From the beginning, the SOS distress signal has actually consisted of a continuous sequence of three-dits/three-dahs/three-dits, all run together without letter spacing. In International Morse Code, three dits form the letter S, and three dahs make the letter O, so "SOS" became an easy way to remember the correct order of the dits and dashes. In modern terminology, SOS is a Morse "procedural signal" or "prosign", and the formal way to write it is with a bar above the letters, i.e. SOS.
In popular usage, SOS became associated with phrases such as "Save Our Ship" and "Save Our Souls". These were a later development, most likely used to help remember the correct letters (something known as a backronym).
Before the development of radio communication in the early 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 dit-and-dah 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. 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, each ship was left to develop its own practices. For example in 1905 the crew of a sinking lightship off Nantucket transmitted the word "HELP" to call for rescue. Perhaps the first international 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 or a cry for help.
SOS was developed from the general German radio call "SOE", with the 3 dits of a "S" easier to hear in under noisy conditions than the one dit of an "E". Also, the otherwise meaningless string of letters was selected because it is easily recognizable and can be sent rapidly. Comparing SOS (di-di-di-dah-dah-dah-di-di-dit) with the older CQD (dah-di-dah-dit dah-dah-di-dah dah-di-dit) (— · — · / — — · — / — · ·) it is obvious how much simpler the new code was. Also, it would not be mistaken for CQ, the radio code for "calling anyone" used in casual circumstances.
In 1906, at the second International Radiotelegraphic Convention in Berlin, an extensive collection of Service Regulations were developed to supplement the main agreement, which was signed on November 3, 1906, becoming 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 to the adoption of the new signal, and, as late as the April, 1912 sinking of the , the ship's Marconi operators intermixed CQD and SOS distress calls. However, in the interests of consistency and public safety, the use of CQD appears to have died out 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-dits/three-dahs/three-dits, with no mention of any alphabetic equivalents. However, in International Morse, three dits comprise the letter S, and three dahs the letter O. It therefore 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 dahs 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 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 which contain the same dits-and-dahs in the same order, with a bar atop the character sequence to indicate that there should not be any internal spaces in the transmission. Thus, under the modern notation, the distress signal becomes SOS. (In International Morse, VTB, IJS and SMB, among others, 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 became important for visual recognition if viewed from above.
During the Second World War, additional codes were employed to include immediate details about attacks by enemy vessels, especially in the Battle of the Atlantic. The signal SSS signalled attacked by submarines, whilst 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 code. All of these codes later switched from three repeats of the letter to four repeats ("RRRR", etc.).
None of these signals were used on their own. Sending SOS as well as other warning signals (TTT, XXX etc.) used similar procedures for effectiveness. These were always followed correctly. Here is an example of an SOS signal the portions in brackets are an explanation only.
SOS SOS SOS de (this is) GBTT GBTT GBTT (call sign of the QE2 repeated 3 times) Queen Elizabeth 2 (name of ship) psn (position) 49.06.30 North, 04.30.20 West. Ship on fire, crew abandoning ship (nature of distress) AR (end of transmission) K (invitation to reply).
Many merchant vessels carried only one or two radio operators in which case the SOS may not be heard by operators off duty. Eventually equipment was invented to summon off-duty operators by ringing an alarm in the operators berth. This was triggered by the operator of the ship in distress transmitting twelve long dashes of four seconds duration each. These were sent prior to the SOS hopefully ringing the automatic alarm in ships so equipped. If possible a short delay was given before transmission of the SOS proper. This was to give those off watch operators time to get to their radio office.
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