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	SOS Listen to the Morse code of SOS
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SOS is the International Morse code distress signal (▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄); the bar over it indicates that the normal gaps between the letters should be omitted. It is used as a start-of-message mark for transmissions requesting help when loss of life or catastrophic loss of property is imminent, as opposed to CT = KA which precedes a routine message. Other prefixes are assigned for mechanical breakdowns and requests for medical assistance.

This distress signal was first adopted by the German government radio regulations effective 1 April 1905, and became the worldwide standard under the second International Radiotelegraphic Convention, which was signed on 3 November 1906, and became effective on 1 July 1908. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System.^[1]SOS is still recognized as a visual distress signal.^[2]

The SOS distress signal is a continuous sequence of three dots, three dashes, and three dots, with no spaces between the letters (notated by the overbar). In International Morse Code, three dots form the letter S, and three dashes make the letter O, so "S O S" became a way to remember the order of the dots and dashes. In modern terminology, SOS is a Morse "procedural signal" or "prosign",^[3] and the formal way to write it is with a bar above the letters or enclosed in angle brackets: SOS or <SOS>.

In popular usage, SOS became associated with such phrases as "Save our Souls" and "Save our Ship". SOS is only one of several ways that the combination could have been written; IWB or V7, for example, would both produce exactly the same sound; SOS is just easier to remember.

Formalization

The use of the SOS signal was first introduced in Germany as part of a set of national radio regulations, effective 1 April 1905. These regulations introduced three new Morse code sequences, including the SOS distress signal.

In 1906, at the second International Radiotelegraphic Convention in Berlin, an extensive collection of Service Regulations was developed to supplement the main agreement, which was signed on 3 November 1906, becoming effective on 1 July 1908. Article XVI of the regulations adopted Germany's Notzeichen (distress signal) as the international standard, reading: "Ships in distress shall use the following signal: ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄ repeated at brief intervals". The first ship to transmit an SOS distress call appears to have been either the Cunard liner RMS Slavonia on 10 June 1909, according to "Notable Achievements of Wireless" in the September 1910 Modern Electrics, or the steamer SS Arapahoe on 11  1909.^[4] The signal of the Arapahoe was received by the United Wireless Telegraph Company station at Hatteras, North Carolina, and forwarded to the steamer company's offices.^[5] However, there was some resistance among the Marconi operators to the adoption of the new signal, and, as late as the April 1912 sinking of the RMS Titanic, the ship's Marconi operators intermixed CQD and SOS distress calls. However, in the interests of consistency and water safety, the use of CQD appears to have died out thereafter.

In both the 1 April 1905, German law, and the 1906 International regulations, the distress signal was specified as a continuous Morse code sequence of three-dots/three-dashes/three-dots, with no mention of any alphabetic equivalents. However, in International Morse, three dots comprise the letter "S", and three dashes the letter "O". It therefore soon became common to refer to the distress signal as "S O S". An early report on "The International Radio-Telegraphic Convention" in the 12 January 1907, Electrical World stated that "Vessels in distress use the special signal, SOS, repeated at short intervals." (In American Morse code, which was used by many coastal ships in the United States through the first part of the twentieth century, three dashes stood for the numeral "5", so in a few cases the distress signal was informally referred to as "S 5 S".)

In contrast to CQD, which was sent as three separate letters with spaces between each letter, the SOS distress call has always been transmitted as a continuous sequence dits and dahs, and not as individual letters. There was no problem as long as operators were aware that "SOS" was technically just a convenient way for remembering the proper sequence of the distress signal's total of nine dits and dahs. In later years, the number of special Morse symbols increased. In order to designate the proper sequence of dits and dahs for a long special symbol, the standard practice is to list alphabetic characters that contain the same dits and dahs in the same order, with a bar atop the character sequence to indicate that the sequence is a digraph and there should not be any internal spaces in the transmission. Thus, under the modern notation, the distress signal becomes SOS (In International Morse Code, VTB, IJS, VGI, SMB, and VZE (among others) would all correctly convert to the ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄ distress-call sequence, but traditionally only SOS is used.)

It has also sometimes been used as a visual distress signal—consisting of three short, three long, and three more short flashes of light, such as from a survival mirror, or with "S O S" spelled out in individual letters (for example, stamped in a snowbank or formed out of logs on a beach). The fact that the letters "S O S" can be read right side up as well as upside down (as an ambigram) became important for visual recognition if viewed from above.

Later developments

Additional warning and distress signals followed the introduction of SOS. On January 20, 1914, the London International Convention on Safety of Life at Sea adopted the Morse code signal TTT (▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄), three letter Ts (▄▄▄▄▄▄) spaced correctly as three letters so as not to be confused with the letter O (▄▄▄▄▄▄▄▄▄▄▄▄▄▄), as the "Safety Signal", used for messages to ships "involving safety of navigation and being of an urgent character".

"Mayday" voice code

With the development of audio radio transmitters, there was a need for a spoken distress phrase, and "Mayday" (from French m'aidez "help me") was adopted by the 1927 International Radio Convention as the equivalent of SOS. For TTT, the equivalent audio signal is "Sécurité" (from French sécurité "safety") for navigational safety, while "Pan-pan" (from French panne "breakdown") signals an urgent but not immediately dangerous situation. French was the international language at the time that these were formalized.

