SSR Mode S Improvements in SSR Techniques and ATC Aspects of Air/Ground Data Links

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SSR Mode S Improvements in SSR Techniques and ATC Aspects of Air/Ground Data Links

21ST ANNUAL CONFERENCE, Amsterdam, Netherlands, 3-7 May 1982

WP No. 57

SSR Mode S Improvements in SSR Techniques and ATC Aspects of Air/Ground Data Links


The improvements to SSR techniques have naturally had a major impact on ATC procedures and methods. These changes, brought about as the result of improved equipment with enhanced capabilities, have been tempered by equipment and system limitations and deficiencies. It is considered by SC 1, that IFATCA policy has kept pace with technical developments from which ATC techniques have stemmed in this area. In addition, there is sufficient IFATCA adopted guidance material.

The subject of Air/Ground Data Links has been actively considered by IFATCA since 1964 and much guidance material has been adopted (see references) in addition to the policy statement B.11 Reykjavik 1973 (amended 1978 ) .

ICAO has recently defined SSR Mode S to include not only all data link functions but also discrete addressing and therefore mono-pulse techniques. The ATC techniques which will no doubt evolve and be developed to take advantage of new facilities will also have to allow for the fact that these systems are not always perfect.

The development of SSR

The development of the use of Secondary Surveillance Radar (SSR) has been a gradual process, which in some areas is still taking place. With the introduction of Mode S and air/ground data transfer on the operational horizon, it is timely to consider the lessons learned so far and to apply these lessons to Mode S where possible.

Although the theory of enhanced aircraft identification was well founded, the early days of SSR as a development of IFF was often a trial and error period for controllers to develop their own techniques. These techniques have been associated with parallel development of equipment enhancements in the air ( e.g. 4096 codes and Mode C altitude) and on the ground (e.g. digital labels associated with the targets).

It could be argued that SSR has only come into its own with the provision of these latter equipment enhancements. In this case, the changes in ATC techniques and the limitations imposed by the equipment should assist in assessing the possible impact of Mode S.

ATC Developments

One of the mayor changes in ATC techniques has been the change in emphasis in monitoring aircraft. With a primary only radar picture , every controlled aircraft must be regularly monitored in order to retain memory identification of targets and to refresh assessment of the total air situation to compare against strategy. With digital labels, especially those associated with tracked or code/callsign paired targets, continuous monitoring can be relaxed. This is because identity of targets is no longer the prime mental function, and the air situation can be reassessed with identity maintained on the display. The ability to remember the identity and predicted positions of a number of moving radar targets has never been acquired by many controllers and may now have been lost by many others who have worked with labelled displays for some time. However, labelling is a very positive advantage because the controller’s mind has been partially freed of one responsibility to concentrate more on other control functions and thus handle more traffic.

The dependence for vital control functions on equipment must not be overlooked. With each advantage provided by electronic equipment there are usually disadvantages, and SSR is no exception. Some of these disadvantages have only recently become of direct concern to the controller. For a controller , consistent data is of more importance than technically perfect and precise data, i.e. technical limitations because of greater precision should not be apparent to the observer. With the reduction of constant surveillance, the occasional incidence garbling or incorrect data has not proved impossible to tolerate. However, such deficiencies as lack of azimuth resolution of digitised SSR targets can be intolerable in areas of high aircraft density, especially where positive radar control is being exercised, i.e. the monitoring function is a high priority. Even in this situation the occasional incorrect Mode C or Mode A transmission is tolerable, because the information is easily disregarded as not fitting into an established pattern that is known to be safe ( controllers have often denied that such incorrect transmissions occur). However, false indications of target heading changes resulting from poor azimuth resolution and track jitter can immediately affect the controllers belief that a safe situation is being monitored.

The impact of Mode S

The improved surveillance service provided by Mode S will hopefully alleviate these difficulties. However, the final objective of providing a data link alternative to voice RTF channels seems to be receiving undue priority over the many intermediate development uses. Because of the high cost of development and some recent accidents, there are inevitably strong political and emotional pressures to justify this objective. If this is the general attitude then ATC may be asked to absorb technical difficulties associated with all such equipment in early stages of its development, before ATC techniques have been evolved to make best use of its advantages. Because of the technical difficulties yet to be overcome, or even to be discovered, the lengthy systems integration necessary and the total time before full operation, it is suggested that as much advantage should be taken of less critical AREAS of Mode S development potential as is possible. In this way Mode S will be able to provide benefit for the widest possible range of users, and experience so gained will benefit the longer term objectives.

One example of such use of Mode S is to automatically report aircraft heading, rate of turn (roll angle) and rate of climb and descent, to ground based radar data processing systems. This application of Mode S does not feature significantly in some of the proposals for mode S, Mode S probably because of the difference in radar target reporting and different track prediction methods in use in different states. In order to improve target position display and provide more accurate prediction. Especially for conflict detection, many ground systems would benefit from data received directly from the aircraft. Improved surveillance would provide controllers with the additional advantages and enable them to develop the new techniques required to take advantage of the full potential of Mode S, hand in hand with the technical developments. Some of these techniques will require regulation, as with SSR. For example strict regulation of SSR code allocation, code use and verification procedures has been necessary. Even so, code duplication still occurs. In some areas code duplication is still a normal practice (e.g. all departures in a particular direction assigned the same code) where it is not practicable to use code allocation procedures which conform with the basic ICAO principles that an aircraft should be assigned only one code per flight, and that this code should not be assigned to any other flight.

The final objective of Mode S appears to be the best the system can best offer at best performance. It is known that SSR cannot be relied upon to provide always perfect performance, and that occasionally there are procedural difficulties. Controller interaction is deemed to be a vital part of Mode S operations, especially where Mode S is a backup or replacement for voice channels. ATC systems vary considerably and not all have the same level of sophistication. It is therefore necessary that simple procedures and principles be developed which can be applied to as many ATC systems as possible. The possibility of confusion, malfunction and human error should not go undetected, yet be easy to overcome.

To conclude

In order to achieve these goals it is suggested that Mode S should be regarded as enhancing ground ATC systems. In this way it should be able to provide benefit at many intermediate levels of sophistication. As the air to air capabilities are developed, these should be regarded as a back-up providing an additional level of safety in case of ground-based ATC system errors or failures. This built in level of safety is becoming increasingly more significant as total systems become larger and more reliant on each subsystem functioning correctly. Mode S appears to offer a solution to assist in overcoming the increasingly difficult problem of technical failure of ground subsystems.

In order that a wider approach to Mode S can be presented, it is strongly suggested that IFATCA take up any option to be involved in forming Mode S policy with ICAO, when the political and emotional pressures associated with Mode S (especially in the areas of ATC instruction, conformation and conflict avoidance) can hopefully be put into perspective.

It is recommended that:

The following general policy statement on Mode s Development be adopted to supplement IFATCA’s present Air/Ground Data Link policy:

1. Mode S developments should be planned to facilitate their application to ATC Systems at all levels of sophistication.

2. Mode S should be developed primarily as a ground system enhancement, designed to improve ATC – Pilot interaction.

3. Mode S air-to-air link capability should be used in a support role for the main ground system, providing a back-up to cover ground system errors and failures.

That, since ATC aspects of the development and use of SSR Mode S are of direct concern to the Federation, IFATCA seek early representation on the ICAO Mode S SSR Improvements and Collision Avoidance System (SICASP) Study Group.

Last Update: September 20, 2020  

November 27, 2019   714   Jean-Francois Lepage    1982    

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