Emergency position-indicating radiobeacon

The 406-MHz beacons with GPS track with a precision of 100 m in the 70% of the world closest to the equator, and send a serial number so the responsible authority can look up phone numbers to notify the registrant (e.g., next-of-kin) in four minutes.

The GPS system permits stationary, wide-view geosynchronous communications satellites to enhance the Doppler position received by low Earth orbit satellites. EPIRB beacons with built-in GPS are usually called GPIRBs, for GPS position-indicating radio beacon or global position-indicating radio beacon.

However, rescue cannot begin until a Doppler track is available. The COSPAS-SARSAT specifications say[11] that a beacon location is not considered "resolved" unless at least two Doppler tracks match or a Doppler track confirms an encoded (GPS) track. One or more GPS tracks are not sufficient.

An intermediate technology 406-MHz beacon (now mostly obsolete in favor of GPS-enabled units) has worldwide coverage, locates within 2 km (12.5 km² search area), notifies kin and rescuers in 2 hours maximum (46 min average), and has a serial number to look up phone numbers, etc. This can take up to two hours because it has to use moving weather satellites to locate the beacon. To help locate the beacon, the beacon's frequency is controlled to 2 parts per billion, and its power is five watts.

Both of the above types of beacons usually include an auxiliary 25-milliwatt beacon at 121.5 MHz to guide rescue aircraft.

The oldest, cheapest beacons are aircraft ELTs that send an anonymous warble on the aviation band distress frequency at 121.5 MHz. The frequency is often routinely monitored by commercial aircraft, but has not been monitored by satellite since Feb. 1, 2009.[12]

These distress signals could be detected by satellite over only 60% of the earth, required up to 6 hours for notification, located within 20 km (12 mi) (search area of 1200 km²), were anonymous, and could not be located well because their frequency is only accurate to 50 parts per million and the signals were broadcast using only 75–100 milliwatts of power. Coverage was partial because the satellite had to be in view of both the beacon and a ground station at the same time; the satellites did not store and forward the beacon's position. Coverage in polar and Southern Hemisphere areas was poor.

False alarms were common, as the beacon transmitted on the aviation emergency frequency, with interference from other electronic and electrical systems. To reduce false alarms, a beacon was confirmed by a second satellite pass, which could easily slow confirmation of a 'case' of distress to as much as 4 hours (although in rare circumstances, the satellites could be positioned such that immediate detection becomes possible.)

The Cospas-Sarsat system was made possible by Doppler processing. Local-user terminals (LUTs) detecting nongeostationary satellites interpret the Doppler frequency shift heard by LEOSAR and MEOSAR satellites as they pass over a beacon transmitting at a fixed frequency. The interpretation determines both bearing and range. The range and bearing are measured from the rate of change of the heard frequency, which varies both according to the path of the satellite in space and the rotation of the earth. This triangulates the position of the beacon. A faster change in the Doppler indicates that the beacon is closer to the satellite's orbit. If the beacon is moving toward or away from the satellite track due to the Earth's rotation, it is on one side or other of the satellite's path. Doppler shift is zero at the closest point of approach between the beacon and the orbit.

If the beacon's frequency is more precise, it can be located more precisely, saving search time, so modern 406-MHz beacons are accurate to 2 parts per billion, giving a search area of only 2 km², compared to the older beacons accurate to 50 parts per million that had 200 km² of search area.

To increase the useful power, and handle multiple simultaneous beacons, modern 406-MHz beacons transmit in bursts, and remain silent for about 50 seconds.

Russia developed the original system, and its success drove the desire to develop the improved 406-MHz system. The original system was a brilliant adaptation to the low-quality beacons, originally designed to aid air searches. It used just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listened to each satellite as long as it was above the horizon. Doppler shift was used to locate the beacon(s). Multiple beacons were separated when a computer program analysed the signals with a fast fourier transform. Also, two satellite passes per beacon were used. This eliminated false alarms by using two measurements to verify the beacon's location from two different bearings. This prevented false alarms from VHF channels that affected a single satellite. Regrettably, the second satellite pass almost doubled the average time before notification of the rescuing authority. However, the notification time was much less than a day.

Receivers are auxiliary systems mounted on several types of satellites. This substantially reduces the program's cost. The weather satellites that carry the SARSAT receivers are in "ball of yarn" orbits, inclined at 99 degrees. The longest period that all satellites can be out of line-of-sight of a beacon is about two hours. The first satellite constellation was launched in the early 1970s by the Soviet Union, Canada, France and the United States.

