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 detected 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 parameters 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 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 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.
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. Mayday signals must only be used where there is grave and imminent danger to life. Otherwise, urgent signals such as pan-pan can be sent. Most jurisdictions have large penalties for false, unwarranted or prank distress signals.
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 Digital Selective Calling (DSC) over the VHF (channel 70) and/or HF frequency bands.
- Transmitting a digital distress signal by activating (or pressing) the distress button (or key) on an Inmarsat-C satellite internet device
- Sending the Morse code group SOS by light flashes or sounds
- Burning a red flare (either hand-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)
- Launching distress rockets
In addition, a distress can be signaled using automated radio signals such as a Search and Rescue Transponder (SART) which responds to 9 GHz radar signal, or an Emergency Position-Indicating Radio Beacon (EPIRB) which operates in the 406 MHz radio frequency. EPIRB signals are received and processed by a constellation of satellites known as COSPAS-SARSAT.
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 are 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.
For hundreds of years inverted national flags were commonly used as distress signals. However, some countries' flags are difficult (e.g., Spain, South Korea, the UK) or impossible (e.g., Japan, Turkey and Scotland) to determine whether they are inverted. Other countries have flags that are inverses of each other; for example, the Polish flag is white on the top half and red on the bottom, while Indonesia's and Monaco's flags are the opposite—i.e., top half red, bottom half white. A ship flying no flags may also be understood to be in distress.
Device loss and disposal 
To avoid pointless searches some devices must be reported when lost. This particularly applies to EPIRBs, life buoys, rafts and devices marked with the vessel's name and port.
Expired flares should not be set off, as this indicates distress. Rather, most port authorities offer disposal facilities for expired distress pyrotechnics. In some areas special training events are organised, where the flares can be used safely.
Aviation distress signals 
Modulation of a radio beacon of distress on 121,5 MHz and 243 MHz. (Radio triangulation)
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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, or six in the UK and the European Alps. 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.
In practice either signal pattern is likely to be recognised in most popular mountainous areas as nearby climbing teams are likely to include Europeans or North Americans.
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". 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.
See also 
- 500 kHz
- COSPAS-SARSAT Search And Rescue Satellite Aided Tracking
- Digital Selective Calling DSC
- Emergency Alert System
- Emergency telephone number
- Global Maritime Distress Safety System GMDSS
- International distress frequency
- Maritime mobile amateur radio
- Mountain rescue
- Search and Rescue Transponder
- Vessel emergency codes
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- 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