Mode S – Review of Policy

Mode S – Review of Policy

40TH ANNUAL CONFERENCE, Geneva, Switzerland, 19-23 March 2001

WP No. 91

Mode S – Review of Policy


The current IFATCA policy on Mode S was adopted in 1982 and 1994 see Annex 1. Since then progress on the subject has been slow, however in Europe the shortage of SSR codes has reached critical status and Eurocontrol have made Mode S the enabler to resolve this problem. This paper has drawn information from the EATMP Mode S Programme “ Functional Requirements for System Support for the Transition to elementary Mode S Surveillance” as the basis for discussion as it from there that the contentious issues for IFATCA have come.

The objectives of the EATMP Mode S programme is:

  • to resolve the shortage of Mode A codes, by removing the necessity to assign a discrete code to every flight;
  • to reduce SSR radio frequency congestion, by reducing the number of transmissions per flight.

The current IFATCA policy does not address these specific points with sufficient clarity, this paper will look at the issues and propose policy to reinforce the current policy.


Use of ICAO 24-bit aircraft address

There is some debate as to whether ATC should receive this and in which format. The EATMP MOFR proposal argues that ATC does not require this information.

A Mode S SSR response from a flight includes the unique 24 bit address of the aircraft In order to alleviate the shortage of Mode A codes, it had been proposed that, in principle, the discrete aspects of the function be achieved by the Mode S address in place of the Mode A code.

The Mode S address is fixed for an individual airframe whereas Mode A code assignment and correlation as currently performed relate to a flight.

In addition, unlike a discrete Mode A code, the airframe (and thus the address) is not unique to a single flight plan; it may be present in a series of flights with the same or different RTF callsigns to be performed by a particular aircraft. E.g in the case of a business-jet that is flying from Brussels to Paris, picking up one passenger, going to London and immediately back to Geneva, the pilots have 3 valid flight plans with 3 valid codes in hand in their cockpit. It would be quite possible that in a moment of inattention the second code is used instead of the first. This would be a failure to identify or would identify on a different FPL.

The address may be included in an ICAO flight plan in the CODE element of item 18 as specified in PANS-RAC but will not be available for flights filed as repetitive flight plans (RPL’s), military flights and many other flight plans which do not include data regarding the specific airframe that is to perform the flight.

Error Situations

The aircraft address is entered in the flight plan as a group of six hexadecimal characters, each consisting of a digit 0 to 9 or a letter A to F (representing the numbers 10 to 15), e.g. 45DF9B. Where the data is entered manually, there is a high probability of error (10% ?) which, in the case of a transposition error, would result in an apparently valid, although incorrect code. This would result in a failure to correlate with the correct flight plan.

Recovery from an incorrect address would require the manual correlation of the track and the flight plan, allowing the association of the correct aircraft address. This would represent an unwelcome increase in controller workload. There is also the remote possibility that an erroneous address could be assigned to a different uncorrelated flight in the system, resulting in an incorrect correlation.

A critical situation could occur if an operator elected to reverse the assignment of two airframes to two flights without notifying the change and the flights were to take off successively. Both flights would then correlate incorrectly and a hazardous situation could easily occur.

Eurocontrol believe that :

  • the availability of the address will be low;
  • in many cases the address will not provide unambiguous correlation with a single flight plan;
  • there is a risk of correlation failure which would cause unnecessary controller distraction.

IFATCA contends that ATC is interested in receiving the 24-bit individual aircraft address. This address, combined with an updated database of the world aircraft fleet, will provide ATC with correct and updated information about aircraft equipage, e.g. RVSM or MLS.

This information will reduce R/T workload and speed up co-ordination and aircraft handling.

There is another risk associated with distribution of codes. How does the ATSP’s receive the correct codes to identify the flights FPL? Should there be an IFPS failure when no data is available or that due to computer software problems the old codes remain active in the FDPS/RDPS identification using 6 alpha numeric characters may prove very difficult. With more and more airlines using alpha numeric callsigns the difficulties in retention on a radar display will be impossible.

Identification procedures for Mode S

PANS-RAC specifies that equipage with Mode S with callsign reporting capability be identified in an ICAO flight plan by the inclusion of letter I or S in element (b) of item 10 (Equipment).

