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Mode S - Assignment of 24-bit aircraft addresses to State aircraft |
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Military Authorities need to take due account of the principles for assignment of 24-bit aircraft addresses as established by ICAO and ensure the uniqueness of the aircraft addresses used at any one time by State aircraft.
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 NAAs, must use a correctly assigned and unique ICAO 24 bit aircraft address at all times during flight.
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What are the principles set forward by ICAO? |
ICAO convention, Annex 10, Volume III, Chapter 9: Aircraft Addressing System. |
Aircraft addresses shall be assigned to aircraft in accordance with 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 equipment on board.
- The address shall not be changed except under exceptional circumstances and shall not be changed during flight.
- The addresses composed of 24 ZEROs or 24 ONEs shall not be assigned to aircraft.
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Military use of 24-bit aircraft addresses |
In NATO member States, authorities have apportioned part of their overall allocation of 24-bit aircraft addresses for military use. The relatively large number of aircraft addresses for military use allows rotating the assignments of 24-bit addresses on military aircraft on a frequent basis.
The purpose of this random changing of addresses is to conceal information on military airframes allowing building up an Air Order of Battle.
A controlled rotation of 24-bit addresses is therefore not considered in contradiction with the above principles.
This rotation of 24-bit addresses however must not be done during flight.
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Only one aircraft can use a particular address at any time |
When using the Mode S transponder in flight, unique and correctly assigned aircraft addresses must be used.
Assignment authorities need to take special care in order to avoid the use of duplicate aircraft addresses at any time.
The actual effects on CNS-systems are quite unpredictable and range from SSR tracks being dropped, aircraft remaining undetected to misdirected communication messages between controllers and pilots.
Some examples:
- SSR Mode S Interrogators & Radar trackers.The uniqueness property of the 24-bit aircraft address is important for the unambiguous identification of the aircraft. Effects of duplicate addresses are unpredictable. This can cause synchronous garbling, radar track swapping or dropping.
- ACAS II. ACAS II systems use Mode S protocols and transponders to maintain safe separation between aircraft. Duplicate 24-bit addresses can have serious safety implications i.e. one or more of the ‘threat’ aircraft can remain partially or totally undetected.
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The occurrence of duplicate 24-bit addresses can jeopardise the safety of aircraft and the operational efficiency of ATM functions and therefore must be avoided at all times.
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Marc
DE CAT
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Military Expert
- DSS/CM/CNS
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| Email:
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| Phone : |
+32.2.729.3405 |
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Last validation: 14/01/2011
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