Today BBC News online had an interesting article on the history of the use of SOS as a call for help.
Early ships had no radio communication options and relied on flags, flares or signal lights to communicate with other ships. In a disaster situation ships were usually had to fend for themselves. When the radio was invented it was a great help, allowing communication over the horizon to other ships or land, and could be used to call for help in disasters. The early radio system could not modulate a voice signal and instead just produced a steady carrier wave than could only switched on or off, so morse code was used in a similar way to land based telegraph systems. In fact most ship radio operators came straight from land telegraph systems. The letters “CQ” were used to first call for attention when beginning a transmission. (as is still the case with HAM radio operators using morse code). In cases of emergency operators added a D and used “CQD”, which did not mean “come quick disaster” as some imagined, just “attention, disaster”. The problem was that in all the noise the D could easily be missed, and as “CQ” calls were so common many CQD calls were just missed. In 1906 the international telegraphy community got together to try to solve the problem in some way that was both internationaly acceptable, and impossible to mistake. The Germans suggested “SOE” (dot-dot-dot, dash-dash-dash, dot) which was considered, but it was feared the single dot for the E could be too easily missed. Eventually the conference agreed upon “SOS” (dot-dot-dot, dash-dash-dash, dot-dot-dot) which is hard to mistake for anything else. Interestingly enough, since the pattern was repeated without a pause it can just as easily be read as IJS, SMB, or VTB. SOS became the official call for assistance on July 1, 1908. Apparently the expressions “Save our Souls” or “Save Our Ship” are really just “backronyms” constructed later.
“It is believed the first ship to have sent out an SOS signal was the American steamer Arapahoe in 1909. When the Titanic was sinking in 1912, its operator first sent out CQD and then SOS, alternating. CQD persisted, particularly among British operators, for many years.”

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