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130-504: In air traffic control , an area control center ( ACC ), also known as a center or en-route center , is a facility responsible for controlling aircraft flying in the airspace of a given flight information region (FIR) at high altitudes between airport approaches and departures. In the US, such a center is referred to as an air route traffic control center ( ARTCC ). A center typically accepts traffic from — and ultimately passes traffic to —

260-780: A   a ( t ) + M d cos ⁡ ( 2 π ∫ 0 t ( F s + F d cos ⁡ ( 2 π F n t ) ) d t ) g ( A , t ) = M n cos ⁡ ( 2 π F n t − A ) {\displaystyle {\begin{array}{rcl}e(A,t)&=&\cos(2\pi F_{c}t)(1+c(t)+g(A,t))\\c(t)&=&M_{i}\cos(2\pi F_{i}t)~i(t)\\&+&M_{a}~a(t)\\&+&M_{d}\cos(2\pi \int _{0}^{t}(F_{s}+F_{d}\cos(2\pi F_{n}t))dt)\\g(A,t)&=&M_{n}\cos(2\pi F_{n}t-A)\\\end{array}}} The doppler signal encodes

390-1075: A   a ( t ) + M n cos ⁡ ( 2 π F n t ) g ( A , t ) = ( M d / 2 ) cos ⁡ ( 2 π ( F c + F s ) t + ( A , t ) ) + ( M d / 2 ) cos ⁡ ( 2 π ( F c − F s ) t − ( A , t ) ) {\displaystyle {\begin{array}{rcl}t&=&t_{+}(A,t)-(R/C)\sin(2\pi F_{n}t_{+}(A,t)+A)\\t&=&t_{-}(A,t)+(R/C)\sin(2\pi F_{n}t_{-}(A,t)+A)\\e(A,t)&=&\cos(2\pi F_{c}t)(1+c(t))\\&+&g(A,t)\\c(t)&=&M_{i}\cos(2\pi F_{i}t)~i(t)\\&+&M_{a}~a(t)\\&+&M_{n}\cos(2\pi F_{n}t)\\g(A,t)&=&(M_{d}/2)\cos(2\pi (F_{c}+F_{s})t_{+}(A,t))\\&+&(M_{d}/2)\cos(2\pi (F_{c}-F_{s})t_{-}(A,t))\\\end{array}}} where

520-470: A ' Flight Information Service ', which is similar to flight following. In the United Kingdom, 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

650-451: A 'Single European Sky', hoping to boost efficiency and gain economies of scale. The primary method of controlling the immediate airport environment is visual observation from the airport control tower. The tower is typically a tall, windowed structure, located within the airport grounds. The air traffic controllers , usually abbreviated 'controller', are responsible for separation and efficient movement of aircraft and vehicles operating on

780-433: A DVOR uses an omnidirectional antenna. These are usually Alford Loop antennas (see Andrew Alford ). Unfortunately, the sideband antennas are very close together, so that approximately 55% of the energy radiated is absorbed by the adjacent antennas . Half of that is re-radiated, and half is sent back along the antenna feeds of the adjacent antennas . The result is an antenna pattern that is no longer omnidirectional. This causes

910-400: 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 centre or a neighbouring terminal or approach control may co-ordinate directly with

1040-433: 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

1170-400: A circular array of typically 48 omni-directional antennas and no moving parts. The active antenna is moved around the circular array electronically to create a doppler effect, resulting in frequency modulation. The amplitude modulation is created by making the transmission power of antennas at e.g. the north position lower than at the south position. The role of amplitude and frequency modulation

1300-427: A combination of factors. Most significant is that VOR provides a bearing from the station to the aircraft which does not vary with wind or orientation of the aircraft. VHF radio is less vulnerable to diffraction (course bending) around terrain features and coastlines. Phase encoding suffers less interference from thunderstorms. VOR signals offer a predictable accuracy of 90 m (300 ft), 2 sigma at 2 NM from

1430-528: A comparable level. As of 2008 in the United States, GPS-based approaches outnumbered VOR-based approaches but VOR-equipped IFR aircraft outnumber GPS-equipped IFR aircraft. There is some concern that GNSS navigation is subject to interference or sabotage, leading in many countries to the retention of VOR stations for use as a backup. The VOR signal has the advantage of static mapping to local terrain. The US FAA plans by 2020 to decommission roughly half of

