Communication Between ATS Units

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Communication Between ATS Units

53RD ANNUAL CONFERENCE, Gran Canaria, Spain, 5-9 May 2014

WP No. 91

Communication Between ATS Units

Presented by TOC


Different ways of communication are possible between adjacent ATS Units. Purpose of the paper is to study different methods of voice communication, data exchange and coordination between neighboring ATS units and to review the state of the question in different regions as well as conflicts that may arise related to this subject.

This paper proposes new policy in regards to ground to ground communication.


1.1.  To maintain an orderly, efficient and safe flow of air traffic it is essential to have verified and exact data about each and every flight operating inside controlled airspace.

1.2.  The coordination between adjacent ATC units should therefore be clear, unambiguous, fast and reliable. Rules on how and what is the subject of coordination between ATC units are to be laid down by individual units in accordance with procedures prescribed by local and global organizations. Also, the amount of information contained in the flight plan has increased to reflect advances in technology.

1.3.  Diversity in regard to technology used for exchange of flight data is spreading from simple ground telephone lines to state of the art data communications and network systems for information sharing.

1.4.  Despite the diversity in possibilities for exchange, there are still places in which there isn’t any or only minor coordination at all. This could be due to political matters, technical difficulties, economic reasons, etc.

1.5.  Since sophisticated equipment is not available for every ATC unit, the focus here will not be on the state of the art systems that are used in some parts of the world, but on the basic means which one can communicate with and examples of incidents concerning coordination.


2.1. The first recorded transfer of flight data dates back to 1920 when the first rules were established about mutual exchange of weather information between aerodromes and using separated inbound and outbound tracks over the English Channel.

2.1.1 From then on both the amount and the quality of data transferred and the technical means used for the coordination have made an enormous progress. However, there are still many areas in the world where coordination problems occur.

2.2 In environments where automated exchange of flight data is not available, there are a number of other methods to improve the accuracy of coordination exchange.

2.2.1  Reliable communications links, such as dedicated telephone lines can assist with timely coordination by voice between units. Where dedicated lines are not practicable, using domestic or international telephone systems is an option. In this case, it is preferable to have a communication system configured for automatic dialling, and to have backup phone numbers for the adjacent centre in the case of network congestion or an engaged line.

2.2.2  High levels of controller workload sometimes contribute to not coordinating in a timely fashion, or neglecting to coordinate changes to essential flight information. In these cases a coordinator, or planner controller can be tasked with acquitting coordination. Additionally, supervisors should be provided with equipment to contact adjacent ATS units, and should assist controllers with coordination when necessary.

2.2.3  Standardised and robust methods of recording and managing flight data within an ATS unit can assist in planning coordination and ensuring that it is completed by the required time. For example, when using paper strips, if they are ordered based on time over the boundary waypoint, it is easy to see which flight is due next for coordination. When using integrated ATS systems with electronic strips, it may assist to set up automated alerts or manually set timer alerts to remind the controller of the need to perform coordination. ATC check and training departments should be responsible for ensuring that appropriate procedures are applied and standardised within an ATS unit.

2.3 ICAO has extensive and detailed provisions regarding transfer of flight data between ATS units. Rules for coordination in air traffic control are prescribed within ICAO PANS-ATM Doc 4444 in chapter 10.1. “Coordination in respect of the provision of Air Traffic Control Service”.

2.3.1 They state that, in order to reduce the need for verbal coordination, ATC units should, to the extent possible, establish and apply standardized procedures for the coordination and transfer of control of flights. Such coordination shall be specified in Letters of Agreement (LoA) and, if applicable, in local instructions. The Letters of Agreement must specify the way a flight is handed over between two adjacent units. Doc 4444, paragraph lists what such LoA ́s and local instructions should include. For example definition of areas of responsibility and common interest, significant points for transfer of control and communications, separation minima, procedures for exchange of flight plan and control data, and SSR code assignment.

2.3.2 Paragraph 6.2 “Aeronautical fixed service (ground-ground communications)”, from ICAO Annex 11 “Air Traffic Services”, sets standards and recommended practices for direct speech and/or data link communications in air traffic services. SARP ́s regarding Communications within a Flight Information Region require that there are appropriate facilities for communication between, for example, ACC, APP and TWR units. Communication facilities to other units, like appropriate military units and meteorological office are also required. In regard to communications between FIR ́s, Annex 11 states: Flight information centers and area control centers shall have facilities for communications with all adjacent flight information centers and area control centers Examples of system-automated coordination are ATS interfacility data communication (AIDC) and On-line Data Interchange (OLDI). The global air navigation plan includes several concepts to improve the use of automation in this area. By these methods, flight plan data are exchanged between the two automated systems of the concerned ATC units. Human intervention is only required when specific conditions need to be agreed for a particular flight (i.e. non-standard flight levels, separation to be agreed between two particular flights, etc.) and when the automatic data exchange has failed. ICAO Doc 9694 – “Manual of Air Traffic Services Data Link Applications” includes, besides provisions on data link communications between air and ground, a chapter and a detailed part on AIDC. AIDC is a data link application that exchanges information including notification of flights approaching a FIR boundary (notify phase), coordination or boundary- crossing conditions (coordinate phase) and transfer of control and communications (transfer phase). The data communication can also include negotiation on conditions of transfer and exchange of free text messages.