Like the aircraft address, an aircraft identification is not necessarily unique to a single flight plan, particularly for business and general aviation aircraft using the ‘tail number’. Where fast ‘turn rounds’ are planned and/or one or more of the flights are of short duration, a number of flight plans may be present in an FDPS with a matching aircraft identification (tail number). The problem also occurs to a lesser degree for flights using the flight number. Therefore correlation using the aircraft identification requires additional filtering where ambiguity could occur.

In the situation where a flight is marked as Mode S equipped in the flight plan and a non-discrete mode A (conspicuity) code has been assigned but the aircraft is not so equipped, the surveillance data for the flight will fail to correlate with the flight plan and a labelled data block will not be displayed.

Establishment of correlation based on the use of callsign reporting capability specified in the flight plan could cause significant problems with RPLs. As correlation would not otherwise take place, it would be important that callsign reporting capability be indicated only when the whole fleet eligible for use for the repetitive flight concerned was suitably equipped. This could result in a significant reduction in the use of the facility for many years.

The above problem could be resolved by flight plan modification (CHG) messages being sent where callsign reporting capability exists for a particular flight but is not so declared in the RPL or vice-versa. IATA have stated that the implementation of the function should not depend on the filing of such messages therefore the use of this data cannot be considered a sound basis for correlation using Mode S.

For a transition period, mixed fleet equipage is to be expected. Therefore, to ensure that correlation takes place for all flights, non-equipage would have to be assumed for all RPLs where 100% fleet equipage could not be safely assumed.

The identification of aircraft, or better flights, using the ”aircraft address” or callsign reporting capability of Mode S clearly poses many problems.

It may be noted that these constraints are not a problem for Eastbound North Atlantic flights entering the European domestic area so dependency on the use of the mode S reported callsign would be acceptable in such circumstances. In general, it may be argued that a non-discrete conspicuity code should be assigned to all flights entering a PA in the mode S area which are indicated to be callsign reporting equipped in the flight plan; a correlation failure is not as critical as in the departure phase and action can be taken by the controller to request assignment of a discrete mode A code if necessary. It should be noted that flights already within the PA when entering the mode S area will already have a discrete mode A code assigned by a previous unit so current correlation procedures need not be changed.

In summary, correlation solely based on aircraft identification reporting:

  • Is dependent on correct flight plan filing;
  • Is inefficient (at best) for RPL’s where fleet equipage is mixed;
  • Requires additional processing to handle ambiguity, where it exists;
  • May be acceptable for flights entering the mode S area at a PA boundary.

The conclusion from the above is that, whilst possible in many cases, the initiation of correlation based solely on Mode S aircraft identification reporting will result in limited reduction of Mode A code use for many years and will lead to instances where correlation will not take place and remedial action will be necessary by the controller. Controller acceptance of non-correlation will depend on the circumstances but it is likely to be unacceptable for departures from major airports but acceptable for flights entering the mode S area at a PA boundary. Furthermore the aircraft operators may be expected to object strongly if equipment fitted at significant cost is not used for one of the principal reasons for its installation.

Use of Modes A and S in Combination

As described above, the dependency of correlation initiation by Mode S on flight plan derived data has some risks and/or is likely to give very limited benefit, even in the medium term. On the other hand, the current method using Mode A is efficient in achieving 100% (or very, very near it) initiation and maintenance of correlation. If correlation of a Mode S equipped flight were first to be achieved by Mode A, its actual Mode S status, including the address, could then be determined unambiguously, independently from the flight plan.

An alternative to correlation initiation using the callsign downlinked by the mode S transponder is therefore proposed, particularly where immediate correlation (and the provision of callsign labeled surveillance data) is required. In such circumstances, it is possible to continue to assign a discrete Mode A code to flights (including code retention for flights entering the mode S area) and initiate correlation (after departure or on entering the area) in accordance with current procedures.

The Mode S status of the flight will then be determined. For eligible mode S equipped flights with callsign reporting, the Mode A code can then be released for re-assignment although the flight would continue to squawk the same code for conspicuity purposes, i.e. to ensure that it is visible to units which are not equipped with Mode S. The track would utilize the Mode S data to maintain correlation throughout the Mode S area.

If the flight is not equipped with mode S callsign reporting capability or a discrepancy with the flight plan callsign is identified, the code will retain its discrete status.

