Air traffic control

Air traffic control (ATC) is a service provided by ground-based

air traffic controllers (also called control tower operators (CTO))
who direct aircraft on the ground and through a given section of
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.[1]

Air traffic controllers monitor the location of aircraft in their

assigned airspace by radar and communicate with the pilots by
radio.[2] To prevent collisions, ATC 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, ATC may issue instructions that pilots are required to
obey, or advisories (known as flight information in some countries)
that pilots may, at their discretion, disregard. The pilot in command
is the final authority for the safe operation of the aircraft and may, Air traffic control tower of Mumbai
in an emergency, deviate from ATC instructions to the extent International Airport (India)
required to maintain safe operation of their aircraft.[3]

Language
Pursuant to requirements of the International Civil Aviation Organization (ICAO), ATC operations are
conducted either in the English language or the language used by the station on the ground.[4] In practice,
the native language for a region is used; however, English must be used upon request.[4]

History
In 1920, Croydon Airport, London, was the first airport in the world to introduce air traffic control.[5] The
"aerodrome control tower" was a wooden hut 15 ft (4.6 m) high with windows on all four sides. It was
commissioned on February 25, 1920 and provided basic traffic, weather and location information to
pilots.[6][7]

In the United States, air traffic control developed three divisions. The first of several air mail radio stations
(AMRS) was created in 1922 after World War I when the U.S. Post Office began using techniques
developed by the Army to direct and track the movements of reconnaissance aircraft. Over time, the AMRS
morphed into flight service stations. Today's flight service stations do not issue control instructions, but
provide pilots with many other flight related informational services. They do relay control instructions from
ATC in areas where flight service is the only facility with radio or phone coverage. The first airport traffic
control tower, regulating arrivals, departures and surface movement of aircraft at a specific airport, opened
in Cleveland in 1930. Approach/departure control facilities were created after adoption of radar in the
1950s to monitor and control the busy airspace around larger airports. The first air route traffic control
center (ARTCC), which directs the movement of aircraft between departure and destination, was opened in
Newark in 1935, followed in 1936 by Chicago and Cleveland.[8] Currently in the U.S., the Federal
Aviation Administration (FAA) operates 22 ARTCCs.

After the 1956 Grand Canyon mid-air collision, killing all 128 on board, the FAA was given the air-traffic
responsibility over the United States in 1958, and this was followed by other countries. In 1960, Britain,
France, Germany and the Benelux countries set up Eurocontrol, intending to merge their airspaces. The
first and only attempt to pool controllers between countries is the Maastricht Upper Area Control Centre
(MUAC), founded in 1972 by Eurocontrol and covering Belgium, Luxembourg, the Netherlands and
north-western Germany. In 2001, the EU aimed to create a "Single European Sky", hoping to boost
efficiency and gain economies of scale.[9]

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. A controller must carry out the
job using the precise and effective application of rules and
procedures that, however, need flexible adjustments according to
differing circumstances, often under time pressure.[10] In a study
that compared stress in the general population and this kind of
system markedly showed more stress level for controllers. This
variation can be explained, at least in part, by the characteristics of
the job.[11] São Paulo–Guarulhos International
Airport's control tower
Surveillance displays are also available to controllers at larger
airports to assist with controlling air traffic. Controllers may use a
radar system called secondary 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, altitude, and other information
described in local procedures. In adverse weather conditions, the
tower controllers may also use surface movement radar (SMR),
surface movement guidance and control system (SMGCS), or
advanced surface movement guidance and control system
(ASMGCS) to control traffic on the maneuvering area (taxiways
and runway).

The areas of responsibility for tower controllers fall into three

general operational disciplines: local control or air control, ground
control, and flight data/clearance delivery—other categories, such
as airport apron control or ground movement planner, may exist at
extremely busy airports. While each tower may have unique Control tower at Birmingham Airport,
airport-specific procedures, such as multiple teams of controllers England
(crews) at major or complex airports with multiple runways, the
following provides a general concept of the delegation of
responsibilities within the tower environment.

Remote and virtual tower (RVT) is a system based on air traffic

controllers being located somewhere other than at the local airport
tower and still able to provide air traffic control services. Displays
for the air traffic controllers may be live video, synthetic images
based on surveillance sensor data, or both.