2.4.1.  IFATCA currently has no policy regarding issues communication between ATS Units.

2.4.2.  TOC considers ground-to-ground communication as important as ground-to-air communication. Coordination between ATS Units should be fast and reliable. Automated coordination is preferred but other means of communications might be more appropriate in some cases.

2.4.3.  There are cases where diplomatic issues harm the establishment of a proper way of coordination. IFATCA believes these issues should have no detrimental impact on the provision of air traffic services.

2.5. Local problems and applications

2.5.1. Oceanic One of the sectors where coordination problems occur is the Australian Oceanic sector, which has boundaries with Jakarta, Colombo, Maldives, Mauritius and Johannesburg. Anecdotally, controllers in oceanic control areas indicate coordination errors are generally related to incorrect flight level, incorrect boundary time, or failure to provide coordination information in a timely manner. Some regions report significant errors multiple times per week. Unfortunately accurate statistics are hard to get and generally unreliable, but there is a reasonable amount of incidents regarding incorrect flight level. This is a significant problem as one of the major boundary waypoints has 3 bi-directional airways converging on it. Late/No coordination is rare, and when it occurs, it’s usually only 2-3 minutes late – this is not a significant problem when the coordination parameter is 30 minutes. There are some ways to address this, like using departure times to determine boundary times, or getting data link logons. Usually there are a number of contributing factors/causes for the above:

  • Workload – Controllers were managing too much traffic, this may be due to staffing levels or inadequate sectorisation.
  • Equipment – The system has little or no automation to provide alerts and support decision making.
  • Screen display – often doesn’t display the whole airspace, and the system was very inflexible in reconfiguring the display. As a result, sometimes aircraft symbols would just leave the screen and be forgotten about.
  • Poor procedures – each controller is using a different technique to sort and annotate their strips, sometimes information on the (paper) strips and the data on the CWP isn’t the same, which caused confusion and may also have been a cause of incorrect coordinated level.
  • Communication links – the lines between international units are often unreliable or of poor quality. It is easy to mishear coordination. International phone lines take a long time to connect.

2.5.2 Africa  Africa is a developing part of the world with many countries. Issues such as a poor communication infrastructure, language differences and conflicts causing diplomatic issues between neighboring countries, affect ATC coordination.
The obtainment of equipment is often limited or uncoordinated so new equipment cannot link to neighboring countries.  Most of the communication infrastructure is provided by external service providers who fail to provide reliable communication links and are affected by the general economic environment. LoA’s are nonexistent in conflict areas. In areas where LoA’s do apply, they are often not reviewed enough to address any shortcomings. As a result a significant number of incidents in Africa are related to coordination.  An example of an incident caused by a lack of coordination is in the area at the borders of Zimbabwe, Zambia, Botswana and Namibia. The airports of respectively Victoria Falls, Livingstone, Kasane and Katima Mulilo are close to each other; the first 3 are within a 30nm radius of each other, while Katima Mulilo is about 20 minutes away. There are no direct telephone lines and a VHF link was put in place as alternative means of communication. However due to terrain, the VHF link sometimes fails. A light aircraft departed from Victoria Falls to Kasane. Due to lack of communication possibilities the pilot was requested to contact Kasane and forward his estimate on their frequency. However, while still relaying estimates to Kasane, the pilot had to make an emergency landing in Zimbabwe. Kasane tower didn’t have any possibilities to contact Victoria Falls tower, who lost contact with the aircraft, about the emergency so the rescue vehicles that Kasane dispatched were held up at the border, by the Zimbabwean border authorities who were not aware of the emergency. Coordination incidents have been noted to increase significantly when new procedures are introduced. These procedures are often introduced without any simulation for controllers beforehand or with very little coordination with neighboring ANSPs. Better-prepared introductions of new procedures or the use of any type of communication mentioned below, would reduce the number of incidents.

