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Distress signal
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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.[1]: PCG D−3  Use of distress signals in other circumstances may be against local or international law. An 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

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Distress signals at sea are defined in the International Regulations for Preventing Collisions at Sea and in the International Code of Signals.[2] 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).[3]

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

Distress signals
Smoke signal
  • Transmitting a spoken voice Mayday message by radio over very high frequency channel 16 (156.8 MHz) or medium frequency on 2182 kHz[2]
  • Transmitting a digital distress signal by activating (or pressing) the distress button on a marine radio equipped with Digital Selective Calling (DSC) over VHF channel 70 or over another designated DSC frequency in the maritime MF and HF bands.[2]
  • Transmitting a digital distress signal by activating (or pressing) the distress button (or key) on an Inmarsat-C satellite internet device[2]
  • Sending the Morse code group SOS ( ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄ ) by light flashes or sounds[2]
  • Burning a red flare (either hand-held or aerial parachute flare)[2]
  • Launching distress rockets[2]
  • Emitting orange smoke from a canister[2]
  • Showing flames on the vessel (as from a burning tar barrel, oil barrel, etc.)[2]
  • Raising and lowering slowly and repeatedly both arms outstretched to each side[2]
  • Making a continuous sound with any fog-signaling apparatus[2]
  • Firing a gun or other explosive signal at intervals of about a minute[2]
  • Flying the international maritime signal flags NC [2]
  • 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)[2]

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

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In addition, distress can be signaled using automated radio signals such as a Search and Rescue Transponder (SART) which response 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. Older EPIRBs that use 121.5 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 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). Marine safety authorities recommend the use of GPS-equipped EPIRBs.[4]

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

[edit]

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

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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 signaling.

Inverted flags

[edit]

For hundreds of years inverted national flags were commonly used as distress signals.[5] 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 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.[6] For one country, the Philippines, an inverted flag is a symbol of war rather than distress.[7]

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 wheft.[8]

Examples of inverted flags as distress signals

Device loss and disposal

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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.

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 MHz EPIRB beacons.[9]

Aviation

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Radio beacon of distress
Modulation of a radio beacon of distress on 121.5 MHz and 243 MHz. (Radio triangulation)
Problems playing this file? See media help.

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). 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 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 without radio communications. The standard pattern is a series of 120° turns.

Ground Air Emergency Codes

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Visual code used by survivors in the U.S.
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.[10][11]

Schwarzwald

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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 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.

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 semaphore flags are available, they can be used to communicate with rescuers.

Ground beacons

[edit]

The COSPAS-SARSAT 406 MHz radiofrequency 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

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Distress signal

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A distress signal is an internationally recognized communication used to indicate that a vessel, , person, or other entity is threatened by grave and imminent danger and requires immediate assistance. These signals are governed by global standards to ensure clarity and urgency in emergencies, distinguishing them from routine communications through specific protocols and formats. Distress signals encompass diverse methods, including visual, auditory, radio, and electronic transmissions, as outlined in key international agreements such as Annex IV of the International Regulations for Preventing Collisions at Sea (COLREGS) and Chapter VII of the . Visual signals include red rocket parachute flares, orange smoke, or a square flag with a ball above or below it; auditory signals feature a continuous or explosive sounds at one-minute intervals; and radio signals involve the sequence (···---···) or the spoken word "MAYDAY" repeated three times on designated frequencies like VHF Channel 16 (156.8 MHz). Modern systems also incorporate (DSC) alerts and satellite-based transmissions via services like , integrated into the Global Maritime Distress and Safety System (GMDSS) under the . The use of distress signals is strictly regulated to prevent false alarms, with prohibitions on non-emergency employment enforced by bodies like the (IMO) and the (ITU). Notable historical developments include the adoption of in 1908 by the International Radiotelegraphic Convention as a simple, unambiguous signal, and "" in 1923, derived from the French "m'aider" (help me), to address growing radio traffic in and maritime contexts. These protocols have evolved to enhance rescue coordination, prioritizing distress over urgency ("") or safety ("") messages in communication hierarchies.

General Principles

Definition and Purpose

A distress signal is any standardized form of communication employed to indicate that a mobile unit or person is threatened by grave and imminent danger requiring immediate assistance, encompassing scenarios such as maritime vessels, , or ground operations facing life-threatening perils. This contrasts with urgency signals, denoted by "," which signify serious difficulties that are not immediately life-threatening and thus warrant priority but not absolute precedence. The primary purpose of a distress signal is to rapidly alert relevant rescue authorities and nearby entities, enabling swift of the distressed party and coordination of an effective response. For instance, in maritime contexts, it notifies coast guards to initiate operations, while in aviation, it prompts to provide immediate support and clear . By conveying critical details like position and nature of the , these signals minimize response times and enhance survival chances without specifying the transmission methods involved. Distress signals adhere to core principles ensuring their effectiveness across domains. Universality mandates global standardization through bodies like the (ITU) and (IMO), allowing consistent recognition worldwide. Immediacy grants them absolute priority over all routine communications, often invoking to prevent interference. Redundancy incorporates multiple signal types and transmission means to bolster reliability, as seen in systems like the Global Maritime Distress and Safety System (GMDSS), which requires independent alerting methods to counter single-point failures. Historically, distress signals evolved from ad-hoc visual and telegraphic methods, such as flags and early radio calls like "," to regulated international systems following major incidents. The 1912 sinking of the RMS Titanic, which highlighted communication gaps during its distress transmissions, prompted the International Radiotelegraph Conference to establish common frequencies and protocols, laying the foundation for modern standardization. This progression culminated in comprehensive frameworks like the 1974 International Convention for the Safety of Life at Sea (SOLAS), integrating radio, satellite, and automated technologies for enhanced global efficacy.