Ground control

Ground control (sometimes

known as ground Small control tower at Räyskälä
movement control) is Airfield in Loppi, Finland
responsible for the airport
movement areas, as well as
areas not released to the airlines or other users. This generally
includes all taxiways, inactive runways, holding areas, and some
transitional aprons or intersections where aircraft arrive, having
Inside Pope Field air traffic control
tower
vacated the runway or departure gate. 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 ground control. This is normally done via
VHF/UHF radio, but there may be special cases where other procedures are used. Aircraft or vehicles
without radios must respond to ATC instructions via aviation light signals or else be led by vehicles with
radios. People working on the airport surface normally have a communications link through which they can
communicate with 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 impacts the sequencing of departure
aircraft, affecting 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 ground control as an additional
tool to control ground traffic, particularly at night or in poor visibility. There is 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 targets, data blocks,
and safety alerts, and to interface with other systems such as digital flight strips.

Air control or local control

Air control (known to pilots as tower or tower control) is responsible for the active runway surfaces. Air
control clears aircraft for takeoff or landing, ensuring that prescribed runway separation will exist at all
times. If the air controller detects any unsafe conditions, a landing aircraft may be instructed to "go-around"
and be re-sequenced into the landing pattern. This re-sequencing will depend on the type of flight and may
be handled by the air controller, approach, or terminal area controller.

Within the tower, a highly disciplined communications process between the air control and ground control
is an absolute necessity. Air control must ensure that ground control is aware of any operations that will
impact the taxiways, and work with the approach radar controllers to create gaps in the arrival traffic to
allow taxiing traffic to cross runways and to allow departing aircraft to take off. Ground control needs to
keep the air controllers aware of the traffic flow towards their runways to maximise runway utilisation
through effective approach spacing. Crew resource management (CRM) procedures are often used to
ensure this communication process is efficient and clear. Within ATC, it is usually known as TRM (team
resource management) and the level of focus on TRM varies within different ATC organisations.

Flight data and clearance delivery

Clearance delivery is the position that issues route clearances to aircraft, typically before they commence
taxiing. These clearances contain details of the route that the aircraft is expected to fly after departure.
Clearance delivery or, at busy airports, ground movement planner (GMP) or traffic management
coordinator (TMC) will, if necessary, coordinate with the relevant radar center or flow control unit to
obtain releases for aircraft. At busy airports, these releases are often automatic and 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 clearance delivery is to ensure
that the aircraft has the correct aerodrome information, such as weather and airport conditions, the correct
route after departure, and time restrictions relating to that flight. This information is also coordinated with
the relevant radar center or flow control unit and ground control to ensure that the aircraft reaches the
runway in time to meet the time restriction provided by the relevant unit. At some airports, clearance
delivery also plans aircraft push-backs and engine starts, in which case it is known as the ground movement
planner (GMP): this position is particularly important at heavily congested airports to prevent taxiway and
apron gridlock.

Flight data (which is routinely combined with clearance delivery) is the position that is responsible for
ensuring that both controllers and pilots have the most current information: pertinent weather changes,
outages, airport ground delays/ground stops, runway closures, etc. Flight data may inform the pilots using a
recorded continuous loop on a specific frequency known as the automatic terminal information service
(ATIS).

Approach and terminal control

Many airports have a radar control facility that is associated with
the airport. In most countries, this is referred to as terminal control
and abbreviated to TMC; in the U.S., it is referred to as a
TRACON (terminal radar approach control). 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 close together, one consolidated terminal
control center may service all the airports. The airspace boundaries
and altitudes assigned to a terminal control center, which vary
widely from airport to airport, are based on factors such as traffic Potomac Consolidated TRACON in
flows, neighboring airports and terrain. A large and complex Warrenton, Virginia, United States
example was the London Terminal Control Centre, which
controlled traffic for five main London airports up to 20,000 feet
(6,100 m) and out to 100 nautical miles (190 km).

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.

In the U.S., TRACONs are additionally designated by a three-digit alphanumeric code. For example, the
Chicago TRACON is designated C90.[12]

Area control center/en-route 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 and Canada 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
The training department at the
flight restrictions, as well as allowing pilots into the ATC system
Washington Air Route Traffic Control
prior to the need to a clearance into certain airspace. Across
Center, Leesburg, Virginia, United
Europe, pilots may request for a "Flight Information Service",
States
which is similar to flight following. In the UK it is known as a
"basic service".

