By ShipCalculators.com · Published 9 May 2026

EPIRB: 406 MHz Emergency Position-Indicating Radio Beacon

The Emergency Position-Indicating Radio Beacon (EPIRB) is the 406 MHz satellite distress beacon that anchors the Global Maritime Distress and Safety System (GMDSS) for one-button distress alerting from any ship, anywhere in the world, with no human intervention beyond the initial activation or the automatic float-free release that follows the ship under in a sinking. Mandated by Regulation 7 of Chapter IV of the SOLAS Convention on every cargo ship of 300 gross tonnage and above and every passenger ship engaged on international voyages, the 406 MHz EPIRB transmits a digitally encoded distress message via the international Cospas-Sarsat satellite system, which routes the alert through a Local User Terminal (LUT) ground station and a Mission Control Centre (MCC) to the Rescue Coordination Centre (RCC) responsible for the search-and-rescue region in which the beacon activated. The current performance standard is IMO Resolution MSC.471(101) of 2019, replacing Resolution A.810(19) of 1995, with the IEC 61097-2 testing standard layered on top of the IEC 60945 general requirement for shipborne radio equipment and the IMO Resolution A.694(17) horizontal baseline. The Cospas-Sarsat space segment has expanded from the original LEOSAR low-earth-orbiting payloads through the GEOSAR geostationary segment to the MEOSAR medium-earth-orbit constellation hosted on Galileo, GPS L1 and GLONASS navigation satellites, fully operational since December 2018, delivering typical detection times below 10 minutes with GPS-equipped position accuracy of approximately 100 metres. Each beacon carries a unique 15-character hexadecimal Beacon Identification Code (HEX ID) registered in the flag-State or country database (US NOAA, UK MCA, Australian AMSA), with 5-year battery shelf life and 48-hour minimum operating time after activation. This article maps the EPIRB regime to the GMDSS overview, SOLAS Chapter III life-saving appliances, the Voyage Data Recorder Float-Free Capsule, LRIT long-range identification and tracking, AIS and ECDIS, the Polar Code, the ISM Code, STCW Chapter VI emergency training, SOLAS Chapter I general provisions, the COLREGs Convention and the ISPS Code; ShipCalculators.com hosts the calculator catalogue covering EPIRB carriage thresholds, battery and HRU expiry tracking, and Cospas-Sarsat coverage geometry.

Contents

Background: pre-1995 121.5 + 243 MHz analog beacons

The first generation of maritime emergency beacons emerged from aviation in the late 1960s on the 121.5 MHz civil aviation distress frequency with a parallel 243 MHz military second harmonic. These analog beacons transmitted a swept-tone signal received by passing aircraft, by direction-finding vessels and, from 1982 onward, by the original Cospas-Sarsat low-earth-orbiting payloads. Three intrinsic limits drove the digital replacement: the analog signal carried no identification; satellite Doppler position accuracy was approximately 15 to 20 km; and the false-alert rate exceeded 97 per cent of activations.

The Cospas-Sarsat partners (the Soviet Union, the United States, Canada and France) settled on 406 MHz through an ITU World Radiocommunication Conference allocation. The 406 MHz signal carries a digitally encoded message with beacon identity, country code, beacon-type indicator and optional GNSS position, with Doppler accuracy near 2-5 km and ±100 m when GPS-equipped. The first 406 MHz IMO performance standard was A.696(17) of 1991, superseded by A.810(19) of 1995. International satellite monitoring of 121.5 MHz and 243 MHz was terminated on 1 February 2009, leaving 121.5 MHz operational only as a homing frequency for SAR aircraft.

SOLAS IV/7 application: ≥300 GT + passenger ships

Regulation 7 of SOLAS Chapter IV requires every cargo ship of 300 gross tonnage and above and every passenger ship engaged on an international voyage to carry a satellite EPIRB capable of transmitting a distress alert through the polar-orbiting satellite service in the 406 MHz band. The EPIRB shall be capable of manual deployment, capable of floating free on sinking through automatic activation by a hydrostatic release unit (HRU) at approximately 1.5 to 4 m, capable of manual activation while attached to its bracket, capable of being carried by one person into a survival craft, and fitted with adequate inadvertent-activation safeguards. It shall be tested annually under controlled conditions that do not transmit a distress signal, with the test logged in the radio log.

Every covered ship carries an EPIRB regardless of sea area, because the EPIRB alerts independently of the primary radio installation and of ship’s electrical power. The passenger-ship category covers all sizes. Regulation 7 is reinforced by Regulation 14 (performance standards) and Regulation 13 (maintenance). Application extends to non-SOLAS-flagged ships in SOLAS ports through the no-more-favourable-treatment principle in the Paris and Tokyo MoUs, with non-compliance attracting deficiency code 04102.

IMO performance standards: A.810(19) → MSC.471(101) 2019

Resolution A.810(19) was adopted at the 19th IMO Assembly in November 1995, replacing A.696(17) of 1991, and remained the reference standard for over two decades. A.810(19) defined: 406 MHz transmit frequency with the Cospas-Sarsat-specified modulation and message format; 121.5 MHz homing frequency for line-of-sight DF; automatic float-free release through an HRU; manual activation provision; 48-hour minimum continuous transmit duration; 5-year minimum battery shelf life; environmental survival to IEC 60945; antenna-upright buoyancy; and inadvertent-activation safeguards.

