Spacing, Separation and Segregation – Use in Future ATM Systems

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Spacing, Separation and Segregation – Use in Future ATM Systems

47TH ANNUAL CONFERENCE, Arusha, Tanzania, 10-14 March 2008

WP No. 97

Spacing, Separation and Segregation – Use in Future ATM Systems

Presented by TOC


While investigating the terms separation, spacing and segregation the IFATCA Technical and Operations Committee (TOC) identified that the ICAO Doc 9854 Global Air Traffic Management Operational Concept contained a high level of information pertaining to the three terms. TOC decided to review this information separately from the definitions paper and create a separate paper looking specifically at the future use of separation, spacing and segregation. This paper is of particular relevance as IFATCA has recently produced its own Statement on the Future of Air Traffic Management.

This paper intends to provide an overview of the use of the terms as detailed in ICAO Doc 9854 Global Air Traffic Management Operational Concept, highlight where IFATCA may have issue with the ICAO proposed usage and provide a reference point for any future study related to the three terms. As such the paper is presented as information material only.


1.1.  ICAO Doc 9854 Global Air Traffic Management Operational Concept outlines the concepts required to operate the global air traffic system up to and beyond 2025. The changing air traffic environment envisioned by this document has the potential to impact significantly on the established understanding of separation, spacing and segregation.

1.2.  In 2007 IFATCA released the Statement on the Future of Global Air Traffic Management in part as a reaction to ICAO’s concept document. IFATCA’s statement considered the changing environment from more of an air traffic control (ATC) perspective and considered the ICAO Concept document as a whole. This paper looks at the sections of the ICAO concept document specific to separation, spacing and segregation. The intention is to provide Member Associations (MAs) with information and guidance as States devise and apply applications based on such concepts.

1.3.  This working paper will look at each term separately though will consider the relationship between the terms subject to the proposed changes. Throughout the working paper, when the terms ‘ICAO concept document’ or ‘ICAO document’ are used or any bracketed chapter reference are made these refer to the ICAO Doc 9854 Global Air Traffic Management Operational Concept.


2.1. Separation

2.1.1  ICAO’s concept document introduces some significant changes in the understanding of separation particularly its application. These changes facilitate the new ground and airborne tools and concepts available to the air traffic community.

2.1.2  In the ICAO concept there is no stand alone definition for separation. This is deliberate to emphasise the change to a more strategic environment under the proposed Air Traffic Management (ATM) system and to avoid confusion with current applications. The ICAO concept has instead introduced the term conflict management. Conflict management is one of seven concept components required for an ATM system to function and is:

“…to limit, to an acceptable level, the risk of collision between aircraft and hazards”

Conflict is any situation involving aircraft and hazards in which the applicable separation minima may be compromised. Therefore conflict management takes a very strategic view in that a conflict may happen and is not something that is happening.

2.1.3  There are three layers to conflict management:

  • Strategic conflict management
  • Separation Provision
  • Collision avoidance

The three layers of conflict management clearly define where separation lies. Firstly, separation provision is distinct to collision avoidance (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Chapter 2.7.31:

“Collision avoidance is the third layer of conflict management and must activate when the separation mode has been compromised. Collision avoidance is not part of separation provision…”

Separation provision is the second layer of conflict management and represents the tactical application of separation. Above this strategic conflict management represents separation in the strategic sense. Therefore a change in separation terminology is required.

2.1.4 The ICAO concept document also sees a quantification of separation provision so its level can be set and only used when strategic conflict management measures are inefficient (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Chapter 2.7.12).

“Strategic conflict management measures aim to reduce the need to apply the second layer – separation provision – to an appropriate level as determined by the ATM system design and operation.”

This emphasis on strategic measures has the potential of reducing the application of separation provision by controllers. The controller’s role should evolve into that of greater strategic decision making (managing the system) and should not deteriorate into only reactionary system monitoring

2.1.5  Separation minima is defined (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Appendix B Glossary):

“Separation Minima: The minimum displacements between an aircraft and a hazard which maintain the risk of collision at an acceptable level of safety”

This definition expands on previous ICAO definitions by the use of the term hazard. Before the production of ICAO Doc 9854 Global Air Traffic Management Operational Concept, separation minima where identified in only an aircraft vs. aircraft perspective but now a greater range of hazards are identified. ICAO Global Air Traffic Management Operational Concept Doc 9854 Appendix B Glossary:

“Hazards: The objects or elements an aircraft can be separated from. These are: other aircraft, terrain, weather, wake turbulence, incompatible airspace activity and, when the aircraft is on the ground, surface vehicles and other obstructions on the apron and manoeuvring area.”

