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Global Maritime Distress and Safety System

The Global Maritime Distress and Safety System (GMDSS) is the mandatory international framework of radio and satellite communications that replaced the long-standing Morse code watch on 500 kHz as the primary means by which ships in distress alert rescue authorities. Adopted through amendments to the International Convention for the Safety of Life at Sea (SOLAS) in 1988, phased in from 1992 and made fully mandatory for cargo ships of 300 gross tonnage and above on international voyages and for all passenger ships on 1 February 1999, GMDSS integrates digital selective calling (DSC) on VHF, MF and HF radio bands, satellite communications via Inmarsat and (since 2020) Iridium Certus, the COSPAS-SARSAT beacon network centred on the 406 MHz emergency position-indicating radio beacon (EPIRB), radar and AIS search-and-rescue transponders (SART), and the NAVTEX maritime safety information broadcast service. The system divides the world’s oceans into four sea areas (A1 through A4) and prescribes the minimum equipment complement for each, so that any ship can transmit a distress alert and receive rescue coordination from a maritime rescue coordination centre (MRCC) anywhere in the world. The ShipCalculators.com calculator catalogue includes tools for GMDSS sea area classification, EPIRB coverage and related equipment checks to support compliance planning.

Contents

Background and history

The Morse era and its limitations

From the 1900s until the late twentieth century, the dominant paradigm for maritime distress communications was the manual Morse telegraphy watch maintained by radio officers on the international distress frequency of 500 kHz. The 1927 International Radiotelegraph Convention codified this watch, and successive Safety of Life at Sea conventions preserved it. Ships of a certain tonnage were required to carry at least one certificated radio officer, who maintained listening watches during fixed periods throughout the day. The system worked tolerably well in coastal waters and on busy shipping lanes, but it depended entirely on the watch officer’s alertness, imposed no automatic detection capability, required fluency in Morse, and offered no rapid means of identifying the precise location of a vessel in distress. False alerts were difficult to filter, and the 500 kHz band had a limited range under adverse propagation conditions.

The inadequacy of the Morse system became starkly apparent in a series of maritime disasters during the 1960s and 1970s where ships disappeared without transmitting any distress signal, or where signals were transmitted but not received in time to mount an effective rescue. Parallel developments in satellite technology and digital communications pointed toward an architecture in which distress alerting would be automatic, position-embedded, and monitored continuously without human intervention. ShipCalculators.com provides tools to support compliance planning under the GMDSS framework.

COSPAS-SARSAT origins

The first element of what would become GMDSS was the COSPAS-SARSAT satellite system. In 1979, the Soviet Union, the United States, Canada, and France signed a memorandum of understanding to develop a cooperative satellite-aided search and rescue system. The acronym COSPAS derives from the Russian phrase for “space system for the search of vessels in distress”; SARSAT stands for Search and Rescue Satellite-Aided Tracking. The system was designed around payloads carried on low-earth-orbit meteorological satellites that would detect radio transmissions on 406 MHz and relay their position to ground stations - local user terminals - and onward to MRCCs. A demonstration phase began in 1982, and the international COSPAS-SARSAT organisation was formalised by agreement in 1988, the same year as the SOLAS amendments that created GMDSS. By the late 1980s the system had demonstrated location accuracies of the order of five kilometres for 406 MHz beacons, a dramatic improvement over the earlier 121.5 MHz class of beacons whose position accuracy was measured in tens of kilometres.

The 1988 SOLAS amendments

At the 1988 Conference of Contracting Governments to SOLAS, the maritime community agreed to a wholesale revision of Chapter IV (Radio Communications) to create GMDSS. The amendments entered into force on 1 February 1992, initiating a seven-year transition during which ships could comply either with the old radio officer system or with GMDSS requirements. The old 500 kHz Morse watch ceased on 1 February 1999, the date on which GMDSS became fully mandatory. The 1988 amendments introduced sea area definitions, required DSC controllers for radio equipment, mandated the carriage of 406 MHz EPIRBs with float-free hydrostatic release, and recognised NAVTEX as the primary means of broadcasting maritime safety information (MSI) in coastal waters.

The concept underlying GMDSS is described in SOLAS Chapter IV, Regulation 1 as providing for: automatic transmission of distress alerts by the ship; shore-to-ship distress alert relay; ship-to-ship distress alert; SAR coordination communications; on-scene communications; locating signals (radar and AIS transponders); transmission and receipt of MSI; and general radio communications. The system is thus not merely a distress alerting tool but an integrated communications architecture spanning normal operations, safety broadcasts, and emergency response.