Resolution MSC.471(101) was adopted at MSC 101 in June 2019 as the revised performance standard, replacing A.810(19) for new installations while existing equipment to A.810(19) remains acceptable until replacement. The 2019 revision aligns the standard with the MEOSAR segment fully operational since December 2018, with the second-generation beacon (SGB) message specification and with the integration of internal GNSS receivers. Principal revisions are explicit requirements for GNSS position-encoded transmission, alignment with the Cospas-Sarsat T.001 specification, provision for AIS-encoded homing on 161.975 / 162.025 MHz alongside legacy 121.5 MHz analog homing, tightened EMC and environmental specifications, and revised labelling and registration for the 15-character HEX ID. Both standards sit on top of Resolution A.694(17) of 1991, the horizontal general requirement for shipborne radio equipment.

IEC 61097-2 + IEC 60945 testing standards

The international electrotechnical testing standard for the 406 MHz EPIRB is IEC 61097-2 (Maritime navigation and radiocommunication equipment, Part 2, Cospas-Sarsat EPIRB operating on 406 MHz), providing the test methods and pass/fail criteria for type approval. IEC 61097-2 implements the IMO performance standard at the laboratory level, covering: transmit frequency stability on the 406.025, 406.028 and 406.037 MHz channel allocations; modulation conformance with the Cospas-Sarsat-specified bit rate; message-format conformance to T.001; 121.5 MHz homing characteristics; the 48-hour duration and 5-year shelf-life battery tests; buoyancy and float-free behaviour; HRU release-depth verification; manual-activation force and sequence; the self-test function; and inadvertent-activation safeguards.

The horizontal IEC 60945 standard provides the environmental and EMC envelope for all GMDSS and bridge electronics: temperature cycling between -25 and +55 °C for exposed equipment, vibration sweep, mechanical shock, drop test from 1 m, water spray and immersion, salt mist, solar radiation, and EMC immunity and emission limits. The EPIRB falls under the exposed-equipment envelope. The class-society type-approval certificate is issued under the manufacturer’s IEC 61097-2 plus IEC 60945 test report by an IACS member society on behalf of a flag State, satisfying SOLAS IV/14 and underpinning PSC verification.

Cospas-Sarsat: 406 MHz primary + 121.5 MHz homing

The international Cospas-Sarsat programme is the cooperative satellite-aided SAR system established by the 1979 memorandum among the Soviet Union, the United States, Canada and France, formalised by the 1988 Cospas-Sarsat Programme Agreement and now operating with 45 participating States and organisations. The system uses two distinct frequencies. The 406 MHz frequency is the primary distress frequency detected by the satellite space segment: payloads receive the digital transmission, downlink to a Local User Terminal, and the LUT extracts beacon identity, GNSS position (if encoded) and Doppler-derived position. The 121.5 MHz frequency is the homing frequency for SAR aircraft, helicopters and rescue craft equipped with DF receivers for final approach.

The 406 MHz transmission is a 0.5-second burst repeated every 50 seconds at approximately 5 W. The 121.5 MHz transmission is a continuous swept-tone at approximately 50 mW. Both begin within 50 seconds of activation and continue for the 48-hour minimum. The 406.0-406.1 MHz band is allocated by the ITU Radio Regulations exclusively to mobile-satellite distress and safety, enabling the typical satellite detection threshold near -156 dBm without adjacent-band interference.

GMDSS framework: EPIRB + DSC + Inmarsat-C

The GMDSS combines three principal distress-alerting paths, each redundant to the others. The first is Digital Selective Calling (DSC) on VHF channel 70 (156.525 MHz) in sea area A1, on MF 2187.5 kHz in A2 and on the HF DSC frequencies (4207.5, 6312, 8414.5, 12577 and 16804.5 kHz) in A3 / A4, carrying a digital alert containing MMSI, position, nature of distress and acknowledgement request. The second is satellite communication through Inmarsat-C or, since 2020, Iridium Certus. The third is the EPIRB itself, transmitting on 406 MHz through Cospas-Sarsat independently of the ship’s radio installation and electrical power.

The triple-redundancy is intentional. DSC requires the radio installation intact and powered; Inmarsat-C / Iridium requires the ship-earth-station intact and powered. The EPIRB is the last-resort path: self-contained, internally powered, self-buoyant and float-free. In a rapid sinking with no opportunity for a manual alert, the HRU-actuated float-free release detaches the EPIRB at 1.5-4 m, the unit rises and activates through a water-immersion sensor, and the 406 MHz transmission begins within 50 seconds. The relationship is complementary redundancy, not substitution: SOLAS IV/7 requires the EPIRB regardless of sea area or other satellite-service equipment.

Space segment 1: GEOSAR (3 geostationary)

The GEOSAR segment consists of search-and-rescue payloads on geostationary meteorological satellites at approximately 35,786 km altitude. The current operational constellation comprises at least 3 satellites: typically GOES-East and GOES-West (NOAA), MSG-3 or MSG-4 (EUMETSAT), INSAT-3D / 3DR (India) and Electro-L or successors (Russia), each covering roughly a third of the Earth’s surface between latitudes 70° N and 70° S.

The GEOSAR mechanism is a bent-pipe relay: the payload receives the 406 MHz transmission, downconverts to L-band and rebroadcasts to the LUT ground stations within the satellite footprint. The typical GEOSAR detection time from activation to LUT alert is less than 5 minutes. The principal limitation is that the geostationary geometry provides no Doppler shift between beacon and satellite, so GEOSAR cannot derive a Doppler position fix. A GEOSAR-only detection provides the alert and beacon identity but no position unless the beacon is GPS-equipped, with the position refined by LEOSAR or MEOSAR while SAR response begins.

Space segment 2: LEOSAR (4 polar low-earth-orbit)

The LEOSAR segment is the original Cospas-Sarsat layer, dating from the 1982 demonstration phase, based on payloads on low-earth-orbiting polar satellites at approximately 850 to 1000 km altitude in near-polar sun-synchronous orbits (typically 99° inclination). The current constellation has at least 4 operational satellites: historically the NOAA POES (USA) and Cospas (Russia) series, with MetOp (Europe) providing additional coverage. Each satellite views a swath approximately 6,000 km wide.