Types of Signals

Distress signals are categorized primarily by their transmission medium, encompassing audio, visual, electronic and radio-based methods, as well as less conventional approaches. These classifications ensure adaptability to various environments and conditions, allowing for effective communication of emergencies across land, sea, and air without relying on specific operational domains. Audio signals employ sound devices to convey urgency over distances, particularly in low-visibility scenarios. Common examples include the continuous sounding of fog horns, bells, whistles, or sirens, which indicate a need for assistance when repeated without interruption. In certain emergency contexts, such as signaling a person overboard, three prolonged blasts on a horn or whistle are repeated at intervals to alert nearby vessels or rescuers. These auditory methods are standardized internationally to prevent confusion with routine navigation signals. Visual signals utilize light, color, or physical markers for detection, often serving as primary alerts during daylight or clear conditions. These encompass flags, such as an inverted national ensign or the international code flags "N" (representing "I am in distress and require assistance") hoisted with flag "C" below it; shapes like a square flag with a above or below it; and lights, including flashing patterns or searchlights. Pyrotechnic devices form a key subset, featuring hand-held , parachute , or orange signals that produce highly visible bursts. For instance, a hand-held burns with intense light for visibility up to several miles. Electronic and radio signals leverage electromagnetic transmission for precise, long-range alerting, often incorporating coded messages. , particularly the sequence ··· −−− ··· representing "," is transmitted via radio , light flashes, or sound for immediate recognition. Voice procedures over radio telephony involve repeating "" three times followed by the vessel or party's identification and situation details. Modern digital variants include satellite-linked beacons operating on 406 MHz, such as Emergency Position-Indicating Radio Beacons (EPIRBs), Personal Locator Beacons (PLBs), and Emergency Locator Transmitters (ELTs), which automatically broadcast distress alerts with GPS coordinates to global search-and-rescue systems. These electronic bursts enable rapid location accuracy within kilometers. Other methods, though less common, provide supplementary options in isolated or resource-limited situations. Ground markers involve arranging natural or available materials—such as rocks, branches, , or —into large, contrasting symbols or letters on open terrain, visible from ; examples include "" for general distress or an "X" indicating inability to proceed. Such signals are designed to be at least 3 meters () in size for aerial detection. Olfactory signals remain exceptionally rare and typically incidental, lacking formal standardization. Hybrid combinations integrate multiple signal types to enhance reliability and confirmation, such as pairing a radio voice call with a simultaneous visual or to verify the alert across mediums. This approach mitigates risks from single-method failures, like radio interference or obscured visuals. The effectiveness of distress signals varies significantly based on environmental factors, including visibility range, weather conditions, and operational duration. For example, pyrotechnic flares offer a detection range of up to 10 nautical miles (about 18.5 km) for parachute types in ideal conditions but are curtailed by , , or high winds that disperse or dim light. Audio signals propagate effectively over 1-2 kilometers in calm air but attenuate rapidly in wind or over water. Electronic beacons maintain high reliability with global coverage, though battery life and activation timing influence success; flares, by contrast, endure only 30-60 seconds, necessitating timely use.

Historical Development

Early Examples

Distress signals in ancient and medieval periods primarily relied on rudimentary visual and auditory methods employed by sailors and explorers to attract attention over distances. Smoke from fires, often lit on hilltops or ship decks, served as one of the earliest known forms for conveying urgency, with ancient seafarers arranging controlled blazes to transmit messages to coastal watchers or nearby vessels. Cannon shots emerged in the late medieval era as ships adopted weaponry, where volleys alerted others to peril, though their interpretation varied by context. The 19th century brought innovations that enhanced these methods, beginning with the semaphore system invented by in 1792, an using pivoting arms on towers to relay messages rapidly across land, later adapted for signaling to include distress calls. Early electric telegraphs, pioneered by in the 1840s, enabled wired distress transmissions on land and short-range shipboard use, marking a shift toward more reliable communication. In maritime contexts, gunfire became a traditional signal by the , with volleys used to call for assistance. The British under the Merchant Shipping Act of 1876 authorized the use of socket signals (rockets) for distress. On land, explorers like and during their 1804–1806 expedition across used bonfires for nighttime alerts to native groups and mirror flashes to signal over rivers, adapting survival techniques from indigenous practices they observed, including smoke signals. These early methods suffered from a critical lack of global standardization, often leading to tragic miscommunications; for instance, during the 1854 sinking of the after a collision with the off Newfoundland, the Arctic's crew fired guns and rockets, but the Vesta's officers mistook them for a customary , delaying aid and contributing to over 300 deaths. Such incidents underscored the need for uniform protocols, paving the way for later international agreements. An early radio precursor was the Marconi company's adoption of in 1904 as a distress call for .