2.1.6  Separation mode is a new term introduced and is (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Appendix B):

“An approved set of rules, procedures and conditions of application associated with separation minima.”

This term describes the criteria required for the application of separation. Separation modes must consider (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Chapter 2.7.17):

“…the safety level required, the nature of the activity and hazard, the qualifications and the roles of the actors, and other conditions of application, if applicable such as weather conditions and traffic density.”

2.1.7  The greatest change for separation proposed in the ICAO document is in its provision. That is who applies separation modes and minima and then monitors their application to ensure they are maintained. The ICAO concept document defines separation provision as (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Appendix B):

“The tactical process of keeping aircraft away from hazards by at least the appropriate separation minima.”

For separation to be applied there needs to be a separator that is a responsible agent. The ICAO concept document in defining separator identifies two agents which may act as the provider (ICAO Doc 9854 Global Air Traffic Management Operational Concept, Appendix B):

“Separator: The agent responsible for separation provision for a conflict, being either the airspace user or separation provision service provider”

A separation provision service provider is air traffic control.

For each hazard there must be a predetermined separator that is the overall arbiter of separation. The ICAO concept document specifies the airspace user by default as the predetermined separator from all hazards. (ICAO Doc 9854 Global Air Traffic Management Operational Concept, chapter 2.1.8:

“a) the ATM system will minimise restrictions on user operations; therefore, the pre- determined separator will be the airspace user, unless safety or ATM system design require a separation provision service.”

At first glance this appears to represents a major change from the current thinking of the Air Traffic Service Provider (ATSP) being the default provider of separation provision. However controlled airspace is today established to meet safety and ATM system requirements and so the change is perhaps not as great as first it seems. It is possible though that in light of the ICAO statement, the application of enhanced onboard systems encompassing concepts such as Airborne Separation Assistance Systems (ASAS) coupled with accurate navigation could significantly alter the threshold at which a separation provision service is required. The ICAO concept introduces the component of ATM service delivery management which includes trajectory management and allocation of the predetermined separator.

2.1.8  The ICAO concept document goes on to state that if the airspace user is responsible for separation from all hazards then they are practicing full self separation (Chapter 2.7.24). If the separation service provider is responsible for separation from all hazards then they are practising a full separation provision service (Chapter 2.7.28). If, for the range of hazards, allocation of separation responsibility is divided then it is distributed separation (ICAO Doc 9854 Global Air Traffic Management Operational Concept, chapter 2.7.25):

“Distributed separation occurs when, for an airspace user’s activity, there are different separators for different hazards. This can be because different predetermined separators have been defined or because delegation of separation has occurred.”

2.1.9  Predetermination of separation is also important as it provides a starting point for the process of separation delegation or co-operative separation (chapter 2.7.26). Before delegation can occur it must be known who the predetermined separator is so that separation provision can be handed from – at the start of delegation, and back to at the end of delegation (chapter 2.7.21). Delegation can be affected for types of hazard or for specified hazards (such as specific aircraft vs. specific aircraft separation). When carried out the delegated separator must use appropriate separation modes for the period of the delegation.

2.1.10  The chapters highlighting the rules for separation delegation are important as there is currently no other high level ICAO document highlighting these requirements beyond specific applications. These statements equate to several of the statements within the IFATCA Manual Page 4124 para 2.7. – Cooperative Separation:

“From a human factors aspect, IFATCA has strong concerns over the transfer of control responsibility to the cockpit for the following reasons:

–  If separation functions are transferred to the cockpit, the situation awareness and skills base of the controller will be degraded to the point when intervention will not be possible, and

–  Aircrew workload will increase by fulfilling additional tasks which are currently carried out by ATC. This might lead to overload situations in cockpit workload when other, higher priority tasks have to be taken care of by the crew. Responsibility for the control function cannot simply be handed back to the controller.

Delegation of separation shall be thoroughly described and defined in ATC and aircrew procedures.

Airspace within which Co-operative Separation is used must be so designated.

Before establishing a single airspace continuum over different States, all legal issues regarding liability and protection of staff should be addressed.

ATC and aircraft utilizing such delegated separation airspace shall be certified with minimum equipment.

