Airborne Separation Assurance

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Airborne Separation Assurance

37TH ANNUAL CONFERENCE, Toulouse, France, 30 March – 3 April 1998

WP No. 84

Airborne Separation Assurance


The concept of Airborne Separation Assurance has been discussed in detail ever since technology realistically promised to deliver practical Airborne Collision Avoidance Systems (ACAS). As early as 1977 ACAS (or at least its predecessors) was seen by the FAA as having the potential to provide a significant part of an integrated aircraft separation assurance system. The concept of ‘Distributed Management’ of air traffic or the apportionment of the responsibility for aircraft separation between the ground and the cockpit, has been part of the FAA lexicon since the 1960’s.

Within a few years the concept of providing at least part of the aircraft separation assurance system in the cockpit was gaining momentum. In 1995, a working paper was presented to a working group of the ICAO SSR Improvements and Collision Avoidance Systems Panel (SICASP) which explored the ASAS concept in some detail for the first time.

This paper will examine the technical aspects of ASAS and CDTI from the viewpoint of the Air Traffic Management system in general and the Air Traffic Controller in particular.


Collision Avoidance – was defined by Lester and Quan as – ‘the immediate evasive action taken by a pilot to avoid imminent collision with another aircraft’. TCAS provides a mechanism which initiates such action using miss distance parameters which are of necessity much finer than any acceptable ATC separation standard.

Separation Assurance – Has no single definition but is frequently taken to refer to the design and application of airspace and procedures that actively maintain the appropriate and predefined margins of separation around, above and below each aircraft. Transport Canada’s Manual of Air Traffic Services is written to define the correct application of separation standards as requiring:

  • Planning to ensure separation;
  • Execution of the plan so as to achieve separation;
  • Monitoring of the situation to ensure that the plan and execution are effective.

Alternative definitions of separation assurance concentrate on the provision of systems such as conflict alert and ACAS used to provide backup to the controller and pilot in the performance of the aircraft separation assurance function. See and Avoid is cited in these cases as the simplest form of separation assurance. It is this later definition which has given substance to the ASAS concept.

Situational Awareness – A human factors concept central to the performance of Air Traffic Control as well as many other ‘mission critical’ functions. It is often referred to by controllers as ‘the picture’. A more precise definition was framed by Dr Mica Endsley and quoted in the Controller 1/97 edition:

“… The perception of the elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future.”

Airborne Separation Assurance System – The ICAO SSR Improvement and Collision Avoidance Systems Panel (SICASP) has used the following partial definition of ASAS:

“The equipment, communications protocols, airborne surveillance and other aircraft state data, flight crew and Air Traffic Control procedures which permits the pilot to exercise responsibility, in agreed and appropriate circumstances, for separation of his aircraft from one or more other aircraft.”

Casaux elaborates the ASAS concept by identifying two phases of evolution:

Phase 1:The responsibility for aircraft separation remains on the ground. New ATC clearances are created to allow an aircraft to base its flight path in some way on that of a conflicting aircraft in a similar manner to that employed currently in VMC in terminal areas.

Phase 2: The responsibility for maintaining aircraft separation will be split between ATC and the aircraft along the following lines:

Cockpit Display of Traffic Information – A method of providing flight crews with the means to improve there situational awareness in respect of other air traffic. The simplest definition of CDTI to hand is ‘When traffic information is added to an electronic horizontal situation indicator (HSI)’. It is worth noting that some of the current TCAS systems use the HSI to display traffic information. Others utilise the rate of climb and descent indicator for this purpose.

Free Flight – There are two notable definitions of Free Flight. In the U.S. the RTCA Task Force 3 defines Free Flight as:

A safe and efficient flight operating capability under IFR in which the operators have the freedom to select their path and speed in real time. Air traffic restrictions are only imposed to ensure separation, to preclude exceeding airport capacity, to prevent unauthorised flight through special use airspace and to ensure safety of flight.

Of more relevance to the current discussion is the definition employed in the Eurocontrol EATMS Operational Concept Document:

Airspace without any fixed route structures in which suitably equipped aircraft will be able to fly user preferred 3D or 4D routings and may take responsibility for their own separation.


Why ASAS? – Undoubtedly the major aim of the ASAS concept is to reduce required separation standards and as a result increase the capacity of airspace and progress towards the ideal of Free Flight. Whether or not ASAS, even in concept has this potential is questionable.

Why CDTI? – The Cockpit Display of traffic Information has been seen since before TCAS was developed as being the method by which flight crew could be given a degree of situational awareness sufficient to facilitate self separation from other aircraft in certain limited circumstances. Current CDTI implementations are, for civilian applications, limited to the displays utilised by TCAS II systems. These are, as previously mentioned, of the HSI (Nav Display) type or utilise the Rate of Climb and Descent Indicator to provide a rudimentary display of proximate traffic.

