Monitor the Developments in the Operational use of ASAS

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Monitor the Developments in the Operational use of ASAS

42ND ANNUAL CONFERENCE, Buenos Aires, Argentina, 17-21 March 2003

WP No. 93

Monitor the Developments in the Operational use of ASAS

Presented by SC1


1.1  WP 78 presented in Committee B at IFATCA’s 41st Annual Conference, Report of Separation and Airspace Safety Panel (SASP), identified the need to monitor developments on Airborne Separation Assurance Systems (ASAS) and as such was put on the work programme of SC1.

1.2  ICAO’s Surveillance and Conflict Resolution System Panel (SCRSP), the successor to SICASP, was given the following task on ASAS:

Develop, as necessary, ICAO provisions to meet the operational and technical requirements for an airborne separation assurance system (ASAS).

1.3  IFATCA has developed several policies and provisional policies on ASAS in respect of “Transfer of control functions – Legal aspects”, “Transfer of separation function – Human factors aspects” and “ASAS & CDTI”.

1.4  SC1 has presented information material on ASAS at the Marrakech 2000 and Geneva 2001 conferences.


2.1  To fulfil the task as referenced in para 1.2, SCRSP WGA has developed a draft ICAO ASAS circular with the main objective:

“to inform the aviation community about existing activities related to Airborne Situational Awareness and Airborne Spacing and Separation Provision, with a view to promoting a common understanding”

“to promote a common understanding in the aviation community of the main issues to be addressed before ASAS applications could be considered for implementation”

2.2  Discussion in this working paper is limited to the draft ASAS circular, version 1.2. The circular addresses the implementation issues that would be required for standardisation through ICAO if ASAS were to be implemented. Although it is expected that ICAO will only publish the ASAS Circular in 2004, it will be presented as an information paper to ANC/11 in September 2003.

2.3  IFATCA had the opportunity to comment the draft ASAS circular through its Panel member in SASP.


2.4  During the development of the document, the ASAS acronym changed from “Airborne Separation Assurance System” to “Airborne Separation Assistance System” and removal of any occurrence of the word “assurance” in the document.

2.5  The document provides a high-level overview and concept of the Airborne Separation Assistance System and identifies the main operational and technical issues related to ASAS.

2.6  Four broad categories of ASAS applications with a precise allocation of responsibility for separation between the ground and the air have been identified :

a) Traffic situational awareness:

No change to current roles and responsibilities.

No significant changes in separation tasks and no change in separation responsibility.

e.g. Enhanced traffic situational awareness on the airport surface, Enhanced traffic situational awareness during flight operations, Enhanced visual acquisition for see-and-avoid for IFR and VFR.

b) Airborne spacing:

Execution of new tasks without any transfer of responsibility.

Flight crews would be given new tasks, but separation provision would remain controller’s responsibility and applicable separation minima would be unchanged.

e.g. Enhanced sequencing and merging operations.

c) Airborne separation:

Tactical transfer of responsibility.

Delegation of separation responsibility and transfer of corresponding separation tasks from the controller to the flight crew, limited to designated aircraft and in time and space. Requirement to define airborne separation standards.

e.g. ASAS crossing procedure.

d) Airborne self-separation (from all traffic):

Strategic transfer of responsibility.

Flight crews are required to separate their flight from all traffic in accordance with airborne separation minima and rules of flight applicable for the airspace occupied.

e.g. Autonomous aircraft operations in dedicated airspace.

2.7  The document clearly states that:

“whenever a controller retains a responsibility for the provision of separation, the airborne separation must be greater than the ATS separation minima, so that the controller is able to monitor the procedure and, if necessary, take corrective action to maintain separation”.

2.8  The document addresses criticality and safety issues and states that safety objectives need to be specified and proven when introducing ASAS. This includes a safety assessment for each individual ASAS application.

2.9  An overview is given on the operational considerations: responsibilities during ASAS operations, ASAS procedural and human factors issues, ASAS and the “party-line”, training considerations and ASAS transitional issues.

2.10  The ASAS circular also gives a brief résumé of the trials and projects related to ASAS and its applications that are in progress in the US (e.g. the Capstone program in Alaska) and in Europe (e.g. Mediterranean Free Flight project).

2.11  Attachment B of the Circular includes a template to be used to define the application and provide information for its technical analysis.


2.12  In general, the document has improved much from the previous version, particularly a better presentation of the operational aspects, including a more precise allocation of the responsibility for separation.

2.13  The document should clearly mention the intent to maintain the independence of ACAS. Although a number of paragraphs highlight the importance of ACAS as an independent safety net, others suggest that ACAS and ASAS could share some components provided the loss of the ASAS functions is not detrimental to the ACAS function.

ACAS is a last-resort safety function and should remain independent of the primary means of separation. Failure in the primary means of separation could be erroneous data, flight crew misinterpreting displayed data followed by an inappropriate course of action.