Controllers and aircrew shall be provided with special training and certification to operate in delegated separation airspace.

The delegation of separation clearance shall be of a temporary nature, and shall be terminated either at a fix, a specified level, a specified time, or when standard ATC separation has been re-established, or when one of the aircraft has landed.

All aircraft and controller functions in Co-operative Separation shall be synchronized to the same time reference.

“Loss of separation” warning systems shall be incorporated in the application at ATC facilities and on aircraft.

Standard escape procedures shall be established for aircraft not being able to maintain separation assurance.”


2.1.11  The requirements of cooperative separation highlight the requirement for the delegated party (the party that has accepted the delegation of separation) to know the separation minima and rules of application before they can assume responsibility. This has particular implications for any automation support tools for ATC and any airborne systems such as those related to the ASAS concept. The IFATCA Statement on the future of global ATM highlights this:

“Future work will be defining these modes, include work on separation from weather and air to air separation. This work will also be necessary for the automation expected in future ATM systems because a conflict free trajectory cannot be determined unless the applicable separation modes and minima are known to the automation.”

As a result separation provision has the potential of becoming complex. Systems, both on the ground and in the air will need to be designed with functions and parameters that can accommodate several things. The hazards requiring separation, who the predetermined separator is for the hazard type or specific hazard, the separation modes applicable and any changes in the previously mentioned brought about management by trajectory.

2.2 Spacing

2.2.1  ICAO’s concept document defines spacing as:

“Any application of a distance or time between an aircraft and a hazard at or above separation minima in order to maintain a safe and orderly flow of traffic”

IFATCA reflects much of this definition within chapter 4 of its future ATM statement:

“Spacing is not the same as separation. Spacing needs to be in excess of the separation minima, so that failure of spacing can result in action before failure of separation”

2.2.2  The difference between the ICAO concept definition and the section from Chapter 4 of the IFATCA statement is important. IFATCA is of the view that when spacing reaches the separation minimum then spacing ceases and there is only the separation minimum applied.

2.2.3  The ICAO concept document places spacing in the context of traffic synchronisation. Traffic synchronisation is one of the seven ATM concept components that must be present in the ATM system and is (Chapter 2.5.1):

“…the tactical establishment and maintenance of a safe, orderly and efficient flow of air traffic.”

Further to this the ICAO document identifies the relationship between traffic synchronization and conflict management by classifying traffic synchronization as a strategic method of achieving conflict management as per chapter 2.7.10:

“Strategic conflict management is the first layer of conflict management and is achieved through the airspace organization and management, demand and capacity balancing and traffic synchronization components”

2.2.4 From an ATC perspective, spacing faces several changes with the application of the new ATM environment proposed by the ICAO concept document. Chapter 2.5 of the document highlights:

  • Dynamic 4D trajectory control;
  • Collaborative modification of sequences;
  • Delegation of spacing to the flight deck; and
  • Increased use of integrated automation both on the ground and in the air to achieve the above.

The above have the potential to mean less human (ATC) applied spacing and less ground based monitoring of spacing (via greater delegation to the cockpit using concepts like ASAS).

2.2.5  With regards to the reduction of tactical conflict management and controller workload, ICAO’s concept document chapter 2.5.4 states:

“Traffic synchronization, together with the other ATM components, will contribute to the efficient handling of traffic from gate to gate. There will be dynamic 4-D trajectory control and negotiated conflict-free trajectories. These techniques will reduce the need for traditional path stretching in high traffic density areas and will reduce the adverse impact this has on economy and efficiency.”

The above mentions dynamic 4D trajectories and it is true that, spacing’s dynamic nature will remain. However as onboard navigation, time keeping and ground surveillance become more accurate, communication more diverse and of greater capacity, the spacing buffers that currently exist in the system will probably be reduced. Via these factors, aircraft compliance to spacing commands or requirements will need to be more rigidly adhered to so as to preserve the aircraft displacement above the applicable separation minimum.