Modern Standardization

The establishment of modern international distress signal protocols began in the early with the 1906 International Radiotelegraph Convention in , where 27 nations agreed to adopt the sequence "...---..." () as the universal distress signal for maritime radio communications, granting it absolute priority over other transmissions. This convention, signed on November 3, 1906, and effective from July 1, 1908, marked the first global standardization of a radio distress call, replacing varied national signals and requiring ships to monitor for it continuously. The sinking of the RMS Titanic on April 15, 1912, exposed gaps in radio distress procedures and lifeboat provisions, prompting the convening of the first International Conference for the Safety of Life at Sea (SOLAS) in , which adopted the on January 20, 1914. This treaty mandated continuous radio watches, sufficient lifeboats, and international cooperation in distress responses, laying the groundwork for coordinated maritime safety. The (ITU), evolving from the 1906 convention's framework, formalized as the distress standard in its radio regulations by 1908, while the (IMO), established in 1948, assumed oversight of SOLAS implementation. In 1927, at the International Radiotelegraph Convention in Washington, D.C., the phonetic distress call ""—derived from the French "m'aider" (help me)—was adopted for voice radio communications to reduce misunderstandings in multilingual operations. Post-World War II advancements shifted toward integrated systems, culminating in the Global Maritime Distress and Safety System (GMDSS), adopted under SOLAS amendments in and fully implemented on February 1, 1999, by the IMO. GMDSS mandates automated and digital communications on SOLAS vessels over 300 gross tons, including stations for global coverage and terrestrial radio for coastal areas, enabling distress alerts without operator intervention. Subsequent updates emphasized digital transitions, such as the introduction of (DSC) in the 1990s via ITU Recommendation M.493, which allows ships to transmit pre-formatted distress messages with vessel identification and position data over VHF, MF, and HF bands. The integration of Global Navigation Satellite Systems (GNSS), like GPS, into GMDSS equipment from the 1990s onward provides precise positioning in distress alerts, enhancing accuracy through devices such as Emergency Position-Indicating Radio Beacons (EPIRBs). SOLAS, now in its 1974 consolidated version with amendments, has 167 contracting states representing over 99% of global merchant tonnage, ensuring widespread adoption of these standards. However, challenges persist in developing regions, where high costs, limited , and outdated equipment hinder full GMDSS compliance, leading to reliance on legacy analog systems and delayed distress responses.

Maritime Signals

Radio and Voice Procedures

In maritime distress situations, radio and voice procedures provide a structured means for vessels to alert nearby ships, coast stations, and rescue coordination centers of immediate danger, ensuring prioritized communication on designated frequencies. These protocols, governed by international standards, emphasize clarity, repetition, and brevity to overcome environmental noise and interference at sea. The primary voice distress signal is "Mayday," transmitted phonetically via radiotelephony on VHF Channel 16 (156.800 MHz) or (MF) 2182 kHz, following an initial (DSC) alert if equipped. The Mayday procedure begins with the signal "MAYDAY" repeated three times, followed by "this is," the vessel's name or call sign repeated three times, its position (latitude and longitude), the nature of the distress (e.g., fire, flooding, or man overboard), the number of persons on board, and the specific assistance required, such as medical aid or evacuation. This format allows responders to quickly assess and act on the urgency, with transmissions continuing until acknowledged or assistance arrives. For example, a vessel might broadcast: "Mayday, Mayday, Mayday, this is Motor Vessel Example, position 40 degrees North 30 degrees West, engine failure and taking water, five persons aboard, require immediate tow." The procedure originated from the French phrase "m'aider" (help me) and was first adopted internationally at the 1927 International Radiotelegraph Convention in Washington, D.C., as a voice equivalent to Morse code signals. For vessels equipped with Morse code capability, the international distress signal "SOS" (...---... in Morse) can be transmitted continuously via radiotelegraphy, historically on the MF calling frequency of 500 kHz until its phase-out, and in modern contexts on high frequency (HF) bands such as 4207.5 kHz or 6312 kHz. This signal holds absolute priority over all other traffic, requiring all stations to cease transmissions and listen upon hearing it, facilitating uninterrupted distress calls in areas without voice coverage. Although largely superseded by digital systems, SOS remains valid under the Global Maritime Distress and Safety System (GMDSS) for HF communications. Distinguishing from full distress, the urgency signal "" (pronounced ) is used for serious situations that do not immediately threaten life or the vessel, such as a or navigational hazard, and is also repeated three times in the same format on VHF Channel 16 or 2182 kHz. During any distress or urgency transmission, all non-essential communications must be silenced to maintain clear channels, with stations monitoring for at least five minutes after the call before resuming routine traffic. alerts prompt assistance but do not invoke the same level of mandatory response as . Upon receiving a distress call, the nearest or ship station acknowledges it immediately via radiotelephony on the same frequency, repeating the vessel's identity and confirming receipt, then relays the message to a rescue coordination center (RCC) if unable to assist directly. If no acknowledgment is heard within five minutes, other stations may relay the alert on appropriate frequencies to broaden the response. A brief silence period follows the initial call to allow for acknowledgments without overlap. These protocols were standardized under GMDSS, which became mandatory for SOLAS vessels of 300 gross tons and above on international voyages starting February 1, 1999, requiring VHF radiotelephony equipment for reliable distress initiation.

