services to aircraft in flight between airports as well. Pilots fly under one of two sets of rules for separation: Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Air traffic controllers have different responsibilities to aircraft operating under the different sets of rules. While IFR flights are under positive control, in the US VFR pilots can request flight following, which provides traffic advisory services on a time permitting basis and may also provide assistance in avoiding areas of weather and flight restrictions.

En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots are required to comply with these instructions. En-route controllers also provide air traffic control services to many smaller airports around the country, including clearance off of the ground and clearance for approach to an airport. Controllers adhere to a set of separation standards that define the minimum distance allowed between aircraft. These distances vary depending on the equipment and procedures used in providing ATC services.

General characteristics

En-route air traffic controllers work in facilities called Area Control Centers, each of which is commonly referred to as a "Center". The United States uses the equivalent term Air Route Traffic Control Center (ARTCC). Each center is responsible for many thousands of square miles of airspace (known as a Flight Information Region) and for the airports within that airspace. Centers control IFR aircraft from the time they depart an airport or terminal area's airspace to the time they arrive at another airport or terminal area's airspace. Centers may also "pick up" VFR aircraft that are already airborne and integrate them into the IFR system. These aircraft must, however, remain VFR until the Center provides a clearance.

Center controllers are responsible for climbing the aircraft to their requested altitude while, at the same time, ensuring that the aircraft is properly separated from all other aircraft in the immediate area. Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight. This effort is complicated by crossing traffic, severe weather, special missions that require large airspace allocations, and traffic density. When the aircraft approaches its destination, the center is responsible for meeting altitude restrictions by specific points, as well as providing many destination airports with a traffic flow, which prohibits all of the arrivals being "bunched together". These "flow restrictions" often begin in the middle of the route, as controllers will position aircraft landing in the same destination so that when the aircraft are close to their destination they are sequenced.

As an aircraft reaches the boundary of a Center's control area it is "handed off" or "handed over" to the next Area Control Center. In some cases this "hand-off" process involves a transfer of identification and details between controllers so that air traffic control services can be provided in a seamless manner; in other cases local agreements may allow "silent handovers" such that the receiving center does not require any co-ordination if traffic is presented in an agreed manner. After the hand-off, the aircraft is given a frequency change and begins talking to the next controller. This process continues until the aircraft is handed off to a terminal controller ("approach").

Radar coverage

Since centers control a large airspace area, they will typically use long range radar that has the capability, at higher altitudes, to see aircraft within 200 nautical miles (360 km) of the radar antenna. They may also use TRACON radar data to control when it provides a better "picture" of the traffic or when it can fill in a portion of the area not covered by the long range radar.

In the U.S. system, at higher altitudes, over 90% of the U.S. airspace is covered by radar and often by multiple radar systems; however, coverage may be inconsistent at lower altitudes used by unpressurized aircraft due to high terrain or distance from radar facilities. A center may require numerous radar systems to cover the airspace assigned to them, and may also rely on pilot position reports from aircraft flying below the floor of radar coverage. This results in a large amount of data being available to the controller. To address this, automation systems have been designed that consolidate the radar data for the controller. This consolidation includes eliminating duplicate radar returns, ensuring the best radar for each geographical area is providing the data, and displaying the data in an effective format.

Centers also exercise control over traffic travelling over the world's ocean areas. These areas are also FIRs. Due to the fact that there are no radar systems available for oceanic control, oceanic controllers provide ATC services using procedural control. These procedures use aircraft position reports, time, altitude, distance, and speed to ensure separation. Controllers record information on flight progress strips and in specially developed oceanic computer systems as aircraft report positions. This process requires that aircraft be separated by greater distances, which reduces the overall capacity for any given route.

Some Air Navigation Service Providers (e.g Airservices Australia, Alaska Center, etc.) are implementing Automatic Dependent Surveillance - Broadcast (ADS-B) as part of their surveillance capability. This new technology reverses the radar concept. Instead of radar "finding" a target by interrogating the transponder, ADS transmits the aircraft's position several times a second. ADS also has other modes such as the "contract" mode where the aircraft reports a position based on a predetermined time interval. This is significant because it can be used where it is not possible to locate the infrastructure for a radar system (e.g. over water). Computerized radar displays are now being designed to accept ADS inputs as part of the display. As this technology develops, oceanic ATC procedures will be modernized to take advantage of the benefits this technology provides.