2.2.6  Onboard assistance will be needed to make the above reduced spacing displacements viable as the human ability to routinely apply accurate spacing is fallible (thus the present greater spacing buffers). Also without onboard assistance a pilot’s ability to comply accurately in all conditions without knowing the movements of the traffic he or she is to space against is difficult to say the least. ASAS, although known as ‘separation assistance systems’, is indeed the primary assistance that will be used to facilitate new spacing procedures. For example Package 1 of ASAS as defined in the Principles of Operation for the use of Airborne Separation Assistance Systems (PO-ASAS) contains three airborne spacing concepts:

  • Enhanced sequencing and merging operations (ASPA-S&M)
  • In-trail procedure in oceanic airspace (ASPA-ITP)
  • Enhanced crossing and passing operations (ASPA-C&P)

The net result of more accurate navigation coupled with onboard spacing systems could mean a reduction of spacing distance. Eventually it may be difficult to differentiate between spacing distance and separation minima with regards to the actual distance used.

2.2.7 Although conformance and monitoring of spacing is envisaged as moving more to the cockpit, the agent responsible for spacing is not clearly defined by the ICAO document. That is ‘who will be the initial decider of what spacing shall be applied?’ It may be that the concept expects that this will be done as part of management by trajectory. For the near future though it is most likely spacing responsibility will remain with the controller as the agent of separation. More strategic spacing concepts such as Collaborative Decision Making (CDM) or Medium Term Conflict Detection (MTCD) systems may affect this though as the initial sequencing and spacing function is moved to a higher level. ICAO Doc 9854 Global Air Traffic Management Operational Concept, chapter 2.5.6:

“Traffic synchronization principles include the following:

a) the ability to tactically and collaboratively modify sequences to optimize aerodrome operations, including gate management and/or airspace user operations.”

2.3 Segregation

2.3.1  The ICAO concept document does not provide a stand alone definition for segregation. Segregation is identified as a function of airspace organization and management by way of examples provided within section 2.2 Airspace Organisation and Management and Appendix 1 The concept – Explanations and Examples. The examples used are the creation of special use airspace, the design of structured route systems and segregated arrival and departure procedures.

Airspace organization and management is defined as a measure used to achieve strategic conflict management within chapter 2.7.10 of the ICAO concept document:

“Strategic conflict management is the first layer of conflict management and is achieved through the airspace organization and management, demand and capacity balancing and traffic synchronization components”

2.3.2  The aim of strategic conflict management measures is (ICAO Doc 9854 Global Air Traffic Management Operational Concept, 2.7.12):

“[…] to reduce the need to apply the second layer – separation provision – to an appropriate level as determined by the ATM system design and operation”

2.3.3  IFATCA identifies this change within the Statement on the Future of Global Air Traffic Management under Performance expectations – Capacity:

“Available ATM Capacity will no longer be managed as “number of aircraft per sector” but in the “number of tactical interventions required” as strategic conflict management establishes orderly flows of air traffic”

And in the same document under Humans and Technology – humans and automation:

“For air traffic controllers this will involve changes such as […], adjustments in working- style to support a more strategic trajectory management traffic flow,”

The above ICAO and IFATCA documents would suggest increasing use of segregation over separation with ATM system design with a ’prevention is better than cure’ method of conflict management. However segregation is currently highlighted by ICAO as an inefficient and detrimental tool with regards to the equitable sharing of airspace by legitimate users. ICAO Doc 9426 Air Traffic Services Planning Manual, Part One Section 1 Chapter 3.1.3:

“In this respect, it has been found that, to resort to segregation of the airspace, i.e. its splitting and subsequent systematic allocation to the exclusive use by a specific party, generally results in the least efficient over-all use of the available airspace as it invariably leads to a sterilization of large portions of the airspace for prolonged periods of time.”

2.3.4 Section 1 chapter 3.1.3 of ICAO Doc 9426 Air traffic Services Planning Manual may appear to create an obstacle to the increased use of segregation but the ICAO document provides solutions by advocating changes in segregation’s application. This is brought about by the vision of harmonious and fully flexible airspace. Within the ICAO concept this high level vision is stated in Chapter 2.1.2 – Airspace Organization and Management:

“a) all airspace will be the concern of ATM and will be a usable resource;

b) airspace management will be dynamic and flexible;

c) any restriction on the use of any particular volume of airspace will be considered transitory; and

d) all airspace will be managed flexibly. Airspace boundaries will be adjusted to particular traffic flows and should not be constrained by national or facility boundaries.”