Visual and Pyrotechnic Signals

Visual and pyrotechnic signals serve as essential non-electronic means of indicating distress at sea, relying on line-of-sight to alert nearby vessels or when radio communication is unavailable or ineffective. These signals are governed by international standards, including those outlined in the International Convention for the Safety of Life at Sea (SOLAS) and the , ensuring uniformity across maritime operations. Flag-based signals include the designation NC, formed by hoisting the N (blue-white checkerboard) flag over the C (blue-white-yellow-blue horizontal stripes) flag, conveying "I am in distress and require immediate assistance." An alternative traditional signal is flying the national ensign upside down from the masthead, a practice recognized internationally for vessels still afloat but in peril; in some navies, such as the British Royal Navy, an upside-down red-white ensign serves a similar purpose. Additionally, an orange distress flag featuring a black ball above or below a black square—or a black square and circle—must be displayed prominently during daylight to denote urgency, as specified in Annex IV of the International Regulations for Preventing Collisions at Sea (COLREGS). For nighttime or low-visibility conditions, a continuous flashing white light signaling the pattern in (··· ––– ···) can be used from a handheld or fixed device to mimic the international distress rhythm. Pyrotechnic devices provide bright, attention-grabbing illumination or smoke plumes for both day and night use. Red parachute flares, launched from a pistol or handheld rocket, ascend to a minimum height of 300 meters before deploying a parachute, burning for at least 40 seconds at an intensity of 30,000 candela to achieve visibility over long distances. Handheld red flares emit a bright red light for a minimum of 60 seconds at 15,000 candela, allowing crew to signal position while remaining on deck. Orange smoke canisters, either buoyant or handheld, release dense orange smoke for 3 to 5 minutes, primarily for daytime use to mark location and indicate wind direction for rescuers. These devices must meet SOLAS performance criteria to ensure reliability in emergencies. These signals are typically deployed as backups when radio or electronic systems fail, with offering a visible range of up to 10 nautical miles under optimal conditions, though effectiveness diminishes in , , or high seas. Devices like flares have expiration dates of 3 to 4 years from manufacture, after which their performance may degrade, necessitating regular inspection and replacement. Under SOLAS Chapter III and the LSA Code, vessels are required to carry not less than 12 rocket parachute flares on or near the navigation bridge. Survival craft, such as lifeboats and liferafts, must each carry 6 hand-held red flares and 4 rocket parachute flares, in addition to smoke signals, to comply with lifesaving appliance standards for distress signaling. Overall, while highly effective in clear weather for attracting attention and guiding rescue efforts, their success depends on prompt deployment and favorable environmental factors.

Automated Systems

Automated systems in maritime distress signaling primarily consist of electronic beacons designed to automatically transmit distress alerts, position , and identification to facilitate rapid operations. These devices operate independently of manual intervention in many cases, enhancing reliability during emergencies at sea. The two key systems are the Emergency Position-Indicating Radio Beacon (EPIRB) for long-range satellite detection and the Search and Rescue Transponder () for short-range radar-assisted location. The EPIRB is a float-free or manual-activation device that transmits on the 406 MHz to the COSPAS-SARSAT system, alerting global coordination centers with the vessel's identity and approximate location derived from Doppler shift calculations. Since the late 1990s, many EPIRBs have incorporated built-in GPS receivers, enabling transmission of precise coordinates with an accuracy of approximately 100 meters, a significant improvement over earlier models that relied solely on . This GPS integration, widely adopted in the 2000s, reduces search areas dramatically and supports faster response times. The functions as a operating in the 9.2–9.5 GHz X-band frequency, responding to interrogating pulses from nearby search vessels or by generating a distinctive signal. When detected, it appears on the rescuer's as a series of 12 equally spaced blips, each separated by about 0.6 , with the closest blip indicating the SART's actual position; as range decreases to under 1 , the display shifts to concentric circles for precise homing. The effective detection range is typically 5–10 for shipborne radars at standard heights, extending to 30–40 from . Both EPIRBs and SARTs activate automatically upon immersion in water or manually via a switch, ensuring deployment even if the is incapacitated. EPIRBs are required to transmit continuously for at least 48 hours at -40°C to 70°C temperatures, while SARTs maintain standby for 96 hours and active response for 8 hours. Registration is mandatory for EPIRBs with national authorities, linking the beacon's unique 15-character code to the vessel's (MMSI) number, owner details, and emergency contacts to aid verification and response. The COSPAS-SARSAT system, formalized by international agreement in 1979 with the first satellites launched in 1982, introduced 406 MHz EPIRBs as a digital alternative to analog 121.5 MHz beacons, with widespread adoption following the 1987 launch of compatible geostationary satellites. The 406 MHz standard became mandatory under the Global Maritime Distress and Safety System (GMDSS) in the 1990s, phasing out 121.5 MHz signals by 2009 due to their high false alert rates and poor accuracy. Recent advancements include integration with the Automatic Identification System (AIS), where modern EPIRBs transmit local VHF AIS distress messages alongside satellite signals, alerting nearby vessels within a 20–40 nautical mile radius. Despite their effectiveness, automated systems face challenges, including false alerts that constitute 95–98% of 406 MHz EPIRB activations, often due to inadvertent triggering or testing errors, straining resources. Early COSPAS-SARSAT implementations had coverage gaps in polar regions due to reliance on low-Earth satellites with limited relays, though full global coverage, including polar areas, was achieved by the 2000s with expanded constellations; supplementary systems like have since enhanced messaging in remote zones. Personal locator beacons represent compact variants for individual use, often integrating similar 406 MHz and GPS features but with shorter battery life.