Flight traffic mapping

All inbound and outbound traffic for Minneapolis/St. Paul (MSP) airport on a weekday at 12:52 PM CDT from Animated Atlas: Flight Traffic over North America
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All inbound and outbound traffic for Minneapolis/St. Paul (MSP) airport on a weekday at 12:52 PM CDT from Animated Atlas: Flight Traffic over North America

The mapping of flights in real-time is based on the air traffic control system. In 1991, data on the location of aircraft was made available by the Federal Aviation Administration to the airline industry. The National Business Aviation Association (NBAA), the General Aviation Manufacturers Association, the Aircraft Owners & Pilots Association, the Helicopter Association International, and the National Air Transportation Association petitioned the FAA to make ASDI information available on a "need-to-know" basis. Subsequently, NBAA advocated the broad-scale dissemination of air traffic data. The Aircraft Situational Display to Industry (ASDI) system now conveys up-to-date flight information to the airline industry and the public. Three companies distribute ASDI information, FlightExplorer, FlightView, and FlyteComm. Each company maintains a website that provides free updated information to the public on flight status. Stand-alone programs are also available for displaying the geographic location of airborne IFR (Instrument Flight Rules) air traffic anywhere in the FAA air traffic system. Positions are reported for both commercial and general aviation traffic. The programs can overlay air traffic with a wide selection of maps such as, geo-political boundaries, air traffic control center boundaries, high altitude jet routes, satellite cloud and radar imagery.

Problems

Traffic

For more information see Air traffic flow management.

The day-to-day problems faced by the air traffic control system are primarily related to the volume of air traffic demand placed on the system, and weather. Several factors dictate the amount of traffic that can land at an airport in a given amount of time. Each landing aircraft must touch down, slow, and exit the runway before the next crosses the end of the runway. This process requires at least one and up to four minutes for each aircraft. Allowing for departures between arrivals, each runway can thus handle about 30 arrivals per hour. A large airport with two arrival runways can handle about 60 arrivals per hour in good weather. Problems begin when airlines schedule more arrivals into an airport than can be physically handled, or when delays elsewhere cause groups of aircraft that would otherwise be separated in time to arrive simultaneously. Aircraft must then be delayed in the air by holding over specified locations until they may be safely sequenced to the runway. Up until the 1990s, holding, which has significant environmental and cost implications, was a routine occurrence at many airports. Advances in computers now allow the sequencing of planes hours in advance. Thus, planes may be delayed before they even take off (by being given a "slot"), or may reduce power in flight and proceed more slowly thus significantly reducing the amount of holding.

Weather

Beyond runway capacity issues, weather is a major factor in traffic capacity. Rain or ice and snow on the runway cause landing aircraft to take longer to slow and exit, thus reducing the safe arrival rate and requiring more space between landing aircraft. Fog also requires a decrease in the landing rate. These, in turn, increase airborne delay for holding aircraft. If more aircraft are scheduled than can be safely and efficiently held in the air, a ground delay program may be established, delaying aircraft on the ground before departure due to conditions at the arrival airport.

In Area Control Centers, a major weather problem is thunderstorms, which present a variety of hazards to aircraft. Aircraft will deviate around storms, reducing the capacity of the en-route system by requiring more space per aircraft, or causing congestion as many aircraft try to move through a single hole in a line of thunderstorms. Occasionally weather considerations cause delays to aircraft prior to their departure as routes are closed by thunderstorms.

Much money has been spent on creating software to streamline this process. However, at some ACCs, air traffic controllers still record data for each flight on strips of paper and personally coordinate their paths. In newer sites, these flight progress strips have been replaced by electronic data presented on computer screens. As new equipment is brought in, more and more sites are upgrading away from paper flight strips.

Call signs

A prerequisite to safe air traffic separation is the assignment and use of distinctive call signs. These are permanently allocated by ICAO on request usually to scheduled flights and some air forces for military flights. They are written callsigns with 3-letter combination like KLM, AAL, SWA , BAW , DLH followed by the flight number, like AAL872, BAW018. As such they appear on flight plans and ATC radar labels. There are also the audio or Radio-telephony callsigns used on the radio contact between pilots and Air Traffic Control not always identical with the written ones. For example BAW stands for British Airways but on the radio you will only hear the word Speedbird instead. By default, the callsign for any other flight is the registration number (tail number) of the aircraft, such as "N12345" or "C-GABC". The term tail number is due to the fact that a registration number is usually painted somewhere on the tail of a plane, yet this is not a rule. Registration numbers may appear on the engines, anywhere on the fuselage, and often on the wings. The short Radio-telephony callsigns for these tail numbers is the first letter followed by the last two, like C-BC spoken as Charlie-Bravo-Charlie for C-GABC or the last 3 letters only like ABC spoken Alpha-Bravo-Charlie for C-GABC or the last 3 numbers like 345 spoken as tree-fower-fife for N12345.