The LEOSAR mechanism is store-and-forward: a polar-orbiting satellite passes over a beacon, receives and stores the digital transmission with the time and frequency information needed for Doppler position derivation, and downloads to the next visible LUT. Doppler position is derived from the frequency-shift profile of the closing-velocity signature, accurate to approximately 2-5 km without GPS. The principal limitation is revisit time: a beacon in mid-latitudes is visible for about 10 minutes per pass, with passes separated by 90 minutes to 2 hours. The typical LEOSAR detection time is approximately 90 minutes worst-case in mid-latitudes, faster in polar regions where orbital convergence produces multiple passes per hour.

Space segment 3: MEOSAR (Galileo + GPS L1 + GLONASS) since 2018

The MEOSAR segment is the most recent layer, consisting of search-and-rescue payloads on medium-earth-orbit GNSS satellites at approximately 20,000 to 24,000 km altitude. The constellation aggregates payloads on three GNSS systems: SAR/Galileo on each of the 30 European Galileo satellites, SAR/GPS on the US GPS Block IIIA L1 frequency, and SAR/GLONASS on the Russian GLONASS-K satellites. Combined MEOSAR provides continuous global coverage with multiple satellites in view from any point on Earth.

MEOSAR was declared fully operational by Cospas-Sarsat in December 2018 following deployment of the supporting MEOLUT ground stations. The detection mechanism combines bent-pipe relay with multi-satellite time-difference-of-arrival (TDOA) and frequency-difference-of-arrival (FDOA) processing at the MEOLUT: each transmission is received by multiple satellites simultaneously, and the differences in arrival time and Doppler frequency enable the MEOLUT to compute position to approximately 2 km without GPS encoding or ±100 m with GPS encoding. The Galileo Return Link Service (RLS) added in 2020 provides a return-link acknowledgement to the second-generation beacon, confirming to the casualty that the alert has been received.

Detection times: GEOSAR <5min, LEOSAR ~90min, MEOSAR <10min

GEOSAR provides the fastest alerting at less than 5 minutes from activation to LUT alert but no position unless the beacon is GPS-equipped. LEOSAR provides slowest alerting at approximately 90 minutes worst-case in mid-latitudes (better than 30 minutes in polar regions) but independent Doppler position to 2-5 km without GPS. MEOSAR provides the best combined performance: less than 10 minutes from activation to MCC alert with independent position to ~2 km without GPS or ±100 m with GPS encoding.

The three layers function as a coordinated alert envelope. Typical sequence: beacon activates; GEOSAR alert routes to the MCC and on to the RCC within minutes; MEOSAR alert with position arrives shortly afterwards; LEOSAR alert provides redundant confirmation if GEOSAR or MEOSAR coverage is degraded; SAR craft are tasked and use 121.5 MHz homing for the final approach.

Local User Terminal (LUT) ground stations

The Local User Terminal (LUT) is the ground station that receives the satellite downlink and processes the 406 MHz beacon signal. The LEOLUT is a steerable parabolic antenna tracking the polar satellite during its overhead pass. The GEOLUT is a fixed antenna pointed at a geostationary satellite, receiving the bent-pipe downlink continuously. The MEOLUT is a more complex station with multiple steerable antennas tracking several MEOSAR satellites simultaneously to support the TDOA and FDOA processing.

The LUT performs signal reception and decoding (extracting the 406 MHz message, validating T.001 conformance), position determination (Doppler, TDOA / FDOA or GNSS-encoded) and alert routing to the associated Mission Control Centre (MCC). The current Cospas-Sarsat ground segment comprises approximately 65 LEOLUTs, 30 GEOLUTs and a growing network of MEOLUTs worldwide, each under a service-level agreement defining coverage, routing protocol and the associated MCC.

Mission Control Centre (MCC) routing

The Mission Control Centre (MCC) receives alerts from one or more LUTs and routes them to the responsible Rescue Coordination Centre (RCC) based on beacon position. There are approximately 30 MCCs worldwide, each operated by a Cospas-Sarsat participating State and assigned a service area. MCC functions are alert receipt; alert validation through cross-checking against the International Beacon Registration Database (IBRD) and national registries; position resolution when multiple fixes arrive from different segments; alert routing to the responsible RCC; inter-MCC routing when the beacon position falls outside the MCC service area; and beacon-type categorisation distinguishing maritime EPIRBs, aviation Emergency Locator Transmitters (ELTs) and Personal Locator Beacons (PLBs).

Routing follows the IAMSAR Manual: an EPIRB alert routes to the Maritime Rescue Coordination Centre (MRCC), an ELT alert to the Aeronautical Rescue Coordination Centre (ARCC) and a PLB alert to the appropriate national authority.

~150 Rescue Coordination Centres globally

The Rescue Coordination Centre (RCC) receives the alert from the MCC and coordinates the rescue. The world’s oceans are divided into approximately 150 SAR regions under the IMO International Convention on Maritime Search and Rescue, 1979 (SAR Convention). The largest correspond to ocean-spanning national responsibilities: the United States Coast Guard for the North Atlantic, North Pacific and US Arctic; the Russian Federation for the Russian Arctic; AMSA for one of the world’s largest single SAR regions covering most of the southern Indian and southern Pacific Oceans; HM Coastguard for the UK SAR region.

