Safe Use of TCAS/ACAS and the Impact of Mode S Development

  • Home 1984 Safe Use of TCAS/ACAS and the ....

Safe Use of TCAS/ACAS and the Impact of Mode S Development

23RD ANNUAL CONFERENCE, Estoril, Portugal, 26-30 March 1984

WP No. 78

Safe Use of TCAS/ACAS and the Impact of Mode S Development


This paper expands on the paper presented last year and takes account of current development work. It is not intended to describe TCAS in full, but to provide a background against which Conference may evaluate the Draft Recommendations. It is hoped that Conference will decide that more detailed IFATCA policy is now due.


Mode S is now being defined by ICAO in terms of compatibility with existing secondary surveillance radar (SSR), the technical characteristics of transmissions, the format of transmissions, the content and use of these transmissions (including operational techniques). Because of the rate of progress on Mode S development, and the increasing demand for its capabilities, the formulation of Standards and Recommended Practices is likely to be rapid.

Without Mode S there is no positive method of identifying a radar return as a particular aircraft seen on the previous radar scan, without additional communication. For example communication by R/T to agree the code to be set on the transponder, or by an input to the ground computer system to identify the position of a particular aircraft. Basic Mode S provides this additional communication and gives target identity. Identity codes will not be changeable as with current SSR. This allows information to be addressed to and interpreted by each aircraft individually, not only ground to air , but also air to air. Although flight plan and aircraft information may not be present, this ability to be certain of updating a previous radar return with an identified target on the next radar scan is most significant. The additional capability of Mode S to carry data is equally significant. Both advances are made possible by progress in the field of computer technology.

Mode S is used to advantage in conjunction with monopulse radar techniques. The continuous target identity facility is enhanced, and the transponder can be ‘turned off’ for a particular radar site when the target position is established using signal comparison techniques, before the radar beam has fully passed across the target. The continuous identification of a target using Mode S, together with its Mode C altitude, provides the basis for the Traffic Alert and Collision Avoidance System (TCAS). Current development in the USA has established a two tier Mode S system, an economy version for VFR general aviation (TCAS I) , and a fully equipped collision avoidance version (TCAS II). This combination is expected to provide adequate collision avoidance for all airborne environments involving controlled aircraft and those on scheduled services.

Because the fitment of Mode S and TCAS is likely to be rapid, at least in the USA, procedures must be developed to accommodate a mixture of Mode S and basic SSR – equipped aircraft. In addition, procedures must be developed to permit co-ordination of air and ground action in mixed control environments. Whilst ‘self-protection’ cannot be denied to any Captain responsible for his aircraft and passengers, there are circumstances where uncoordinated action may increase his risk instead of reducing it.

Although TCAS is being proposed as a collision avoidance system i.e. last resort action in the case of failure of all other methods of separation, it does not mean to say that the Traffic Alert function is merely the advance warning capability of collision avoidance advisories. There are many additional proposals for this function. For example an enhanced TCAS could be used , in areas of poor navigational or communication resources, to ensure separation. In addition, it could possibly be used to maintain separation between two aircraft. It could also be argued that TCAS provides a means whereby positive control is not required over all traffic in some areas where TCAS is fitted to all aircraft. It has yet to be shown that TCAS is sufficiently effective or reliable to compensate for a reduced air traffic control service.

TCAS is only as good as the software algorithms it uses, and these are only as good as the data available. For example, the TCAS paper presented last year to Conference stated that Mode C altitudes to the nearest 100ft. Were not adequate for determining rates of climb and descent or changed thereto. A Working Group of the SICAS Panel is now investigating 50 and 25 feet resolution with Mode S, in order to improve the situation. Azimuth resolution has always been poor, and while improvements are being made, it is likely that air to air azimuth resolution will remain poor for some time to come , without extensive on-board computer systems. However, any additional azimuth data will increase the effectiveness of TCAS and assist in reducing the number of false alerts. Even if it could be assumed that TCAS is an airborne system not directly related to ATC operations, it cannot be assumed that there will be no ATC impact. The introduction of many ‘aircraft only’ systems, such as VOR, inertial navigation, ILS/autopilot coupling have all resulted in changes to ATC. There will be inevitable changes in ATC techniques, procedures and phraseologies to effect quick and safe action to resolve conflicts reported by airborne equipment. Where positive air traffic control is being provided, it is natural that action must be orderly and fully co-ordinated. Uncoordinated action may result in a worsening situation.

