SOS: The Universal Distress Signal

The SOS signal is an internationally recognized means for obtaining help. A distress signal indicates that a person or group of people, watercraft, aircraft, or other vehicle is threatened by a serious or imminent danger and requires immediate assistance. Use of distress signals in other circumstances may be against local or international law.

Origins and History of SOS

Radio (initially known as "wireless telegraphy") was developed in the late 1890s and was quickly recognized as an important aid to maritime communication. Previously, seagoing vessels had adopted a variety of standardized visual and audio distress signals, using such things as semaphore flags, signal flares, bells, and foghorns.

In 1903, an Italian representative at the Berlin Preliminary Conference on Wireless Telegraphy, Captain Quintino Bonomo, discussed the need for common operating procedures, including the suggestion that "ships in distress…" Without international regulations, individual organizations were left to develop their practices. For instance, the British Marconi International Marine Communication Company that outfitted ships with telegraph equipment preferred the distress code "CQD," or "-.-. --.- -.." in Morse code.

Germany was the first country to adopt the SOS distress signal, which it called the Notzeichen signal, as one of three Morse code sequences included in national radio regulations which became effective on 1 April 1905. In 1906, the first International Radiotelegraph Convention met in Berlin, which produced an agreement signed on 3 November 1906 that became effective on 1 July 1908.

In both the 1 April 1905 German law and the 1906 international regulations, the distress signal is specified as a continuous Morse code sequence of three dots / three dashes / three dots, with no mention of any alphabetic equivalents.

What is SOS?

SOS is a Morse code distress signal (▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄), used internationally, originally established for maritime use. In formal notation SOS is written with an overscore line (SOS), to indicate that the Morse code equivalents for the individual letters of "SOS" are transmitted as an unbroken sequence of three dots / three dashes / three dots, with no spaces between the letters. In International Morse Code three dots form the letter "S" and three dashes make the letter "O", so "S O S" became a common way to remember the order of the dots and dashes.

SOS, when it was first agreed upon by the International Radio Telegraphic Convention in 1906, was merely a distinctive Morse code sequence and was initially not an abbreviation. The letters don’t actually stand for anything. Not "Save Our Ship," not "Save Our Souls" - it wasn't even originally designed to stand for letters of the English alphabet. Although the letter sequence "SOS" stands for nothing, it came from somewhere as a replacement for a visual distress signal.

In modern terminology, SOS is a Morse "procedural signal" or "prosign", used as a start-of-message mark for transmissions requesting assistance when loss of life or catastrophic loss of property is imminent. Other prefixes are used for mechanical breakdowns, requests for medical assistance, and a relayed distress signal originally sent by another station.

Adoption and Early Use of SOS

The first ships that have been reported to have transmitted an SOS distress call were the Cunard oceanliner RMS Slavonia on 10 June 1909 while sailing the Azores, and the steamer SS Arapahoe on 11 August 1909 while off the North Carolina coast. 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 Marconi operators to adopting 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.

SOS as a Versatile Signal

SOS is still recognized as a standard distress signal that may be used with any signaling method. It has been used as a visual distress signal, consisting of three short/three long/three short flashes of light, such as from a survival mirror. In some cases the individual letters "S O S" have been spelled out, for example, stamped in a snowbank or formed out of logs on a beach. You can write them on the beach, send them via Morse code or, if you have a pen, write them on your hand and flash them at passersby.

Visual SOS signals are among the most effective ways to attract rescuers’ attention, especially in daylight or when you have no radio equipment. A small reflective object-such as a mirror, piece of metal foil, or even a polished knife blade-can become a powerful signaling tool. Fire has always been a classic method of signaling for help.

When visibility is low-at night, in fog, dense forest, or underground-sound becomes the most effective way to attract attention. A whistle is one of the most efficient emergency tools. Avoid screaming continuously, as it quickly drains your energy.

Additional Maritime Distress Signals

Additional warning and distress signals followed the introduction of SOS. During World War II, 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, while "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 signal. None of these signals was used on its own.

Sending SOS as well as the urgency signal ("XXX" in CW, and "PAN-PAN" in voice) and safety signal ("TTT" in CW, and "SECURITE" in voice) used similar procedures for effectiveness.