Some geosynchronous satellites have beacon receivers. Since the end of 2003, there are four such geostationary satellites (GEOSAR) that cover more than 80% of the surface of the earth. As with all geosynchronous satellites, they are located above the equator. The GEOSAR satellites do not cover the polar caps. Since they see the Earth as a whole, they see the beacon immediately, but have no motion, and thus no Doppler frequency shift to locate it. However, if the beacon transmits GPS data, the geosynchronous satellites give nearly instantaneous response.

RCCs are responsible for a geographic area, known as a "search-and-rescue region of responsibility" (SRR). SRRs are designated by the International Maritime Organization and the International Civil Aviation Organization. RCCs are operated unilaterally by personnel of a single military service (e.g. an air force, or a navy) or a single civilian service (e.g. a national police force, or a coast guard).

These international search-and-rescue points of contact[13] receive SAR alerts from the USMCC.[14]

The U.S. NOAA operates the U.S. Mission Control Center (USMCC) in Suitland, Maryland. It distributes beacon signal reports to one or more of these RCCs:[14]

The US Coast Guard web page for EPIRBs states: "You may be fined for false activation of an unregistered EPIRB. The US Coast Guard routinely refers cases involving the nondistress activation of an EPIRB (e.g., as a hoax, through gross negligence, carelessness, or improper storage and handling) to the Federal Communications Commission. The FCC will prosecute cases based upon evidence provided by the Coast Guard, and will issue warning letters or notices of apparent liability for fines up to $10,000."[16]

The Canadian Mission Control Centre receives and distributes distress alerts.

In Canada, the Canadian Coast Guard and Canadian Forces Search and Rescue (Royal Canadian Air Force and Royal Canadian Navy) are partners in Joint Rescue Co-ordination Centres; CCG operates Maritime Rescue Subcentres to offload work from JRCC.

The United Kingdom, the Department for Transport, Maritime and Coastguard Agency operates the Mission Control Centre (UKMCC), which receives and distributes distress alerts.

In the UK, the Distress and Diversion Cell of the Royal Air Force provides continuous monitoring of 121.5 MHz and 243.0 MHz, with autotriangulation from a network of terrestrial receivers on both frequencies.

In Russia, operations are supported by the Federal State Unitary Enterprise Morsvyazsputnik. [17]

In Hong Kong, operations are supported by the Hong Kong Marine Department's[17] Hong Kong Maritime Rescue Co-ordination Centre (MRCC)

In India, operations are supported by the Indian Space Research Organisation (ISRO)[17] and by the Indian Coast Guard's Maritime Rescue Coordination Centre Mumbai (MRCC)

In China, operations are supported by the Maritime Safety Administration, Bureau of Harbour Superintendency.[17]

In Japan, operations are supported by the Japan Coast Guard[17]

In Vietnam, operations are supported by the Ministry of Transport, Vietnam Maritime Administration (VINAMARINE).[17]

In Singapore, operations are supported by the Civil Aviation Authority of Singapore.[17]

In the Republic of Korea, operations are supported by the Korea Coast Guard.[17]

In Indonesia, operations are supported by the National SAR Agency of Indonesia (BASARNAS).[17]

In Taiwan, operations are supported by the International Telecommunication Development Company (ITDC)[17]

In a Safety Recommendation released September 2007, the U.S. National Transportation Safety Board once again recommended that the U.S. FAA require all aircraft have 406 MHz ELTs.[20] They first recommended this back in 2000 and after vigorous opposition by AOPA, the FAA declined to do so. Citing two recent accidents, one with a 121.5 MHz ELT and one with a 406 MHz ELT, the NTSB concludes that switching all ELTs to 406 MHz is a necessary goal to work towards.[21]

NASA has conducted crash tests with small airplanes to investigate how ELTs perform.[22][23][24]

Automatic ELTs have impact monitors activated by g-force.

Emergency locator transmitters (ELTs) for aircraft may be classed as follows:[36]

• A: automatically ejected
• AD: automatic deployable
• F: Fixed
• AF: automatic fixed
• AP: automatic portable
• W: water activated
• S: survival

Within these classes, an ELT may be either a digital 406 MHz beacon, or an analog beacon (see below).

• Any ELT that is not a 406 MHz ELT with a Hex Code became obsolete February 1, 2009.

According to the U.S. Federal Aviation Administration, ground testing of A-, B-, and S-type ELTs is to be done within the first 5 minutes of each hour. Testing is restricted to three audio sweeps.[37] Type I and II devices (those transmitting at 406 MHz) have a self test function and must not be activated except in an actual emergency.

