A maritime call sign is a unique identifier assigned to ships and boats. All radio transmissions must be individually identified by the call sign. Merchant and naval vessels are assigned call signs by their national licensing authorities.
One of the earliest applications of radiotelegraph operation, long predating broadcast radio, were marine radio stations installed aboard ships at sea. In the absence of international standards, early transmitters constructed after Guglielmo Marconi’s first transatlantic message in 1901 were issued arbitrary two-letter calls by radio companies, alone or later preceded by a one-letter company identifier. These mimicked an earlier railroad telegraph convention where short, two-letter identifiers served as Morse code abbreviations to denote the various individual stations on the line.
For example, “N” and two letters would identify U.S. Navy; “M” and two letters would be a Marconi station. On April 14, 1912, the RMS Titanic station MGY, busily delivering telegram traffic from ship’s passengers to the coastal station at Cape Race, Newfoundland, would receive warnings of ice fields from Marconi stations aboard the SS Mesaba and the SS Californian. Its distress call would be answered by a station aboard the RMS Carpathia.
Later that same year, an international conference standardized radio call signs so that the first two letters would uniquely identify a transmitter’s country of origin. Merchant and naval vessels are assigned call signs by their national licensing authorities. 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. United States merchant vessels are given call signs beginning with the letters “W” or “K” while U.S. naval ships are assigned call signs beginning with “N”.
Originally, both ships and broadcast stations were given call signs in this series consisting of three or four letters, but as demand for both marine radio and broadcast call signs grew, gradually American-flagged vessels were given longer call signs with mixed letters and numbers. As broadcast stations became commonplace in the 1920s, some original three- and four-letter call signs were reassigned as the corresponding ships were removed from U.S. registry.
The WSB call sign had been held by two ships before being assigned to The Atlanta Journal for use by its Atlanta, Georgia, broadcast radio station in 1922. The SS Francis H. Leggett was shipwrecked off Oregon’s coast on September 18, 1914, and later the Firwood, a ship destroyed by fire near Peru on December 18, 1919.
In New Zealand, it is a legal requirement to have a Maritime VHF Callsign if operating a Maritime VHF Radio. Vessels may be issued with either a VHF or a VHF/SSB callsign. In New Zealand, callsigns commonly begin with the letters ZM followed by another letter and 4 numbers. For example ZMR7777. New Zealand registered vessels heading offshore may be recognized through their ZM callsign. An MMSI is issued by Radio Spectrum Management and is permanently attached to a callsign. They must not be separated. It is an offence to use a false MMSI or Callsign for a radio transmission.