Automated Distress Signals

Since many merchant vessels carried only one or two radio operators, no one might hear a distress signal when both operators were off-duty. Eventually, equipment was invented to summon operators by ringing an alarm in the operator's cabin, and on the bridge, and the only switch able to disable the alarm was only permitted to be in the wireless telegraph room.

The alarm was sent by the operator on the ship in distress transmitting the radiotelegraph alarm signal (auto-alarm) signal-twelve extra-long dashes, each lasting four seconds with a one-second gap between them, and transmitted in A2 (modulated CW). The alarm signal was normally sent with a mechanical or electronic timing circuit to ensure it was sent accurately. However, ships radio room clocks typically had markings on the dial to guide operators in sending the signal manually. The Auto Alarm receivers were designed to activate upon receiving four such dashes. Once four valid dashes are detected, the automatic alarm is activated.

The radiotelephony equivalent of the radiotelegraph alarm signal is the radiotelephony alarm signal, which is the transmission of alternating tones of 2200 Hz and 1300 Hz, with each tone having a duration of 250 ms. Automatic alarm systems aboard ships must activate when such a signal is received and the receiving vessel is within 500 nmi (930 km) of the transmitting vessel's position, or if the distress position is in the polar areas (latitude greater than 70° N or 70° S).

Modern Distress Signals and Technologies

Modern technology has greatly increased the chances of being rescued quickly in an emergency. Devices like smartphones, GPS trackers, and satellite beacons can automatically send your location and distress signals, even when traditional communication methods fail.

A Search and Rescue Transponder (SART) responds to 9 GHz radar signals. An Emergency Position-Indicating Radio Beacon (EPIRB) operates in the 406 MHz radio frequency. EPIRB signals are received and processed by a constellation of satellites known as Cospas-Sarsat. Older EPIRBs that use 121.5 MHz are obsolete. Many EPIRBs have an in-built Global Positioning System receiver. When activated these EPIRBs rapidly report the latitude and longitude of the emergency accurate to within 120 m (390 ft). The position of non-GPS EPIRBs is determined by the orbiting satellites, this can take ninety minutes to five hours after activation and is accurate to within 5 km (3.1 mi).

A miniaturized EPIRB capable of being carried in crew members' clothing is called a Personal Locator Beacon (PLB). Regulators do not view them as a substitute for a vessel's EPIRB. In situations with a high risk of "man overboard", such as open ocean yacht racing, PLBs may be required by the event's organizers.

EPIRBs and PLBs have a unique identification number (UIN or "HexID"). A purchaser should register their EPIRB or PLB with the national search and rescue authority; this is free in most jurisdictions.

Other Distress Signals

A single aerial flare alerts observers to the existence of a vessel in distress somewhere in the general direction of the flare sighting on the horizon but extinguishes within one minute or less. A hand-held flare burns for three minutes and can be used to localize or pinpoint more precisely the exact location or position of the party in trouble.

During daylight hours when the sun is visible, a heliograph mirror can be used to flash bright, intense sunlight. For hundreds of years inverted national flags were commonly used as distress signals.

The civilian aircraft frequency for voice distress alerting is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well).

The recognized mountain distress signals are based on groups of three, or six in the UK and the European Schwarzwald. A distress signal can be three fires or piles of rocks in a triangle, three blasts on a whistle, three shots from a firearm, or three flashes of light, in succession followed by a one-minute pause and repeated until a response is received.

To communicate with a helicopter in sight, raise both arms (forming the letter Y) to indicate "Yes" or "I need help", or stretch one arm up and one down (imitating the letter N) for "No" or "I do not need help".

The "Signal for Help" Campaign

Intimate partner violence increased in severity during the onset of the COVID-19 pandemic. The combination of increased isolation in quarantine, and increase in the use of video communication, created a critical need for a widely recognized, discrete, way a survivor could reach out for help.

During this time, the Canadian Women’s Foundation wanted to do more. Tapping into their trusted relationships with intimate partners violence advocates and providers, they were able to quickly co-create the Signal for Help. However, they knew the signal’s effectiveness was tied to its ability to be recognized, and appropriately acted upon.

The signal is simple yet effective: raise one hand with the palm facing outward and the thumb tucked into the palm, forming a fist with the thumb inside. This discreet gesture can be used in situations where speaking out loud or drawing attention could be risky or challenging but the hand signal is discreet and universal. It can be employed in various settings, from public spaces to private encounters, without drawing unwanted attention.

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