RCC functions are alert receipt; SAR-asset tasking (coast guard cutters, naval ships, merchant vessels under SOLAS V/33 obligation to render assistance, helicopters, fixed-wing SAR aircraft and shore parties); on-scene coordination through a designated On-Scene Coordinator (OSC); inter-RCC liaison near SAR-region boundaries; telemedicine; and post-rescue reporting. The RCC is staffed 24/7 with access to the SAR-asset database, the maritime AIS feed and meteorological data.

Category I: automatic float-free, hydrostatic release

The Category I EPIRB is the SOLAS-mandated configuration under Regulation 7: an EPIRB fitted in a float-free bracket with a Hydrostatic Release Unit (HRU) that automatically releases the unit from the bracket when submerged to approximately 1.5 to 4 m. The HRU is a small spring-loaded cutter actuated by a water-pressure-sensitive diaphragm, severing the retaining strap. Once released, the EPIRB rises under its own buoyancy, the water-activation switch detects immersion, and the 406 MHz transmission begins within 50 seconds.

The bracket must be installed where the EPIRB can float free without obstruction: typical locations are an upper deck position, a bridge wing or a monkey island. The bracket and HRU must be tested and certified, with the HRU expiry date typically set at 2 years from manufacture and clearly marked. PSC inspectors check HRU expiry under deficiency code 04102. Category I is the default for SOLAS-class ships.

Category II: manual activation

The Category II EPIRB is a manually activated configuration without an HRU and without automatic float-free release, fitted in a fixed bracket from which it must be removed and activated by a crew member. Category II is not acceptable as the SOLAS-mandated EPIRB under Regulation 7, but is widely used on non-SOLAS small craft, fishing vessels under 300 GT, recreational vessels and as secondary EPIRBs on SOLAS ships (for example, on individual life rafts).

Typical use cases are coastal fishing vessels where the crew has time to activate before abandoning ship, recreational sailing vessels with the EPIRB in the cabin or cockpit, and survival-craft beacons carried into the life raft. The 406 MHz transmission characteristics, satellite detection mechanism and SAR response are identical for Category I and Category II: both transmit the same digital message, are detected by the same space segment and alert the same RCC. The distinction is purely in the activation mechanism.

15-character HEX ID registration

Every 406 MHz EPIRB carries a unique 15-character hexadecimal Beacon Identification Code (the HEX ID) encoded into the digital message, identifying the beacon globally. The HEX ID is structured per Cospas-Sarsat T.001 and contains the 3-digit Maritime Identification Digit (MID) country code, the beacon-type identifier, the manufacturer code and the unique serial number. For maritime EPIRBs, the HEX ID is typically derived from the ship’s 9-digit MMSI plus additional identifier bits, so that the alert can be cross-referenced to AIS, DSC and Inmarsat databases.

The HEX ID is registered in the country database of the flag State, mandatory under Cospas-Sarsat policy and enforced through flag-State regulations. The record contains beacon HEX ID; ship name, IMO number, MMSI and call sign; operator’s contact details including a 24-hour emergency telephone; home port and typical operating area; manufacturer, model and serial number; HRU and battery expiry dates. The principal benefit is rapid false-alert resolution: the MCC cross-checks the HEX ID, calls the registered emergency contact, and verifies whether the alert is genuine in approximately 5 to 15 minutes.

Country databases: NOAA, MCA, AMSA, etc.

Each Cospas-Sarsat participating State maintains a national beacon registry. Principal registries are the NOAA SARSAT Beacon Registration Database (USA, beaconregistration.noaa.gov), the UK MCA 406 MHz Beacon Registry, the Australian AMSA Beacon Registration Service, the Transport Canada Beacon Registry, the French CNES Beacon Registry, the Russian Cospas-Sarsat MCC records, and equivalent registries in approximately 45 States.

For flag States without a national registry, the Cospas-Sarsat International Beacon Registration Database (IBRD) in Montreal serves as the registration channel and as a backup verification source for the MCC alert-validation process. Registration cost is typically free or nominal. Validity is typically 2 years, after which the operator must confirm or update the record; non-renewed registrations are flagged in the IBRD as potentially out of date.

Battery life: 5 years standby + 48 hours operating

Battery characteristics are specified by MSC.471(101) (and previously A.810(19)) and tested under IEC 61097-2. The battery must provide 5 years minimum shelf life in standby and 48 hours minimum continuous transmit duration at rated power and duty cycle once activated. The cell is typically a lithium-manganese-dioxide primary battery (non-rechargeable).

The 5-year shelf life dictates the 5-year replacement cycle, with the expiry date marked on the casing and encoded in the self-test electronics. Replacement is performed by the manufacturer or an authorised service agent: the battery is sealed inside to maintain the IP rating, requiring opening the casing, replacing the pack, resealing and re-testing. The typical cost is USD 200-400, often a substantial fraction of a new EPIRB, and many operators replace the entire unit instead. Modern lithium EPIRBs typically deliver closer to 60-72 hours at typical temperatures, with the 48-hour figure representing worst-case low-temperature performance.

Homing accuracy: ±100m via GPS + DF + doppler

Modern EPIRBs deliver position accuracy through three independent mechanisms used in combination. The first is encoded GNSS position: a GPS-equipped EPIRB (the modern default for SOLAS-class beacons) contains an internal GNSS receiver that acquires a fix during the first 50 seconds and encodes it into the digital message, typically accurate to approximately ±100 m.

The second is satellite Doppler / TDOA / FDOA position computed at the LUT: LEOSAR Doppler ±2-5 km; MEOSAR multi-satellite TDOA / FDOA approximately ±2 km; GEOSAR (single-satellite, no Doppler) provides no position fix without GNSS encoding. The satellite position is independent of GNSS encoding, providing redundancy in the case of receiver failure or poor satellite visibility. The third is 121.5 MHz radio direction-finding (DF) by SAR aircraft or rescue craft on final approach, providing a bearing to the beacon with accuracy of approximately ±2°. Combined, the three mechanisms typically deliver SAR craft within visual range of the casualty within minutes of arrival in the search area.

