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Maritime call signs arecall signs assigned as unique identifiers toships andboats. 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 ofradiotelegraph operation, long predatingbroadcast radio, were marine radio stations installed aboard ships at sea. In the absence of international standards, early transmitters constructed afterGuglielmo 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 asMorse code abbreviations to denote the various individual stations on the line (for instance, AX could represent Halifax). "N" and two letters would identifyU.S. Navy; "M" and two letters would be aMarconi station.
On April 14, 1912, theRMS Titanic station MGY, busily delivering telegram traffic from ship's passengers to the coastal station at Cape Race, Newfoundland (call sign MCE), would receive warnings of ice fields from Marconi stations aboard theSS Mesaba (call sign MMV) and theSS Californian (call sign MWL).[1] Its distress callCQD CQD CQD CQD CQD CQD DE MGY MGY MGY MGY MGY MGY POSITION 41.44 N 50.24 W would be answered by a station aboard theRMS Carpathia (call sign MPA).[2] Later that same year, an international conference standardised 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 areflags 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, i.e. 3LXY2). 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. TheWSB call sign had been held by two ships (theSS Francis H. Leggett, shipwrecked off Oregon's coast on September 18, 1914, and later theFirwood, a ship destroyed by fire near Peru on December 18, 1919[3]) before being assigned toThe Atlanta Journal for use by its Atlanta, Georgia, broadcast radio station in 1922. Similarly, WEZU, the international radio call sign of the shipSS Lash Atlantico, was assigned in 1997 to a broadcast station.[4] Additional call signs would be reassigned to coastal stations or moved from marine radio to terrestrial broadcast radio when ships were sold for registration to foreign nations, as the new owners would obtain new, local call signs for any existing shipboard radio stations.
In the U.S., leisure craft withVHF radios may not be assigned call signs, in which case the name of the vessel is used instead. Ships in the U.S. wishing to have a radio licence anyway are under FCC Radio Service Code SA: "Ship Recreational or Voluntarily Equipped".[5] Those calls follow the land mobile format of the initial letter K or W followed by 1 or 2 letters followed by 3 or 4 numbers (such as KX0983 or WXX0029).
U.S. Coast Guard small boats have a number that is shown on both bows (i.e. port and starboard) in which the first two digits indicate the nominal length of the boat in feet. For example, Coast Guard 47021 refers to the 21st in the series of 47-foot motor lifeboats. The call sign might also be abbreviated to the final two or three numbers during operations, for example: Coast Guard zero two one.