Co-ordination is effected by communication. The better the means of communication and its attendant procedures, the better the co-ordination and the efficient the result. In order to provide a better co-ordination of collision avoidance, the same or similar information should be provided to all those concerned, that is , to the controller as well as the pilots. Mode S should therefore contain alert data for the ground organisation. TCAS II equipped aircraft could trigger TCAS I aircraft involved in the collision alert advisory to include an alert feature in their transmissions. This could also be of advantage to those facilities where other ground based automatic conflict alert systems are not available.


Airborne collision avoidance development is continuing to progress in software and hardware. Mode S is the basis for TCAS, which is currently under evaluation. TCAS provides a pilot with an additional sensory aid to maintain the safety of his aircraft, but this will have an effect on ATC operating techniques, procedures and phraseologies. The need for the collision avoidance element of TCAS is already established and bearing in mind that it is intended for last second action, IFATCA should address itself to defining any conditions for its use ( especially in a mixed traffic environment) in relation to ATC procedures. The Traffic Alert function of TCAS alone requires as much consideration as collision avoidance systems. The pilot will now have a better appreciation of other aircraft nearby. As yet there are no procedures or guidelines for pilots to request confirmation of separation from other aircraft. These requests could now be more frequent and interrupt the controllers work. Although responsible use of traffic alert may be assumed, the conditions and relative responsibilities of pilots and controllers have yet to be defined for its use.

The legislative requirement for MODE S fitted aircraft to carry TCAS equipment will, at least initially, probably only apply in USA airspace. Nevertheless, as has been stressed in previous conference WP’s the main problem for IFATCA is that, like it or not, all aircraft engaged on international operations involving flights in USA airspace will have, and use, this equipment in all other parts of the world as well -in areas in which no operational requirement for the carriage of TCAS is as yet envisaged and where the majority of other flying will almost certainly not be TCAS equipped. Mode S itself could provide a better airborne service because of its additional data capacity.

In addition, Mode S could integrate airborne collision avoidance with ground systems. Mode S is capable of providing ground systems, especially computers attempting to predict future movement, with the aircraft’s current intentions. For example. Turn rate, limit of turn, climb and descent.


It is recommended that IFATCA adopt the following statements on TCAS and Mode S to supplement IFATCA’s present basic policy statement on airborne collision avoidance systems and Mode S developments.

(a) The use of automatic airborne collision avoidance systems should allow for safe operation within different types of airspace, with different ATC procedures and with different aircraft equipment capabilities, without detriment to the ATC service or to aircraft not fully equipped.

(b) The inevitable changes in ATC procedures, techniques and phraseologies, as the result of airborne collision avoidance and traffic alert systems, should be compatible, not only with a controller’s responsibilities for providing positive separation, but also with a controller’s ability to discharge them.

(c) Mode S should automatically provide an indication to the ground service, at least, when an aircraft is the subject of a collision avoidance advisory, even if that aircraft is not fully equipped. (I.e. TCAS I only).

(d) IFATCA encourages any development which automatically provides the ground service with the same or similar indications of conflict or potential conflict as provided to a pilot.

(e) Collision avoidance transmissions should not exclude, by virtue of their priority, transmissions directly concerned with establishing and maintaining tactical separation.

(f) Co-ordination between pilot and ATC should have been made, wherever possible, before a manoeuvre contrary to ATC clearance is made as the result of a collision avoidance advisory.

That SC 1 should continue to monitor developments through the IFATCA Observer on the SICAS Panel.

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

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

Mode S Air to Air link capability should be used in a support role for the main Ground system, providing a back up cover for Ground system errors and failures.”

Last Update: September 20, 2020  

November 30, 2019   544   Jean-Francois Lepage    1984    

Comments are closed.

  • Search Knowledgebase