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1560-416: A controller can review the last radar returns from the aircraft to determine its likely position. For an example, see the crash report in the following citation. RAS is also useful to technicians who are maintaining radar systems. 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

1690-403: A cumulative nine months on strike between 2004 and 2016. VHF omnidirectional range A VOR beacon radiates via two or more antennas an amplitude modulated signal and a frequency modulated subcarrier . By comparing the fixed 30 Hz reference signal with the rotating azimuth 30 Hz signal the azimuth from an aircraft to a (D)VOR is detected. The phase difference is indicative of

1820-486: A different ICAO code. Pilots typically use high frequency radio instead of very high frequency radio to communicate with a center when flying over the ocean, because of HF's relatively greater propagation over long distances. Military aircraft, however, are typically equipped with ARC-231 SATCOMs that allow over-the-horizon communication. Area control centers (ACCs) control IFR air traffic in their flight information region (FIR). The current list of FIRs and ACCs

1950-400: A distance of 100 nautical miles (185 kilometres; 115 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

2080-450: A first-come, first-served basis. Aircraft passing from one sector to another are handed off and requested to change frequencies to contact the next sector controller. Sector boundaries are specified by an aeronautical chart . Air traffic controllers working within a center communicate via radio with pilots of instrument flight rules ( IFR ) aircraft passing through the center's airspace. A center's communication frequencies (typically in

2210-432: 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 kilometres; 230 miles) 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

2340-414: 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. Centres also exercise control over traffic travelling over

2470-516: A localizer frequency is selected, the OBS is not functional and the indicator is driven by a localizer converter, typically built into the receiver or indicator. A VOR station serves a volume of airspace called its Service Volume. Some VORs have a relatively small geographic area protected from interference by other stations on the same frequency—called "terminal" or T-VORs. Other stations may have protection out to 130 nautical miles (240 kilometres) or more. It

2600-417: 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

2730-453: A morse code identifier, optional voice, and a pair of navigation tones. The radial azimuth is equal to the phase angle between the lagging and leading navigation tone. The conventional signal encodes the station identifier, i ( t ) , optional voice a ( t ) , navigation reference signal in c ( t ) , and the isotropic (i.e. omnidirectional) component. The reference signal is encoded on an F3 subcarrier (colour). The navigation variable signal

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2860-714: A new area control centre into service at the London Area Control Centre (LACC) 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 the London Area Control Centre. However, the centre was initially troubled by software and communications problems causing delays and occasional shutdowns. Some tools are available in different domains to help

2990-468: A number of airlines, particularly in Europe, have started using alphanumeric call signs 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' call sign for the period the flight is in his sector if there

3120-401: A pair of VOR beacons; as compared to the accuracy of unaugumented Global Positioning System (GPS) which is less than 13 meters, 95%. VOR stations, being VHF, operate on "line of sight". This means that if, on a perfectly clear day, you cannot see the transmitter from the receiver antenna, or vice versa, the signal will be either imperceptible or unusable. This limits VOR (and DME ) range to

3250-477: A plane's arrival and intentions from its pre-filed flight plan . Some centers have ICAO-designated responsibility for airspace located over an ocean such as ZNY and ZOA, the majority of which is international airspace . Because substantial volumes of oceanic airspace lie beyond the range of ground-based radars, oceanic airspace controllers have to estimate the position of an airplane from pilot reports and computer models ( procedural control ), rather than observing

3380-510: A radar control facility that is associated with that specific 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 'terminal radar approach control' or TRACON. While every airport varies, terminal controllers usually handle traffic in a 30-to-50-nautical-mile (56 to 93 km; 35 to 58 mi) radius from the airport. Where there are many busy airports close together, one consolidated terminal control centre may service all

3510-428: A seamless manner; in other cases, local agreements may allow 'silent handovers', such that the receiving centre 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 its pilot begins talking to the next controller. This process continues until the aircraft is handed off to a terminal controller ('approach'). Since centres control

3640-688: A single facility. For example, NATS combines the London Terminal Control Centre (LTCC) and London Area Control Centre (LACC) in Swanwick, Hampshire . The United States Federal Aviation Administration (FAA) defines an ARTCC as: [a] facility established to provide air traffic control service to aircraft operating on IFR flight plans within controlled airspace, principally during the en-route phase of flight. When equipment capabilities and controller workload permit, certain advisory/assistance services may be provided to VFR aircraft. An ARTCC