Inherent Limitations of CDTI – A 1980 Human Factors Journal article gives some interesting insight into the particular problems associated with developing a useful situational awareness from a CDTI. This article draws on an earlier work vi to make the following important point:

“The CDTI display is perceptually a more complicated display than those used by air traffic controllers. On an ATC display the map is fixed and all of the aircraft symbols move with respect to the fixed background. With a CDTI, pilots view a display having a continually moving background as well as moving aircraft. A parallel may be drawn between the perceptual problems found with conventional ship board radar and potential CDTI problems. Perceptual errors caused by misleading apparent motion of other ships due to ownership’s rotation have occasionally led to what are referred to as ‘radar assisted collisions”

The same paper deals in some detail with experiments to examine the effectiveness of various display symbologies and aids that could be employed on a CDTI. In particular the use of ‘predictor’ lines to indicate the future position of the target aircraft. The paper arrives at a general conclusion that:

“…this experimental task proved to be more difficult than the authors expected. Without predictors, pilots could not accurately predict the relative position of the intruder aircraft. If either aircraft was turning, pilots could not make accurate judgements unless the predictor showed the future effect of each aircraft’s current turn rate.”

In all of the material to hand on CDTI there is no importance placed on the fact that in all cases of development described, predictor information is derived solely from the target aircraft’s current velocity. As any Air Traffic Controller will attest, proper situational awareness comes from a comprehensive knowledge of aircraft intent (by way of flight plan or clearance) as well as observations of current course, speed and altitude.

Knowledge of Aircraft Intent – The Importance a knowledge of aircraft intent is supported in work that deals more generally with the concept of ASAS rather than any particular technology or application. Smith Billings et al refer to the sharing of intent information as being critical if free flight is to be safely supported with acceptably small separation distances.

Even this work sees only the intent of aircraft in the immediate vicinity as being of relevance to the ASAS concept. The ATC role of using proactive intervention to achieve optimum flight trajectories is foreseen as diminishing to a system monitoring role with intervention only on a reactive basis described in the paper as ‘management by exception’.

Candidate Technologies – There is little doubt that current communications and display technology has the ability to provide a comprehensive Air Situation Display, however derived, in the cockpit. The development of automated traffic management systems is on the other hand still very much in its infancy. Even the most modern ATC automation systems have trouble implementing an effective error free conflict alert function. Automated conflict resolution is still very much a pipe dream.

ACAS however, enjoys a degree of success and general acceptance. In spite of its rudimentary ability to provide some cockpit display of traffic information ACAS is still very much the last line of defence as far as separation assurance is concerned.

Two related technologies present as having the greatest potential for development as the backbone of future ASAS. Both of these rely to some extent on Mode S transponder equipment:

TCAS – Relies on the airborne interrogation of the transponder of proximate aircraft. If both aircraft are fitted with Mode S transponders then co-ordinated avoidance manoeuvres are possible based on the threat perception of one or both aircraft in a conflicting pair.

TCAS has limited surveillance integrity and was designed as an implementation of the collision avoidance function rather the separation assurance function. Nevertheless, the FAA in particular see TCAS as having a potential to facilitate the implementation of at least some ASAS functions.

Avoidance action specified by TCAS is confined to manoeuvres in the vertical plane. This limitation is likely to remain as long as the current technology is employed.

ADS Broadcast (ADS – B) – This system uses the Mode S transponder background response signal (SQUITTER) to broadcast position information derived from aircraft navigation systems such as GPS. ADS B is seen by some as the cornerstone of any future CNS/ATM/Free Flight environment since it appears to offer a relatively low cost surveillance solution. Since the SQUITTER reply contains aircraft identification information ADS B has the potential to provide a better standard of situational awareness via a CDTI than is currently possible with TCAS. It does not however provide the comprehensive information on aircraft intent that is provided within a conventional ground based ATC system or more recently in a standard ADS environment.

It is worth noting that ADS B is also the leading candidate technology for ACAS II/TCAS IV. Thus we are likely to see a reduction in discrimination between the Separation Assurance and the Collision Avoidance function. Rockwell Collins have said that its latest TCAS offering (TCAS 4000) doubles the surveillance range of existing systems (to 100nm) and can be expanded to include ADS-B.

Target Applications – No agency has yet seriously suggested that all forms of separation can be achieved effectively from the cockpit using current or developing technology. Instead a series of target applications are being progressively identified. For example:

En-Route Airspace

Reduction in oceanic separation standards – Love & McFarland proposed a number of methods for reducing separations using CDTI. Their preferred strategy was a means of implementing reduced vertical separation minima.

In Trail Climb – The FAA In Trail Climb program proposes the use of TCAS to allow a following aircraft to monitor its own separation with preceding traffic during a climb to a higher level.

Terminal Area

Parallel approach station keeping – ADS B is seen as a cost effective alternative for the E Scan Parallel Runway approach Monitor (PRM). PRM is under development to allow independent instrument approaches to medium spaced parallel runways.

Self separation during approach sequencing – This is simply an extension of the existing procedure in which an Air Traffic Controller may require a pilot in VMC to base his flight path on that of another aircraft with which he has and can be expected to maintain, visual contact.

Uncontrolled (Class F, G) Airspace

As an adjunct to existing collision avoidance systems an enhanced CDTI is seen by some administrations as a means to provide an enhanced level of safety in uncontrolled airspace thus deferring the need for a higher level of Air Traffic Service.