The flight number part is decided by the aircraft operator. In this arrangement, an identical call sign might well be used for the same scheduled journey each day it is operated, even if the departure time varies a little across different days of the week. The call sign of the return flight often differs only by the final digit from the outbound flight. Generally, airline flight numbers are even if eastbound, and odd if westbound. In order to reduce the possibility of two callsigns on one frequency at any time sounding too similar, a number of airlines, particularly in Europe, have started using alphanumeric callsigns that are not based on flight numbers. For example DLH23LG, spoken as lufthansa-two-tree-lima-golf. Additionally it is the right of the air traffic controller to change the 'audio' callsign for the period the flight is in his sector if there is a risk of confusion, usually choosing the tail number instead.

Before around 1980 IATA and ICAO were using the same 2-letter callsigns. Due to the larger number of new airlines after deregulation ICAO established the 3-letter callsigns as mentioned above. The IATA callsigns are currently used in aerodromes on the announcement tables but never used any longer in Air Traffic Control. For example, AA is the IATA callsign for the ICAO - ATC equivalent AAL. Other examples include LY/ELY for El Al, DL/DAL for Delta Air Lines, LH/DLH for Lufthansa etc.

Technology

Many technologies are used in air traffic control systems. Primary and secondary radar are used to enhance a controller's "situational awareness" within his assigned airspace — all types of aircraft send back primary echoes of varying sizes to controllers' screens as radar energy is bounced off their skins, and transponder-equipped aircraft reply to secondary radar interrogations by giving an ID (Mode A), an altitude (Mode C) and/or a unique callsign (Mode S). Certain types of weather may also register on the radar screen.

These inputs, added to data from other radars, are correlated to build the air situation. Some basic processing occurs on the radar tracks, such as calculating ground speed and magnetic headings.

Other correlations with electronic flight plans are also available to controllers on modern operational display systems.

Some tools are available in different domains to help the controller further:

  • Conflict Alert (CA): a tool that checks possible conflicting trajectories and alerts the controller. The most common used is the STCA (Short Term CA) that is activated about 2 minutes prior the loss of separation. The algorithms used may also provide in some systems a possible vectoring solution, that is, the way to turn or descend/climb the aircraft in order to avoid infringing the minimum safety distance or altitude clearance.
  • Minimum Safe Altitude Warning (MSAW): a tool that alerts the controller if an aircraft appears to be flying too low to the ground or will impact terrain based on its current altitude and heading.
  • System Coordination (SYSCO) to enable controller to negotiate the release of flights from one sector to another.
  • Area Penetration Warning (APW) to inform a controller that a flight will penetrate a restricted area.
  • Arrival and Departure manager to help sequence the takeoff and landing of aircraft.
  • Converging Runway Display Aid (CRDA) enables Approach controllers to run two final approaches that intersect and make sure that go arounds are minimized
  • Final Approach Spacing Tool (FAST) gives aircraft a runway assignment that the Approach Controller will give to the aircraft. FAST can also suggest vectors for downwind and base with the correct timing. In Europe the equivalent system is known under the term metering system and predicts the future spacing of approaching aircraft on the runway
  • User Request Evaluation Tool (URET) takes paper strips out of the equation for En Route controllers at ARTCCs By providing a display that shows all aircraft that are either in or currently routed into the sector. URET provides conflict advisories up to 30 minutes in advance and has a suite of assistance tools that assist in evaluating resolution options and pilot requests.
  • Mode S: provides a data downlink of flight parameters via Secondary Surveillance Radars allowing radar processing systems and therefore controllers to see various data on a flight, including airframe unique id, indicated airspeed and flight director selected level, amongst others.
  • CPDLC: Controller Pilot Data Link Communications - allows digital messages to be sent between controllers and pilots, avoiding the need to use radiotelephony. It is especially useful in areas where difficult-to-use HF radiotelephony was previously used for communication with aircraft, e.g oceans. This is currently in use in various parts of the world including the Atlantic and Pacific oceans.
  • ADS-B: Automatic Dependent Surveillance Broadcast - provides a data downlink of various flight parameters to air traffic control systems via the Transponder (1090 MHz) and reception of those data by other aircraft in the vicinity. The most important is the aircraft's latitude, longitude and level: such data can be utilized to create a radar-like display of aircraft for controllers and thus allows a form of pseudo-radar control to be done in areas where the installation of radar is either prohibitive on the grounds of low traffic levels, or technically not feasible (e.g. oceans). This is currently in use in Australia and parts of the Pacific Ocean and Alaska.
  • The Extended Computer Display System (EXCDS): A system of electronic flight strips replacing the old paper strips developed by NAV CANADA. EXCDS allows controllers to manage electronic flight data online using touch-sensitive display screens resulting in fewer manual functions and a greater focus on safety. The system has also been sold to the Air Navigation Services Providers in the United Kingdom and Denmark.