3770-520: 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

3900-480: 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. 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

4030-447: A target by interrogating the transponder, the ADS-B equipped aircraft 'broadcasts' 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

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4160-1287: A transmitter closes on and recedes from the receiver results in F3 modulation (colour). The pairing of transmitters offset equally high and low of the isotropic carrier frequency produce the upper and lower sidebands. Closing and receding equally on opposite sides of the same circle around the isotropic transmitter produce F3 subcarrier modulation, g ( A , t ) . t = t + ( A , t ) − ( R / C ) sin ⁡ ( 2 π F n t + ( A , t ) + A ) t = t − ( A , t ) + ( R / C ) sin ⁡ ( 2 π F n t − ( A , t ) + A ) e ( A , t ) = cos ⁡ ( 2 π F c t ) ( 1 + c ( t ) ) + g ( A , t ) c ( t ) = M i cos ⁡ ( 2 π F i t )   i ( t ) + M

4290-415: A two or three 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 radio-telephony call signs 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 call sign would be 'Speedbird 832', instead of

4420-419: 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,

4550-434: A variable signal. One of them is amplitude modulated, and one is frequency modulated. On conventional VORs (CVOR), the 30 Hz reference signal is frequency modulated (FM) on a 9,960 Hz subcarrier . On these VORs, the amplitude modulation is achieved by rotating a slightly directional antenna exactly in phase with the reference signal at 30 revolutions per second. Modern installations are Doppler VORs (DVOR), which use

4680-541: Is a risk of confusion, usually choosing the aircraft registration identifier instead. Many technologies are used in air traffic control systems. Primary and secondary radars are used to enhance a controller's situational awareness within their 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

4810-468: Is a vast simplification. The primary complication relates to a process that is called "blending". Another complication is that the phase of the upper and lower sideband signals have to be locked to each other. The composite signal is detected by the receiver. The electronic operation of detection effectively shifts the carrier down to 0 Hz, folding the signals with frequencies below the Carrier, on top of

4940-416: Is a wide range of capabilities on these systems as they are being modernised. 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 (known to pilots as tower or tower control ) is responsible for

5070-510: Is also coordinated with the relevant radar centre 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 aircraft parking area gridlock. Flight data (which

5200-461: 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

5330-402: Is brought in, more and more sites are upgrading away from paper flight strips. Constrained control capacity and growing traffic lead to flight cancellation and delays : 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 . The military takes 80% of Chinese airspace, congesting

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5460-411: Is called a VORTAC . A VOR co-located only with DME is called a VOR-DME. A VOR radial with a DME distance allows a one-station position fix. Both VOR-DMEs and TACANs share the same DME system. VORTACs and VOR-DMEs use a standardized scheme of VOR frequency to TACAN/DME channel pairing so that a specific VOR frequency is always paired with a specific co-located TACAN or DME channel. On civilian equipment,

5590-671: Is encoded by mechanically or electrically rotating a directional, g ( A , t ) , antenna to produce A3 modulation (grey-scale). Receivers (paired colour and grey-scale trace) in different directions from the station paint a different alignment of F3 and A3 demodulated signal. e ( A , t ) = cos ⁡ ( 2 π F c t ) ( 1 + c ( t ) + g ( A , t ) ) c ( t ) = M i cos ⁡ ( 2 π F i t )   i ( t ) + M

5720-602: Is itself amplitude modulated with a 60 Hz amplitude modulation (also some 30 Hz as well). This distortion can add or subtract with the above-mentioned 60 Hz distortion depending on the carrier phase. In fact one can add an offset to the carrier phase (relative to the sideband phases) so that the 60 Hz components tend to null one another. There is a 30 Hz component, though, which has some pernicious effects. DVOR designs use all sorts of mechanisms to try to compensate these effects. The methods chosen are major selling points for each manufacturer, with each extolling

5850-539: Is maintained by the International Civil Aviation Organization (ICAO). Note that the cited ICAO source gives the shapefile coordinates for each FIR, and also its page source gives a list of current ACCs in text form. The following is the alphabetic list of all ACCs and their FIRs as of October 2011: Air traffic control Air traffic control ( ATC ) is a service provided by ground-based air traffic controllers who direct aircraft on