Aviation Signals

Air-to-Ground Communications

In , air-to-ground communications for distress situations primarily involve voice transmissions between pilots and (ATC) to alert authorities of imminent danger to the or its occupants, enabling rapid coordination of assistance. These protocols emphasize clear, standardized to ensure priority handling and minimize miscommunication during high-stress scenarios. The distress signal "," derived from the French "m'aider" meaning "help me," is the international voice call for such emergencies, distinct from the less urgent "" used for situations requiring assistance but not immediate peril. The standard procedure for issuing a call begins with the pilot transmitting on the current ATC frequency in use, repeating "" three times, followed by the aircraft's callsign repeated three times, and then key details of the situation. This format includes the aircraft's position, altitude or , the nature of the (e.g., engine failure or fire), intentions (e.g., diversion or landing), number of souls on board, and any other pertinent information such as fuel remaining or weather conditions. If no response is received on the primary frequency, the pilot switches to the international aeronautical frequency of 121.5 MHz VHF (or 243.0 MHz UHF for military operations) and repeats the transmission, broadcasting to "any station" if necessary. These procedures are internationally standardized under ICAO Annex 10, Volume II, which mandates absolute priority for distress communications over all other air-ground transmissions. Upon receiving a , ATC immediately acknowledges the call, often with "Roger, Mayday [callsign]" to confirm receipt, and takes control of the frequency by imposing if needed using "SEELONCE MAYDAY." Controllers then clear the around the distressed by vectoring other traffic away, providing priority handling such as direct routing to the nearest suitable , and coordinating services including if the is lost. This response facilitates a safe diversion, with ATC offering vectors, altitude adjustments, and weather updates to support the pilot's declared intentions. In parallel, pilots may activate the to code 7700 to visually indicate the on scopes. These voice protocols trace their origins to the early , formalized in response to a surge in aviation accidents following the rapid growth of commercial air travel after , with the "" signal specifically introduced at London's in 1923 to standardize distress calls amid increasing cross-Channel flights. Adoption of (VHF) radio for air-ground communications in the further enhanced reliability by reducing interference common in earlier medium-frequency systems, enabling clearer transmissions over longer ranges. Looking forward, digital alternatives like Controller-Pilot Data Link Communications (CPDLC) allow transmission of messages as pre-formatted text uplinks (e.g., " "), supplementing voice procedures in equipped for more precise data exchange. Military aviation adapts these protocols slightly, often using "Declare 7700" over voice to signal an while setting the to that code, prioritizing brevity in tactical environments without fully replacing the call.

Transponder and Electronic Codes

In , and electronic locator devices serve as automated systems for signaling distress without requiring voice communication, enabling rapid detection by (ATC) and search-and-rescue (SAR) authorities. These systems include Mode A/C that broadcast specific codes to interrogations and Emergency Locator Transmitters (ELTs) that emit radio signals upon activation. They provide critical data such as identity, altitude, and position, facilitating prioritized response to emergencies. The squawk code 7700 is the universal emergency transponder code designated for general distress situations in . When a pilot sets the transponder to 7700 in Mode 3A, it immediately alerts ATC radars, causing the aircraft's blip to appear prominently on screens, often with audio alarms for controllers. This code prompts ATC to offer priority handling, including vectors to the nearest or assistance, while the transponder continues to display the aircraft's identification and altitude if Mode C is active. ELTs are self-contained, battery-powered transmitters installed on aircraft, designed to automatically activate upon severe impact, such as a crash, to aid in locating wreckage and survivors. They primarily operate on 406 MHz for digital satellite detection via the COSPAS-SARSAT system, which has integrated ELTs since 1982, and secondarily on 121.5 MHz as a homing signal for nearby aircraft or ground rescuers. The 406 MHz signal encodes unique device identification, registered to the aircraft owner, allowing SAR teams to quickly verify genuine distress calls and reduce false alarms. Modern 406 MHz ELTs, particularly those in models introduced after 2000, often incorporate built-in GPS receivers to transmit precise coordinates, enabling SAR forces to pinpoint the within 100 or better, compared to the broader Doppler-based positioning of earlier units. These devices include self-test modes that allow pilots to verify functionality without transmitting a full distress signal, typically limited to short bursts during ground checks to prevent accidental alerts; airborne testing is prohibited to avoid interfering with real emergencies. Registration with national authorities, such as the NOAA in the , is mandatory for 406 MHz ELTs to encode owner details and facilitate rapid contact during activations. The development of transponder codes originated in the 1950s from military (IFF) systems, which evolved into civil applications for ATC radar identification. The (ICAO) standardized Mode A codes, including designations like 7700, in the 1970s through Annex 10, ensuring global interoperability for . ELTs trace back to post-World War II military needs but became widespread in after the 1973 FAA mandate requiring them on most U.S.-registered , with certain exemptions including aircraft used in air transportation with a maximum capacity exceeding 18,000 pounds after January 1, 2004; the shift to 406 MHz as the primary frequency was completed by 2009, when COSPAS-SARSAT ceased monitoring 121.5 MHz signals to focus on more reliable digital transmissions. These systems demonstrate high effectiveness in distress scenarios, with 406 MHz ELTs achieving activation rates of approximately 81-83% in survivable crashes and near-100% satellite detection when signals are transmitted, significantly reducing SAR response times compared to older 121.5 MHz models. ELTs have been mandatory on most U.S.-registered civil since 1973 under FAA regulations (14 CFR § 91.207), contributing to thousands of successful rescues by enabling precise localization even in remote areas.