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 air traffic control centers, each of which is commonly
referred to as a "center". The United States uses the equivalent term air route traffic control center. Each
center is responsible for a given flight information region (FIR). Each flight information region covers
many thousands of square miles of airspace and the airports within that airspace. Centers control IFR
aircraft from the time they depart from 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 system. These aircraft must continue under VFR flight rules until the center provides
a clearance.

Center controllers are responsible for issuing instructions to pilots to climb their aircraft to their assigned
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 issuing instructions to pilots so that they will meet 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 (370 km) of the radar antenna. They may also use
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 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 flight information regions (FIRs).
Because 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 Unmanned radar on a remote
reduces the overall capacity for any given route. See for example mountain
the North Atlantic Track system.

Some air navigation service providers (e.g., Airservices Australia, the U.S. Federal Aviation Administration,
Nav Canada, etc.) have implemented 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, the ADS-B equipped aircraft sends a position report as determined
by the navigation equipment on board the aircraft. ADS-C is another mode of automatic dependent
surveillance, however ADS-C operates in the "contract" mode where the aircraft reports a position,
automatically or initiated by the pilot, based on a predetermined time interval. It is also possible for
controllers to request more frequent reports to more quickly establish aircraft position for specific reasons.
However, since the cost for each report is charged by the ADS service providers to the company operating
the aircraft, more frequent reports are not commonly requested except in emergency situations. ADS-C 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-C inputs as part of the
display.[13] This technology is currently used in portions of the North Atlantic and the Pacific by a variety
of states who share responsibility for the control of this airspace.

Precision approach radars (PAR) are commonly used by military controllers of air forces of several
countries, to assist the pilot in final phases of landing in places where instrument landing system and other
sophisticated airborne equipment are unavailable to assist the pilots in marginal or near zero visibility
conditions. This procedure is also called talkdowns.

A radar archive system (RAS) keeps an electronic record of all radar information, preserving it for a few
weeks. This information can be useful for search and rescue. When an aircraft has 'disappeared' from radar
screens, a controller can review the last radar returns from the aircraft to determine its likely position. For
example, see this crash report.[14] RAS is also useful to technicians who are maintaining radar systems.

Flight traffic mapping

The mapping of flights in real-time is based on the air traffic control system, and volunteer ADS-B
receivers. 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 and 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. Some companies that distribute ASDI
information are 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

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 approach 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
Intersecting contrails of aircraft over
handle about 60 arrivals per hour in good weather. Problems begin London, an area of high air traffic
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 speed in
flight and proceed more slowly thus significantly reducing the amount of holding.

Air traffic control errors occur when the separation (either vertical or horizontal) between airborne aircraft
falls below the minimum prescribed separation set (for the domestic United States) by the US Federal
Aviation Administration. Separation minimums for terminal control areas (TCAs) around airports are lower
than en-route standards. Errors generally occur during periods following times of intense activity, when
controllers tend to relax and overlook the presence of traffic and conditions that lead to loss of minimum
separation.[15]

Weather

Beyond runway capacity issues, the weather is a major factor in

traffic capacity. Rain, ice, snow or hail 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.
Airplane taking off from Dallas/Fort
Worth International Airport with the
In Area Control Centers, a major weather problem is
ATC tower in the background
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.

Congestion

Constrained control capacity and growing traffic lead to flight cancellation and delays:

In America, delays caused by ATC grew by 69% between 2012 and 2017.[9] ATC staffing
issues were a major factor in congestion.[16]
In China, the average delay per domestic flight spiked by 50% in 2017 to 15 minutes per
flight.
In Europe, en route delays grew by 105% in 2018, due to a lack of capacity or staff (60%),
weather (25%) or strikes (14%), costing the European economy €17.6bn ($20.8bn), up by
28% on 2017.

