Definition
A 24-bit aircraft address is a unique identifier assigned to an aircraft for air-ground communications, navigation, and surveillance. This identifier consists of 24 bits and is represented as an alphanumerical code made up of six hexadecimal characters, where each character accounts for 4 bits. In total, this allows for 16,777,214 unique identifiers.
Description
The primary role of the 24-bit aircraft address is identification. It does not convey specific details about the aircraft, such as its type or operator. Importantly, at any moment, a particular address can be assigned to only one aircraft, and each aircraft is limited to one address. If a removable transponder is used across multiple aircraft, it must be capable of updating the address each time it is installed in a new aircraft.
Changing an aircraft address is rare and only occurs under exceptional circumstances, never during flight. According to ICAO regulations outlined in Annex 10, Volume III, Part I, Appendix to Chapter 9, member states are responsible for assigning these addresses. The first several bits of the address signify the member state, while the remaining bits indicate the specific number within that state.
To maintain efficient use of address blocks, states should carefully choose how they assign these identifiers. When 75% of the allocated addresses are in use, a state can request more. If an aircraft changes its registration to a new state, the new state will assign a new address from its own block, making the old address available for reuse.
Applications
24-bit aircraft addresses are crucial for routing information to individual aircraft in various applications, including:
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Aeronautical telecommunication network (ATN), like CPDLC used in the European Union.
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Mode S Airborne Collision Avoidance System (ACAS).
When completing a flight plan, the 24-bit address is filled in to link the aircraft to its specific flight, noted in field 18 under the “CODE/” designator (e.g., “CODE/F00001”). This step is vital for ensuring that uplinked clearances reach the intended aircraft.
Interestingly, 24-bit addresses are not exclusive to aircraft. Transponders on aerodrome vehicles, obstacles, or fixed Mode S target detection devices also receive these identifiers. In such cases, the term “aircraft” is broadened to include both aircraft and vehicles (A/V). The same address can be assigned to multiple vehicles, provided they operate at aerodromes that are at least 1,000 km apart.