5980-451: Is not discontinuous. The amplitude of the next antenna rises as the amplitude of the current antenna falls. When one antenna reaches its peak amplitude, the next and previous antennas have zero amplitude. By radiating from two antennas, the effective phase center becomes a point between the two. Thus the phase reference is swept continuously around the ring – not stepped as would be the case with antenna to antenna discontinuous switching. In

6110-641: Is popularly thought that there is a standard difference in power output between T-VORs and other stations, but in fact the stations' power output is set to provide adequate signal strength in the specific site's service volume. In the United States, there are three standard service volumes (SSV): terminal, low, and high (standard service volumes do not apply to published instrument flight rules (IFR) routes). Additionally, two new service volumes – "VOR low" and "VOR high" – were added in 2021, providing expanded coverage above 5,000 feet AGL. This allows aircraft to continue to receive off-route VOR signals despite

6240-518: 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 official airport 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

6370-480: Is 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 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

6500-438: 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). Many airports have

6630-517: Is that the EIRP provides in spite of losses, e.g. due to propagation and antenna pattern lobing, for a sufficiently strong signal at the aircraft VOR antenna that it can be processed successfully by the VOR receiver. Each (D)VOR station broadcasts a VHF radio composite signal, including the mentioned navigation and reference signal, and a station's identifier and optional additional voice. The station's identifier

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6760-680: Is the U.S. equivalent of an area control center (ACC). There are 22 ARTCCs located in nineteen states. The flight information region controlled by a center may be further administratively subdivided into areas comprising two to nine sectors. Each area is staffed by a set of controllers trained on all the sectors in that area. Sectors use distinct radio frequencies for communication with aircraft. Each sector also has secure landline communications with adjacent sectors, approach controls, areas, ARTCCs, flight service centers, and military aviation control facilities. These landline communications are shared among all sectors that need them and are available on

6890-493: Is the only facility with radio or phone coverage. The first airport traffic control tower, regulating arrivals, departures, and surface movement of aircraft in the US 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

7020-606: 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 centre 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

7150-457: Is then fed over an analog or digital interface to one of four common types of indicators: In many cases, VOR stations have co-located distance measuring equipment (DME) or military Tactical Air Navigation ( TACAN ) – the latter includes both the DME distance feature and a separate TACAN azimuth feature that provides military pilots data similar to the civilian VOR. A co-located VOR and TACAN beacon

7280-470: Is thus swapped in this type of VOR. Decoding in the receiving aircraft happens in the same way for both types of VORs: the AM and FM 30 Hz components are detected and then compared to determine the phase angle between them. The VOR signal also contains a modulated continuous wave (MCW) 7 wpm Morse code station identifier, and usually contains an amplitude modulated (AM) voice channel. This information

7410-498: Is typically a three-letter string in Morse code . While defined in Annex 10 voice channel is seldomly used today, e.g. for recorded advisories like ATIS . A VORTAC is a radio-based navigational aid for aircraft pilots consisting of a co-located VHF omnidirectional range and a tactical air navigation system (TACAN) beacon. Both types of beacons provide pilots azimuth information, but

7540-466: 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

7670-835: 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

7800-567: The bearing from the (D)VOR station to the receiver relative to magnetic north. This line of position is called the VOR "radial". While providing the same signal over the air at the VOR receiver antennas. DVOR is based on the Doppler shift to modulate the azimuth dependent 30 Hz signal in space, by continuously switching the signal of about 25 antenna pairs that form a circle around the center 30 Hz reference antenna. The intersection of radials from two different VOR stations can be used to fix

7930-733: The pilots by radio . To prevent collisions, ATC enforces traffic separation rules, which ensure each aircraft maintains a minimum amount of 'empty space' around it at all times. It is also common for ATC to provide services to all private , military , and commercial aircraft operating within its airspace; not just civilian aircraft. 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 of an aircraft always retains final authority for its safe operation, and may, in an emergency, deviate from ATC instructions to

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8060-512: The very high frequency aviation bands , using amplitude modulation (AM) 118 MHz to 137 MHz, for overland control) are published in aeronautical charts and manuals, and are also announced to a pilot by the previous controller during a hand-off. Most VHF radio assignments also have a UHF (225 to 380 MHz) paired frequency used for military flights. In addition to radios to communicate with aircraft, center controllers have access to communication links with other centers and TRACONs . In

