SOS is the commonly used description for the international Morse code distress signal.
This distress signal was first adopted by the German government in radio regulations
effective April 1, 1905, and became the worldwide standard under the second International Radiotelegraphic
Convention, which was signed on November 3, 1906 and became effective on July 1, 1908.
SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global
Maritime Distress Safety System. SOS is still recognized as a visual distress signal.
The SOS distress signal is a continuous sequence of three dits, three dahs, and 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 order of the dits and dahs. 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: SOS.
In popular usage, SOS became associated with such phrases as "save our ship", "save our
souls" and "send out succour". These may be regarded as mnemonics, but SOS does not actually
stand for anything and is not an abbreviation, acronym or initialism. In fact, SOS is only
one of several ways that the combination could have been written; VTB, for example, would
produce exactly the same sound, but SOS was chosen to describe this combination. SOS is
the only 9-element signal in Morse code, making it more easily recognizable, as no other symbol
uses more than 8 elements.
Formalization The use of the SOS signal was first introduced
in Germany as part of a set of national radio regulations, effective April 1, 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 November
3, 1906, becoming effective on July 1, 1908. Article XVI of the regulations adopted Germany's
Notzeichen 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 Slavonia on June 10,
1909, according to "Notable Achievements of Wireless" in the September, 1910 Modern Electrics,
or the steamer SS Arapahoe on August 11, 1909. 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. 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 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-ditsthree-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 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.. 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 "SOS" spelled out in individual letters.
The fact that SOS can be read right side up as well as upside down 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." With the development of audio radio transmitters,
there was a need for a spoken distress phrase, and "Mayday" was adopted by the 1927 International
Radio Convention as the equivalent of SOS. For TTT, the equivalent audio signal is "Sécurité"
for navigational safety. It is interesting to note that "Mayday" actually originated
as the French m'aidez and that there is a third, lesser distress call: "panne, spelled
"pan" in English. French was the international language at the time that these were formalized.
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 signaled attacked by submarines, whilst RRR warned of an attack by a surface raider,
QQQ warned of an unknown raider, 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. None of these signals were used on their own.
Sending SOS as well as other warning signals 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 de GBTT GBTT GBTT Queen Elizabeth 2 psn 49.06.30 North, 04.30.20 West. Ship
on fire, crew abandoning ship AR K.
Ships and coastal stations would normally have required quiet times twice an hour to
listen for priority signals. However, 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. Famous SOS calls
HMHS Britannic RMS Lusitania
RMS Titanic SS Andrea Doria
See also Morse code
500 kHz 2182 kHz
Distress signal GMDSS
Mayday Pan-pan
Sécurité Vessel emergency codes
References
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. Manual of Wireless Telegraphy
for the Use of Naval Electricians. 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. Dilks, John H. III, "Why SOS?" in OSS, June,
2007, pages 88–89.