By then the market for air-traffic services was worth $14bn. More efficient ATC could save 5-10% of
aviation fuel by avoiding holding patterns and indirect airways.[9]
The military takes 80% of Chinese air space, congesting the thin corridors open to airliners. Britain is
closing military air space only during air-force exercises.[9]

Callsigns
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 and other
military services for military flights. There are written callsigns with a 3-letter combination followed by the
flight number such as AAL872 or VLG1011. As such they appear on flight plans and ATC radar labels.
There are also the audio or Radiotelephony callsigns used on the radio contact between pilots and air traffic
control. These are not always identical to their written counterparts. An example of an audio callsign would
be "Speedbird 832", instead of the written "BAW832". This is used to reduce the chance of confusion
between ATC and the aircraft. By default, the callsign for any other flight is the registration number (tail
number) of the aircraft, such as "N12345", "C-GABC" or "EC-IZD". The short Radiotelephony callsigns
for these tail numbers is the last 3 letters using the NATO phonetic alphabet (e.g. ABC spoken alpha-
bravo-charlie for C-GABC) or the last 3 numbers (e.g. three-four-five for N12345). In the United States,
the prefix may be an aircraft type, model or manufacturer in place of the first registration character, for
example, "N11842" could become "Cessna 842".[17] This abbreviation is only allowed after
communications have been established in each sector.

Before around 1980 International Air Transport Association (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 are no longer used in air traffic control. For example, AA is the IATA callsign for American
Airlines; the ATC equivalent is AAL. Flight numbers in regular commercial flights are designated by the
aircraft operator and identical callsign might 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 callsign 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 (e.g. DLH23LG, spoken as Lufthansa-two-three-lima-golf, to
prevent confusion between incoming DLH23 and outgoing DLH24 in the same frequency). 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.

Technology
Many technologies are used in air traffic control systems. Primary and secondary radar are used to enhance
a controller's situation 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.

Usually, a flight data processing system manages all the flight plan related data, incorporating – in a low or
high degree – the information of the track once the correlation between them (flight plan and track) is
established. All this information is distributed to modern operational display systems, making it available to
controllers.
The FAA has spent over US$3 billion on software, but a fully automated system is still yet to be achieved.
In 2002 the UK brought a new area control centre into service at the London Area Control Centre,
Swanwick, Hampshire, relieving a busy suburban centre at West Drayton, Middlesex, north of London
Heathrow Airport. Software from Lockheed-Martin predominates at the London Area Control Centre.
However, the centre was initially troubled by software and communications problems causing delays and
occasional shutdowns.[18]

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

Flight data processing systems: this is the system (usually one per center) that processes all
the information related to the flight (the flight plan), typically in the time horizon from gate to
gate (airport departure/arrival gates). It uses such processed information to invoke other
flight plan related tools (such as e.g. MTCD), and distributes such processed information to
all the stakeholders (air traffic controllers, collateral centers, airports, etc.).
Short-term conflict alert (STCA) that checks possible conflicting trajectories in a time horizon
of about 2 or 3 minutes (or even less in approach context – 35 seconds in the French Roissy
& Orly approach centres[19]) and alerts the controller prior to the loss of separation. The
algorithms used may also provide in some systems a possible vectoring solution, that is, the
manner in which to turn, descend, increase/decrease speed, or 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.
The departure manager (DMAN): A system aid for the ATC at airports, that calculates a
planned departure flow with the goal to maintain an optimal throughput at the runway,
reduce queuing at holding point and distribute the information to various stakeholders at
the airport (i.e. the airline, ground handling and air traffic control (ATC)).
The arrival manager (AMAN): A system aid for the ATC at airports, that calculates a
planned arrival flow with the goal to maintain an optimal throughput at the runway,
reduce arrival queuing and distribute the information to various stakeholders.
Passive final approach spacing tool (pFAST), a CTAS tool, provides runway assignment
and sequence number advisories to terminal controllers to improve the arrival rate at
congested airports. pFAST was deployed and operational at five US TRACONs before
being cancelled. NASA research included an active FAST capability that also provided
vector and speed advisories to implement the runway and sequence advisories.
Converging runway display aid (CRDA) enables approach controllers to run two final
approaches that intersect and make sure that go arounds are minimized.
Center TRACON automation system (CTAS) is a suite of human centered decision support
tools developed by NASA Ames Research Center. Several of the CTAS tools have been
field tested and transitioned to the FAA for operational evaluation and use. Some of the
CTAS tools are: traffic management advisor (TMA), passive final approach spacing tool
(pFAST), collaborative arrival planning (CAP), direct-to (D2), en route descent advisor (EDA)
and multi-center TMA. The software is running on Linux.[20]
Traffic management advisor (TMA), a CTAS tool, is an en route decision support tool that
automates time based metering solutions to provide an upper limit of aircraft to a TRACON
from the center over a set period of time. Schedules are determined that will not exceed the
 specified arrival rate and controllers use the scheduled times to provide the appropriate
delay to arrivals while in the en route domain. This results in an overall reduction in en route
delays and also moves the delays to more efficient airspace (higher altitudes) than occur if
holding near the TRACON boundary, which is required in order to prevent overloading the
TRACON controllers. TMA is operational at most en route air route traffic control centers
(ARTCCs) and continues to be enhanced to address more complex traffic situations (e.g.
adjacent center metering (ACM) and en route departure capability (EDC))
MTCD & URET
In the US, 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.
In Europe, several MTCD tools are available: iFACTS (NATS), VAFORIT (DFS), new
FDPS (MUAC). The SESAR[21] programme should soon launch new MTCD concepts.

