Call sign

A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a station's identity.

In the case of states such as Liberia or Panama, which are flags of convenience for ship registration, call signs for larger vessels consist of the national prefix plus three letters (for example, 3LXY, and sometimes followed by a number, e.g. 3LXY2).

Originally, both ships and broadcast stations were assigned call signs in this series consisting of three or four letters.

Maritime coast stations on high frequency (both radiotelegraphy and radiotelephony) were assigned three-letter call signs.

Leisure craft with VHF radios may not be assigned call signs, in which case the name of the vessel is used instead.

Originally aviation mobile stations (aircraft) equipped with radiotelegraphy were assigned five-letter call signs (e.g. KHAAQ).

Land stations in aviation were assigned four-letter call signs (e.g. WEAL – Eastern Air Lines, NYC.)

Currently, all signs in aviation are derived from several different policies, depending upon the type of flight operation and whether or not the caller is in an aircraft or at a ground facility.

In this case, the call sign is spoken using the International Civil Aviation Organization (ICAO) phonetic alphabet.

For example, all British civil aircraft have a five-letter registration beginning with the letter G, which can also serve for a call sign.

One exception to the parallelism between registration and call sign is ultralight airplanes in France, who are not obliged to carry a radio and indeed often don't.

Radio call signs used for communication in crewed spaceflight are not formalized or regulated to the same degree as for aircraft.

The first amateur radio call sign assigned to the International Space Station was NA1SS by the United States.

While broadcast radio stations will often brand themselves with plain-text names, identities such as "Cool FM", "Rock 105" or "the ABC network" are not globally unique.

All new call signs have been four-character for some decades, though there are historical three-character call letters still in use today, such as KSL in Salt Lake City; KOA in Denver; WHO in Des Moines; WWJ and WJR in Detroit; WJW-TV in Cleveland; WBT in Charlotte; WBZ in Boston; WSM in Nashville; WGR in Buffalo; KFI; KNX and KHJ in Los Angeles; and WGN, WLS and WLS-TV in Chicago.

In Canada, the publicly owned Canadian Broadcasting Corporation uses the prefix CB; privately owned commercial broadcast stations use primarily CF and CH through CK prefixes; and four stations licensed to St. John's by the Dominion of Newfoundland government retain their original VO calls.

Amateur radio call signs are in the international series and normally consist of a one or two character prefix, a digit (which may be used to denote a geographical area, class of license, or identify a licensee as a visitor or temporary resident), and a 1-, 2-, or 3-letter suffix.

Examples include VO1S (VO1 as a Dominion of Newfoundland call sign prefix, S to commemorate Marconi's first trans-Atlantic message, a single-character Morse code S sent from Cornwall, England to Signal Hill, St. John's in 1901) and GB90MGY (GB as a Great Britain call sign prefix, 90 and MGY to commemorate the 90th anniversary of historic 1912 radio distress calls from MGY, the Marconi station aboard the famed White Star luxury liner RMS Titanic).

When identifying a station by voice, the call sign may be given by simply stating the letters and numbers, or using a phonetic alphabet.

Also, wireless network routers or mobile devices and computers using Wi-Fi are unlicensed and do not have call signs.

Some wireless networking protocols also allow SSIDs or MAC addresses to be set as identifiers, but with no guarantee that this label will remain unique.

Callbooks were originally bound books that resembled a telephone directory and contained the name and addressees of licensed radio stations in a given jurisdiction (country).

Callbooks have evolved to include on-line databases that are accessible via the Internet to instantly obtain the address of another amateur radio operator and their QSL Managers.