which three initials comprised the distress call prior to sos?

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Transmission procedures, or procedures which govern the sending and receipt of radio messages, recommend that calls for help include one’s own three letters call sign prefixed by “mayday” (pronounced “m’aidez” in French). An example: “Pan-pan Pan Am 123, mayday mayday.” Transmissions should be as brief and concise as possible.
It is not clear at this time which three letters that comprised the distress call prior to S.O.S., though many have speculated it could be CQD – Code de la Navigation Radioelectrique (French for Radio Signaling Code).
“CQD” is a distress call. The distress signal CQD was used from about 1907 to approximately 1929 by radio operators in communication with shipboard radio telephones. Its most famous use was at the sinking of the RMS Titanic on April 15, 1912 as well as several other maritime catastrophes.

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Mayday is not the correct term for anyone experiencing grave and imminent danger requiring immediate assistance; it should be used only when ” your own life [or that of your companions] is in danger and you require immediate assistance . ” However, it is unambiguously identified internationally as being a distress signal by Annex 10 to the International Telecommunication Union’s (ITU) Radio Regulations. The ITU is the international organization in charge of setting radio frequency spectrum, technical standards for radio equipment and procedures that govern the sending and receipt of messages.
Mayday transmissions are finite in number. The first two digits indicate the urgency or priority of the message and not its location. The digits were chosen so that every country with an alphabet had at least one-third of the code available: there would be 18 (three groups of six), which would allow almost every country to have a single letter designated as “coded” for use by rescue services. For example:
ROM, means “Rescue Me”;
QTH, means “Quartermaster”;
NMO, means “Naval Mobile Operator”;
MME, means “Maritime Mobile Operator”, etc.
An example of a message that should be clearly identified as a distress signal is: “CQD Rom 9475.” (“Rome 9475”) The meaning should be obvious to everyone. The last 4 digits indicate the address of the person calling in these messages; there is no need to indicate the name of the person calling. This is one reason why we do not see French language search and rescue broadcasts using this system; note that in English Caen Radio uses CQD instead.

All aircraft are required to carry a radio capable of transmitting and receiving the distress frequency, 243.0 MHz. This is universally referred to as the “Mayday” frequency. In addition, all aircraft must carry operating instructions indicating how to use their radio equipment in order to transmit and receive on this frequency.
The International Civil Aviation Organization (ICAO), an agency of the United Nations with responsibility for matters relating to international aviation, recommends that Mayday and Pan-Pan signals be initiated at 0000Z, 1200Z, and 0000 Universal Coordinated Time (UTC) daily. This recommendation is set forth in Annex 2 (Radio) of the Convention on International Civil Aviation (Chicago 7 December 1944). This recommendation may be changed by ICAO “after consultation with states and the international organizations concerned” (ICAO Resolution No. 449).

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The following signals are defined at Annex 10 of the Radio Regulations:

a) In the event of an accident or emergency in flight, and if such circumstances render it necessary to call for immediate assistance, every aircraft shall transmit her three-letter call sign and a distress signal using international Morse Code (see paragraph 5 below). For this purpose all aircraft shall exhibit a distinctive flashing light visible from the air. If aircraft fitted with radio sets capable of transmitting on two or more frequencies are involved, they should be requested to broadcast simultaneously all their transmissions using any available frequency.