8190-655: The 967 VOR stations in the US, retaining a "Minimum Operational Network" to provide coverage to all aircraft more than 5,000 feet above the ground. Most of the decommissioned stations will be east of the Rocky Mountains , where there is more overlap in coverage between them. On July 27, 2016, a final policy statement was released specifying stations to be decommissioned by 2025. A total of 74 stations are to be decommissioned in Phase 1 (2016–2020), and 234 more stations are scheduled to be taken out of service in Phase 2 (2021–2025). In

8320-528: 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

8450-837: The Global Positioning System ( GPS ) are increasingly replacing VOR and other ground-based systems. In 2016, GNSS was mandated as the primary needs of navigation for IFR aircraft in Australia. GNSS systems have a lower transmitter cost per customer and provide distance and altitude data. Future satellite navigation systems, such as the European Union Galileo , and GPS augmentation systems are developing techniques to eventually equal or exceed VOR accuracy. However, low VOR receiver cost, broad installed base and commonality of receiver equipment with ILS are likely to extend VOR dominance in aircraft until space receiver cost falls to

8580-407: 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 centre 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

8710-485: The U.S., TRACONs are additionally designated by a three-digit alphanumeric code. For example, the Chicago TRACON is designated C90. Air traffic control also provides services to aircraft in flight between airports. 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

8840-548: The UK, 19 VOR transmitters are to be kept operational until at least 2020. Those at Cranfield and Dean Cross were decommissioned in 2014, with the remaining 25 to be assessed between 2015 and 2020. Similar efforts are underway in Australia, and elsewhere. In the UK and the United States, DME transmitters are planned to be retained in the near future even after co-located VORs are decommissioned. However, there are long-term plans to decommission DME, TACAN and NDBs. The VOR signal encodes

8970-903: The United States, centers are electronically linked through the National Airspace System , which allows nationwide coordination of traffic flow to manage congestion. Centers in the United States also have electronic access to nationwide radar data. Controllers use radar to monitor the progress of flights and instruct aircraft to perform course adjustments as needed to maintain separation from other aircraft. Aircraft with center contact can be readily distinguished by their transponders . Pilots may request altitude adjustments or course changes for reasons including avoidance of turbulence or adverse weather conditions. Controllers can assign routing relative to location fixes derived from latitude and longitude , or from radionavigation beacons such as VORs . Typically, centers have advance notice of

9100-476: The VHF frequency is tuned and the appropriate TACAN/DME channel is automatically selected. While the operating principles are different, VORs share some characteristics with the localizer portion of ILS and the same antenna, receiving equipment and indicator is used in the cockpit for both. When a VOR station is selected, the OBS is functional and allows the pilot to select the desired radial to use for navigation. When

9230-469: The VOR station located on the grounds of John F. Kennedy International Airport has the identifier JFK. VORs are assigned radio channels between 108.0 MHz and 117.95 MHz (with 50 kHz spacing); this is in the very high frequency (VHF) range. The first 4 MHz is shared with the instrument landing system (ILS) band. In the United States, frequencies within the pass band of 108.00 to 111.95 MHz which have an even 100 kHz first digit after

9360-556: The VOR system is generally used by civil aircraft and the TACAN system by military aircraft. However, the TACAN distance measuring equipment is also used for civil purposes because civil DME equipment is built to match the military DME specifications. Most VOR installations in the United States are VORTACs. The system was designed and developed by the Cardion Corporation. The Research, Development, Test, and Evaluation (RDT&E) contract

9490-465: The active runway surfaces. Air control gives clearance for aircraft takeoff or landing, whilst 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

9620-471: 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 aircraft hours in advance. Thus, aircraft 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

9750-465: 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 'team resource management' (TRM), and the level of focus on TRM varies within different ATC organisations. Clearance delivery

9880-900: 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 aircraft 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 aircraft arrivals per hour. A large airport with two arrival runways can handle about 60 arrivals per hour in good weather. Problems arise 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

10010-454: The air, a ground delay programme may be established, delaying aircraft on the ground before departure due to conditions at the arrival airport. In Area Control Centres, a major weather problem is thunderstorms , which present a variety of hazards to aircraft. Airborne 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

10140-412: The aircraft approaches its destination, the centre 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

10270-481: The aircraft operator, and identical call sign 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 call sign of the return flight often differs only by the final digit from the outbound flight. Generally, airline flight numbers are even if east-bound, and odd if west-bound. In order to reduce the possibility of two call signs on one frequency at any time sounding too similar,

