ICAO DOC 8585 Designators for Aircraft Operating Agencies Aeronautical Authorities and Services Ed 183.
The designators consist of: a) three-letter designators for use on the international aeronautical telecommunication service; and b) telephony designators for aircraft operating agencies and for authorities and services operating aircraft, which may be used as appropriate. Parts 1, 2 and 3 consist of encode and decode tables linking aircraft operating agencies, aeronautical authorities and services with their assigned ICAO three-letter and telephony designators. Addresses of government civil aviation authorities are listed in Part 4. New edition quarterly.