The released code can be made available immediately for re-assignment for flights which are to remain within the Mode S area for the whole of their flight or leave it on entry to an area where Mode A is not used for surveillance purposes (such as the North Atlantic Oceanic Control Area). Although such an assessment may not be performed accurately by an individual unit (as most units are is not normally concerned with the whole route of flight after leaving their airspace), it is suggested that the great majority of eligible intra- European flights could be determined from the departure and destination airports and the route pertinent to the unit. It would also be possible for IFPS to determine eligibility.

In respect of inter-unit notification and co-ordination procedures, the question of mode A status notification requires further discussion. It is suggested that in some cases it may be necessary to advise the next unit that the Mode A code is not being used for correlation purposes but for conspicuity only in order to avoid an erroneous correlation with a following non-equipped flight squawking the same code.

Further discussion is required in relation to the effect on non-mode S units adjacent to the area.

Where an equipped flight is to land at an airport within coverage of the adjacent unit, rules may be required to protect the disassigned code with respect to subsequent flights that plan to leave the Mode S area.

Reduced protection may need to be applied to flights that leave the Mode S area at the PA boundary and rules for code optimization as specified in [SSR FS] should be applied.

Under this method the number of Mode A codes required by a unit using Mode S may be expected to reduce considerably, except that the potential depends on a relatively short assignment time before release; if discrete codes are assigned a long time, e.g. an hour before take off, the potential for code savings is greatly diminished.


IFATCA contends that ATC is interested in receiving the 24-bit individual aircraft address and that there should be established a coorrelation utilising a suitable database.

IFATCA believes that the use of ”Conspicuity Codes” of a Mode S identified aircraft within the Mode S area must be studied in depth as other implications such as level 3 co-ordination with military units, possibly having no Mode S equipment, must be studied with care. It could well be that such a positive identification of each flight, as nowadays with SSR, cannot be achieved and so risk of miss co-ordination and miss-identification is much greater. A safety validation and studies of all operational implications is urgently necessary.

The correlation of a departing flight using Mode S must be as safe and easy as the procedures used nowadays with SSR. It is very worrying to hear of identification procedures where a individual SSR code is used for a mode S flight, and once identification is achieved, ATC must re-assign this same SSR-code again without delay in order to save codes (becoming a conspicuity code = non individual code). Workload of ATC must be assessed and it must be ascertained that a rapid re-use of the same SSR-code is really safe and without any risk of miss-correlation. Workload issues and last minute airframe changes are just some of the problems that must be solved.

Transfer of Mode S flights to non-Mode S units is another problem. The current operations is that the receiving unit is responsible of assigning the new individual ATC-transponder code to an aircraft and in sequence establish positive radar identification. The proposal suggests now that the last Mode S unit before transfer to a SSR-only unit, is to assign to the aircraft an individual SSR-code, presumably using the ORCAM concept, and identify it using the 4096 bit response of mode A, before it is sent over to the receiving SSR unit having no mode S equipment.

It is acknowledged that there is a potential safety risk at the transfer area between the Mode S zone and the non-Mode S units using SSR, and associated with additional workload on ATC units.

In areas of mixed mode operations operational input and risk analysis are required, as it must be ascertained that the proposed procedures are safe and workable.

Procedures involving civil and military coordination in the same area eg a military unit is not Mode S equipped and the aircraft are using conspicuity codes and are mode S identified. Must be fully investigated.

It must be ascertained that well-identified and acknowledge problems such as lack of SSR-codes and uncoordinated over-interrogation of transponders by SSR-antennas, is not corrected by immature, potentially unsafe and quick fixes lowering the current safety standard and bringing in even more safety hazards.

It is recommended that:

Prior to the operational use of Mode S that ATC have in place the necessary database correlating 24 bit address with airframes. This database must be continuously updated, including airborne equipment fitted.

That ATC receives the 24 bit address.

The use of ”Conspicuity Codes” of a Mode S identified aircraft within the Mode S area must ensure that the safe operations of all other non mode S units in that airspace eg military operations are not compromised.

The correlation of a departing flight using Mode S must be as safe and easy as the procedures used nowadays with SSR.

Where Mixed Mode operations are conducted simulations and training must be provided to all concerned.

Last Update: September 29, 2020  

March 12, 2020   112   Jean-Francois Lepage    2001    

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