URET and MTCD provide conflict advisories up to 30 minutes in advance and have 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 (24-bits encoded), 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, Canada and parts of the Pacific
Ocean and Alaska.
The electronic flight strip system (e-strip):

A system of electronic flight strips replacing the old paper strips is

being used by several service providers, such as Nav Canada,
MASUAC, DFS, DECEA. E-strips allows controllers to manage
electronic flight data online without paper strips, reducing the need
for manual functions, creating new tools and reducing the ATCO's
workload. The firsts electronic flight strips systems were
independently and simultaneously invented and implemented by
Nav Canada and Saipher ATC in 1999. The Nav Canada system
known as EXCDS[22] and rebranded in 2011 to NAVCANstrips
and Saipher's first generation system known as SGTC, which is Electronic flight progress strip
now being updated by its 2nd generation system, the TATIC TWR. system at São Paulo Intl. control
DECEA in Brazil is the world's largest user of tower e-strips tower – ground control
system, ranging from very small airports up to the busiest ones,
taking the advantage of real time information and data collection
from each of more than 150 sites for use in air traffic flow management (ATFM), billing and statistics.
 Screen content recording: Hardware or software based recording function which is part of
most modern automation system and that captures the screen content shown to the ATCO.
Such recordings are used for a later replay together with audio recording for investigations
and post event analysis.[23]
Communication navigation surveillance / air traffic management (CNS/ATM) systems are
communications, navigation, and surveillance systems, employing digital technologies,
including satellite systems together with various levels of automation, applied in support of a
seamless global air traffic management system.[24]