Major manufacturers: ACR, Jotron, McMurdo, Kannad, GME, Ocean Signal

Principal manufacturers of 406 MHz Cospas-Sarsat EPIRBs under SOLAS type approval include ACR Electronics (USA, GlobalFix series widely fitted on US-flagged vessels and Pacific operators), Jotron (Norway, Tron series on Scandinavian, North Sea and offshore vessels), McMurdo (UK, Orolia / Safran group, SmartFind G8 and G5 series), Kannad Marine (UK / France, also Orolia / Safran, SafePro and SafeLink Solo series), GME (Australia, Accusat MT600 series on Australian and New Zealand vessels) and Ocean Signal (UK, ACR Electronics group, rescueME EPIRB1 series). Other manufacturers under type approval include Cobham SATCOM, JRC (Japan Radio Company), Furuno (Japan), Sailor, Hangzhou Junda and Zhonghai Communication Equipment (China). Each typically offers a Category I float-free line and a Category II manual line, with class-society type-approval certificates issued by IACS member societies; modifications typically require re-approval.

Typical price: USD 600-1,500 retail

Retail price of a 406 MHz Cospas-Sarsat EPIRB at SOLAS specification ranges from approximately USD 600 for a basic Category II manually activated unit to approximately USD 1,500 for a top-of-the-line Category I float-free GPS-enabled unit: USD 600-800 for Category II GPS-enabled (non-SOLAS small craft); USD 800-1,000 for Category I float-free non-GPS; USD 1,000-1,300 for Category I float-free GPS-enabled; USD 1,300-1,500+ for advanced units with AIS-encoded homing on 161.975 / 162.025 MHz or second-generation beacon support including the Galileo Return Link Service.

Lifecycle costs include HRU replacement at USD 100-200 every 2 years, battery replacement at USD 200-400 every 5 years (often replaced by a new EPIRB instead), the annual self-test included in the GMDSS radio survey, and registration renewal at typically zero cost. The total 5-year cost of ownership for a SOLAS-class Category I EPIRB is approximately USD 1,200-2,000.

1 February 2009 phase-out of 121.5/243 MHz analog satellite monitoring

The 121.5 MHz and 243 MHz analog satellite monitoring by Cospas-Sarsat was terminated on 1 February 2009, ending satellite detection of the original analog beacons that pre-dated the 406 MHz era. The termination was decided by the Cospas-Sarsat Council in 2000 with a 9-year transition period, on three grounds: the very high false-alert rate (>97 per cent) of analog beacons; poor satellite Doppler position accuracy (15-20 km); and the lack of identification in the analog signal.

The termination did not affect the 121.5 MHz homing function of modern 406 MHz EPIRBs: the 121.5 MHz transmission continues to be required by IMO performance standards for line-of-sight DF homing by SAR craft, but is no longer processed by Cospas-Sarsat. From 1 February 2009, only 406 MHz beacons are detected by satellite, and SAR response to a 121.5 MHz signal depends entirely on terrestrial reception. The transition required replacement of all analog 121.5 MHz EPIRBs on SOLAS-class ships and is widely regarded as a successful regulatory phase-out, with the post-transition false-alert rate falling to approximately 95 per cent (still high in absolute terms but much more resolvable through HEX ID registration).

2024 GMDSS modernisation: Inmarsat-C → Iridium Certus

The GMDSS Modernisation Plan, adopted by IMO MSC at its 98th session in 2017 and refined through subsequent sessions, addressed the long-running concern that the 1988 framework rested on technology that had been substantially superseded. The principal changes affecting the satellite-service framework are the recognition of Iridium as a GMDSS satellite-service provider under IMO Resolution MSC.428(98) and the subsequent acceptance of the Iridium Certus maritime safety service in 2020, providing global coverage including the polar regions where Inmarsat geostationary coverage is limited; the continued availability of Inmarsat-C alongside Iridium; and the 2024 introduction of the GMDSS-2 service class in some jurisdictions, recognising the dual Inmarsat-C plus Iridium Certus fit.

The benefit for sea area A4 (polar) operators is that Iridium Certus provides A4 coverage that Inmarsat-C cannot reach (Inmarsat is limited to approximately 70° N to 70° S due to the geostationary geometry). Polar Code-applicable ships typically now carry Iridium Certus alongside the EPIRB and the standard GMDSS fit. The EPIRB is unaffected: the 406 MHz Cospas-Sarsat system is independent of Inmarsat and Iridium, and the EPIRB performance standards (MSC.471(101)) were updated in 2019 on the separate Cospas-Sarsat track.

Personal Locator Beacon (PLB) variant for crew

The Personal Locator Beacon (PLB) is a smaller, person-sized variant of the 406 MHz Cospas-Sarsat beacon, carried by an individual rather than fitted to a vessel. The PLB transmits the same 406 MHz digital message, is detected by the same space segment and triggers the same SAR response, but is registered to the individual. The PLB is not SOLAS-mandated: SOLAS IV/7 requires the ship-mounted EPIRB only, and the PLB is supplementary.

Typical use is on commercial fishing vessels, offshore installations as crew-overboard locators, and in the recreational marine and outdoor sectors. A typical PLB has a smaller battery (~24-hour operating duration), pocket-sized form factor and retail price approximately USD 250-500. The PLB is not certified for the SOLAS float-free or hydrostatic-release functions. In some jurisdictions, flag-State or industry regulations require crew members to carry PLBs in addition to the ship’s EPIRB (some North Sea offshore operators, some Australian commercial fishing fleets and certain expedition cruise operators in polar waters). The EU Marine Equipment Directive (MED) and harmonised EX-class approvals cover certain PLBs for hazardous-area applications.