World War II suffix codes

During World War II, additional codes were employed to include immediate details about attacks by enemy vessels, especially in the Battle of the Atlantic. The signal SSS signaled attacked by submarines, while RRR warned of an attack by a surface raider, QQQ warned of an unknown raider (usually an auxiliary cruiser), and AAA indicated an attack by aircraft. They were usually sent in conjunction with the SOS distress signal. All of these codes later switched from three repeats of the letter to four repeats, e.g., "RRRR".

None of these signals were used on their own. Sending SOS as well as the urgency signal (XXX in CW, and PAN-PAN in voice) and safety signal (TTT in CW, and SECURITE in voice) used similar procedures for effectiveness. These were always followed correctly. Here is an example of an SOS signal; the portions in parentheses are an explanation only.

SOS SOS SOS (urgent distress call follows)

DE (from) GBTT GBTT GBTT (GBTT identifies the Queen Elizabeth 2 radio room, repeated 3 times)

(Ship) QUEEN ELIZABETH 2 PSN (position) 49 06 30 N (North latitude) 04 30 20 W (West longitude)

(Our ship is) ON FIRE (and the crew is) ABANDONING SHIP
AR (End of Message) K (reply, anyone)

Audio tone signals and automatic alarms

Ships and coastal stations would normally have required quiet times twice an hour to listen for priority signals, for 3 minutes, at different times for 500 kHz and 2182 kHz.

Ship's radio room clock, with 4 second long red bands with 1 second white gaps around the circumference, so the CW alarm signal could be sent manually. The red and green wedges denote compulsory 3 minute silent periods for receiving weak distress signals.

Since many merchant vessels carried only one or two radio operators, no one might hear a distress signal when both operators were off-duty. Eventually, equipment was invented to summon operators by ringing an alarm in the operator's cabin, and on the bridge, and the only switch able to disable the alarm was only permitted to be in the wireless telegraph room.^[6] The alarm was sent by the operator on the ship in distress transmitting the radiotelegraph alarm signal (auto-alarm) signal—twelve extra-long dashes, each lasting four seconds with a one-second gap between them, and transmitted in A2 (modulated CW).^[7] The alarm signal was normally sent with a mechanical or electronic timing circuit to ensure it was sent accurately. However, ships radio room clocks typically had markings on the dial to guide operators in sending the signal manually. The regulations for the auto-alarm were defined in the 1927 Safety Of Life At Sea (SOLAS) international maritime regulations, and in Article 19, § 21, of the General Regulations annexed to the International Radiotelegraph Convention, 1927.5 5.^[6]

The Auto Alarm receivers were designed to activate upon receiving four such dashes. Once four valid dashes are detected, the automatic alarm is activated. The distressed ship's operator would then delay sending the SOS message itself to give off-watch radio operators time to reach their radio room.

The radiotelephony equivalent of the radiotelegraph alarm signal is the radiotelephony alarm signal, which is the transmission of alternating tones of 2200 Hz and 1300 Hz, with each tone having a duration of 250 ms. Automatic alarm systems aboard ships must activate when such a signal is received and the receiving vessel is within 500 nmi (930 km) of the receiving vessel's position, or if the distress position is in the polar areas (latitude greater than 70° N or 70° S). The alarm should also activate when the call is received and the distance between the vessel in distress and the receiving vessel cannot be determined.^[8]

Historical SOS calls

• 
  Steamship Kentucky sank in 1910, early use of SOS which saved all 46 lives on board.


• 
  RMS Titanic (which used CQD as well), sank in 1912


• 
  RMS Lusitania, torpedoed in 1915


• 
  HMHS Britannic, sank in 1916


• 
  SS Andrea Doria, sank in 1956

See also

Look up SOS in Wiktionary, the free dictionary.

• 500 kHz


• 2182 kHz


• Distress signal


• 
  Global Maritime Distress and Safety System (GMDSS)


• Mayday


• Pan-pan


• Prosigns for Morse code


• CQD


• Save Our Shores


• Sécurité


• Vessel emergency codes

References

This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (September 2009) (Learn how and when to remove this template message)

Further reading

• 
  "The Wireless Telegraph Conference". The Electrician: 157–160, 214. 27 November 1903. 
  


• 
  Final Protocol, First International Radio Telegraphic Conference, Berlin, 1903.


• 
  "Regelung der Funkentelegraphie im Deutschen Reich". Elektrotechnische Zeitschrift: 413–414. 27 April 1905. 
  


• 
  "German Regulations for the Control of Spark Telegraphy". The Electrician: 94–95. 5 May 1905. 
  


• 
  Robison, Samuel (1906). Manual of Wireless Telegraphy for the Use of Naval Electricians (1st ed.). 
  


• 
  1906 International Wireless Telegraph Convention, U.S. Government Printing Office.


• 
  "The International Radio-Telegraphic Convention". Electrical World: 83–84. 12 January 1907. 
  


• 
  "'S 5 S' Rivals 'C Q D' for Wireless Honors", Popular Mechanics, February, 1910, page 156.


• 
  "Notable Achievements of Wireless", Modern Electrics, September, 1910, page 315.


• Collins, Francis A., "Some Stirring Wireless Rescues", from The Wireless Man, 1912, pages 104–141.


• Turnball, G. E., "Distress Signalling", The Yearbook of Wireless Telegraphy and Telephony, 1913, pages 318–322 (includes text of "Circular 57").


• Dilks, John H. III, "Why SOS?" in OSS, June, 2007, pages 88–89.