Air navigation service providers (ANSPs) and air traffic service

providers (ATSPs)
Azerbaijan – AzərAeroNaviqasiya
Albania – Albcontrol
Algeria – Etablissement National de la Navigation Aérienne (ENNA)
Argentina - Empresa Argentina de Navegación Aérea (EANA)
Armenia – Armenian Air Traffic Services (ARMATS)
Australia – Airservices Australia (Government owned Corporation) and Royal Australian Air
Force
Austria – Austro Control
Bangladesh- Civil Aviation Authority, Bangladesh
Belarus – Republican Unitary Enterprise "Белаэронавигация (Belarusian Air Navigation)"
Belgium – Skeyes - Authority of Airways
Bosnia and Herzegovina – Agencija za pružanje usluga u zračnoj plovidbi (Bosnia and
Herzegovina Air Navigation Services Agency)
Brazil – Departamento de Controle do Espaço Aéreo (ATC/ATM Authority) and ANAC –
Agência Nacional de Aviação Civil (Civil Aviation Authority)
Bulgaria – Air Traffic Services Authority
Cambodia – Cambodia Air Traffic Services (CATS)
Canada – Nav Canada – formerly provided by Transport Canada and Canadian Forces
Cayman Islands – CIAA Cayman Islands Airports Authority
Central America – Corporación Centroamericana de Servicios de Navegación Aérea
Guatemala – Dirección General de Aeronáutica Civil (DGAC)
El Salvador
Honduras
Nicaragua – Empresa Administradora Aeropuertos Internacionales (EAAI)
Costa Rica – Dirección General de Aviación Civil
Belize
Chile – Dirección General de Aeronáutica Civil (DGAC)
Colombia – Aeronáutica Civil Colombiana (UAEAC)
Croatia – Hrvatska kontrola zračne plovidbe (Croatia Control Ltd.)
Cuba – Instituto de Aeronáutica Civil de Cuba (IACC)
Czech Republic – Řízení letového provozu ČR
Cyprus - Department of Civil Aviation
Denmark – Naviair (Danish ATC)
Dominican Republic – Instituto Dominicano de Aviación Civil (IDAC) "Dominican Institute of
Civil Aviation"
Eastern Caribbean – Eastern Caribbean Civil Aviation Authority (ECCAA)
Anguilla
Antigua and Barbuda
British Virgin Islands
Dominica
Grenada
Saint Kitts and Nevis
Saint Lucia
Saint Vincent and the Grenadines
Ecuador – Dirección General de Aviación Civil (DGAC) "General Direction of Civil Aviation"
Government Body
Estonia – Estonian Air Navigation Services
Europe – Eurocontrol (European Organisation for the Safety of Air Navigation)
Fiji - Fiji Airports (fully owned Government Commercial Company)
Finland – Finavia
France – Direction Générale de l'Aviation Civile (DGAC) : Direction des Services de la
Navigation Aérienne (DSNA) (Government body)
Georgia – SAKAERONAVIGATSIA, Ltd. (Georgian Air Navigation)
Germany – Deutsche Flugsicherung (German ATC – State-owned company)
Greece – Hellenic Civil Aviation Authority (HCAA)
Hong Kong – Civil Aviation Department (CAD)
Hungary – HungaroControl Magyar Légiforgalmi Szolgálat Zrt. (HungaroControl Hungarian
Air Navigation Services Pte. Ltd. Co.)
Iceland – ISAVIA
Indonesia – AirNav Indonesia
Iran - Iran Civil Aviation Organization (ICAO)
Ireland – Irish Aviation Authority (IAA)
India – Airports Authority of India (AAI) (under Ministry of Civil Aviation, Government of India
and Indian Air Force)
Iraq – Iraqi Air Navigation – ICAA
Israel – Israeli Airports Authority (IIA)
Italy – ENAV SpA and Italian Air Force
Jamaica – JCAA (Jamaica Civil Aviation Authority)
Japan – JCAB (Japan Civil Aviation Bureau)
Kenya – KCAA (Kenya Civil Aviation Authority)
Latvia – LGS (Latvian ATC)
Lithuania – ANS (Lithuanian ATC)
Luxembourg – Administration de la navigation aérienne (ANA – government administration)
Macedonia – DGCA (Macedonian ATC)
Malaysia – Civil Aviation Authority of Malaysia (CAAM)
Malta – Malta Air Traffic Services Ltd
Mexico – Servicios a la Navegación en el Espacio Aéreo Mexicano
Morocco - Office National Des Aeroports (ONDA)
Nepal – Civil Aviation Authority of Nepal
 Netherlands – Luchtverkeersleiding Nederland (LVNL) (Dutch ATC) Eurocontrol (Maastricht
Upper Area Control Centre)
New Zealand – Airways New Zealand (State owned enterprise)
Nigeria - Nigeria Civil Aviation Authority (NCAA)
Norway – Avinor (State-owned private company)
Oman – Directorate General of Meteorology & Air Navigation (Government of Oman)
Pakistan – Civil Aviation Authority (under Government of Pakistan)
Peru – Centro de Instrucción de Aviación Civil CIAC Civil Aviation Training Center
Philippines – Civil Aviation Authority of the Philippines (CAAP) (under the Philippine
Government)
Poland – Polish Air Navigation Services Agency (PANSA)
Portugal – NAV (Portuguese ATC)
Puerto Rico – Administracion Federal de Aviacion
Romania – Romanian Air Traffic Services Administration (ROMATSA)
Russia – Federal State Unitary Enterprise "State ATM Corporation"
Saudi Arabia – Saudi Air Navigation Services (SANS)
Seychelles – Seychelles Civil Aviation Authority (SCAA)
Singapore – Civil Aviation Authority of Singapore (CAAS)
Serbia – Serbia and Montenegro Air Traffic Services Agency Ltd. (SMATSA)
Slovakia – Letové prevádzkové služby Slovenskej republiky
Slovenia – Slovenia Control
South Africa – Air Traffic and Navigation Services (ATNS)
South Korea – Korea Office of Civil Aviation
Spain – AENA now AENA S.A. (Spanish Airports) and ENAIRE (ATC & ATSP)[25]
Sri Lanka – Airport & Aviation Services (Sri Lanka) Limited (Government owned company)
Sweden – LFV (government body)
Switzerland – Skyguide
Taiwan – ANWS (Civil Aeronautical Administration)
Thailand – AEROTHAI (Aeronautical Radio of Thailand)
Trinidad and Tobago – Trinidad and Tobago Civil Aviation Authority (TTCAA)
Turkey – General Directorate of State Airports Authority (DHMI)
United Arab Emirates – General Civil Aviation Authority (GCAA)
United Kingdom – National Air Traffic Services (NATS) (49% State owned public-private
partnership)
United States – Federal Aviation Administration (FAA) (government body)
Ukraine – Ukrainian State Air Traffic Service Enterprise (UkSATSE)
Venezuela – Instituto Nacional de Aeronautica Civil (INAC)
Zambia - Zambia Civil Aviation Authority (ZCAA)[26]
Zimbabwe - Zimbabwe Civil Aviation Authority[27]