2.14  The following detailed comments have been communicated to SCRSP :

  • The term “visual separation” is being used throughout the document but this is for IFR flights no longer in line with current ICAO developments (e.g. withdrawal of the ICAO procedure “Clearance to fly maintaining own separation while in visual meteorological conditions above 10.000 ft”), it was proposed to use other terminology. Some States (e.g. UK) do no permit visual separation when radar is available.
  • Responsibility for separation has been addressed and a precise allocation of responsibility for separation between the ground and the air has been identified for the 4 ASAS applications. Yet in an airborne spacing application, an ATC instruction to maintain a given spacing with designated aircraft is considered “a new task” for the flight crew and not a responsibility. Here it is unclear where the legal responsibility lies. This should be more clearly defined.
  • Suggesting that the “new task” streamlines a controller’s task, thus implying a potential capacity increase is premature. The overall responsibility remains with the controller, his/her surveillance and workload is not changing.
  • More attention should be given to what is “dedicated airspace” and when exemptions and mixed-mode operations exist, including failures in-flight. What is envisaged with regard to airspace classification and flight rules?
  • The Airborne Surveillance and Separation Assurance Processing (ASSAP) function is likely to perform correlation and data fusion of surveillance reports. It is assumed that these surveillance data requirements provide positive unambiguous identification and intent information. More attention is required to the “positive identification” of other aircraft. Flight ID and R/T call-sign are very often not the same and could introduce error on the flight-deck if not handled correctly in design and implementation (e.g. is every pilot aware that EIN is called ‘Shamrock’ and vice versa?).
  • “Intent information” would be required for all ASAS applications to make the information on the CDTI meaningful for pilots. Transfer of responsibility from ATC to the flight crew does not only involve separation from aircraft but should also include knowledge of airspace constraints.
  • What is the risk that the air-air crosslink would exacerbate frequency congestion?
  • There will be a limit to the number of new tasks or transfer of responsibilities flight crews can accept. As an example, the reduction of multiple conditional clearances to only one condition can be referenced.
  • There is a limit to the maximum number of aircraft with transferred responsibility a controller can handle when an abrupt return of responsibility would happen due to e.g. technical reasons (outage/degradation of GNSS signal, saturation of Mode S, etc.).
  • In the résumé of the trials and projects related to ASAS, it’s quite astonishing to see that the ITC procedure in US Oceanic airspace is mentioned as an ASAS-like application where this procedure uses ACAS as a separation tool. The Circular clearly states that ACAS is not an option for ASAS (note that IFALPA strongly opposes the use of ITC, ITC is not an approved ICAO procedure (no SARPS) and even contradicts the current ICAO approved radar identification procedures).
  • It was proposed that the operational procedures should also include :
    • the proposed procedure to be applied for transfer of control from one sector or unit to another (i.e. reference the small sectors in the core area of Europe);
    • the objective of the transfer of responsibility e.g. separation responsibility to one or more aircraft. 

2.15  The document does not mention the need for downlinking of ASAS. Since ASAS is constrained in range and time horizons, the ATC ground system should retain the overall airspace picture, therefore an ATC/ASAS interface would be required.

2.16  Consideration should be given with regard to military aircraft controlled by military ATC units. As an example: silent co-ordination often relies on the fact that Air Defence will separate military aircraft from civil traffic with a minimum separation of 2000’ or 5NM. At present the civil pilot is not informed. What will be the requirement under ASAS?


3.1  Compared to previous ASAS documents and compared to the previous versions of this document, the proposed ASAS circular has improved much.

3.2  Shared responsibility and undefined transfer of responsibility is no longer part of the concept. It has been realised that a clear division in responsibility between controller and pilot-in-command is a must. On the other hand giving “new tasks” to the aircrew implies some sort of responsibility. IFATCA will have to monitor carefully the developments with respect to these “new tasks”.

3.3  IFATCA will have to monitor that ACAS, as a last-resort safety function should remain independent of the primary means of separation. How far the mentioned sharing of “some components” has an impact on ACAS independency needs to be investigated.

3.4  Provided the ACAS system remains independent of the ASAS application, the ASAS circular is not contradicting IFATCA policy.


It is recommended that:

4.1 Until the change of the term of “Assurance” to “Assistance” in the acronym ASAS has been ratified by ICAO, IFATCA policy on ASAS should have the acronym expanded to include both terms and to read “Airborne Separation Assurance/Assistance System”.

This change relates to IFATCA policy page 3276, para 7.5. and page 4126, para 2.9.8.


Professional and Technical Manual of IFATCA (2002).

Draft ICAO Airborne Separation Assistance System (ASAS) circular, Version 1.1E, 6 august 2002.

Last Update: September 29, 2020  

March 21, 2020   531   Jean-Francois Lepage    2003    

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