2.5.3. Europe  Although European ATS Units often have suitable equipment for data distribution, there are examples from both the past and nowadays where a lack of coordination caused incidents.  The airports of Berlin Tempelhof and Berlin Schonefeld are situated only 6nm from each other. During the Cold War, Tempelhof, being in West-Berlin and Schonefeld in East-Berlin, had no coordination or ways of communicating at all. However Tempelhof traffic would sometimes cross Schonefeld final. The only way for Schonefeld to know about the traffic was seeing it on radar, if radar was available. This resulted in several incidents.  Since 1974, several parties are trying to find a solution for the political issues in Cyprus. Eurocontrol and ICAO have knowledge about the problems and ICAO doesn’t recognize Turkish side as a country. The UN, Northern Cyprus Turkish side and Cyprus are trying to solve it. These issues also affect Air Traffic Control. In order to minimize the problem, the Cyprus Civil Aviation Authority has issued instructions to aircraft operators so that when an aircraft is flying over Turkey on a route that will bring it over Cyprus, the pilot must call Nicosia ACC 10 minutes prior to entering Cyprus FIR, giving details of its flight.  In the Uberlingen accident in 2002, an air traffic controller from an adjacent ATC unit, who saw the upcoming crash, was unable to reach the air traffic controller working the two colliding aircraft, because the phone line was not working due to maintenance. Other means of communication, which could serve as a backup, where not available.

2.5.4. North America The 3 geographically largest countries in North America, Canada, the USA and Mexico used the North American Aviation Trilateral to establish a process for development of a seamless interface between automation systems, focusing on the automated exchange of ICAO flight data. Meetings began in 1998, and the first interconnection occurred in 2005 between the USA and Mexico. By 2009, the Canadian, US, and Mexican operational automation flight interfaces were complete. In 2011, Havana successfully implemented the automation Interface with Miami to expand the interchange into the Caribbean and Gulf of Mexico.

2.6  While nations with fluid diplomatic relations and similar technical level can easily agree to establish common goals for the fluidity of air traffic across their common boundary, the situation can be much more difficult between two countries maintaining some disputes. In such a case even the drafting of a LoA can be a complicated issue. However in the US/Cuba situation, there is still an economic barrier, but the automation interface was established between the countries in a collaborative way.

2.7  Within a country there could also be a problem with communication between adjacent centers, for example between military and civil units. This could be due to technology that does not match, but also for confidential or security reasons.

2.8  It seems clear that the preferred order for methods of coordination should be: automated coordination, phone coordination and finally self-transfer of flight data. However some times voice may be preferred, e.g. weather diversions or emergencies. The proper way has to be established in the corresponding LoA in view of the technical and operational factors of the particular units concerned.

2.9 Positive intervention

2.9.1  The potential causes and negative consequences of coordination errors have been noted, however it is also important to note that good coordination procedures and facilities achieve more than simply reducing ‘errors’.

2.9.2  Where facilities, procedures and the working relationship between ATS units is such that accurate flight information is transferred readily and comprehensively, it helps to create a consistent and seamless level of service for operators. Where pilot requests and intentions are able to be transferred to the next ATS unit, there is a greater level of predictability and reduced workload for pilots. Efficiency can be increased by more readily coordinating and granting pilot requests.

2.9.3  Potentially unsafe situations are sometimes detected by a controller in an adjacent ATS unit. ATCOs are well aware of their professional duty to bring unsafe situations immediately to the attention of their colleagues. Between ATSUs this is only possible with a reliable means of communication. For example: a controller, either by use of radar or other available flight plan data, may detect a conflict in adjacent airspace that appears to be unresolved. The controller that detects the conflict can call the adjacent controller to advise them of the problem in order for it to be


3.1.  Accurate and timely flight information is essential to maintain safe air traffic control. Incorrect or insufficient coordination can lead to safety related events.

3.2.  The coordination between different ATS units should be clear, unambiguous, fast and reliable in order to have verified and exact data about all flights operating inside the airspace.

3.3.  A Letter of Agreement is most widely used for prescribing the conditions for the transfer of operationally valuable data between ATS units concerned.

3.4.  Automated coordination is preferred. However, in some circumstances it may be more appropriate to use another means of coordination.

3.5.  ICAO has a number of provisions regarding the exchange of flight data between ATS units. In addition to detailed definition of proper facilities and procedures for basic coordination, the application of data link coordination (AIDC) has been described in ICAO manuals. AIDC is only possible when automation is in place in both ATS units.

3.6.  There are cases where diplomatic issues harm the establishment of a proper way of coordination. IFATCA believes these issues should have no detrimental impact on the provision of air traffic services.


4.1 IFATCA policy is:

Ground-to-ground communication is as important as air-to-ground communication.

and is included in the IFATCA Technical and Professional Manual.


ICAO Doc 4444 – Air Traffic Management.

ICAO Annex 11 – Air Traffic Services.

ICAO Doc 9694 – Manual of Air Traffic Services Data Link Applications.

North American (NAM) Common Coordination Interface Control Document (ICD) – January 2012.

Air Traffic Management Circular 12/2011, AIDC – Airports Authority of India.

Last Update: September 30, 2020  

May 6, 2020   1246   Jean-Francois Lepage    2014    

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