Proposed changes
In the United States, some alterations to traffic control procedures are being examined:

The Next Generation Air Transportation System examines how to overhaul the United States
national airspace system.
 Free flight is a developing air traffic control method that uses no centralized control (e.g. air
traffic controllers). Instead, parts of airspace are reserved dynamically and automatically in a
distributed way using computer communication to ensure the required separation between
aircraft.[28]

In Europe, the SESAR[21] (Single European Sky ATM Research) programme plans to develop new
methods, technologies, procedures, and systems to accommodate future (2020 and beyond) air traffic
needs. In October 2018, European controller unions dismissed setting targets to improve ATC as "a waste
of time and effort" as new technology could cut costs for users but threaten their jobs. In April 2019, the
EU called for a "Digital European Sky", focusing on cutting costs by including a common digitisation
standard and allowing controllers to move to where they are needed instead of merging national ATCs, as it
would not solve all problems. Single air-traffic control services in continent-sized America and China does
not alleviate congestion. Eurocontrol tries to reduce delays by diverting flights to less busy routes: flight
paths across Europe were redesigned to accommodate the new airport in Istanbul, which opened in April,
but the extra capacity will be absorbed by rising demand for air travel.[9]

Well-paid jobs in Western Europe could move east with cheaper labour. The average Spanish controller
earn over €200,000 a year, over seven times the country average salary, more than pilots, and at least ten
controllers were paid over €810,000 ($1.1m) a year in 2010. French controllers spent a cumulative nine
months on strike between 2004 and 2016.[9]

Privatization

Many countries have also privatized or corporatized their air navigation service providers.[29] There are
several models that can be used for ATC service providers. The first is to have the ATC services be part of a
government agency as is currently the case in the United States. The problem with this model is that
funding can be inconsistent and can disrupt the development and operation of services. Sometimes funding
can disappear when lawmakers cannot approve budgets in time. Both proponents and opponents of
privatization recognize that stable funding is one of the major factors for successful upgrades of ATC
infrastructure. Some of the funding issues include sequestration and politicization of projects.[30]
Proponents argue that moving ATC services to a private corporation could stabilize funding over the long
term which will result in more predictable planning and rollout of new technology as well as training of
personnel.

Another model is to have ATC services provided by a government corporation. This model is used in
Germany, where funding is obtained through user fees. Yet another model is to have a for-profit corporation
operate ATC services. This is the model used in the United Kingdom, but there have been several issues
with the system there including a large-scale failure in December 2014 which caused delays and
cancellations and has been attributed to cost-cutting measures put in place by this corporation. In fact,
earlier that year, the corporation owned by the German government won the bid to provide ATC services
for Gatwick Airport in the United Kingdom. The last model, which is often the suggested model for the
United States to transition to is to have a non-profit organization that would handle ATC services as is used
in Canada.[31]

The Canadian system is the one most often used as a model by proponents of privatization. Air traffic
control privatization has been successful in Canada with the creation of Nav Canada, a private nonprofit
organization which has reduced costs and has allowed new technologies to be deployed faster due to the
elimination of much of the bureaucratic red tape. This has resulted in shorter flights and less fuel usage. It
has also resulted in flights being safer due to new technology. Nav Canada is funded from fees that are
collected from the airlines based on the weight of the aircraft and the distance flown.[32]
ATC is operated by national governments with few exceptions: in the European Union, only Italy has
private shareholders. Privatisation does not guarantee lower prices: the profit margin of MUAC was 70% in
2017, as there is no competition, but governments could offer fixed terms concessions. Australia, Fiji and
New Zealand run the upper-air space for the Pacific islands' governments. HungaroControl offers remote
airport tower services from Budapest, and since 2014 provides upper air space management for Kosovo.