Visual Ground Markers

Visual ground markers are essential for survivors of downed aircraft to communicate their location and needs to (SAR) aircraft overhead, particularly when electronic systems like emergency locator transmitters fail. These markers rely on simple, standardized symbols created from available materials such as orange signal panels, rocks, branches, clothing, or aircraft wreckage to ensure visibility from the air. The (ICAO) outlines these in Annex 12, Search and Rescue, which specifies that symbols must be at least 2.5 meters (8 feet) in length and made as conspicuous as possible against the terrain to facilitate detection during aerial searches. The core of visual ground signaling is the Ground-Air Emergency Code, a set of universal symbols for survivors to convey specific messages. For instance, a large "V" indicates "require assistance," an "X" signals "cannot proceed" or "require medical assistance," an "N" means "no" or "negative," and a "Y" affirms "yes." Directional arrows point to the intended path or location of the aircraft, while parallel lines denote "information received that aircraft is in this direction" or "nothing found, will continue search." A triangle marks a safe landing or camp site, and a plus sign (+) requests a doctor. These codes, detailed in the appendix to ICAO Annex 12, were developed post-World War II from military air recognition panels used during the war, with formal standardization occurring in the 1950s as aviation SAR protocols evolved under ICAO's first edition of Annex 12 in 1955. Survivors arrange these symbols in open, flat areas, aiming for heights of up to 6 meters for letters like "SOS" or arrows to enhance aerial visibility, often using contrasting colors or materials from survival kits. Pyrotechnic devices complement static markers by providing dynamic visual cues. Daytime smoke grenades, typically emitting dense orange for 3-5 minutes, mark positions clearly against the and are standard in aviation survival kits for their high visibility in daylight SAR operations. At night, red flares or lights from the same devices signal location over similar durations. Signal mirrors, another key tool, reflect sunlight to create a concentrated flash visible up to 10 miles away under clear conditions, allowing survivors to direct attention toward passing by aiming the reflection through a built-in sighting hole. These pyrotechnics and mirrors adhere to ICAO Annex 12 guidelines for ground-to-air communication and are included in mandatory survival equipment, with pilots receiving training on their deployment as part of ICAO-recommended SAR procedures in documents like the Manual on Search and Rescue (Doc 7333).

Land-Based Signals

Vehicle and Personal Devices

In land-based scenarios, distress signals for vehicles and personal use primarily rely on visual, auditory, and electronic methods to alert nearby drivers, pedestrians, or rescuers during breakdowns, accidents, or off-road emergencies. These signals emphasize portability and immediacy, allowing individuals to communicate hazards without fixed . Common devices include built-in vehicle features and handheld tools designed for quick deployment. Vehicle-mounted distress signals include hazard lights, which activate all turn signals simultaneously to indicate a stopped or disabled . In the United States, federal regulations require commercial motor vehicle drivers to immediately engage these four-way flashers upon stopping on a roadway and maintain them until the vehicle is moved or warning devices are placed. Air horns serve as auditory alerts, producing loud blasts to warn approaching traffic of hazards; they are standard on trucks and heavy for emergency signaling, often integrated with air brake systems for volumes up to 110 decibels. Reflective triangles are portable visual markers placed behind a stalled to enhance , typically folding into a compact form and meeting standards for reflectivity and stability. These must be positioned within 10 minutes of stopping, at distances of 10 feet, 100 feet, and 200 feet from the on highways. Personal devices extend these capabilities for individuals outside or away from vehicles. Personal Locator Beacons (PLBs) are handheld, GPS-enabled transmitters operating on the 406 MHz frequency, designed for global satellite detection by search-and-rescue systems like COSPAS-SARSAT. They provide location accuracy of approximately 100 meters and transmit for at least 24 hours in immersion conditions, with batteries offering 48 hours of operational life and a of 5 to 7 years (some models up to 10 years as of 2023); many models are buoyant for use. Road flares, typically red and burning for 30 minutes at high intensity, offer temporary illumination in low-visibility conditions, while high-visibility vests with reflective strips ensure wearer detectability from up to 1,000 feet, often made to EN ISO 20471 standards. Modern advancements include satellite messengers such as the inReach, which uses the satellite network for two-way , location sharing, and interactive alerts without cellular coverage. Launched in 2011, these devices connect to a 24/7 response center that coordinates with local emergency services, including 911 in the United States, facilitating rescue operations. Since the , integration with emergency apps has evolved, allowing satellite-enabled smartphones to text 911 directly in remote areas (e.g., via Apple's Emergency since 2022), building on text-to-911 services introduced around 2012. Regulations govern these devices to ensure reliability and accountability. In the United States, the mandates registration of all 406 MHz PLBs with the , including owner details and updates for changes in information, to aid in distress response. In the , many member states require vehicles to carry breakdown kits featuring at least one reflective compliant with ECE R27 standards and a high-visibility vest for each occupant, to be used during roadside emergencies.

Ground Beacons and Markers

Ground beacons and markers encompass fixed and semi-fixed installations, as well as improvised visual aids, designed to facilitate operations in terrestrial environments, particularly remote mountainous or alpine regions. These systems provide passive or active signaling to alert rescuers to the location of distressed individuals, complementing personal devices like PLBs by offering stationary reference points or environmental cues. Avalanche transceivers, operating at the frequency of 457 kHz adopted in , serve as essential ground beacons in snowy terrains for detecting and locating buried victims during . These devices transmit and receive signals to enable rapid partner , with rescuers switching between search and transmit modes to pinpoint signals amid multiple burials. The was selected for its penetration through and minimal interference, supporting organized alpine protocols. In ski resorts and mountainous areas, the system employs passive reflectors integrated into clothing and gear, detectable via harmonic radar technology operational since 1983. Rescuers use handheld detectors transmitting at 917 MHz, which the reflectors double to 1834 MHz for location up to 80 meters in air and 20 meters through snow, aiding in burials and lost person searches without requiring battery power from the reflector. This two-part system has been adopted by over rescue organizations worldwide, enhancing fixed infrastructure in high-risk recreational zones. Visual markers include trail-based SOS indicators and reflective cairns, which provide durable, low-tech signaling in hiking areas. Improvised markers, such as rock piles arranged in a "V" shape, universally denote a need for assistance and are visible from the air, often combined with contrasting materials for emphasis during daylight searches. These ground-to-air symbols, along with three rock mounds in a triangle, align with international distress conventions to guide aerial or ground teams. Deployment and maintenance of these s follow established protocols, with the International Commission for Alpine Rescue (ICAR) issuing recommendations for beacon functionality and in avalanche-prone areas. In national parks, routine inspections ensure signal reliability, though specific fixed installations vary by region, emphasizing integration with broader safety . During the , rescuers relied on improvised visual cues amid rubble to identify potential survivor locations despite challenging urban terrain.