ICAO SSR Improvements and Collision Avoidance Systems Panel (SICASP) – In its work on ASAS Applications Development Framework and Approval Guidelines SICASP makes some significant points pertinent to this discussion, for example:

“It is important that an ASAS should not be viewed solely as an improved ACAS even when it is based on the enhanced ACAS surveillance proposed for ACAS III. It is essential to retain the integrity of the ACAS safety Benefit”

SICASP follows the previously highlighted approach to ASAS in that it sees development proceeding with specific applications rather than a system wide approach as would be required to further the European version of Free Flight. It details several generic areas to be addressed in the assessment of the implications of each application.

These include:

  • Precise definition of the application;
  • Requirements for surveillance:
    • Range and accuracy;
    • Data content;
    • Is there a need for data exchange or interface with ATC.
  • Requirements for the cockpit display:
    • Display characteristics including range and resolution and label symbology;
    • Display control and aural indications.
  • Failure mode selection and indications;
  • Is the airborne data required to be downlinked to ATC?
  • Should ATC be able to initiate, terminate or modify the application selection;
  • ACAS Collision Avoidance Function integrity:
    • The impact on the Collision Avoidance function must be understood.
  • Operational Procedures:
    • Each application will require a clear statement regarding the proposed division of responsibility for separation between the Air Traffic Controller and the Flight Crew.

IFALPA – The International Federation of Airline Pilots Associations has expressed considerable opposition to the conceptual development of ASAS for a number of reasons chiefly based on workload, legal issues and the traditional demarcations between the pilot and ATC professions.


Accepting the fact that a fully automated Air Traffic Management System could possibly be developed in the future, The question then arising is should such a system be operated from the ground or from the cockpit.

In considering that question it is important to have a clear picture of the essential elements of any separation assurance system be it airborne or on the ground. For the purposes of this discussion, these could be listed simply as follows:

Identification – Separation assurance cannot be achieved unless the systems supporting it provide the operator (pilot or ATC) with a guaranteed method of positively identifying relevant traffic.

Knowledge of intent – Separation Assurance depends, in the established definition of the term, on planning. Planning effectively, in the Air Traffic Control sense, requires a comprehensive knowledge of the intent of all of the traffic in a particular system. This knowledge comes from an awareness of flight plan and clearance details.

Neither of these essential elements are given proper importance in any of the available material on ASAS.

We have then a fundamental difference between the classical definitions of separation assurance and what is developing as ASAS.

It is however arguable that many of the target applications being investigated and developed as part of the evolution of the ASAS concept have a potential to improve the safety and productivity of ATC systems.

IFATCA has a need to develop policy on the development and use of these applications in order to provide guidance to our representatives on the various standard setting forums. That policy should be based on a clear set of minimum requirements necessary for the transfer of specific separation responsibility from the ground to the cockpit.

Comprehensive policy needs to be developed (by SC1V & SC VII) on the transfer of separation responsibility from ATC to the cockpit. This policy should take account of developing technology that will theoretically extend the feasibility of such transfer beyond the current visual limits.


Report of the FAA Task Force on Aircraft Separation Assurance – January 1977 FAA EM- 78- 19,1.

U.S. DoT – System 4 study, Distributed Air Traffic Control – Report to the ATC Advisory Committee – June 1969.

Airborne Separation Assurance System – SICASP WG meeting Sydney 1995 – Francis Casaux – DGAC France.

The Cockpit Display of Traffic Information and the Threat Alert and Collision Avoidance System Integration: A review – P.T. Lester and E.E. Quan, Jr. 1984.

Perception of Horizontal Aircraft Separation on a Cockpit Display of Traffic Information – Palmer, Jago, Baty & O’Connor – Human Factors Society Journal 1980, 22(5), 605 – 620.

Will the ATSD precipitate display assisted collisions – R.E. Curry – Lincoln Labs report #PB- 215-714 – 1972.

The Shape of the Future Air Traffic Management System – P. Smith, C. Billings, D. Woods, E. McCoy, N. Sarter, N. Denning and S. Dekker – 9th International Symposium on International Psychology April 1997 Reproduced in The Controller 3/97.

Oceanic In Trail Climb Using TCAS II – Program Plan – FAA Office of Airspace Capacity and Requirements and R & D service – 1994.

ATC Market Report 30/10/97.

Reduction in Oceanic Separation Standards Through the Use of a TCAS derived CDTI – Love & McFarland – The MITRE Corporation CH2359-8/86/0000-0294 – 1986 IEEE.

Self Separation in Terminal Areas Using CDTI – D.H. Williams. – Proceedings of the HUMAN FACTORS SOCIETY – 27th Annual Meeting 1983.

ICAO PANS-RAC, Doc 4444 Appendix C 13 & 14.2.

ICAO ASAS Applications Development Framework and Approval Guidelines. Draft 4 – Aug 1996.

Last Update: September 28, 2020  

March 5, 2020   691   Jean-Francois Lepage    1998    

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