ATC regulations in the United States

The United States airspace is divided into 21 zones (centers), and each zone is divided into sectors. Also
within each zone are portions of airspace, about 50 miles (80.5 km) in diameter, called TRACON
(Terminal Radar Approach Control) airspaces. Within each TRACON airspace are a number of airports,
each of which has its own airspace with a 5-mile (8-km) radius. FAA control tower operators (CTO) / air
traffic controllers use FAA Order 7110.65 as the authority for all procedures regarding air traffic.[33]

See also
Air traffic service
Flight information service officer
Flight planning
ICAO recommendations on use of the International System of Units
Forward air control
Global air-traffic management
RMCDE
Tower en route control (TEC)
List of tallest air traffic control towers in the United States

References
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8. FAA HISTORICAL CHRONOLOGY, 1926–1996
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13. "Automatic Dependent Surveillance - Contract (ADS-C) - SKYbrary Aviation Safety" (https://
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15. Breitler, Alan; Kirk, Kevin (September 1996). "Effects of Sector Complexity and Controller
Experience on Probability of Operational Errors in Air Route Traffic Control Centers". Center
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16. Gilbert, Trish (June 15, 2016). "Air traffic control staffing shortage must be addressed" (http
s://thehill.com/blogs/congress-blog/technology/283328-air-traffic-control-staffing-shortage-m
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17. "What is an Abbreviated Aircraft Call Sign?*" (http://atccommunication.com/what-is-an-abbre
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19. "Le filet de sauvegarde resserre ses mailles" (https://web.archive.org/web/20090327181434/
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olutions%5Cproducts%5CIIDS%5Cexcds%5Cdefault.xml). NAV CANADA. Archived from
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ao/en/ro/rio/execsum.pdf) (PDF). icao.int. p. 10. Archived from the original (http://www.icao.in
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25. "Acerca de ENAIRE – ENAIRE – Información corporativa" (https://web.archive.org/web/201
50704122232/http://www.enaire.es/csee/Satellite/Enaire/es/Enaire.html). Archived from the
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26. "Zambia Civil Aviation Authority - Home" (http://www.caa.co.zm/). www.caa.co.zm. Retrieved
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27. "Civil Aviation Authority of Zimbabwe" (https://web.archive.org/web/20190629142236/https://
www.caaz.co.zw/). www.caaz.co.zw. Archived from the original (https://www.caaz.co.zw/) on
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28. Leslie, Jacques. "Wired 4.04: Free Flight" (https://www.wired.com/wired/archive/4.04/es.faa.
html). Wired. Retrieved July 3, 2015.
29. McDougall, Glen; Roberts, Alasdair S (August 15, 2007). "Commercializing Air Traffic
Control: Have the Reforms Worked?". Canadian Public Administration: Vol. 51, No. 1, pp.
45–69, 2009. SSRN 1317450 (https://papers.ssrn.com/sol3/papers.cfm?abstract_id=131745
0).
30. American Federation of Government Employees; et al. "FAA Labor Unions Oppose ATC
Privatization" (https://www.passmember.org/images/congressional_letters/FAA%20Labor%2
0Unions%20Oppose%20ATC%20Privatization.pdf) (PDF). Professional Aviation Safety
Specialists. Retrieved November 25, 2016.
31. Rinaldi, Paul (2015). "Safety and Efficiency Must Remain the Main Mission". The Journal of
Air Traffic Control. 57 (2): 21–23.
32. Crichton, John (2015). "The NAV CANADA Model". The Journal of Air Traffic Control. 57 (2):
33–35.
33. "Air Traffic Plans and Publications" (http://www.faa.gov/documentlibrary/media/order/atc.pdf)
(PDF). FAA. Archived (https://web.archive.org/web/20090510140116/http://www.faa.gov/doc
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External links
U.S. Centennial of Flight Commission – Air Traffic Control (https://web.archive.org/web/2006
0524043438/http://www.centennialofflight.gov/essay/Government_Role/Air_traffic_control/P
OL15.htm)
The short film A TRAVELER MEETS AIR TRAFFIC CONTROL (1963) (https://archive.org/d
etails/gov.archives.arc.43449) is available for free download at the Internet Archive.
NASA video of US air traffic (https://www.youtube.com/watch?v=8pYiC7bTUxQ)

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