False alarm rate: ~95% of activations non-distress

The false-alert rate for 406 MHz EPIRBs is high: approximately 95 per cent of global EPIRB activations are non-distress events, broken down approximately as inadvertent activation during maintenance or testing (~35 per cent), where the unit is removed from the bracket, the test mode is misused or the activation switch is contacted by mistake; water ingress to the activation switch (~25 per cent), from heavy rain, deck washing or rough-weather seawater; HRU malfunction (~15 per cent); disposal of obsolete units without battery deactivation (~10 per cent), where retired EPIRBs activate inadvertently in landfill; training events (~10 per cent); and other (~5 per cent), including theft, vandalism and equipment failure.

The high rate is the principal operational challenge of the Cospas-Sarsat system: each false alert consumes MCC and RCC operator time. The system relies on HEX ID registration for rapid resolution: the MCC contacts the registered emergency contact and verifies the situation before tasking SAR assets. Genuine distress alerts (the remaining 5 per cent) are resolved within minutes.

Cospas-Sarsat false-alert resolution protocol

The Cospas-Sarsat false-alert resolution protocol is the standardised MCC procedure for validating each alert before tasking SAR resources. An alert is received from a LUT; the MCC cross-checks the HEX ID against the IBRD and national registry; the MCC contacts the registered emergency telephone (typically a 24-hour ship-management or SAR-coordination line) and asks whether the EPIRB has been activated; if the operator can confirm the activation is non-distress (the EPIRB is being serviced ashore, or the master is reached via VHF or satellite and confirms no distress), the alert is closed as confirmed false; if the operator cannot confirm the situation or there is doubt, the alert is treated as genuine distress and SAR response is initiated.

The MCC keeps records of false alerts by HEX ID, and repeat-offender beacons are flagged for follow-up by the registering State. The protocol balances rapid response to genuine distress against minimising wasted SAR effort: typical resolution time for a non-distress alert is 5 to 30 minutes, with SAR tasking occurring as soon as validation either confirms distress or fails to confirm the alert is non-distress.

Relationship to AIS-SART (different technology)

The AIS-SART (Automatic Identification System Search and Rescue Transmitter) is a SOLAS-mandated locating device under SOLAS III/6 and IV/7, distinct from the EPIRB. The AIS-SART uses AIS technology on the 161.975 MHz and 162.025 MHz VHF AIS frequencies to transmit the location of survival craft to other AIS-equipped vessels and coast stations within VHF range. The AIS-SART is not detected by satellite and does not connect to Cospas-Sarsat; its range is limited to VHF line-of-sight, typically 5-20 nautical miles to nearby vessels and longer to high-altitude SAR aircraft.

The AIS-SART complements the EPIRB, not replaces it. The EPIRB provides global satellite alerting to the responsible RCC; the AIS-SART provides on-scene tactical locating to SAR responders and to other ships, who see the survival craft on their AIS display once within VHF range. SOLAS III/6 requires AIS-SARTs for survival craft, with carriage varying by ship size. The AIS-SART supplements the older 9 GHz radar SART; both technologies remain acceptable, with the radar SART detected by 3 GHz and 9 GHz marine radars.

Relationship to VDR Float-Free Capsule

The Voyage Data Recorder Float-Free Capsule (FFC) is the orange “black box” capsule on the upper deck of a VDR-fitted ship, designed to float free through HRU release in the same manner as the Category I float-free EPIRB. Both use a deck-mounted bracket, HRU at 1.5-4 m, automatic detachment on submersion, surface buoyancy and post-release activation. Both are mandatory under separate SOLAS chapters: the EPIRB under SOLAS IV/7, the VDR under SOLAS V/20.

The functions are different: the EPIRB transmits a 406 MHz distress alert to bring rescue forces; the FFC contains the 15-item VDR data (bridge audio, radar image, position, ECDIS, alarms, engine and rudder orders) for post-casualty investigation and is recovered by salvors after the casualty is located. The FFC carries a 37.5 kHz pinger for underwater location by survey vessels but is not a distress alerting device. The two devices are complementary: the EPIRB initiates the SAR response, the FFC preserves casualty data for the investigation.

Class society type-approval: DNV, LR, ABS, BV, NK, RINA, KR, CCS, RS, IRS

The class-society type-approval of the 406 MHz EPIRB is issued by an IACS member society on behalf of a flag State. The IACS societies are DNV (Norway), Lloyd’s Register (LR) (UK), American Bureau of Shipping (ABS) (USA), Bureau Veritas (BV) (France), ClassNK (Japan), RINA (Italy), Korean Register (KR) (South Korea), China Classification Society (CCS) (China), Russian Maritime Register of Shipping (RS) (Russia) and Indian Register of Shipping (IRS) (India). Each maintains a type-approval programme covering the IEC 61097-2 plus IEC 60945 envelope.

The certificate is the primary evidence of conformance to the IMO performance standard and satisfies SOLAS IV/14. It identifies the specific model and revision of the EPIRB plus its bracket and HRU, the IMO performance standard (MSC.471(101) for new approvals, A.810(19) for legacy) and the IEC testing standards. Modifications require re-approval. Recognised classification societies outside IACS include Polski Rejestr Statkow (PRS), Croatian Register of Shipping (CRS) and Turkish Lloyd, alongside various national flag-State authorities operating their own type-approval programmes.