• Automatic SOS radios were developed as early as the 1930s.[38]
• The United States Air Force institutes development of a "Crash-Locator Beacon" and a "Crash-Locator Bearing Recorder" in the early 1950s.[39]
• In the UK, by 1959 the first automatic beacon for liferafts had been produced by Ultra Electronics, and at the same time Burndept produced the TALBE (Talk and Listen Beacon Equipment) - VHF, and SARBE - Search-And-Rescue-Beacon Equipment (UHF) range of beacons which were used by the Fleet Air Arm and later, Royal Air Force. Later, SARBE beacons included a radio for voice communication by the survivor with the rescuing personnel.[40]
• Jan 9 1964: FAA Advisory Circular 170-4 investigated ELTs
• Mar 17 1969: FAA Advisory Circular 91-19 advised pilots to install ELTs
• A Saturday Evening Post article covered the death of 16-year-old Carla Corbus, who survived, though badly injured, along with her mother, for 54 days after the plane her step-dad was flying crashed in the Trinity Alps of California in March 1967. He was lost and died in the woods looking for rescue.
• The winter 1969 search for the Hawthorne Nevada Airlines Flight 708 "Gamblers' Special" DC-3 that crashed on February 18, 1969 in the Sierra Nevada Mountains. Five aircraft crashed and five searchers were killed while trying to find Flight 708.[41]
• Carriage requirements for emergency locator beacons on most US non-jet powered fixed-wing civil aircraft became law on December 29, 1970, with the signing of Senate bill S.2193, "The Occupational Safety and Health Act of 1970," Public Law 91-596.[42][43] as a last-minute rider to the Occupational Safety and Health Act. Senator Peter Dominick (R-Colorado) added the unrelated beacon language as a rider to the bill, which became section 31 of the law. (Earlier in the session he tried to add the requirements as an amendment to House bill H.R. 14465, the "Airport and Airways Development Act of 1969," but was unsuccessful.[44]) It required most general aviation aircraft to install ELTs by Dec. 30, 1973, and it preempted all the state ELT laws. The federal ELT law left the matter of alerting vague, although the initial idea was alerting by over flying aircraft which could receive an ELT's 75-milliwatt signal from 50 nautical miles away. The law set the compliance dates as one year after passage for newly manufactured or imported aircraft (December 30, 1971), and three years for existing aircraft (December 30, 1973). In response to the law, the Federal Aviation Administration (FAA) published on March 13, 1971, Notice of Proposed Rule Making (NPRM) 71–7 with the proposed amendments to the Federal Aviation Regulations (FAR).[45] After public comment, the final rules were published in the Federal Register on September 21, 1971.[46]
• The disappearance of U.S. Congressmen Hale Boggs and Nick Begich in a general aviation aircraft on October 16, 1972 sparked the then largest ever search and rescue effort, which proved fruitless. This high-profile event further hastened the mandating of ELTs aboard aircraft.[47]
• The RTCA published DO-145, DO-146, and DO-147, which the FAA then adopted the three DO documents as Technical Standard Order TSO C91.
• After problems with the C-91 ELTs, The FAA responded to the defective early ELTs by outlawing the installation of C-91 ELTs and certifying C91a ELTs with an improved gravity switch, improved crash and fire-worthy casing, and batteries that work in colder temperatures.
• March 16, 1973: AC 20–85, Emergency Locator Transmitters and Receivers
• Dec 23, 1992: TSO-C126, 406 MHz Emergency Locator Transmitter (ELT)[48] defines the 406 MHz ELT

Emergency position-indicating radio beacons (EPIRBs) are sub-classified as follows:[16]

Recognized categories:

• Category I – 406/121.5 MHz. Float-free, automatically activated EPIRB. Detectable by satellite anywhere in the world. Recognized by GMDSS.
• Category II – 406/121.5 MHz. Similar to Category I, except is manually activated. Some models are also water activated.