Sea area definitions and equipment requirements

The four sea areas

GMDSS divides ocean space into four contiguous sea areas designated A1 through A4, with each successive area encompassing greater ocean distance from shore-based VHF or MF coverage. The definitions appear in SOLAS Chapter IV, Regulation 2. An online GMDSS sea area calculator allows operators to classify voyages and determine applicable equipment obligations.

Sea area A1 covers the area within the radiotelephone coverage of at least one VHF coast station in which continuous Digital Selective Calling alerting is available. The nominal range of VHF propagation is approximately 20 to 30 nautical miles from the coast station, though actual coverage depends on antenna height, terrain, and atmospheric conditions. A ship operating exclusively in A1 carries, at minimum, a VHF DSC radio, a NAVTEX receiver, and a 406 MHz EPIRB (or an Inmarsat ship earth station as the primary alerting means). In A1, the EPIRB may be replaced by a VHF DSC EPIRB capable of operating through a VHF coast station.

Sea area A2 extends beyond A1 to the limit of coverage of at least one MF coast station providing continuous DSC alerting on 2187.5 kHz. This area typically covers waters out to approximately 150 to 400 nautical miles from the coast, again subject to propagation conditions. Ships in A2 must carry all A1 equipment plus an MF DSC radio capable of maintaining a continuous watch on 2187.5 kHz and a radiotelephone watch on 2182 kHz.

Sea area A3 covers waters within the footprint of a geostationary communication satellite that provides continuous alerting. In practice, A3 corresponds to the combined coverage of the Inmarsat constellation, which covers latitudes approximately 70°N to 70°S. Ships in A3 carry all A2 equipment plus either an Inmarsat ship earth station (the C, Mini-C, or Fleet series terminals approved under the relevant IMO performance standards) or an HF DSC radio covering the international distress and calling frequencies on the 4, 6, 8, 12, and 16 MHz bands.

Sea area A4 is a residual category comprising all sea areas not included in A1, A2, or A3. In practice, A4 corresponds to polar waters above approximately 70°N and below approximately 70°S, beyond the elevation angle at which geostationary satellites are visible. Ships trading in A4 must carry HF DSC radio as the primary means of long-range distress alerting, in addition to all equipment for the lower sea areas. The Iridium low-earth-orbit (LEO) constellation, which provides pole-to-pole coverage, was recognised as an alternative provider by MSC.526(106) adopted in June 2021, making LEO-based alerting a viable option in A4 waters. The polar communications via Iridium calculator and the polar communications via HF DSC tool support route planning in these regions.

Minimum equipment summary

The minimum equipment tables in SOLAS Chapter IV, Regulation 7 (passenger ships) and Regulation 9 (cargo ships) set out the full carriage requirements by sea area. Common elements across all areas include a two-way VHF radio for on-scene coordination (usually portable, SOLAS-approved), a satellite EPIRB or equivalent alerting device, a 9 GHz radar transponder (SART), and a NAVTEX receiver where broadcasts are available. All DSC controllers must include an integrated GPS or other GNSS receiver so that the vessel’s position is automatically included in the DSC distress alert. The EPIRB coverage calculator and the system-level EPIRB equipment reference support equipment compliance checks.

Digital selective calling

Principles and ITU-R M.493

Digital Selective Calling is the automated digital encoding protocol that enables a radio to transmit or receive a formatted address call specifying the purpose (distress, urgency, safety, or routine), the called or calling vessel’s maritime mobile service identity (MMSI), the position of the vessel, and the subsequent working frequency or mode. The standard governing DSC formats and procedures is ITU-R Recommendation M.493, first issued in 1982 and periodically revised. The current version in force for GMDSS type-approval is M.493-15 (2019). DSC controllers integrated into VHF, MF, and HF radios automatically watch the designated calling channel and decode incoming calls without operator attention.

The MMSI is a nine-digit number issued by the ship’s administration as part of the ship station licence. It encodes the Maritime Identification Digits (MID) of the flag state, followed by a sequential vessel identifier. EPIRBs and AIS-SARTs carry a 15-digit encoded MMSI derived from the vessel’s MMSI. Coast stations and MRCCs hold databases mapping MMSIs to vessels, enabling rapid identification when a DSC distress alert is received.

VHF DSC - channel 70 and channel 16

VHF DSC operates on channel 70 (156.525 MHz), which under the Radio Regulations of the International Telecommunication Union is reserved exclusively for DSC distress, urgency, safety, and calling. Voice distress and safety traffic on VHF uses channel 16 (156.800 MHz). The regulatory model therefore separates the digital alert initiation (channel 70) from the subsequent voice coordination (channel 16). SOLAS Regulation IV/15 requires all VHF radios fitted under Chapter IV to maintain a continuous watch on both channel 70 (DSC) and channel 16 (voice), unless the radio is in use on another channel. Portable two-way VHF radios approved for GMDSS purposes under IMO Resolution MSC.149(77) do not themselves require DSC capability but must be capable of operation on channel 16. A GMDSS-approved handheld VHF tool is available for on-scene communication planning.