PSC inspection: validity + battery + HRU expiry + registration

Port State Control (PSC) inspection of the EPIRB is a routine element of PSC visits under the Paris and Tokyo MoUs and other regional regimes. The inspector typically checks the physical presence of the EPIRB in its float-free bracket; the type-approval certificate (verifying MSC.471(101) or A.810(19) and IACS type-approval); the battery expiry date on the casing; the HRU expiry date on the HRU itself; the annual self-test record in the radio log; the HEX ID registration through IBRD or national registry; and the EPIRB visual condition including bracket integrity, antenna condition and absence of damage.

Deficiencies are recorded under code 04102 with an action code ranging from minor (rectify before next port) to detention. Common deficiencies are battery expired or close to expiry, HRU expired, missing or unreadable HEX ID label, registration not current or not verifiable, physical damage to antenna or casing, bracket corrosion, missing annual self-test entries, and EPIRB type-approval certificate missing or out of date.

Tokyo MoU + Paris MoU deficiency code 04102

The Paris MoU (27 European and North Atlantic States) and the Tokyo MoU (21 Asia-Pacific States) operate the principal PSC regimes outside the Mediterranean and other regional MoUs. Both use the harmonised IMO PSC deficiency-code system with the EPIRB-related code 04102 (“Satellite EPIRB 406 MHz”). Related codes are 04101 (“VHF radio installation”), 04103 (“Inmarsat ship earth station”), 04104 (“MF/HF radio installation”), 04105 (“DSC system”) and 04106 (“Search and Rescue locating equipment”, covering AIS-SART and radar SART). The codes are listed in IMO Resolution A.1138(31) Procedures for Port State Control 2019.

Action codes range from 17 (rectify at next port) through 17 + RO (recognised organisation to verify rectification) to 30 (detention). An expired HRU or battery typically attracts action code 17 (rectify within 14 days); a missing or non-functional EPIRB attracts action code 30 (detained until rectification). EPIRB deficiencies are typically among the top 30 deficiency codes by frequency in the annual PSC reports of both MoUs.

Formula, assumptions, and limits

Formula

The principal numerical envelope is set by IMO MSC.471(101) and Cospas-Sarsat T.001:

f_{\text{satellite}} = 406 \text{ MHz primary}

f_{\text{homing}} = 121.5 \text{ MHz (line-of-sight DF)}

T_{\text{battery, standby}} = 5 \text{ years}

T_{\text{battery, operating}} \geq 48 \text{ hours}

T_{\text{detection, MEOSAR}} < 10 \text{ minutes}

\text{Position accuracy} \approx \pm 100 \text{ m (GPS-equipped EPIRB)}

\text{GT}_{\min} = 300 \text{ for SOLAS application}

The detection-time hierarchy is GEOSAR <5 min, MEOSAR <10 min, LEOSAR ~90 min worst case in mid-latitudes, with the operational alert envelope dominated by GEOSAR plus MEOSAR.

Derivation

The 406 MHz frequency was selected through ITU World Radiocommunication Conference frequency management in the 1980s; the 406.0-406.1 MHz band is allocated exclusively to mobile-satellite distress and safety, enabling satellite detection at low received power (~-156 dBm threshold) without adjacent-band interference. The 121.5 MHz homing frequency is retained from the legacy aviation distress system as a line-of-sight DF beacon; satellite monitoring ended on 1 February 2009. The 5-year shelf life and 48-hour operating duration derive from a trade-off between battery cost, size and weight, and the requirement to support at least two LEOSAR-pass cycles plus typical SAR response time. The 1.5-4 m HRU release depth balances releasing only after actual sinking (avoiding inadvertent release in heavy seas) against releasing before the ship reaches a depth from which the EPIRB cannot rise in time.

Assumptions

The envelope assumes the EPIRB is correctly installed in an unobstructed float-free bracket, the HRU is within expiry and correctly serviced, the battery is within its 5-year shelf life and tested at the most recent annual test, the HEX ID is correctly registered with current operator contact, the Cospas-Sarsat space segment is operating nominally with at least GEOSAR plus MEOSAR coverage in the operating region, and the responsible RCC has accepted the SAR region assignment and is staffed 24/7.

Worked example

Consider a bulk carrier of 50,000 GT in the South Atlantic, encountering a structural failure at 35° S 25° W, with the master unable to send a manual distress alert before sinking. The Category I EPIRB is on the monkey island. As the ship sinks, the HRU releases the EPIRB at approximately 2 m depth. The unit rises to the surface, the water-activation sensor detects immersion, and the 406 MHz transmission begins within 50 seconds. The first burst is received by the GEOSAR payload on MSG-3, downlinked to a GEOLUT in Europe, and the alert is forwarded to the European MCC within 3 minutes. The MCC cross-checks the HEX ID, identifies the bulk carrier and contacts the operator. Within the next 5 minutes, MEOSAR delivers an independent position fix to within 2 km of the GNSS-encoded position (itself within 100 m of the actual position). The MCC routes the alert to the responsible MRCC, which tasks merchant vessels in the area through AMVER and dispatches SAR aircraft from the nearest coast within 30 minutes. The 121.5 MHz transmission supports the final approach.

Edge cases and limits

Principal edge cases are antenna obstruction by ship structure (polar icing, mast-top equipment, container stacks) blocking the 406 MHz transmission; HRU release failure through corrosion, jamming or expired service date; inverted float orientation if the EPIRB is damaged; rapid sinking deeper than 4 m before HRU release; GNSS signal blockage at the moment of activation preventing GNSS encoding for the first several minutes; battery failure at low temperature or in aged units; registration out of date preventing rapid MCC validation; and HEX ID label illegibility. MEOSAR detection-time performance assumes a working MEOLUT with multiple antennas tracking; degraded operation falls back to GEOSAR-only or LEOSAR-only performance.