Specialized Contexts

Military Applications

In military operations, distress signals are adapted for tactical environments to prioritize , evasion, and rapid recovery while minimizing detection by adversaries. Unlike civilian "Mayday" calls, which indicate immediate danger, military protocols often employ urgency signals like "" for non-life-threatening situations, with variants such as "Pan-Pan Medical" reserved for medical transports involving wounded personnel to ensure protected status under international conventions. These secure codes facilitate encrypted communications, including satellite-based bursts that transmit location data covertly to rescue forces, enhancing survivability in combat zones. Specialized devices form the core of military distress signaling. The AN/PRC-112 survival radio, issued to aircrew, operates on ultra-high frequency (UHF) bands for voice and data transmission, integrating GPS for precise location tracking to aid combat search and rescue (CSAR) missions. Historically, during World War II, blood chits—silk or fabric notices printed with local languages and promises of reward—served as passive signals for downed pilots, appealing to civilians for assistance in regions like the China-Burma-India theater. The AN/PRC-90 series, introduced in the late 1960s but updated into the 1990s and 2000s with variants like the PRC-90-2, provided beacon functionality on 243 MHz for automated distress alerts, remaining a staple for aircrew survival kits. Visual signals in military contexts emphasize covertness and compatibility with advanced . Infrared (IR) strobes, emitting light invisible to the , are standard for night operations, detectable only via goggles (NVGs) to guide rescuers without alerting enemies. Colored panels, such as the VS-17 marker (orange on one side, pink on the other), are deployed by downed pilots to signal positions from the air, contrasting against terrain for visual identification during extraction. Protocols integrate these tools with evasion training to support isolated personnel. (SERE) programs teach aircrew to use signaling devices alongside and blood chits for covert recovery, emphasizing low-profile activation to avoid capture. Historically, during the , colored smoke grenades marked landing zones and distress sites for helicopter extractions, adapting civilian for tactical urgency, with colors varying by mission (e.g., for friendly, for danger). Post-9/11 doctrinal updates have incorporated unmanned aerial vehicles (UAVs) into CSAR operations for enhanced surveillance and faster rescues in asymmetric conflicts.

Space and Remote Environments

In space missions, distress signaling relies on standardized protocols to ensure rapid communication with ground control despite the harsh vacuum environment. NASA's contingency procedures utilize S-band frequencies for emergency transmissions, enabling proximity operations and forward links during critical events. A notable historical example is the Apollo 13 mission in 1970, where the crew's verbal alert—"Houston, we've had a problem"—served as an analog to maritime Mayday calls, initiating a series of contingency responses that ultimately saved the astronauts. Specialized devices enhance these protocols for safety. NASA's contributions to the COSPAS-SARSAT system enable 406 MHz distress signals for terrestrial personal locator beacons (PLBs), while applications rely on dedicated systems like S-band radios and emerging communications. For deep , communication systems offer high-bandwidth alternatives to traditional radio, as demonstrated by NASA's (DSOC) experiment on the Psyche mission, which successfully transmitted data via infrared laser in late 2023 from over 10 million miles away. In extreme terrestrial environments like polar regions and , analogous signaling methods address isolation challenges. In , 406 MHz beacons integrated with networks like provide global coverage for distress alerts, enabling rapid location in areas beyond geostationary reach, such as Sea Area A4 under GMDSS protocols. Historically, desert nomads employed simple visual aids like polished metal mirrors to reflect as signals for assistance, a technique still recommended in survival training for line-of-sight communication over several miles in clear conditions. Key challenges in these environments include constraints and temporal delays. In the of , signals propagate via direct line-of-sight without atmospheric interference, limiting options to radio waves and lasers while excluding refracted visual cues like smoke or flares that rely on air . For Mars missions, one-way signal delays can reach up to 20 minutes due to the varying Earth-Mars distance. Recent advancements emphasize redundancy for reliability. The in the 2020s requires to incorporate multiple links, including the Near Space Network's System alongside direct-to-Earth options and optical communications, to maintain emergency signaling during lunar operations. In private , SpaceX's Crew Dragon vehicle integrates -based emergency signaling using NASA's networks for transmission in low-Earth orbit scenarios.

