The Morse code distress signal is probably one of the most famous messages associated with radio communications.
The SOS message has been included in many films or movies and has many associations with stories like the sinking of the Titanic or other famous ships.
However the reality of the Morse code SOS message is far less romantic and has more associates with people who are in real trouble and wanting assistance.
The sound is very distinctive and there were reserved times on various maritime frequencies that had to be kept clear for distress signals. The signal was also used in many other ways as an alert for a person or group of people in distress.
Video: Morse Code SOS Distress Signal: facts & fiction
Morse code SOS sound
The sound of the Morse code SOS signal is very distinctive with its three short, three long and three short sounds.
One of the key elements is that the Morse code SOS sound is sent not sent as S O S, i.e. three separate characters, but with all the dots and dashes together as one character.
SOS Morse code distress signal basics
The SOS Morse code distress signal was used internationally and it was originally established for maritime use where it was transmitted using on-off keying of a wireless transmitter.
Often called CW for continuous wave or carrier wave, this type of transmission was far easier to transmit as it only required the transmitter to be keyed on and off as opposed to applying a form of audio modulation which was the other option in the time when SOS was widely used.
The other advantage of a Morse signal was that it could be copied at far lower signal strengths than other forms of signal, making it a more effective form of transmission.
SOS signal and message formats
The SOS consist of three dots, three dashes and three dots which are transmitted as a single character - not three letters as is normally seen and heard in films / movies etc.
As a result of the SOS signal being transmitted as a single character, it is often written as a the three letters SOS with an over-score above it.
The character became known as SOS because this was an easy way of remembering it: it could equally well have been designated as "3B", "V7", "IWB" etc as these all make up the same three dots, three dashes and three dots.
When an SOS distress message is being sent, it should follow a set format.
- SOS sent three times
- The word "DE", i.e from, and then the callsign of the vessel three times, and / or other identification of the vessel
- The position
- Other relevant information, e.g. fire, sinking, etc
To make these distress cals, the International distress calling frequency was used. The frequencies for these were 500 kHz, 2182 kHz, and some other frequencies.
There are also various other frequencies used for voice calls, both in the HF and VHF regions of the radio spectrum.
For the International distress frequencies, it was mandatory that when a distress call was heard, all other transmissions should cease. Also a period of silence was maintained on the hour and half hour to ensure any weak signals could be hear.
It was also mandatory that radio silence was maintained on 500 kHz and 2182 kHz channels at the relevant times. Three minute silence periods were used twice every hour on the main primary calling frequencies of 500 kHz (h15 and h45, i.e. 15 and 45 minutes past the hour) and 2182 kHz (h00 and h30).
The clock in a ship's radio room had these periods highlighted. All communication had to stop during these silence periods. If the silence was broken for routine messages the radio operator was likely to be reported, and potentially lose their licence
History of SOS message
SOS was not the first distress signal to be used. With wireless communications being used for many ship communications from around the beginning of the 1900s, the importance of having a set distress signal was realised.
The first distress signal was C Q D, transmitted as three separate letters. The letters CQ were used by radio stations to indicate a general call, indicating they wanted to communicate with someone. The letter D was appended tot his to indicate it was a distress call.
C Q D was adopted by the Marconi International Marine Communication Company, possibly the largest company of the time that was installing radio or wireless equipment onto ships.
On 7 January 1904 the Marconi company issued what was called "Circular 57" which which specified that for the any of the Marconi company ship installations, the new CQD distress called would be used. The starting date for this would be 1 February 1904.
There were also several other distress calls that were adopted by other companies and countries around this time. In fact there was no international agreement on any call and its format.
The famous SOS call was came out of Germany which was the first country to use it. The sequence of three dots, three dashes and three dots was adopted because it was easy to identify and remember. It was very distinctive and could not be confused with other sequences.
The SOS distress call was adopted by Germany from 1 April 1905, and later started to be adopted internationally. The process started in 1906 at the first International Radiotelegraph Convention which met in Berlin. Realising the need to international coordination over distress calls, an agreement was produced and signed on 3 November 1906 to use the SOS call. This became effective on 1 July 1908.
The adoption of SOS as the Morse code distress call did not start to be used by everyone immediately. However, possibly the first reported ship to use the new SOS call was the Cunard liner RMS Slavonia which was shipwrecked off the Azores on 10 June 1909. Two other ships received her signals and went to the rescue.
Even as late as 1912 when the Titanic sank, it was not uniquely used as the wireless operator, Harold Bride, used both CQD and SOS in the distress messages.
The Morse code SOS signal remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System which provided more universal and immediate communications for distress communications.
What does SOS mean
Despite the many ideas, SOS does not stand for anything. It was simply chosen as a set of dots and dashes that was easily identifiable, very memorable and was unlikely to be confused with any other set of dots and dashes.
It was only after its introduction that people started to associate the letters with other words, using it as an abbreviation for a particular phrase.
Although "Save Our Ship" and "Save Our Souls" might seem like an ideal solution to what SOS stands for, neither is true.
However, to many, "Save Our Ship" and "Save Our Souls" can be an easy way of remembering which letters are used in the distress call, they were only made up well after SOS was introduced as the distress signal to be used on the airwaves.
The SOS distress message in Morse code has been used for many years. Although it is not now a mandatory requirement for maritime vessels to be able to send Morse code distress messages, as satellite based systems are now far more reliable.
However, the SOS Morse code distress signal was used for nearly 100 years and resulted in many lives being saved, both at sea and in other situations.
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