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{{Short description|Internationally recognized means for obtaining help}}
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A '''distress signal''', also known as a '''distress call''', is an internationally recognized means for obtaining help. Distress signals are communicated by transmitting [[radio]] signals, displaying a visually observable item or illumination, or making a sound audible from a distance.

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.<ref name=AIM>[http://www.faa.gov/air_traffic/publications/media/aim.pdf Aeronautical Information Manual], U.S. Federal Aviation Administration, 2016</ref>{{rp|PCG D−3}} Use of distress signals in other circumstances may be against local or [[international law]]. An [[pan-pan|urgency signal]] is available to request assistance in less critical situations.

For distress signalling to be the most effective, two parameters must be communicated:
* Alert or notification of an [[emergency]] 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 three 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 at sea are defined in the [[International Regulations for Preventing Collisions at Sea]] and in the [[International Code of Signals]].<ref name="Witherby171">{{cite book | title=COLREGS | publisher=[[Witherby Publishing Group]]| publication-place=Livingston | date=2023 | isbn=978-1-914993-20-6 | page=171}}</ref> [[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. The alerts are of utmost importance in ensuring the safety of life at sea, and are governed by international maritime law, specifically the International Convention for the Safety of Life at Sea (SOLAS).<ref>{{Cite web |title=Distress Alerts |url=https://www.maridict.com/distress_alerts |access-date=2023-11-23 |website=Maritime Dictionary}}</ref>

Distress can be indicated by any of the following officially sanctioned methods:
[[File:Distress Signals.png|thumb|Distress signals]]
[[File: Smoke signal 1.jpg|thumb|Smoke signal]]

* Transmitting a spoken voice [[Mayday (distress signal)|Mayday]] message by radio over [[very high frequency]] [[Marine VHF radio|channel 16]] (156.8 [[Hertz|MHz]]) or [[medium frequency]] on [[2182 kHz]]<ref name="Witherby171"/>
* Transmitting a digital distress signal by activating (or pressing) the distress button on a marine radio equipped with [[Digital Selective Calling|Digital Selective Calling (DSC)]] over VHF channel 70 or over another designated DSC frequency in the maritime MF and HF bands.<ref name="Witherby171"/>
* Transmitting a digital distress signal by activating (or pressing) the distress button (or key) on an [[Inmarsat-C]] satellite internet device<ref name="Witherby171"/>
* Sending the [[Morse code]] group <span style="text-decoration: overline">[[SOS]]</span> ({{Morse|dot|dot|dot|dash|dash|dash|dot|dot|dot}}) by light flashes or sounds<ref name="Witherby171"/>
* Burning a red [[flare]] (either hand-held or aerial parachute flare)<ref name="Witherby171"/>
* Launching [[distress rocket]]s<ref name="Witherby171"/>
* Emitting orange [[smoke signal|smoke]] from a canister<ref name="Witherby171"/>
* Showing flames on the vessel (as from a burning tar barrel, oil barrel, etc.)<ref name="Witherby171"/>
* Raising and lowering slowly and repeatedly both arms outstretched to each side<ref name="Witherby171"/>
* Making a continuous sound with any [[foghorn|fog-signaling apparatus]]<ref name="Witherby171"/>
* Firing a gun or other explosive signal at intervals of about a minute<ref name="Witherby171"/>
* Flying the [[international maritime signal flags]] '''NC''' [[File:ICS November.svg|20px|border]] [[File:ICS Charlie.svg|20px|border]]<ref name="Witherby171"/>
* 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)<ref name="Witherby171"/>

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.

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

===Automated radio signals===
In addition, distress can be signaled using automated radio signals such as a [[Search and Rescue Transponder]] (SART) which response to 9&nbsp;GHz radar signal, or an [[Emergency Position-Indicating Radio Beacon]] (EPIRB) which operates in the 406&nbsp;MHz radio frequency. EPIRB signals are received and processed by a constellation of satellites known as [[Cospas-Sarsat]]. Older EPIRBs that use 121.5&nbsp;MHz are obsolete. Many regulators require vessels that proceed offshore to carry an EPIRB.

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 {{cvt|120|m|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 {{cvt|5|km|mi}}. Marine safety authorities recommend the use of GPS-equipped EPIRBs.<ref>{{cite web|title=GPS versus Non-GPS: A comparison of GPS vs non-GPS 406 MHz distress beacons|url=http://beacons.amsa.gov.au/gps.html|publisher=Australian Maritime Safety Authority|access-date=21 March 2014}}</ref>

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. PLBs are also often carried during risky outdoor activities on the land.

