Review of Technical Policy

Review of Technical Policy

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

WP No. 81

Review of Technical Policy


This paper deals with those policy items which have been the subject of a review by SC 1 during the past year.


After consideration of all current policy , SC 1 concluded that this year the main focus should be, to propose upgrade of those items of “ Provisional Policy” to policy, this in light of developments and the need in the various forums for a definitive position from IFATCA.

The following subjects have been reviewed :

  • “Automatic Dependent Surveillance (ADS)”;
  • “Mixing Datalink Equipages in the Same Airspace”;
  • “Downstream Clearances”;
  • “Direct Controller – Pilot Communications”.


SC 1 have reviewed and amended where required the current provisional policy. To aid discussion and understanding the original rationale has been attached to each proposal. Additionally a reason has been given for any proposed amendment.


It is recommended that the current IFATCA Provisional policy for the following;

  • “Automatic Dependent Surveillance (ADS)”
  • “Mixing Datalink Equipages in the Same Airspace”
  • “Downstream Clearances”
  • “Direct Controller – Pilot Communications”

be upgraded to policy.

Automatic Dependent Surveillance (ADS)

ADS is a service for use by air traffic services in which aircraft automatically provide, via datalink, data derived from on-board navigation and position fixing systems. It is a communication system. Position reports are transmitted automatically by equipment on board the aircraft. The rate at which this data is transmitted can be varied. It is dependent because the position as reported by the aircraft is derived solely from aircraft on-board systems. There is no independent determination or corroboration of the aircraft’s position ( such as radar provides) carried out by ATC.

Proposed Policy

“Before an ADS-ATC service is introduced into operational service, the necessary system components to provide a control service and to support the control task shall be in place. Only pertinent and useful flight data should be supplied to the controller which supports and enhances the construction of a mental model of the traffic situation and supports controller understanding and maintenance of the “picture”.

An ADS-ATC system, must be validated so that it meets the appropriate ICAO safety and certification standards.

In all circumstances where an ADS-ATC service is provided there shall be an alternative independent voice communications capability available.

When entering ADS airspace where an ADS service is provided existing HF and/or VHF voice communications shall be established to confirm their serviceability.

Separation standards in areas using ADS shall never be less than the appropriate non radar minima provided an effect of risks are quantified by an approved ICAO method. Such models should not be used alone but in conjunction with operational judgement to account for those factors that cannot be modelled.

Separation standards for use with ADS shall be global standards, and applied globally.

The separation standards to be applied between radar targets and ADS positions must be subjected to an ICAO approved analysis.

The number of different separation standards available for use should be kept to a minimum. ATC should have the option to limit the number of separation standards in use at any time.

ATC will require the provision of assistance tools for managing airspace where multiple separation standards apply.

The ADS system shall provide a warning to pilot and controller whenever navigation accuracy is degraded below that required to operate in the airspace, and that this will affect separation standards. Procedures must be in place to restore any loss of separation in a timely manner.

Where aeronautical telecommunications services are provided by third party service providers, ATC message traffic must unassailable priority.

Aeronautical telecommunications facilities provided by third party service providers and used in ADS operations must meet the appropriate standards for the operation of a safety critical system.

ADS reporting rates should be determined by the ATS operational requirement.

Procedures for the provision of an ADS- ATC service shall not impose any restriction upon the controller on requesting reports from aircraft under ADS control.

In providing a service for the aeronautical community, the third party service provider must provide assurances on the integrity of information and security from interference.

Graphical or pictorial displays, in conjunction with supporting data displays of ADS data, should be provided to enable the controller to carry out the control tasks.

ADS system design must seek to optimise the interface at the controller workstation.

Control of traffic using position data derived from ADS and radar surveillance can only be used where the control system supports both types of surveillance.

The derivation of position data must be prominently displayed on the situation display continuously.

Transition areas should be established between radar and non- radar areas of responsibility.”

Mixing Datalink Equipages in the Same Airspace

There is one editorial change – In the preamble Line 2 insert “to” after identical.

Additional policy statement to reflect current discussions and developments:

In addition to the interface presented to the controller, procedures and systems events must be operationally identical except where an operational advantage can be achieved by dissimilar implementations and where there is no advert impact on safety or controller/pilot workload.


It is recommended that the following policy be adopted.

“ A fundamental principle in Datalink has been that the message presented to the controller or pilot should be identical to the presentation on the other end. However when multiple airborne datalink types are introduced operational difficulties will be induced. If automation is used to formulate an “approximately similar” message between aircraft Datalink systems that do not share an identical message set, it raises serious questions about the certification of a system that does not show a controller and pilot exactly identical presentations of a common communication.”

IFATCA Policy states :

Controllers must not be required to utilize more than one operationally disparate Datalink system in any unit of airspace. If aircraft equipped with more than one type of Datalink functionality are operating in the same airspace, the interface presented to the controller must be functionally identical.

In addition to the interface presented to the controller, procedures and systems events must be operationally identical except where an operational advantage can be achieved by dissimilar implementations and where there is no advers impact on safety or controller/pilot workload.

Downstream Clearances

The following changes are proposed to the current preamble and provisional policy;

Delete last two lines of pre-amble after downstream clearances.

Replace with:

delivered via datalink are in use i.e. including the current controlling authority that sufficient safeguards are in place.

Editorial: “A” in IFATCA.


It is recommended that the following policy be adopted:

“ One of the fundamental operational principles that has been incorporated IN THE DRAFT ICAO Manual of Air Traffic Services (ATS) Data Link Applications Draft Version 0.4 from the beginning is that any ATS data link system must only allow one ATSU (ATS Unit) to be capable of communicating with a given aircraft at any one time. This requirement was felt to be necessary to ensure that there would be no confusion by the aircrew that a clearance or instruction delivered by the data link system actually came from the proper controller (or controller team). It is therefore essential that when downstream clearances delivered via datalink are in use i.e. including the current controlling authority that sufficient safeguards are in place.”

It is therefore IFATCA Policy that :

Where Downstream Clearance capability is provided via Data Link, sufficient safeguards must be implemented in accordance with the Draft ICAO Manual of Air Traffic Services (ATS) Data Link Applications, Draft Version 0.4, dated 20 September 1996.

Direct Controller – Pilot Communications

As data link becomes an important means of delivering ATS, there are crucial questions that must be answered before this technology is used for safety-critical communications. This need is especially true where a data link communications system is the primary, or even integral, element in allowing the reduction of separation standard minima. This does not prevent Datalink from being provided in other circumstances.

It is therefore IFATCA policy that :

“In any ATS system, where data link is considered a safety-critical element of that system, data link based ATS must be accompanied by direct two-way controller-pilot voice communications which is also safety critical. This direct voice functionality shall be rapid, continuous and static free.”

“Direct voice communications requires that no third human party is involved in the set up and/or delivery of these communications. Any set up procedures by either the pilot or the controller must be minimal and nearly instantaneous.”

Last Update: September 28, 2020  

March 5, 2020   728   Jean-Francois Lepage    1998    

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