callsign

A callsign is a unique designation assigned to a radio transmitter or operator to identify the origin or destination of a transmission. They are allocated by national regulatory authorities and, for international services, by organizations such as the International Telecommunication Union (ITU) and the International Civil Aviation Organization (ICAO). A callsign is distinct from a vessel’s or aircraft’s registration number and from a license, and it is used primarily in voice communications to address and route messages clearly.

In aviation, the callsign identifies an aircraft or flight during air traffic control communications. The callsign typically combines the airline’s designator with the flight number (for example, a British Airways flight might use a designator such as SPEEDBIRD followed by the numeric flight number). General aviation often uses the aircraft’s registration as the callsign when no formal flight number is used, or a local station designator is adopted. The callsign is assigned by the relevant aviation authority and is essential for routing communications and maintaining safety in crowded airspace.

In amateur radio, individuals hold a personal callsign issued by the national regulator, commonly reflecting the operator’s country and a unique sequence. Operators use their callsign to identify themselves in all transmissions, with the callsign appearing in routine and contest traffic. Changes to a callsign or licensing status require regulatory action.

Maritime and other services also use callsigns. Ships and coast stations obtain callsigns registered with their flag state and the ITU, used in radiocommunications at sea and in distress systems. Military and government users may also employ tactical or unit callsigns that differ from civilian identifiers.