10400-501: The aircraft passes over a VOR station or at an intersection in the air defined by one or more VORs. Navigational reference points can also be defined by the point at which two radials from different VOR stations intersect, or by a VOR radial and a DME distance. This is the basic form of RNAV and allows navigation to points located away from VOR stations. As RNAV systems have become more common, in particular those based on GPS , more and more airways have been defined by such points, removing

10530-455: The airports. The airspace boundaries and altitudes assigned to a terminal control centre, which vary widely from airport to airport, are based on factors such as traffic flows, neighbouring airports, and terrain. A large and complex example was the London Terminal Control Centre (LTCC), which controlled traffic for five main London airports up to an altitude of 20,000 feet (6,096 metres) and out to

10660-564: 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

10790-470: 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 Flightradar24 , FlightExplorer, FlightView, and FlyteComm. Each company maintains a website that provides free updated information to the public on flight status. Stand-alone programmes are also available for displaying

10920-528: The centre provides a clearance. Centre 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 its 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

11050-436: The control of a terminal control center or another center. Most centers are operated by the national governments of the countries in which they are located. The general operations of centers worldwide, and the boundaries of the airspace each center controls, are governed by the International Civil Aviation Organization (ICAO). In some cases, the function of an area control center and a terminal control center are combined in

11180-590: The controller further: In the United States, some alterations to traffic control procedures are being examined: In Europe, the Single European Sky ATM Research (SESAR) 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,

11310-403: The decimal point (108.00, 108.05, 108.20, 108.25, and so on) are reserved for VOR frequencies while frequencies within the 108.00 to 111.95 MHz pass band with an odd 100 kHz first digit after the decimal point (108.10, 108.15, 108.30, 108.35, and so on) are reserved for ILS. The VOR encodes azimuth (direction from the station) as the phase relationship between a reference signal and

11440-472: The different sets of rules. While IFR flights are under positive control, in the US and Canada, VFR pilots can request 'flight following' (radar advisories), which provides traffic advisory services on a time permitting basis, and may also provide assistance in avoiding areas of weather and flight restrictions, as well as allowing pilots into the air traffic control system prior to the need to a clearance into certain airspace. Throughout Europe, pilots may request

11570-433: The effective sideband signal to be amplitude modulated at 60 Hz as far as the aircraft's receiver is concerned. The phase of this modulation can affect the detected phase of the sub-carrier. This effect is called "coupling". Blending complicates this effect. It does this because when two adjacent antennas radiate a signal, they create a composite antenna. Imagine two antennas that are separated by their wavelength/2. In

11700-417: The electromechanical antenna switching systems employed before solid state antenna switching systems were introduced, the blending was a by-product of the way the motorized switches worked. These switches brushed a coaxial cable past 50 (or 48) antenna feeds. As the cable moved between two antenna feeds, it would couple signal into both. But blending accentuates another complication of a DVOR. Each antenna in

11830-598: The equivalent term air route traffic control center. Each centre is responsible for a given flight information region (FIR). Each flight information region typically covers many thousands of square miles of airspace, and the airports within that airspace. Centres 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. Centres may also 'pick up' VFR aircraft that are already airborne, and integrate them into their system. These aircraft must continue under VFR flight rules until

11960-471: The extent required to maintain safe operation of the aircraft. Pursuant to requirements of the International Civil Aviation Organization (ICAO), ATC operations are conducted either in the English language, or the local language used by the station on the ground. In practice, the native language for a region is used; however, English must be used upon request. In 1920, Croydon Airport near London, England,

12090-523: The following provides a general concept of the delegation of responsibilities within the air traffic control 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 (sometimes known as ground movement control , GMC)

12220-407: The frequencies above the carrier. Thus the upper and lower sidebands are summed. If there is a phase shift between these two, then the combination will have a relative amplitude of (1 + cos φ). If φ was 180°, then the aircraft's receiver would not detect any sub-carrier (signal A3). "Blending" describes the process by which a sideband signal is switched from one antenna to the next. The switching

12350-432: The geographic location of airborne instrument flight rules (IFR) air traffic anywhere in the FAA air traffic system. Positions are reported for both commercial and general aviation traffic. The programmes can overlay air traffic with a wide selection of maps such as, geo-political boundaries, air traffic control centre boundaries, high altitude jet routes, satellite cloud and radar imagery. The day-to-day problems faced by