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. EPIRB registration allows the authority to alert searchers of the vessel's name, label, type, size, and paintwork; to promptly notify next-of-kin, and to quickly resolve inadvertent activations.

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.

===Mayday===
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
* Its 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

=== 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|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 signaling.

=== Inverted flags ===
For hundreds of years inverted [[national flag]]s were commonly used as distress signals.<ref>For example, 36 [[United States Code|U.S. Code]] [https://web.archive.org/web/20140324220425/http://www.gpo.gov/fdsys/pkg/USCODE-1996-title36/html/USCODE-1996-title36-chap10-sec176.htm §176](a) provides: "The flag should never be displayed with the union down, except as a signal of dire distress in instances of extreme danger to life or property."</ref> However, for some countries' flags it is difficult (e.g., [[Spain]], [[South Korea]], [[United Kingdom]]) or impossible (e.g., [[Japan]], [[Thailand]], and [[Israel]]) to determine whether they are inverted. Other countries have flags that are inverses of each other; for example, the [[Poland|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, the bottom half white. A ship flying no flags may also be understood to be in distress.<ref>[https://www.pbs.org/wnet/secrets/transcripts/slave-ship-mutiny-program-transcript/755/ "Slave Ship Mutiny Program Transcript"] {{Webarchive|url=https://web.archive.org/web/20101115013507/http://www.pbs.org/wnet/secrets/transcripts/slave-ship-mutiny-program-transcript/755/ |date=15 November 2010 }}. Educational Broadcasting Corporation. 2010. Retrieved 2012-02-15.</ref> For one country, the [[Philippines]], an inverted flag is a symbol of war rather than distress.<ref>{{cite news | url=https://www.reuters.com/article/us-philippines-flag/u-s-apologizes-for-flying-philippine-flag-upside-down-idUSTRE68Q4EE20100927 | title=U.S. Apologizes for flying Philippine flag upside down | newspaper=Reuters | date=27 September 2010 }}</ref>

If any flag is available, distress may be indicated by tying a knot in it and then flying it upside-down, making it into a ''[[wiktionary:wheft|wheft]]''.<ref>{{cite web |url=http://www.allstates-flag.com/fotw/flags/xf-flip.html |title=Flying flags upside down |publisher=Allstates-flag.com |access-date=2009-07-27 |url-status=dead |archive-url=https://web.archive.org/web/20091213230400/http://www.allstates-flag.com/fotw/flags/xf-flip.html |archive-date=2009-12-13 }}</ref>

{{Gallery
 |title=Examples of inverted flags as distress signals
 |align=center
 |File: The Loss of the Romney Man of War RMG PU5970.jpg |{{HMS|Romney|1762|6}} aground off the [[Texel]] in 1804. In this [[Richard Corbould]] print, ''Romney''{{'}}s [[blue ensign]] at the stern is shown inverted, as a sign of distress.
 |File: HMS Pique in a gale during her return to England RMG BHC2257.jpg|{{HMS|Pique|1834|6}} in a gale, flying the [[white ensign]] upside down
}}

=== Device loss and disposal ===
To avoid pointless searches some devices must be reported when lost. This particularly applies to EPIRBs, lifebuoys, 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 organized, where the flares can be used safely.<!--regularly arranged in parts of Finland, in cooperation with emergency authorities-->

EPIRBs must not be disposed of into general waste as discarded EPIRBs often trigger at the waste disposal facility. In 2013, the majority of EPIRB activations investigated by the Australian Maritime Safety Authority were due to the incorrect disposal of obsolete 121.5&nbsp;MHz EPIRB beacons.<ref>{{cite web |last=Gaden |first=Phil |title=A 406Mhz beacon is your best chance of being rescued |url=https://www.youtube.com/watch?v=-X8FJmdcmAI |archive-url=https://web.archive.org/web/20131212014747/http://www.youtube.com/watch?v=-X8FJmdcmAI&gl=US&hl=en |archive-date=2013-12-12 |url-status=dead |publisher=Australian Maritime Safety Authority |access-date=21 March 2014}}</ref>

== Aviation ==
{{Listen|filename=Radio beacon of distress on 121,5 MHz.ogg|title=Radio beacon of distress|description=Modulation of a radio beacon of distress on 121.5&nbsp;MHz and 243 MHz. (Radio triangulation)|format=[[0]]}}
The civilian [[aircraft frequency]] for voice distress alerting is 121.5&nbsp;MHz. Military aircraft use 243&nbsp;MHz (which is a [[harmonic]] of 121.5&nbsp;MHz, and therefore civilian beacons transmit on this frequency as well). Aircraft can also signal an emergency by setting one of several special [[Transponder (aviation)#Transponder codes|transponder code]]s, 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&nbsp;MHz radiofrequency.  (Marine EPIRBs are constructed to float, while an aviation ELT is constructed to be activated by a sharp deceleration and is sometimes referred to as a Crash Position Indicator or CPI).