Obsolete classes:

• Class A – 121.5/243 MHz. Float-free, automatically activating. Due to limited signal coverage and possible lengthy delays in signal recognition, the U.S. Coast Guard no longer recommends use of this type. These devices have been phased out by the U.S. Federal Communications Commission (FCC) and are no longer recognized.
• Class B – 121.5/243 MHz. Manually activated version of Class A. These devices have been phased out by the FCC and are no longer recognized.
• Class S – 121.5/243 MHz. Similar to Class B, except it floats, or is an integral part of a survival craft (lifeboat) or survival suit. These devices have been phased out by the FCC and are no longer recognized. Their use is no longer recommended by the U.S. Coast Guard.
• Class C – Marine VHF ch15/16. Manually activated, these beacons operate on maritime channels only, and therefore are not detectable by satellite or normal aircraft. Designed for small crafts operating close to shore, this type was only recognized in the United States. Use of these units was phased out in 1999. These devices have been phased out by the FCC and are no longer recognized.
• Inmarsat-E – This entered service in 1997 and service ended 1 December 2006; all former users have switched to Category I or II 406 MHz EPIRBs. These beacons were float-free, automatically activated EPIRBs operated on 1646 MHz and were detectable by the Inmarsat geostationary satellite system, and were recognized by GMDSS, but not by the United States. In September 2004, Inmarsat announced that it was terminating its Inmarsat E EPIRB service as of December 2006 due to a lack of interest in the maritime community.[49]
• Furthermore, the U.S. Coast Guard recommend that no EPIRB of any type manufactured before 1989 be used.

EPIRBs are a component of the Global Maritime Distress and Safety System (GMDSS). Most commercial off-shore working vessels with passengers are required to carry a self-deploying EPIRB, while most in-shore and fresh-water craft are not.

As part of the United States efforts to prepare beacon users for the end of 121.5 MHz frequency processing by satellites, the FCC has prohibited the use of 121.5 MHz EPIRBs as of January 1, 2007 (47 CFR 80.1051). See NOAA's statement on the 121.5/243 phaseout Archived 2018-02-09 at the Wayback Machine.

Automatic EPIRBs are water activated. Some EPIRBs also "deploy"; this means that they physically depart from their mounting bracket on the exterior of the vessel (usually by going into the water.)

For a marine EPIRB to begin transmitting a signal (or "activate") it first needs to come out of its bracket (or "deploy"). Deployment can happen either manually where someone must physically remove it from its bracket or automatically where water pressure will cause a hydrostatic release unit to separate the EPIRB from its bracket. If it does not come out of the bracket it will not activate. There is a magnet in the bracket which operates a reed safety switch in the EPIRB. This prevents accidental activation if the unit gets wet from rain or shipped seas.

Once deployed, EPIRBs can be activated, depending on the circumstances, either manually (crewman flicks a switch) or automatically (when water contacts the unit's "sea-switch".) All modern EPIRBs provide both methods of activation and deployment, and thus are labelled "Manual and Automatic Deployment and Activation."

A hydrostatic release unit is designed to deploy automatically when submerged to a prescribed depth; the pressure of the water activates a mechanism which releases the EPIRB.

EPIRB hydrostatic release mechanism

There are two kinds of personal locator beacon (PLB):

• PLB with GPS data (internally or externally provided)
• PLB with no GPS data

All PLBs transmit in digital mode on 406 MHz. There are AIS PLBs that transmit on VHF 70.

Personal locator beacons operating on 406 MHz must be registered. PLBs should not be used in cases where normal emergency response (such as 9-1-1) exists.

• U.S. Military forces at one time used 121.5/243.0 MHz beacons such as the "PRC-106," which had a built-in VHF radio. The military is replacing them with modern 406 MHz PLBs.

All ELTs, all PLBs, and most EPIRBs are required to have a low-power homing signal, that is identical to the original 121.500 MHz VHF beacon signal. However, due to the extremely large number of false alarms that the old beacons generated, the transmit power was greatly reduced, and because the VHF transmitter typically uses the same antenna as the UHF beacon, the radiated signal is further reduced by the inherent inefficiencies of transmitting with an antenna not tuned to the transmitted signal.

406 MHz UHF beacons transmit bursts of digital information to orbiting satellites, and may also contain a low-power integrated analog (121.500 MHz) homing beacon. They can be uniquely identified (via GEOSAR). Advanced beacons encode a GPS or GLONASS position into the signal. All beacons are located by Doppler triangulation to confirm the location. The digital data identifies the registered user. A phone call by authorities to the registered phone number often eliminates false alarms (false alarms are the typical case). If there is a problem, the beacon location data guides search and rescue efforts. No beacon is ignored. Anonymous beacons are confirmed by two Doppler tracks before beginning beacon location efforts.

The distress message transmitted by a 406 beacon contains the information such as:

• Which country the beacon originates from.
• A unique 15-digit hexadecimal beacon identification code (a "15-hex ID").
• The encoded identification of the vessel or aircraft in distress, either as an MMSI value, or as, in the case of an ELT, either the aircraft's registration or its ICAO 24-bit address (from its Mode-S transponder).
• When equipped, a GPS position.
• Whether or not the beacon contains a 121.5 MHz homing transmitter.