12480-427: 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. En-route air traffic controllers work in facilities called air traffic control centres, each of which is commonly referred to as a 'centre'. The United States uses

12610-421: 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 is to prevent collisions, organize and expedite the flow of traffic in the air, and provide information and other support for pilots. Personnel of air traffic control monitor aircraft location in their assigned airspace by radar , and communicate with

12740-403: The horizon—or closer if mountains intervene. Although the modern solid state transmitting equipment requires much less maintenance than the older units, an extensive network of stations, needed to provide reasonable coverage along main air routes, is a significant cost in operating current airway systems. Typically, a VOR station's identifier represents a nearby town, city or airport. For example,

12870-408: 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 Federal Aviation Administration (FAA) has spent over US$ 3 billion on software, but a fully automated system is still yet to be achieved. In 2002, the United Kingdom commissioned

13000-559: The infrastructure for a radar system (e.g., over water). Computerised radar displays are now being designed to accept ADS-C inputs as part of their display. 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

13130-424: The last three 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'. 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

13260-517: The manoeuvring area (taxiways and runways). 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 also exist at extremely busy airports. While each tower may have unique airport-specific procedures, such as multiple teams of controllers ( crews ) at major or complex airports with multiple runways,

13390-461: The movement of aircraft between departure and destination, was opened in Newark in 1935, followed in 1936 by Chicago and Cleveland. Currently in the US, the Federal Aviation Administration (FAA) operates 22 Air Route Traffic Control Centers . After the 1956 Grand Canyon mid-air collision , killing all 128 on board, the FAA was given the air-traffic responsibility in the United States in 1958, and this

13520-399: The need for some of the expensive ground-based VORs. In many countries there are two separate systems of airway at lower and higher levels: the lower Airways (known in the US as Victor Airways ) and Upper Air Routes (known in the US as Jet routes ). Most aircraft equipped for instrument flight (IFR) have at least two VOR receivers. As well as providing a backup to the primary receiver,

13650-521: The new airport in Istanbul, which opened in April, but the extra capacity will be absorbed by rising demand for air travel. 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

13780-426: The older range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radiobeacons ( NDBs ). A worldwide land-based network of "air highways", known in the US as Victor airways (below 18,000 ft or 5,500 m) and "jet routes" (at and above 18,000 feet), was set up linking VORs. An aircraft can follow a specific path from station to station by tuning into

13910-438: The overall capacity for any given route. The North Atlantic Track system is a notable example of this method. 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 newer technology reverses the radar concept. Instead of radar 'finding'

14040-482: 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 a 'talk-down'. 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,

14170-515: The position directly ( radar control , also known as positive control). Pilots flying over an ocean can determine their own positions accurately using the Global Positioning System or other means, and can supply periodic updates to a center. A center's control service for an oceanic flight information region may be operationally distinct from its service for one over land, employing different communications frequencies, controllers, and

14300-426: The position of the aircraft, as in earlier radio direction finding (RDF) systems. VOR stations are short range navigation aids limited to the radio- line-of-sight (RLOS) between transmitter and receiver in an aircraft. Depending on the site elevation of the VOR and altitude of the aircraft Designated Operational Coverages (DOC) of at max. about 200 nautical miles (370 kilometres) can be achieved. The prerequesite

14430-544: The presence of traffic and conditions that lead to loss of minimum separation. 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

14560-508: The reduced number of VOR ground stations provided by the VOR Minimum Operational Network. VOR and the older NDB stations were traditionally used as intersections along airways . A typical airway will hop from station to station in straight lines. When flying in a commercial airliner , an observer will notice that the aircraft flies in straight lines occasionally broken by a turn to a new course. These turns are often made as

14690-437: The revolution radius R = F d C / (2 π F n F c ) is 6.76 ± 0.3 m. The transmitter acceleration 4 π F n R (24,000 g) makes mechanical revolution impractical, and halves ( gravitational redshift ) the frequency change ratio compared to transmitters in free-fall. The mathematics to describe the operation of a DVOR is far more complex than indicated above. The reference to "electronically rotated"

14820-400: The same destination so that when the aircraft are close to their destination they are sequenced. As an aircraft reaches the boundary of a centre's control area, it is 'handed off' or 'handed over' to the next area control centre . 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