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

=== Ground Air Emergency Codes ===

[[File:Ground-Air Visual Code for Use by Survivors.png|thumb|Visual code used by survivors in the U.S.]]

[[File:Ground-Air Visual Code for use by Ground Search Parties.png|thumb|Visual code used by ground search parties in the U.S.]]

Ground-Air Emergency Codes are distress signals used by crashed pilots and military personnel to send signals from the ground to an aircraft.<ref>The Handbook Of The SAS And Elite Forces. How The Professionals Fight And Win. Edited by Jon E. Lewis. p.185-Tactics And Techniques, Evasion, Capture And Escape. Robinson Publishing Ltd 1997. {{ISBN|1-85487-675-9}}</ref><ref>{{cite web | url=https://www.offgridweb.com/survival/international-ground-to-air-signaling-code/ | title=International Ground-to-Air Signaling Code | date=December 2017 }}</ref>

== Schwarzwald ==
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. Three blasts or flashes is the appropriate response.

In the [[Schwarzwald]], the recommended way to signal distress is the [[Schwarzwald 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 [[heliograph|signal mirror]]) or audible (shouts, whistles, etc.). The rescuers acknowledge with three signals per minute.

In practice, either signal pattern is likely to be recognized in most popular mountainous areas as nearby climbing teams are likely to include Europeans or North Americans.

[[File:Ground to air signaling - yes.jpg|thumb|upright|Signal for "yes, I need help"]]
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 [[Flag semaphore#Characters|semaphore flags]] are available, they can be used to communicate with rescuers.

== Ground beacons ==
The COSPAS-SARSAT 406&nbsp;MHz radiofrequency distress signal can be transmitted by [[hiker]]s, [[Backpacking (wilderness)|backpackers]], [[trekking|trekkers]], [[mountaineer]]s 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 ==
* [[2182 kHz]]
* [[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
* [[Index of aviation articles]]
* [[International distress frequency]]
* [[Maritime mobile amateur radio]]
* [[Mayday]]
* [[Mountain rescue]]
* [[Search and Rescue Transponder]]
* [[SOLAS Convention]]
* [[SOS]]
* [[TACBE]]
* [[VS-17]]
* [[Vessel emergency codes]]
* [[Emergency locator beacon]]
* [[Air-to-ground communication]]

== References ==
{{Reflist}}

== External links ==
*[http://www.boatsafe.com/nauticalknowhow/060199tip6.htm What is the meaning of SOS?]
*[https://web.archive.org/web/20080314122732/http://www.uscgboating.org/safety/metlife_archived_9-21-2007/distress.htm US Coast Guard: Visual Distress Signals for Recreational Boaters]
*[http://www.uscg.mil/hq/g-m/mse4/vds.htm US Coast Guard: Flares and other Visual Distress Signals]
*[https://web.archive.org/web/20120331125636/http://www.tc.gc.ca/publications/BIL/TP9878/PDF/HR/TP9878.PDF Transport Canada: Radio Distress Procedures Card]
*[https://www.globalsecurity.org/military/library/policy/army/fm/31-70/Appb.htm FM 31-70 Appendix B]
*[http://www.sopravvivere.net/en/signaling/ SIGNALING TECHNIQUES TO ASK HELP • Survive]
*[https://www.stevenhodson.co.uk/4-82-communications-and-equipment/ 4.82 Communications and Equipment – New Zealand Private Pilots Licence Exam Notes]
*[https://survivalmanual.github.io/19 19.md]
*[https://applications.icao.int/postalhistory/annex_12_search_and_rescue.htm The Postal History of ICAO]
*[https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part1_gen_section_3.6.html GEN 3.6 Search and Rescue]
*{{cite web|url=https://static1.squarespace.com/static/5c2f2a6971069951c23cc51d/t/5c807457971a18443f276d71/1551922268506/Ground+to+Air+Visual+Signals.pdf|title=Ground to Air Visual Signals|date=2019-03-07|access-date=2023-04-13}}
*{{cite web|url=https://probablyhelpful.com/survival/afm/sections/19-ground-to-air_emergency_code.html|website=probablyhelpful.com|title=Ground-to-Air Emergency Code |access-date=2023-04-13}}

{{Morse code}}
{{Authority control}}

[[Category:Distress signals| ]]
[[Category:Emergency communication]]
[[Category:Encodings]]
[[Category:Maritime signalling]]
[[Category:Rescue]]
[[Category:Sign systems]]
[[Category:Survival skills]]