SOS

For other uses, see SOS (disambiguation).
SOS
Audio of SOS in Morse code
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SOS is a Morse code distress signal (  ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄ ), used internationally, originally established for maritime use. In formal notation SOS is written with an overscore line (SOS), to indicate that the Morse code equivalents for the individual letters of "SOS" are transmitted as an unbroken sequence of three dots / three dashes / three dots, with no spaces between the letters.[1] In International Morse Code three dots form the letter "S" and three dashes make the letter "O", so "S O S" became a common way to remember the order of the dots and dashes. IWB, VZE, 3B, and V7 form equivalent sequences, but traditionally SOS is the easiest to remember.

SOS, when it was first agreed upon by the International Radio Telegraphic Convention in 1906, was merely a distinctive Morse code sequence and was initially not an abbreviation. Later a backronym was created for it in popular usage, and SOS became associated with mnemonic phrases such as "Save Our Souls" and "Save Our Ship".[2] Moreover, due to its high-profile use in emergencies, the phrase "SOS" has entered general usage to informally indicate a crisis or the need for action.

The phrase "SOS" used on a Belgian emergency telephone traffic sign

SOS originated in German government maritime radio regulations adopted effective 1 April 1905. It became a worldwide standard when it was included in the service regulations of the first International Radiotelegraph Convention signed on 3 November 1906, which became effective on 1 July 1908. In modern terminology, SOS is a Morse "procedural signal" or "prosign",[3] used as a start-of-message mark for transmissions requesting assistance when loss of life or catastrophic loss of property is imminent.[4] Other prefixes are used for mechanical breakdowns, requests for medical assistance, and a relayed distress signal originally sent by another station. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System.[5]

SOS is still recognized as a standard distress signal that may be used with any signaling method.[6] It has been used as a visual distress signal, consisting of three short/three long/three short flashes of light,[7] such as from a survival mirror. In some cases the individual letters "S O S" have been spelled out, for example, stamped in a snowbank or formed out of logs on a beach. "S O S" being readable upside down as well as right side up (as an ambigram) is an advantage for visual recognition.

SOS sent from a flashlight
  1. ^ U.S. Navy, Bureau of Naval Personnel, Radioman 3 & 2, NAVPERS 10228-B, Washington, D.C.: U.S.G.P.O., 1957, pp. 135, 177, 402.
  2. ^ "S O S", The Sailors' Magazine and Seaman's Friend, October 1915, page 158.
  3. ^ Weik, Martin (2012). Communications Standard Dictionary (3rd ed.). Springer Science & Business Media. p. 760. ISBN 9781461304296.
  4. ^ For emergency transmissions, SOS replaces CT = KA =   ▄▄▄ ▄ ▄▄▄ ▄ ▄▄▄ , which is the marker for the start of routine messages.
  5. ^ "Discontinuation of Morse code services in the MF radiotelegraphy band" (PDF). GMDSS. GMDSS Resolution. 10 February 1993. COM/Circ.115. Archived from the original (PDF) on 2020-06-10. Retrieved 2019-08-18.
  6. ^ U. S. Coast Guard, Amalgamated International and U. S. Inland Navigation Rules, Annex IV, Retrieved October 24, 2018
  7. ^ "How to Signal SOS With a Flashlight [Complete Guide]". Lumen Authority. 2021-04-15. Retrieved 2021-07-07.