ICAOs FANS committee addressed the problem of the shortage of capacity in the ATC system in a number of ways. One of the concepts, that forms the foundation of the ICAO CNS/ATM concept, is ADS or Automatic Dependent Surveillance. This working paper introduces the concept of ADS, and presents draft policy for conference debate.

ADS – The Need for Change

One of the major capacity shortfalls in the ATC service provided in some parts of the globe, is the use of procedural control. Non-radar separation standards are necessarily large to accommodate the lack of continuous updates of flight data to the controller and thereby to accommodate the safety of aircraft under control. Additionally a fixed route structure is often linked to operations in such areas where non-radar control is employed. This provides the structure and predictability with which traffic can be controlled and the certainty or predictability necessary for control to be exercised. It is, in effect, a snapshot of time approach to ATC and therefore it is always historic.

Non-radar control is the only type of control possible in some parts of the globe – over the high seas, or in sparsely populated areas , because it is not possible to provide an ATC infrastructure that can support radar and communication systems. The results of this type of operation incur penalties to aircraft operators, especially as in some areas the aircraft operating in these areas have long sector lengths, thus the cost of non-optimal flight is high. For example, in 1992 United Airlines estimated that it would save US$238,000 per month, per aircraft, flying on North Pacific routes by flying user preferred routes utilising ADS and GNSS. Therefore, to increase capacity in these types of operating environments, the limitations of the line of sight restrictions of radar and VHF voice communications must be overcome to provide a more flexible and efficient ATC service.

The FANS committee sought to use a number of emergent technologies that have reached a mature stage in their development to address these problems.

ADS: The Concept

The most notable enabling technology is the use of satellites, in two forms -communications (or SATCOM) and satellite navigation (or SATNAV). It is important to distinguish between the two. Other technologies that will be employed will be the use of digital data communications and networks, on a global scale – the ATN or Aeronautical Telecommunication Network. This uses protocols and an architecture that is able to meld data in an operable way through several levels or ‘layers’. Airlines are already well advanced in utilising digital data communications datalinks such as AIRCOM, ACARS, and AVPAC, based upon ARINC 622 and 722 protocols.

What is ADS? To quote ICAO:

One way of considering ADS is to think of it in terms of a communications system. The aircraft communicates its position, derived from on board systems, to ATC automatically. That is all that ADS is. It is automatic because position reports and other routine messages are transmitted automatically by equipment on board the aircraft. The rates at which these are sent is established prior to entry into ADS airspace, by forming a contract between the ATSU and the aircraft. If the ATSU wants a report at a different interval then this can be obtained. It is dependent because the position of an aircraft that is presented to ATC 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.

To turn ADS into an operable system it requires a lot more than just the communications links. It needs a system: or ADS-ATC system components. An ADS-ATC system is purely an ATC system that uses ADS, wholly or in part as its surveillance medium.

It is important to understand that the way that FANS and the CNS/ATM has evolved is such as to give a ‘menu’ of tools with which to mix and match systems to suit the operational requirements of particular ATS providers. However, given the flexible approach that the FANS committee developed, there is still a basic level of system components that will be required for operational use. Therefore the composition of an ADS-ATC system might take the form depicted and described in Appendix A.

System components

Navigation and flight management systems

On board navigation systems that meet certain performance criteria or Required Navigation Performance – or RNP – criteria ( rather like the minimum navigation performance navigation specification [MNPS] used on the North Atlantic ), and flight management systems that manage flight profiles.

Avionics

On board systems that take the data from on board sensors, formats the data and then routes this data to the appropriate datalink medium.

Pilot interface

It is intended that whilst routine communications will be transmitted automatically, all messages transmitted will be monitored by the flight crew. Additionally, the message transmission rate can be altered, and non-routine messages can be transmitted by the use of the datalink, thus a message entry panel will be required.

Datalink medium or the communications interface

This is the link between the aircraft and the ground system. The datalink medium can be a SATCOM link, but it could also be a VHF or Mode S datalink. In oceanic areas and remote land based areas SATCOM is the most suitable datalink medium. In domestic airspace VHF or Mode S datalinks may be more suitable. In either event it is intended that datalink communications will be carried out in a seamless, and transparent way to controllers and flight crew.

Controller interface

It is envisaged that the data communicated – or downlinked will be presented visually on a controller workstation display. It may include:

• a high resolution monitor
• an input device
• some form of keyboard for composing messages
• computer tools and/or decision aids to meet control task functions;
• voice comms.

ATC automation

The data communicated – or downlinked – to the ground must be used in some way. Several functions have been identified for the control task functions:

• Position monitoring;
• Conflict prediction;
• Data presentation;
• Clearance validation;
• Tracking;
• Conflict resolution;
• Wind grid estimation;
• Flight management (or free route/FLEXTRACK clearance tool).

An important element of the ground based automation will be some degree of flight data processing capability.

There are other potential uses of the data that can be produced using the ADS concept in a radar environment. For example, it could be used to replace radar where it would be economically beneficial, it could be used to fill in known gaps in radar coverage, or between areas where overlapping radar coverage does not exist.

The Progress of Implementation

There are numerous developments, proposed and near to operational implementation, worldwide where ADS is being used or components tested.