Regulatory basis

The framework is anchored in SOLAS IV/7 (carriage), IV/14 (performance standards), IV/13 (maintenance) and IV/15 (records). The IMO performance standard is MSC.471(101) of 2019, replacing A.810(19) of 1995, both layered on A.694(17) general requirements. The IEC testing standards are IEC 61097-2 (EPIRB-specific) and IEC 60945 (general). The Cospas-Sarsat documentation is C/S T.001 for 406 MHz beacons, C/S T.018 for second-generation beacons, and C/S A.001 Administrative Manual. The PSC framework operates under IMO Resolution A.1138(31) with deficiency code 04102.

Common errors

Treating 121.5 MHz as still satellite-monitored: satellite monitoring ended 1 February 2009; 121.5 MHz now serves only line-of-sight DF homing.
Failing to update the HEX ID registration after a change of ship ownership or MMSI: an out-of-date record delays MCC false-alert resolution.
Disposing of an obsolete EPIRB without deactivating the battery: many false alerts originate from retired EPIRBs in landfill. Deactivate the battery or return the unit to the manufacturer.
Allowing the HRU to expire: the HRU must be replaced before its (typically 2-year) expiry date; an expired HRU may not release reliably and is a PSC deficiency under code 04102.
Treating the annual self-test as optional: the annual EPIRB self-test is mandatory under SOLAS IV/13 and must be recorded in the radio log.
Confusing the EPIRB with the AIS-SART: the EPIRB is a 406 MHz satellite distress alerting device; the AIS-SART is a VHF locating device on 161.975 / 162.025 MHz.
Confusing the EPIRB bracket with the VDR Float-Free Capsule: both float free and use HRUs, but the EPIRB alerts and the FFC preserves casualty-investigation data.
Assuming a Category II EPIRB is acceptable for SOLAS IV/7: Regulation 7 requires the float-free Category I configuration; Category II is for non-SOLAS small craft only.
Assuming the GMDSS modernisation affects the EPIRB: the modernisation introduces Iridium Certus in the satellite-service category, but the EPIRB is on the separate Cospas-Sarsat track.
Mounting the bracket in an obstructed location: the EPIRB must float free without obstruction; common errors include positions where it snags on overhead structure, antenna arrays or container stacks.

References

The principal source for SOLAS Chapter IV Regulation 7 carriage of the 406 MHz float-free satellite EPIRB is the IMO consolidated text of the International Convention for the Safety of Life at Sea, 1974, as amended, with Regulation 7 supplying the carriage requirement on every cargo ship of 300 GT and above and every passenger ship engaged on international voyages, Regulation 13 the maintenance regime, Regulation 14 the performance-standard requirement, and Regulation 15 the radio-log record. The IMO performance-standard lineage is set out in two principal instruments: IMO Assembly Resolution A.810(19) of 1995 (the original consolidated performance standard for float-free satellite EPIRBs on 406 MHz, replacing the earlier A.696(17) of 1991) and IMO Resolution MSC.471(101) of 2019 (the current revised standard aligned with the MEOSAR space segment and second-generation beacon specification). The horizontal baseline is supplied by IMO Assembly Resolution A.694(17) of 1991. The international Cospas-Sarsat satellite system documentation is anchored in the 1988 International Cospas-Sarsat Programme Agreement and in the Cospas-Sarsat T.001 Specification for 406 MHz Distress Beacons, the C/S T.018 Specification for second-generation beacons, and the C/S A.001 Administrative Manual, all through the Cospas-Sarsat Secretariat in Montreal at cospas-sarsat.int. The international electrotechnical testing standards are IEC 61097-2 and IEC 60945. The frequency-allocation framework is supplied by the ITU Radio Regulations allocating 406.0-406.1 MHz exclusively to mobile-satellite distress and safety, and by ITU-R Recommendation M.633. The national beacon registration framework is operated through registries including the NOAA SARSAT Beacon Registration Database (USA), the UK MCA 406 MHz Beacon Registry, the Australian AMSA Beacon Registration Service, the Transport Canada Beacon Registry, the French CNES Beacon Registry and the Russian Cospas-Sarsat MCC records, with the Cospas-Sarsat International Beacon Registration Database (IBRD) in Montreal serving flag States without a national registry. Class-society type-approval is operated through the IACS member societies (DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, RINA, Korean Register, China Classification Society, Russian Maritime Register of Shipping and Indian Register of Shipping) for the principal manufacturers (ACR Electronics, Jotron, McMurdo, Kannad Marine, GME, Ocean Signal, JRC, Furuno and others). Port-State-control enforcement of SOLAS IV/7 is operated through the Paris MoU and the Tokyo MoU under the harmonised IMO PSC framework set out in IMO Resolution A.1138(31) Procedures for Port State Control 2019, with EPIRB deficiencies recorded under code 04102. The GMDSS modernisation framework is supplied by the IMO MSC GMDSS Modernisation Plan adopted at MSC 98 in 2017 and the recognition of Iridium through IMO Resolution MSC.428(98) and subsequent MSC decisions, alongside parallel MSC.526(106) updates. The SAR coordination framework is supplied by the IMO International Convention on Maritime Search and Rescue, 1979 (SAR Convention) and the International Aeronautical and Maritime Search and Rescue (IAMSAR) Manual jointly published by IMO and ICAO. The Personal Locator Beacon framework is operated outside the SOLAS-mandatory regime under the same Cospas-Sarsat 406 MHz system, with national flag-State or industry regulations specifying carriage in jurisdictions including some North Sea offshore-vessel operators and certain Australian commercial fishing fleets.