The digital distress message generated by the beacon varies according to the above factors and is encoded in 30 hexadecimal characters. The unique 15-character digital identity (the 15-hex ID) is hard-coded in the firmware of the beacon. The 406.025 MHz carrier signal is modulated plus or minus 1.1 radians with the data encoded using Manchester encoding, which ensures a net zero phase shift aiding Doppler location[54]

• 406 MHz beacons transmit for a quarter of a second immediately when turned on, and then transmit a digital burst once every 50 seconds thereafter. Both GEOSAR and LEOSAR satellites monitor these signals.
• The repetition period shall not be so stable that any two transmitters appear to be synchronized closer than a few seconds over a 5-minute period. The intent is that no two beacons will have all of their bursts coincident. The period shall be randomised around a mean value of 50 seconds, so that time intervals between transmission are randomly distributed on the interval 47.5 to 52.5 seconds. (specification for first-generation beacons)[55]
• Preliminary specification for second-generation beacons. From beacon activation a total of [6] initial transmissions shall be made separated by fixed [5s ± 0.1s] intervals. The first transmission shall commence within [3] seconds of beacon activation. Transmissions shall then occur at nominally [30] second intervals until [30 ± 1] minutes after beacon activation. The repetition period between the start of two successive transmissions shall be randomised around the stated nominal value, so that intervals between successive transmissions are randomly distributed over ± [5] seconds. Subsequent transmissions [TBD].[56]
• 406 MHz beacons will be the only beacons compatible with the MEOSAR (DASS) system.[57]
• 406 MHz beacons must be registered (see below).

Example hex codes look like the following: 90127B92922BC022FF103504422535[58]

• A bit telling whether the message is short (15 hex digits) or long (30 hex digits) format.
• A country code, which lets the worldwide COSPAS/SARSAT central authority identify the national authority responsible for the beacon.
• Embedded 15-Hex ID or 15-hex transmitted distress message, for example, 2024F72524FFBFF The hex ID is printed or stamped on the outside of the beacon and is hard-coded into its firmware. The 15-hex ID can only be reprogrammed by certified distress radiobeacon technicians. The national authority uses this number to look up phone numbers and other contact information for the beacon. This is crucial to handle the large number of false alarms generated by beacons.
• A location protocol number, and type of location protocol: EPIRB or MMSI, as well as all the data fields of that location protocol. If the beacon is equipped with GPS or GLONASS, a rough (rounded) latitude and longitude giving the beacon's current position. In some aircraft beacons, this data is taken from the aircraft's navigation system.
• When a beacon is sold to another country, the purchaser is responsible for having the beacon reprogrammed with a new country code and to register it with their nation's beacon registry, and the seller is responsible to de-register the deprecated beacon ID with their national beacon registry.
• One can use the beacon decoder web page[59] at Cospas-Sarsat to extract the 15-hex ID from the 30-hex distress message.

• see above for transmission schedule
• 406 MHz UHF- carrier signal at 406.025-406.076 MHz ± 0.005 MHz[60]

Channel frequency (status)[61][62]

• Ch-1 A: 406.022 MHz (reference)
• Ch-2 B: 406.025 MHz (in use today)
• Ch-3 C: 406.028 MHz (in use today)
• Ch-4 D: 406.031 MHz
• Ch-5 E: 406.034 MHz
• Ch-6 F: 406.037 MHz (in use today)
• Ch-7 G: 406.040 MHz (in use today)
• Ch-8 H: 406.043 MHz
• Ch-9 I: 406.046 MHz
• Ch-10 J: 406.049 MHz (operational at a future date)
• Ch-11 K: 406.052 MHz (operational at a future date)
• Ch-12 L: 406.055 MHz
• Ch-13 M: 406.058 MHz
• Ch-14 N: 406.061 MHz (operational at a future date)
• Ch-15 O: 406.064 MHz (operational at a future date)
• Ch-16 P: 406.067 MHz
• Ch-17 Q: 406.070 MHz
• Ch-18 R: 406.073 MHz (operational at a future date)
• Ch-19 S: 406.076 MHz (operational at a future date)

• Marine VHF radio channels 15/16 – these channels are used only on the obsolete Class C EPIRBs
• The obsolete Inmarsat-E beacons transmitted to Inmarsat satellites on 1646 MHz UHF.
• 121.5 MHz VHF ± 6 kHz (frequency band protected to ±50 kHz)[63] (Satellite detection ceased on 1 February 2009,[64] but this frequency is still used for short-range location during a search and rescue operation)
• 243.0 MHz UHF ± 12 kHz (frequency band protected to ± 100 kHz)[63][65] (prior to 1 February 2009 – COSPAS-SARSAT Compatible)

In North America and Australasia (and most jurisdictions in Europe) no special license is required to operate an EPIRB. In some countries (for example the Netherlands[66]) a marine radio operators license is required. The following paragraphs define other requirements relating to EPIRBs, ELTs, and PLBs.