14950-542: The same two-letter call signs. Due to the larger number of new airlines after deregulation, the ICAO established the three-letter call signs as mentioned above. The IATA call signs are currently used in aerodromes on the announcement tables, but are no longer used in air traffic control. For example, AA is the IATA call sign for American Airlines ; the ATC equivalent is AAL. Flight numbers in regular commercial flights are designated by

15080-418: The second receiver allows the pilot to easily follow a radial to or from one VOR station while watching the second receiver to see when a certain radial from another VOR station is crossed, allowing the aircraft's exact position at that moment to be determined, and giving the pilot the option of changing to the new radial if they wish. As of 2008 , space-based Global Navigation Satellite Systems (GNSS) such as

15210-404: The station identifier, i ( t ) , optional voice, a ( t ) , navigation variable signal in c ( t ) , and the isotropic (i.e. omnidirectional) component. The navigation variable signal is A3 modulated (greyscale). The navigation reference signal is delayed, t + , t − , by electrically revolving a pair of transmitters. The cyclic doppler blue shift, and corresponding doppler red shift, as

15340-739: The successive stations on the VOR receiver, and then either following the desired course on a Radio Magnetic Indicator, or setting it on a course deviation indicator (CDI) or a horizontal situation indicator (HSI, a more sophisticated version of the VOR indicator) and keeping a course pointer centered on the display. As of 2005, due to advances in technology, many airports are replacing VOR and NDB approaches with RNAV (GNSS) approach procedures; however, receiver and data update costs are still significant enough that many small general aviation aircraft are not equipped with GNSS equipment certified for primary navigation or approaches. VOR signals provide considerably greater accuracy and reliability than NDBs due to

15470-418: The taxiways and runways of the airport itself, and aircraft in the air near the airport, generally 5 to 10 nautical miles (9 to 19 kilometres ; 6 to 12 miles ), depending on the airport procedures. A controller must carry out the job using the precise and effective application of rules and procedures; however, they need flexible adjustments according to differing circumstances, often under time pressure. In

15600-418: The thin corridors open to airliners. The United Kingdom closes its military airspace only during military exercises. 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 call signs with

15730-452: 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

15860-432: 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

15990-429: The transverse direction the two signals will sum, but in the tangential direction they will cancel. Thus as the signal "moves" from one antenna to the next, the distortion in the antenna pattern will increase and then decrease. The peak distortion occurs at the midpoint. This creates a half-sinusoidal 1500 Hz amplitude distortion in the case of a 50 antenna system, (1,440 Hz in a 48 antenna system). This distortion

16120-557: 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 reduces

16250-492: The written 'BAW832'. This is used to reduce the chance of confusion between ATC and the aircraft. By default, the call sign for any other flight is the registration number (or tail number in US parlance) of the aircraft, such as 'N12345', 'C-GABC', or 'EC-IZD'. The short radio-telephony call signs for these tail numbers is the last three letters using the NATO phonetic alphabet (e.g. ABC, spoken alpha-bravo-charlie for C-GABC), or

16380-506: Was awarded 28 December 1981. Developed from earlier Visual Aural Radio Range (VAR) systems. The VOR development was part of a U.S. civil/military program for Aeronautical Navigation Aids. In 1949 VOR for the azimuth/bearing of an aircraft to/from a VOR installation and UHF DME (1950) and the first ICAO Distance Measuring Equipment standard, were put in operation by the U.S. CAA (Civil Aeronautics Administration). In 1950 ICAO standardized VOR and DME (1950) in Annex 10 ed.1. The VOR

16510-503: Was created in 1922, after World War I, when the U.S. Post Office began using techniques developed by the U.S. 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

16640-468: Was designed to provide 360 courses to and from the station, selectable by the pilot. Early vacuum tube transmitters with mechanically rotated antennas were widely installed in the 1950s, and began to be replaced with fully solid-state units in the early 1960s. DVOR were gradually implemented They became the major radio navigation system in the 1960s, when they took over from the older radio beacon and four-course (low/medium frequency range) system . Some of

16770-670: 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 European Union (EU) aimed to create

16900-415: Was the first airport in the world to introduce air traffic control. The 'aerodrome control tower' was a wooden hut 15 feet (5 metres) high with windows on all four sides. It was commissioned on 25 February 1920, and provided basic traffic, weather, and location information to pilots. In the United States, air traffic control developed three divisions. The first of several air mail radio stations (AMRS)

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