These statements are expanded in chapter 2.2 of the ICAO concept document and here we see the greatest change for segregations application. Areas highlighted are:

  • The desire to limit the use of special use or restricted airspace to have the minimum impact on other airspace users. This includes looking at the dimensions of such airspace and the methods and times of activation.
  • Airspace boundaries should be adjusted to traffic patterns and provide for seamless handling irrespective of State boundaries. Such adjustment should be able to accept flexibility in track assignment.
  • Structured route systems will be applied only when required to enhance capacity and avoid areas where access has been limited or where hazardous conditions exist.
  • Airspace designation or access based on equipage can be used but shall not exclude non-compliant users from access to the system and if possible the system shall not permanently preclude mixed mode operations.

That segregation will still be used as a method of creating exclusive subsets of ATM system users is unchanged from the current environment. However what the above shows is that ICAO’s intent, including that within the ICAO concept, is to change the application of segregation from that of today. Segregation will be used, as today, in planning airspace structures but physically to the least extent possible. This can mean the actual size or time span for the application of segregated airspace will be limited to the greatest extent the system can afford. Though still a strategic conflict management tool, segregation will be carried out more tactically within itself. This will require greater co-ordination between ATM users and greater flows of information to allow flexible application. It appears that the ICAO concept function of management by trajectory will play a significant role in achieving this.

2.3.5. A current example of many of the statements of chapter 2.2 of the ICAO concept document is Flexible Use of Airspace (FUA). In simple terms this Eurocontrol concept uses a higher policy body to design airspace, routes and procedures to facilitate the segregation of airspace. Airspace management cells (AMCs) facilitate the day to day running of the system using the high body’s procedures and guidelines to accept, prioritise and notify users of military and other user request for exclusive use of airspace. Civil controllers then use information created by the AMCs and permanently published information (such as the local Aeronautical Information Publication (AIP)) to activate and deactivate airspace, establish routes and act as a basis for co-ordination.

2.3.6  As can be seen from the FUA example, the majority of segregation application is done at the strategic level while work below that, though requiring significant amounts of information sharing and co-ordination, is still largely applied via reference to strategic airspace planning and rules of use. ASAS and improved communication systems if capable of handling the volumes of information required and smart enough to process the data effectively could represent a significant change in the process outlined above in the sense that, the application of managing aircraft trajectories with regards to segregated airspace or routes could pass directly to cockpit.

2.3.7  Finally it is important reiterate that segregation designed to facilitated new technology must not disadvantage non-compliant airspace users and alternatives must be available.

Regulation of airspace for the exclusive use of applicable users is identified in chapter 2.2 of the ICAO document, as undesirable. This is also reflected in the IFATCA Statement on the future of global ATM (Airspace User Operations):

“The ATM system will be in constant change and will be designed for mixed-mode operations; in any case the ATM system is required to accept mixed capabilities of airspace users.”


3.1  ICAO Doc 9854 Global Air Traffic Management Operational Concept introduces several new concepts with regards to the application of separation, spacing and segregation. These changes in part facilitate the proposed future ATM system.

3.2  The ICAO document is specific in highlighting where separation, spacing and segregation are placed in the ATM system. Strategic separation is now encompassed within the first layer of conflict management – strategic conflict management. Separation provision is the second layer of conflict management and is tactical in application. Spacing is a tool of traffic synchronization which is in part a strategic method of conflict management. The ICAO document states spacing must be at or above the separation minima. IFATCA within its future ATM statement highlights spacing as always being above the separation minimum. Segregation is a tool of airspace organization and management which in turn is used to achieve strategic conflict management.

3.3  Realization of the ICAO concept document could see a considerable change in current controller operations with increased emphasis on strategic conflict management methods and for separation provision to be only used when strategic conflict management is no longer efficient. Strategic traffic sequencing concepts such as CDM, dynamic segregated airspace and trajectories could see the controller carrying out more strategic conflict management.

3.4  The ICAO concept document proposes significant change in the provision of separation with the separator increasingly being the airspace user. General rules for the delegation of separation are specifically outlined as are the hazards that can be separated against. These provisions accommodate the increasing use of onboard systems to maintain aircraft spacing. When considering all of these factors together it is possible to see the threshold for the requirement for a separation provision service provider being progressively raised.

3.5  Separation, spacing and segregation all impact on each other and it is possible to view each term as having strategic and tactical applications.

3.6  Spacing displacement will in the future become smaller, and so it will become more difficult to differentiate spacing from separation minima as onboard navigation becomes more accurate and ASAS is increasingly used.


It is recommended that;

4.1. This paper is accepted as information material.

Last Update: September 29, 2020  

April 11, 2020   892   Jean-Francois Lepage    2008    

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