SOS

SOS is the commonly used description for the International distress signal (· · · — — — · · ·) when sent in Morse code. This distress signal was first adopted by the German government in radio regulations effective April 1, 1905, and became the worldwide standard when it was included in the second International Radiotelegraphic Convention, which was signed on November 3, 1906, and became effective on July 1, 1908.

From the beginning, the SOS distress signal has actually consisted of a continuous sequence of three-dots/three-dashes/three-dots, all run together without letter spacing. In International Morse Code, three dots form the letter S, and three dashes make the letter O, so "SOS" became an easy way to remember the correct order of the dots and dashes. In modern terminology, SOS is a "procedural signal" or "prosign", and the formal way to write it with a bar above the letters, i.e. SOS.

In popular usage, SOS became associated with phrases such as "Save Our Souls", "Save Our Ship", and "Stop Other Signals". However, these phrases were a later development, merely used to help remember the correct order of letters.

Early developments

Prior to the development of radio communication in the mid-1890s, seagoing vessels had already adopted a wide variety of visual and audio distress signals, using such things as semaphore flags, signal flares, bells, and foghorns. Radio — which initially was called "wireless telegraphy" — at first employed Morse code, the dot-and-dash system originally developed for landline telegraphy. With the introduction of shipboard radio installations, there was a need for standardized communication, but cooperation was somewhat limited by national differences and rivalries between competing radio companies.

The first International Radiotelegraphic Conference was held in Berlin, Germany in 1903. At the time, Captain Quintino Bonomo, an Italian representative, discussed the need for common operating procedures, including the suggestion that "ships in distress... should send the signal SSS DDD at intervals of a few minutes", according to "The Wireless Telegraph Conference", in the November 27, 1903, issue of The Electrician. However, procedural questions were beyond the scope of the 1903 Conference, so although Article IV of the Conference's Final Protocol, signed August 13, 1903, stated that "Wireless telegraph stations should, unless practically impossible, give priority to calls for help received from ships at sea," no standard signal was adopted at the time.

Because of the absence of international regulations, individual organizations were left to develop their own practices. Perhaps the first radio distress call adopted was "CQD" ( — · — ·    — — · —    — · · ) which was announced on January 7, 1904 by "Circular 57" of the Marconi International Marine Communication Company, and which became effective for Marconi installations beginning February 1, 1904. Another suggestion appeared in the 1906 edition of S. S. Robison's "Manual of Wireless Telegraphy for the Use of Naval Electricians", published for use by the United States Navy. This stated that the standard visual flag signals, known as the International Code of Signals, would likely also be adopted for radio use. Thus, the flag signal "NC" ( — ·     — · — · ), which stood for "In distress; want immediate assistance", would also likely become the radio distress call.

SOS created in Germany

A third standard resulted in the creation of the SOS distress signal. The German government issued a set of national radio regulations, effective April 1, 1905, which introduced three new Morse code sequences, including the SOS distress signal:

1. Ruhezeichen ("Cease-sending signal"), consisting of six dashes ( — — — — — — ), sent by shore stations to tell other local stations to stop transmitting.
2. Suchzeichen ("Quest signal"), composed of three-dots/three dashes/one-dot, all run together (· · · — — — · ), used by ships to get the attention of shore stations.
3. Notzeichen ("Distress signal"), consisting of three-dots/three-dashes/three-dots (· · · — — — · · · ), also in a continuous sequence, "to be repeated by a ship in distress until all other stations have stopped working".

In 1906, the second International Radiotelegraphic Convention was held in Berlin. This convention developed an extensive collection of Service Regulations to supplement the main agreement, which was signed on November 3, 1906, and became effective on July 1, 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 the Cunard liner Slavonia on June 10, 1909, according to "Notable Achievements of Wireless" in the September, 1910 Modern Electrics. However, there was some resistance among the Marconi operators about adopting 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. But with the need for consistency for public safety, the use of CQD largely disappeared after this point.

In both the April 1, 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, and it soon became common to refer to the distress signal as "SOS" — an early report on "The International Radio-Telegraphic Convention" in the January 12, 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 "S5S").

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 of dots-and-dashes, 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 dots and dashes. In later years, the number of special Morse symbols increased with the addition of numerous procedural signals. In order to designate the proper sequence of dots-and-dashes for these special symbols, the standard practice is to list alphabetic characters which contain the same dots-and-dashes in the same order, with the understanding that there are no internal spaces in the transmission of the special signal. It is also common practice to list procedural signals with a bar above the letters, thus, under this notation, the distress signal becomes SOS. (In International Morse, VTB, IJS and SMB, would also correctly translate into the · · · — — — · · ·  distress call sequence, but traditionally only SOS is used).

SOS has also sometimes been used as a visual distress signal, consisting of three-short/three-long/three-short light flashes, or with "SOS" spelled out in individual letters, for example, stamped in a snowbank or formed out of logs on a beach. The fact that SOS can be read right-side-up as well as upside-down makes visual recognition easier.

References

• "The Wireless Telegraph Conference", The Electrician, November 27, 1903, pages 157-160, 214.
• Final Protocol, First International Radio Telegraphic Conference, Berlin, 1903.
• Regelung der Funkentelegraphie im Deutschen Reich, Elektrotechnische Zeitschrift, April 27, 1905, pages 413-414.
• German Regulations for the Control of Spark Telegraphy, The Electrician, May 5, 1905, pages 94-95.
• Robison, S. S., "Manual of Wireless Telegraphy for the Use of Naval Electricians", 1st edition, 1906.
• 1906 International Wireless Telegraph Convention, U.S. Government Printing Office.
• "The International Radio-Telegraphic Convention", Electrical World, January 12, 1907, pages 83-84.
• "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").

See also

• Mayday
• Pan-pan
• Securite
• Distress signal
• 500 kHz
• CQD