The IFATCA Technical Manual contains reference to extensive provisional policy on the technical aspects of Automatic Dependent Surveillance and its application to Air Traffic Control. The purpose of this paper is to provide additional and up to date information on the implementation of ADS that will allow the upgrading of the existing provisional policy.

There are currently no rules or procedures published by ICAO for the use of ADS by controllers. A basis for addressing this issue was presented to the RGCSP Working Group A meeting In Australia in April of 19981 and later transmitted to the ADSP for consideration at the joint WG/A and WG/B meeting in London in June 1998. This paper refers in turn to work being jointly undertaken by ADSP and RGCSP in formulating an amendment proposal to include relevant material in PANS/RAC Doc 4444.

This section of the paper will set out the elements of the existing IFATCA provisional policy on ADS and examine each one in turn against developments and lessons learned since the policy was first formulated. IFATCA policy is quoted first in italics followed by an sub paragraph containing relevant update material.

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 system. 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.

The statement “It is a communication system” may need further consideration in terms of policy. ADS by its simplest definition is a surveillance system which has as one of its components, a communications system.

The current wording of the proposed amendment to Doc 4444 is as follows:

Drafting comments reinforce a need to emphasise the difference between ADS and radar.

Caution is needed here to avoid over specifying. There will undoubtedly be separation standards based on ADS which do not require direct pilot controller communications. For example 50nm lateral separation using RNP 10 can be satisfied with indirect VHF. Policy of this nature may well be best left to that relating to separation minima rather than particular technical systems.

Failures of some components of ADS airborne and or ground systems can occur and still allow CPDLC to remain available for communications.

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

It is acknowledged that more work is required on the communications section of the Doc 4444 ADS draft. In its current form this section contains a statement that ‘Appropriate communications shall be established prior to the provision of ATS services based on ADS’ On the other hand a paragraph requiring an attempt to establish a CPDLC connection at the same time as an ADS connection, has been removed.

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

The draft Doc 4444 material addresses this issue at 3.4 with:

Drafting notes also call for the inclusion of cleared level adherence monitoring which includes the ability to monitor block level clearances.

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.

The draft material is silent on the question of warning to pilot or controller of system degradation except for an editing annotation that calls for distinct symbology or other HMI indication that the navigational Figure of Merit (FOM) has fallen below or does not meet the minimum level for the application of the relevant ADS separation standard.

Section 8 of the draft material is titled Failure of Equipment. This section is still at an early stage of development. It does contain a statement to the effect that in the event of equipment failure and provided that communications are still available, the controller is responsible for taking the necessary action to establish alternative separation.

The draft Doc 4444 material states at 6.3:

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.

This item involves general datalink requirements as well as specific ADS requirements and should be cross referenced accordingly.

The draft Doc 4444 material states at 6.3.4:

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.

The draft Doc 4444 material states at 3.1 that:

However ADSP have suggested a need for a paragraph in the draft Doc 4444 material on ‘non- display based separation. The current draft contains para 4.1 which begins “When a plan view display is used’. Both of these references suggest that there is a suggestion that IFATCA policy on this point may need to be vigorously defended.

IFATCA can expect support from at least the Australian members of both ADSP and RGCSP who have expressed doubt that there would be situations were ADS could or would be used for the provision of ‘non display based separation’.

Nevertheless it must be noted that there are in existence examples of Air Situation Displays that are not pictorial or graphical in nature such as the system used in the control of North Atlantic air traffic. These display systems are considered by many to provide the optimum solution for the route structure, traffic density and traffic mix encountered in their application areas. Policy in this area needs to be flexible if all of the possible benefits of ADS are to be explored.

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

The draft material contains several specifications for the way in which ADS information should be presented to the controller.

This is however a broad issue that applies to the design of any controller interface. It will need to be pursued not only in the ICAO panels but at local and regional level as systems are designed and implemented.

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 draft Doc 4444 material mentions the use of separation between ADS position symbols and radar position symbols when an integrated display system is in use and where authorised by the appropriate ATS authority.

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

A note following para 4.5 of the draft Doc 4444 material states that extreme care must be taken to ensure that the data presented to controllers is easily distinguished as either ADS based, radar based or flight plan based data.

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

The approach from both ADSP and RGCSP appears to be one of integration rather than segregation of ADS and radar services. If this can be achieved safely in the ATC automation system then controllers would benefit from reduced work load and reduced procedural complexity.

As a general principle there is no need for IFATCA policy to restate ICAO documentation.

Once the ICAO Doc 4444 material is approved and published this principal may well make much of the IFATCA policy redundant provided that the direction currently being pursued by the developers of the material is not substantially altered.

Until that time IFATCA through its standing committees will need to continuously monitor the development of ADS and the standards and practices for its use.

By way of a new baseline for that process, the following recommendations propose some revisions to existing provisional policy.