All distress alerting beacons operating on 406 MHz should be registered; all vessels and aircraft operating under International Convention for the Safety of Life at Sea (SOLAS) and International Civil Aviation Organization (ICAO) regulations must register their beacons. Some national administrations (including the United States, Canada, Australia, and the UK) also require registration of 406 MHz beacons.

• There is no charge to register 406 MHz beacons.
• The U.S. Coast Guard warns that a user's "life may be saved as a result of registered emergency information" because it can respond more quickly to signals from registered beacons.[16]
• Unless the national registry authority advises otherwise, personal information contained in a beacon is used exclusively for SAR distress alert resolution purposes.

The Cospas-Sarsat Handbook of Beacon Regulations provides the status of 406 MHz beacon regulations in specific countries and extracts of some international regulations pertaining to 406 MHz beacons.

The following list shows the agencies accepting 406 beacon registrations by country:

• United States – National Oceanic and Atmospheric Administration
• Canada – Canadian Beacon Registry, CFB Trenton for civil beacons, CMCC for military beacons
• Australia – Australian Maritime Safety Authority (AMSA)
• United Kingdom – United Kingdom Maritime and Coastguard Agency (MCA)
• Greece – Ministry of Merchant Marine and Hellenic Civil Aviation Authority
• France – CNES
• Italy – Stazione Satellitare Italiana - Cospas Sarsat
• Netherlands – Agentschap Telecom (NL)
• Denmark - Danish Maritime Authority
• New Zealand - New Zealand Rescue Coordination Centre Archived 2016-12-02 at the Wayback Machine
• Switzerland - Federal Office for Civil Aviation Archived 2018-07-12 at the Wayback Machine
• International – Cospas-Sarsat International 406 MHz Beacon Registration Database (IBRD)

• Safety of Life a Sea Convention
  • SOLAS 74.95
• ISO
  • ISO 15734
• U.S. Federal Regulations
  • CFR title 46 Vol 6 Section 160.062
• U.S. Coast Guard Regulations
  • USCG 160.162[72]
    • Corrosion resistance test
    • Temperature tests
    • Submergence and manual release test
    • Strength tests
    • Technical tests on the membrane
    • Performance test

A Maritime Survivor Locator Device (MSLD) is a man-overboard locator beacon. In the U.S., rules were established in 2016 in 47 C.F.R. Part 95

MOB devices with DSC or AIS are allocated MMSI numbers in the range 972yyzzzz.

A MSLD may transmit on 121.500 MHz, or one of these: 156.525 MHz, 156.750 MHz, 156.800 MHz, 156.850 MHz, 161.975 MHz, 162.025 MHz (bold are Canadian-required frequencies). Although sometimes defined in the same standards as the COSPAS-SARSAT beacons, MSLDs can not be detected by that satellite network, and are instead intended only for short-range Direction finding equipment mounted on the vessel on which the survivor was traveling.

These devices are distinct from traditional SAR radar transponders (SART), as they transmit AIS messages containing accurate GPS position information and include a GPS receiver and a transmitter on VHF AIS channels, so they show up on ship AIS receivers. They are lightweight and can be used to equip inflatable liferafts.

AIS-SART devices are allocated MMSI numbers in the range 970YYxxxx.

These devices are commonly referred to as SEND (Satellite Emergency Notification Device), and examples include SPOT and inReach.

APRS is used by amateur radio operators to track positions and send short messages. Most APRS packets contain a GPS latitude and longitude, so they can be used for both normal and emergency tracking. They also are routed to the Internet, where they are archived for some period of time, and viewable by others. There are several emergency packet types that can indicate distress. Since it is part of the amateur radio service, it costs nothing to transmit on and uses the extensive network, however, one must be a licensed amateur radio operator. There is also no guarantee that an APRS distress packet report would be seen or handled by emergency responders. It would have to be seen by an amateur radio operator and forwarded on.