Major accidents

Failures in the system have caused delays or even, in rare cases, crashes. On July 1, 2002 a Tupolev Tu-154 and Boeing 757 collided above Überlingen near the boundary between German and Swiss-controlled airspace when a Skyguide-employed controller apparently gave instructions to the southbound Tupolev to descend despite an instruction from the on-board automatic Traffic Collision Avoidance System software to climb. The northbound Boeing, equipped with similar avionics, was already descending due to a software prompt. All passengers and crew died in the resultant collision. Skyguide company publicity had previously acknowledged that the relatively small size of Swiss airspace makes real-time cross-boundary liaison with adjoining authorities particularly important. See Bashkirian Airlines Flight 2937 for more on this accident. It is worth noting that currently air traffic controllers have no way of knowing if or when the TCAS system is issuing resolution advisories to pilots. They also do not know what the advisory is telling the pilots. Therefore, pilots are supposed to immediately follow TCAS resolution advisories and report them as soon as possible. Consequently, they should ignore ATC instructions until they have reported to the ground that they are clear of the conflict.

Other fatal collisions between airliners have occurred over India and Yugoslavia. When a risk of collision is identified by aircrew or ground controllers an "air miss" or "air prox" (air proximity) report can be filed with the air traffic control authority concerned. The worst fatal collision between airliners actually took place on the ground, on March 27, 1977, in what is known as the Tenerife disaster.

The FAA has spent over USD$3 billion on software, but a fully-automated system is still over the horizon. In 2002 the UK brought a new area control centre into service at Swanwick, in Hampshire, relieving a busy suburban centre at West Drayton in Middlesex, north of London Heathrow Airport. Software from Lockheed-Martin predominates at Swanwick. The Swanwick facility, however, was initially been troubled by software and communications problems causing delays and occasional shutdowns.


Air navigation service providers (ANSPs) and traffic service providers (ATSPs)

An Air Navigation Service Provider - The air navigation service provider is the authority directly responsible for providing both visual and non-visual aids to navigation within a specific airspace in compliance with, but not limited to, International Civil Aviation Organization (ICAO) Annexes 2, 6, 10 and 11; ICAO Documents 4444 and 9426; and, other international, multi-national, and national policy, agreements or regulations.

An Air Traffic Service Provider is the relevant authority designated by the State responsible for providing air traffic services in the airspace concerned - where airspace is classified as Type A through G airspace. Air traffic service is a generic term meaning variously, flight information service, alerting service, air traffic advisory service, air traffic control service (area control service, approach control service or aerodrome control service).

Both ANSPs and ATSPs can be public, private or corporatized organisations and examples of the different legal models exist throughout the world today. The world's ANSPs are united in and represented by the Civil Air Navigation Services Organisation based at Amsterdam Airport Schiphol in the Netherlands.

The regulatory function remains the responsibility of the State and can be exercised by Government and/or independent Safety, Airspace and Economic Regulators depending on the national institutional arrangements.

In the United States, the Federal Aviation Administration (FAA) provides this service to all aircraft in the National Airspace System (NAS). With the exception of facilities operated by the Department of Defense (DoD), the FAA is responsible for all aspects of U.S. Air Traffic Control including hiring and training controllers, although there are contract towers located in many parts of the country. DoD facilities are generally staffed by military personnel and operate separately but concurrently with FAA facilities, under similar rules and procedures. A contract tower is an Airport Traffic Control Tower (ATCT) that performs the same function as an FAA-run ATCT but is staffed by employees of a private company (Martin State Airport in Maryland is an example). In Canada, Air Traffic Control is provided by NAV CANADA, a private, non-share capital corporation that operates Canada's civil air navigation service.

See also

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