The Pacific is the focus of several developments which will come on line in 1995. Most notable of these, and perhaps the most advanced, is the Pacific Engineering Trial (PET). These trials are led by the CAA’s of the region, airlines and airframe manufacturers. A number of Boeing 747 – 400 aircraft have been, and will be upgraded to the FANS 1 standard and be able to operate, using ARINC 622 protocols, with ADS equipped ATSU’s. New Zealand, Fiji, Tahiti and Australia are all planning equipment upgrades or will be conducting operational trials in the near future. The Australian CAA is near to certification of FANS 1 equipped aircraft. Japan has conducted trials with both a Beech 99, and a JAL B747-400., transmitting position reports via satellite to an ADS workstation.

In the Asia region, India is planning to provide ADS only routes across the Bay of Bengal for suitably equipped aircraft in the very near future. Operational trials have already been carried out in Thailand in conjunction with Alenia using an aircraft fitted with a VHF data link.

Brazil is presently conducting live trials with an aircraft transmitting ADS basic messages over remote areas in the Amazon at low altitudes. Similar trials are being carried out by the administrations in Canada, the United Kingdom, the United States, and by ASCENA. EUROCONTROL are involved in trials with the French, Italian, and Spanish in the Mediterranean. A particular research objective is evaluating the integration of ADS and radar data. Similar work is being carried out in Iceland, who are also evaluating the use of HF datalink as a suitable medium for ADS operations.

These brief details are not comprehensive, but give some indication of the scope and degree of ADS related research and development activity. One important observation is that whilst considerable effort is being expended on system design, little work has been carried out yet into operational procedures for ADS. The ICAO ADS panel ( ADSP ) are near to publishing ADS guidance material. IFATCA have made significant contributions in the ADSP deliberations. ICAOs RGCSP are also involved in establishing separation values for ADS operations. IFATCA is an active member of the RGCSP.

Issues That Arise From The Introduction of ADS

The introduction of ADS into operational service means a completely new generation of surveillance systems entering the controllers inventory of control tools. ADS brings with it a fundamental change in the quality of ATC service that can be provided in some areas of the globe, that can enhance safety and system capacity.

There are a number of issues that arise when considering the use of ADS operationally. These come about as a result of new operating techniques at the level of the controller, and by institutional arrangements that have the potential to influence or affect controllers in the day to day controlling task. These are:

1. General issues relating to the introduction of ADS
2. The use of dependent surveillance systems
3. The use of voice communications in ADS operations
4. The reduction of non-radar separation standards using ADS
5. Provision of services by third party service providers
6. Presentation of ADS data to the controller
7. Mixed and segregated modes of ADS operation
8. Fusion of ADS and radar data

ADS – General Issues

As presented in section 3 of this working paper, in order for an ADS-ATC service to be provided, a number of systems need to be provided to support and enable the control task to be carried out. Therefore the introduction of an operational ADS-ATC service is contingent upon adequate facilities being available to support the control task. The variations of traffic densities around the globe means that ADS-ATC systems will vary from operating domain to operating domain. However, consideration must be given to the traffic densities, traffic mix and other specific operational factors in determining the initial operating configuration and capability. All such systems must be fully validated and verified prior to, and post operational introduction. Full consultation should be undertaken with operational staff when designing the system.

The early introduction of an ADS surveillance system can be expected to occur in areas to overcome the inherent limitations of the procedural mode of control. It is therefore likely that the order of change from one operational mode to the other may be very great. Controllers must therefore be comprehensively trained and prepared for ADS operations with training programmes which equip control staff with the necessary perceptual and cognitive skills and provides training in all airborne, ground based and infrastructure failure modes.

ADS-ATC operations will have an effect upon the tasks performed on the flight deck. There is some concern amongst pilot organisations that pilot workload may become excessive. Flight crews must receive appropriate training for flight where ADS-ATC is being used.

To Conclude

IFATCA recognises and welcomes the potential contribution that Automatic Dependent Surveillance offers to substantially improving the safety of aircraft operations by providing near continuous flight data to the ground organisation, regardless of the position of the aircraft over the globe.

ADS, in conjunction with systems which support an ADS-ATC control service, can potentially improve the quality of the ATC service, particularly offering capacity gains. Significant improvements can be provided to airspace users in terms of greater flexibility in accommodating aircraft preferred routes and short term tactical changes to the flight plan. However, in order for these gains to be realised, the necessary infrastructure must be designed and installed on the ground. Personnel must be appropriately and comprehensively trained to operate with the tools that the new systems can provide. All such systems must be validated to satisfy the appropriate safety and certification standards.

ADS will be implemented in differing operating domains around the globe. Due regard should be given to the specific operating factors when implementing an ADS-ATC system.

The changes that an ADS-ATC service will bring, means that there will be some changes to the operating environment and on the flight deck, as well as new aircrew tasks created. Flight crews must be adequately trained therefore in ADS-ATC techniques. Care must be exercised that flight deck workload is not increased unduly by poor system design.

It is recommended that:

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.

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

Where an ADS-ATC service is introduced ATS staff shall receive comprehensive and appropriate training in ADS-ATC control service.