A distress signal or 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, ship , aircraft , or other vehicles is in danger and threatens immediate danger.  : 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.
In order for distress to be effective, two parameters must be communicated:
- Alert or notification of a distress in progress
- Golden location location (or localization or pinpointing) of the party in distress.
For example, a single aerial flare alert 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 provides information.
Maritime distress signals
The International Regulations for Preventing Collisions at Sea and the International Code of Signals . Mayday signals must be severe and imminent danger to life. Otherwise, pan-pan can be felt. Most jurisdictions have large penalties for false, unwarranted or prank distress signals.
Distress can be indicated by any of the following sanctioned methods:
- Transmitting a voice by Mayday broadcast by radio over very high frequency channel 16 (156.8 MHz ) and / or high frequency on 2182 kHz
- Transmitting a digital distress signal by activating (or pressing) the distress button (or key) was 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 (or hand-held or aerial parachute flare)
- Launching distress rockets
- Lighting a non-pyrotechnic visual distress signal device 
- Emitting orange smoke from a canister
- Showing flames on the vessel (as from burning barrel, oil barrel, etc.)
- Raising and lowering slowly and repeatedly both arms outstretched to each side
- Making a continuous sound with any fog-signaling 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 ball or anything resembling a ball (round or circular in appearance)
A floating man-overboard pole or dan buoy can be used to indicate that a person is in the water and is ordinarily equipped with a yellow flag (international code of signals flag “O”) .
In North America, marine search and rescue agencies in Canada and the United States also recognize some other distress signals:
- Sea marker dye
- White high intensity light strobe at 60 times per minute
Automated radio signals
In addition, a 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 COSPAS-SARSAT . Older EPIRBs which use 121.5 MHz are obsolete. Many regulators require to proceed to EPIRB.
Many EPIRBs have an in-built Global Positioning Receiver System. When activated these EPIRBs rapidly report the latitude and longitude of the emergency accurate to within 120m. The position of non-GPS EPIRBs is determined by the orbiting satellites, which is accurate to within 5 km. Marine safety authorities recommend the use of GPS-equipped EPIRBs. 
A miniaturized EPIRB capable of being conducted 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 conducted during risky outdoor activities upon land.
EPIRBs and PLBs have a unique identification number (UIN or “HexID”). Should buy their EPIRB or PLB with the national search and rescue authority; this is free in most jurisdictions. EPIRB registration allows the authority 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 is manually keyed into the radio or if a GPS-derived position is passed electronically directly into the radio.
Use of Mayday
A Mayday message consists of the word “mayday” 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 in la./long.or in distance / bearing from a specific location)
- Nature of the vessel distress condition or situation (eg on fire, sinking, aground, taking on water, adrift in hazardous waters)
- Number of persons at risk or to be rescued; serious injuries
- Type of assistance needed or being sought
- Any other details to facilitate the resolution of the action (eg abandoning ship, pumping flood water), estimated available time remaining afloat
Unusual or extraordinary appearance
When none of the above-mentioned officially sanctioned signals are available, attention may be drawn to such a thing as to appear ordinary or out of the ordinary, such as to jib sail hoisted upside down.
During the daylight hours when the sun is visible, a mirror heliograph can be used to flash bright, intense sunlight. Battery-powered laser lights (small size) are available for emergency signalling.
For hundreds of years national inverted flags were commonly used as distress signals.  However, for some countries it is difficult (eg, Spain , South Korea , the UK ) or impossible (eg, Japan , Thailand , Israel and Denmark ) 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-ie, top half red, bottom half white. A ship flying no flags can also be understood to be in distress. 
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 . 
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 are organized where the flares can be used safely.
EPIRBs must not be disposed of as often as 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. 
Aviation distress signals
Radio beacon of distress
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). Aircraft can also signal 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 an aviation ELT is engineered 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 radio communication device . 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 three fires or piles of rocks in a triangle, three blasts on a whistle, three shots from a firearm, 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, they are likely to be recognized as being more likely to include Europeans or North Americans.
To communicate with each other, raise both arms (forming the letter Y) to indicate “Yes” or “I need help,” or “one” and “one” (imitating the letter N) for “No” or “I” do not need help “. If semaphore flags are available, they can be used to communicate with survivors.
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-seekers and personal working in isolated backcountry areas using a small, portable Personal Locator Beacon or PLB .
- 2182 kHz
- 500 kHz
- COSPAS-SARSAT Satellite Search & Rescue 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
- SOLAS Convention
- Vessel emergency codes
- Jump up^ Aeronautical Information Manual, US Federal Aviation Administration, 2016
- Jump up^ Sirius Signal http://siriussignal.com/ . Retrieved 16 May 2015 . Missing or empty( help )
- Jump up^ “GPS versus Non-GPS: A comparison of GPS vs non-GPS 406 MHz distress beacons” . Australian Maritime Safety Authority . Retrieved 21 March 2014 .
- Jump up^ For example, 36US Code §176(a) provides: “The flag should not be displayed with the union down, except as a signal of distress in instances of extreme danger to life or property.”
- Jump up^ “Slave Ship Mutiny Program Transcript”. Educational Broadcasting Corporation. 2010. Retrieved 2012-02-15.
- Jump up^ “Flying flags upside down” . Allstates-flag.com. Archived from the original on 2009-12-13 . Retrieved 2009-07-27 .
- Jump up^ Gaden, Phil. “A 406Mhz beacon is your best chance of being rescued” . Australian Maritime Safety Authority . Retrieved 21 March 2014 .