Regulations and Protocols

International Conventions

The International Convention for the Safety of Life at Sea (SOLAS), first adopted in 1914 following the Titanic disaster and substantially revised in 1974, establishes minimum standards for ship construction, equipment, and operations to enhance safety, including mandatory distress signaling systems. The 1974 version, which entered into force on May 25, 1980, requires ships of 300 gross tonnage and above engaged in international voyages to carry equipment compliant with the Global Maritime Distress and Safety System (GMDSS), encompassing satellite-based emergency position-indicating radio beacons (EPIRBs), VHF (DSC), and other automated alerting mechanisms to ensure rapid transmission of distress signals. This framework promotes international cooperation by obligating contracting states to respond to distress calls and maintain rescue coordination centers. The (ITU) , integrated into the ITU Constitution and Convention since their foundational adoption in 1906 and regularly updated through World Radiocommunication Conferences, allocate specific radio frequencies for distress communications to prevent interference and ensure global interoperability. Notably, Article 5 of the Regulations designates the band 406-406.1 MHz exclusively for low-power mobile-satellite service transmissions from distress beacons, such as EPIRBs, supporting the COSPAS-SARSAT satellite detection system and prohibiting other uses to prioritize emergency alerts. These allocations, binding on over 190 ITU member states, facilitate equitable spectrum access and harmonize distress procedures across maritime, aeronautical, and land-mobile services. The , known as the Chicago Convention, signed on December 7, 1944, by 52 states and now ratified by 193 parties, provides the legal foundation for aviation standards, with Annex 10 addressing aeronautical telecommunications including distress signals. Annex 10, Volume II specifies procedures for radiotelephony distress calls using the signal "" and urgency signals "," along with frequency allocations like 121.5 MHz for emergency transmissions, ensuring aircraft can alert air traffic services and coordinate . It harmonizes with (IMO) standards through joint ICAO-IMO initiatives, promoting seamless cross-domain responses. The International Convention on Maritime (SAR), adopted on April 27, 1979, in and entering into force on June 22, 1985, with 124 contracting parties as of 2025, delineates responsibilities for coordinating operations across sea areas and integrates satellite-based distress alerting. Chapter 2 of the Annex establishes a global SAR plan dividing oceans into 13 regions, while promoting the use of the COSPAS-SARSAT system—initiated in 1979 via a multilateral memorandum among , , the , and the —for detecting 406 MHz beacons from vessels, , and personal locators, thereby extending coverage to land, sea, and air emergencies. This convention obligates parties to rescue persons in distress without regard to and to provide post-rescue care. Despite these frameworks, gaps persist in coverage for small craft and unmanned systems; SOLAS and GMDSS requirements apply primarily to larger vessels, leaving many recreational boats without mandatory distress capabilities, which has prompted calls for expanded non-SOLAS equipment standards. Similarly, international conventions lack comprehensive provisions for distress signaling in drones and unmanned aircraft, where emerging ICAO standards for beyond-visual-line-of-sight operations in the emphasize the need for cyber-secure digital protocols to address vulnerabilities in automated transmissions.

Implementation and Training

Implementation of distress signals involves standardized equipment deployment and procedural protocols across maritime, aviation, and terrestrial environments to ensure reliable transmission and response. In maritime contexts, the Global Maritime Distress and Safety System (GMDSS), established under the (IMO) and (ITU), mandates the installation of satellite-based Emergency Position Indicating Radio Beacons (EPIRBs), (DSC) systems, and receivers on ships over 300 . These systems automate distress alerts via 406 MHz frequencies, integrating with the COSPAS-SARSAT satellite network for global coverage, with implementation phased in from 1992 to 1999 and amendments to SOLAS Chapter IV adopted in 2022 entering into force on 1 January 2024. In aviation, the (ICAO) requires Emergency Locator Transmitters (ELTs) on aircraft, operating on 406 MHz to transmit position data during crashes, as part of the Global Aeronautical Distress and Safety System (GADSS), which emphasizes autonomous distress tracking with position updates every minute for aircraft over 27,000 kg. For land-based applications, Personal Locator Beacons (PLBs) and similar devices under COSPAS-SARSAT standards are implemented by registering beacons with national authorities to encode user identification, enabling rapid localization within 5 km accuracy via GPS-integrated models. Training for distress signal implementation focuses on operator certification, equipment proficiency, and coordinated response drills to minimize errors in high-stress scenarios. Under the Standards of Training, Certification and Watchkeeping (STCW) Convention, Chapter IV outlines mandatory minimum requirements for GMDSS radio operators, including at least 18 years of age, approved education covering radio procedures, satellite systems, and survival craft communications, culminating in practical assessments on simulators. Maritime personnel must demonstrate competency in transmitting calls, acknowledging alerts, and using EPIRBs, with every five years to maintain endorsements. In , ICAO Annex 1 Personnel Licensing integrates distress procedures into pilot and training, requiring knowledge of urgency signals like and phases of emergency (uncertainty, alert, distress) per Annex 12, often delivered through flight simulation and recurrent courses. Terrestrial users, such as hikers or vehicle operators, receive training via national programs emphasizing PLB activation, battery testing, and registration, while (SAR) teams undergo COSPAS-SARSAT-specific instruction on beacon detection and triangulation using ground stations. International exercises, like those coordinated by IMO's Sub-Committee on Navigation, Communications and , simulate multi-agency responses to validate protocols.

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  92. https://applications.icao.int/postalhistory/annex_10_aeronautical_telecommunications.htm
  93. https://www.imo.org/en/about/conventions/pages/international-convention-on-maritime-search-and-rescue-%28sar%29.aspx
  94. https://treaties.un.org/Pages/showDetails.aspx?objid=08000002800d43b3
  95. https://wwwcdn.imo.org/localresources/en/About/Conventions/StatusOfConventions/Status%202025.pdf
  96. https://www.navcen.uscg.gov/sites/default/files/pdf/gmdss/taskForce/GMDSS_Modernization_2_3.pdf
  97. https://www.icao.int/emerging-aviation-issues-increased-use-unmanned-aircraft-systems-uas-and-remotely-piloted-1
  98. https://www.imo.org/en/MediaCentre/MeetingSummaries/Pages/MSC-105th-session.aspx
  99. https://www.imo.org/en/OurWork/HumanElement/Pages/STCW-Convention.aspx
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