A distress signal is an internationally recognized means for obtaining help. Distress signals take the form of or are commonly made by using radio signals, displaying a visually dectected item or illumination , or making an audible sound, from a distance.

A distress signal indicates that a person or group of people, ship, aircraft, or other vehicle is threatened by grave and imminent danger and requests immediate assistance. Use of distress signals in other circumstances may be against local or international law.

In order for distress signalling to be the most effective, two functions must be communicated:

• alert or notification of a distress in progress
• position or location (or localization or pinpointing) of the party in distress.

For example, a single aerial flare alerts observers to the existance 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 3 minutes and can be used to localize or pinpoint more precisely the exact location or position of the party in trouble. An EPIRB both notifies or alerts authorities and at the same time provides position indication information (the PI in EPIRB).

Maritime distress signals

Distress signals at sea are defined in the International Regulations for Preventing Collisions at Sea and in the International Code of Signals. They must only be used where there is grave and imminent danger to life. Other urgent signals should be sent using a pan-pan message.

Distress can be indicated by any of the following officially sanctioned methods:

• transmitting a spoken voice Mayday message by radio over very high frequency (shorter range VHF) channel 16 (156.8 MHz) and/or high frequency (longer range HF) on 2182 kHz
• transmitting a digital distress signal by activating (or pressing) the distress button (or key) on a marine radio equipped with Digitial Selective Calling (DSC) over the VHF (channel 70) and/or HF frequency bands.
• sending the Morse code group SOS by light flashes or sounds
• burning a red flares (either hand-held, foot-held or aerial parachute flare)
• emitting orange smoke from a canister
• showing flames on the vessel (as from a burning tar barrel, oil barrel, etc)
• raising and lowering slowly and repeatedly both arms outstretched to each side
• making a continuous sound with any fog-signalling apparatus
• firing a gun or other explosive signal at intervals of about a minute
• flying the international maritime signal flags NC
  
  
• displaying a visual signal consisting of a square flag having above or below it a ball or anything resembling a ball (round or circular in appearance)

In addition, distress can be signalled using automated radio signals, such as from a 406 MHz radio frequency Emergency Position-Indicating Radio Beacon (EPIRB). These signals are received and processed by a constellation of satellites known as SARSAT/COSPAS.

A GPIRB is an EPIRB that is equipped with a global positioning system (GPS) sensor which enables the distress signal to incorporate the latitude and longitude position of the party in distress.

A DSC radio distress signal can include the position if the lat/long are manually keyed into the radio or if a GPS-derived position is passed electronically directly into the radio.

A Mayday message consists of the word "mayday" spoken three times in succession, which is the distress signal, followed by the distress message, which should include:

• name of the vessel or ship in distress
• her position (actual, last known or estimated expressed in lat./long. or in distance/bearing from a specific location)
• nature of the vessel distress condition or situation (e.g. on fire, sinking, aground, taking on water, adrift in hazardous waters)
• number of persons at risk or to be rescued; grave injuries
• type of assistance needed or being sought
• any other details to facilitate resolution of the emergency such as actions being taken (e.g. abandoning ship, pumping flood water), estimated available time remaining afloat

In North America, marine search and rescue agencies in Canada and the United States also recognize certain other distress signals:

• sea marker dye
• white high intensity strobe light flashing at 60 times per minute

A floating man-overboard pole or dan buoy can be used to indicate that a person is in distress in the water and is ordinarily equipped with a yellow and red flag (international code of signals flag "O") and a flashing lamp or strobe light.

Unusual or Extraordinary Appearance

When none of the above-described officially-sanctioned signals is (or is no longer) available, attention for assistance can be attracted by anything that appears unusual or out of the ordinary, such as a jib sail hoisted upside down.

During daylight hours when the sun is visible, a heliograph mirror can be used to flash bright, intense sunlight.

Battery-powered laser lights the size of small flashlights (electric torches) are available for use in emergency signalling.

Flags

Inverted national flags are no longer commonly used as distress signals. It is difficult or impossible to determine when the flags of many countries (such as

Aviation distress signals

The civilian aircraft emergency 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). Aircraft can also signal an emergency by setting one of several special transponder codes, such as 7700.

The COSPAS/SARSAT signal can be transmitted by an Electronic Locator Transmitter or ELT, which is similar to a marine EPIRB on the 406 MHz radio frequency. (Marine EPIRBS are constructed so as to float while aviation an ELT is constructed so as to be activated by a sharp deceleration and is sometimes referred to as a Crash Position Indicator/CPI).

A "triangular distress pattern" is a rarely-used flight pattern flown by aircraft in distress but without radio communications. The standard pattern is a series of 120° turns.

Mountain distress signals

The recognised mountain distress signals are based on groups of three (six in the UK). A distress signal can be 3 fires or piles of rocks in a triangle, three blasts on a whistle, or three flashes of a light, in succession followed by a one minute pause and repeated until a response is received. Three blasts or flashes is the appropriate response.

In the Alps, the recommended way to signal distress is the Alpine distress signal: give six signals within a minute, then pause for a minute, repeating this until rescue arrives. A signal may be anything visual (waving clothes or lights, use of a signal mirror) or audible (shouts, whistles, etc.). The rescuers acknowledge with three signals per minute. To communicate with a helicopter in sight, raise both arms (forming the letter Y) to indicate "Yes" or "I need help" and stretch one arm up and one down (imitating the letter N) for "No" or "I do not need help". If semaphore flags are available, they can possibly be used to communicate with rescuers.

Ground distress beacons

The COSPAS-SARSAT 406 MHz radio frequency distress signal can be transmitted by hikers, backpackers, trekkers, mountaineers and other ground-based remote adventure seekers and personnel working in isolated backcountry areas using a small, portable Personal Locator Beacon or PLB.

As a plot device

In works of fiction false distress calls from a ship are a common plot hook, usually used by pirates or other antagonists to lure would-be rescuers into a trap. This is a clichéd device in space-faring science fiction, notably the various incarnations of Star Trek, in which Starfleet vessels (among others) are required by law, and most independent ship captains tend to feel morally obligated, to render any assistance they can to a vessel in distress.

See also

• Vessel emergency codes
• Global Maritime Distress Safety System GMDSS
• 500 kHz
• Emergency Alert System
• Mountain rescue
• Maritime mobile amateur radio
• COSPAS-SARSAT Search And Rescue Satellite Aided Tracking
• Digital Selective Calling DSC

References

External links

• What is the meaning of SOS?
• US Coast Guard: Visual Distress Signals for Recreational Boaters
• US Coast Guard: Flares and other Visual Distress Signals
• Transport Canada: Radio Distress Procedures Card

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