Mode S compliance support for State aircraft

Military authorities need to take account of the principles for the assignment of 24-bit aircraft addresses as established by ICAO (the International Civil Aviation Organization) and ensure the uniqueness of the aircraft addresses used at any time by State aircraft.

On the other hand, State aircraft carrying and operating a non-compliant Mode S transponder for which an exemption or dispensation for Mode S ELS or EHS requirements has been granted by the NAA, must use a correctly assigned and unique ICAO 24-bit aircraft address at all times during a flight.

Our role

As the implementation of Mode S requires multiple efforts by the military organisations of each member state in terms of technical adaptations on ground and airborne side as well as procedural aspects, we provide support to States and military organisations in helping to clarify technical as well as procedural and legal aspects. For this purpose we are providing respective guidelines e.g. on the safe implementation of the military IFF Mode 5 in the civil Mode S peacetime environment.

Consult the information below and if any additional questions remain, contact us for more details.

Principles laid down by ICAO

In accordance with Annex 10 to the ICAO Convention (Volume III, Chapter 9, entitled Aircraft Addressing System), aircraft addresses are to be assigned to aircraft in accordance with the following principles:

  • at any one time, no address shall be assigned to more than one aircraft;
  • only one address shall be assigned to an aircraft, irrespective of the composition of on-board equipment. The address shall not be changed except under exceptional circumstances and shall not be changed during flight;
  • addresses composed of 24 ZEROs or 24 ONEs shall not be assigned to aircraft.

Military use of 24-bit aircraft addresses

In NATO (North Atlantic Treaty Organization) member States, authorities have earmarked for military use a proportion of the 24-bit aircraft addresses available to them. The relatively large number of aircraft addresses earmarked for military use allows frequent rotation of the assignment of 24-bit addresses to military aircraft. The purpose of this random changing of addresses is to conceal information concerning military airframes which might allow an air order of battle to be accumulate. The controlled rotation of 24-bit addresses is not therefore considered to be contravene the above principles. The 24-bit addresses must not, however, be rotated during flight.

When Mode S transponders are used in flight, unique and correctly assigned aircraft addresses must be used. The presence of duplicate 24-bit addresses can jeopardise the safety of aircraft and the operational efficiency of ATM (air traffic management) functions, and must therefore be avoided at all times.

SSR Mode S (Mode S Selective Address) interrogators and radar trackers

The uniqueness of the 24-bit aircraft address is important for the unambiguous identification of aircraft. The effects of duplicate addresses are unpredictable. They can cause synchronous garbling, radar track swapping or dropping.

Aircraft Collision Avoidance System (ACAS) II equipment uses Mode S protocols and transponders to maintain safe separation between aircraft. Duplicate 24-bit addresses can have serious safety implications. For example, one or more ‘threat’ aircraft can remain partially or totally undetected.

Non-Mode S flights

Mode S equipage programmes for State aircraft may not be completed in line with the deadlines laid down by the Surveillance Performance and Interoperability Implementing Rule (SPI IR). To cover this eventuality, the SPI IR offers certain limited transitional arrangements. At the same time, it requires air navigation service providers (ANSPs) to accommodate State aircraft which are not equipped with Mode S, provided that they can be safely handled within the capacity of the ATM system.

All State aircraft which are not Mode-S-compliant and which are planning to fly in Mode-S airspace have to request dispensation from the relevant national aviation authority (NAA) for any flight.

Dispensation needs to be requested by all State aircraft which are not compliant with Mode S ELS (elementary Mode S surveillance) or with EHS (enhanced Mode S surveillance) requirements for flights in EHS notified airspace. Dispensation does not need to be requested by transport-type State aircraft assessed to be non-EHS-capable for flights in EHS notified airspace. However, they must at least be compliant with Mode S ELS requirements.

More information on the airspace concerned and points of contact for the submission of requests for dispensation can be found in national civil and/or military aeronautical information publications (AIPs).