A vessel wishing to transmit a distress alert by VHF DSC activates the dedicated red distress button, which is protected against accidental operation by a spring-loaded cover. The controller transmits the formatted DSC call on channel 70, automatically including the MMSI, the distress nature code, and the GNSS-derived position and time. Receiving stations - both other ships and coast stations - automatically sound an alarm and display the alert details. The transmitting vessel then switches to channel 16 and calls “Mayday” in voice, per International Telecommunication Union Radio Regulations Appendix 13.

MF DSC on 2187.5 kHz

Medium frequency DSC uses the exclusive distress and calling frequency of 2187.5 kHz, with 2182 kHz as the companion voice distress frequency. The theoretical maximum range of MF ground-wave propagation is of the order of 400 nautical miles under favourable conditions, though nighttime sky-wave propagation can extend this considerably. GMDSS coast stations monitoring 2187.5 kHz exist at intervals along the world’s major coastlines, and their coverage areas define the outer boundary of A2. The watch on 2187.5 kHz must be maintained continuously by an automatic DSC watch receiver; MF radios fitted under Chapter IV must not require the officer of the watch to scan the frequency manually. SOLAS Chapter IV, Regulation 12 sets out the MF watch-keeping obligation.

HF DSC across the bands

High frequency DSC provides the long-range distress alerting capability required in A3 (as an alternative to satellite) and in A4. The distress and calling frequencies designated by the Radio Regulations for DSC on each band are: 4207.5 kHz (4 MHz band), 6312.0 kHz (6 MHz band), 8414.5 kHz (8 MHz band), 12577.0 kHz (12 MHz band), and 16804.5 kHz (16 MHz band). HF DSC controllers fitted under Chapter IV must scan all five frequencies automatically. The choice of working frequency depends on the time of day, season, and propagation conditions; the 8 MHz band is generally reliable for long-range daytime communication, while 4 MHz provides better short-range nighttime performance. The polar HF DSC communications tool covers HF DSC planning in high-latitude waters where Inmarsat coverage is attenuated.

The companion voice frequencies for HF distress coordination are: 4125 kHz, 6215 kHz, 8291 kHz, 12290 kHz, and 16420 kHz. A ship transmitting an HF DSC distress alert identifies the subsequent working voice frequency in the DSC call format, so that any receiving station can immediately switch to the correct channel for coordination.

Satellite communications

Inmarsat-C SafetyNET and FleetNET

Inmarsat-C is a store-and-forward data messaging service operating via the Inmarsat L-band geostationary satellites. Unlike the earlier Inmarsat-A and Inmarsat-B voice and telex terminals, the -C terminal uses an omnidirectional antenna and transmits at low data rates (600 bps uplink, 1200 bps downlink), making it compact and practical for routine carriage on all vessel classes. The terminal maintains a continuous receive watch and can accept incoming messages, including broadcast safety information, without operator action.

SafetyNET is the Inmarsat service through which NAVAREA and METAREA coordinators, hydrographic offices, and other authorities broadcast MSI to ships in specific ocean regions. SafetyNET messages are addressed either to a geographic area code (circular or rectangular) or to a worldwide broadcast code. A vessel with an active Inmarsat-C terminal will receive all SafetyNET broadcasts addressed to its current geographic area without any operator input, provided the area code is configured correctly. FleetNET is the commercial counterpart service used by fleet operators to send business messages to a group of vessels identified by a shared fleet code; it uses the same L-band infrastructure but does not carry MSI.

Inmarsat-C also provides ship-to-shore alerting via the Distress Priority Service, which routes a high-priority message to the appropriate MRCC via the Inmarsat Land Earth Station (LES) network. Inmarsat Fleet Broadband and Fleet One terminals, which operate at higher data rates, are not mandated under GMDSS for distress alerting purposes (except where they are used as the primary alerting means in A3 under the MSC performance standards for Inmarsat ship earth stations), but they may be carried additionally for commercial communications.

The polar communications via Inmarsat (A3) tool maps the elevation angle limitations of geostationary Inmarsat coverage as a function of latitude, illustrating the transition from A3 to A4 obligation. The LRIT terminal reference covers Long-Range Identification and Tracking terminals, which share the Inmarsat-C or Iridium infrastructure.

Iridium Certus and MSC.526(106)

The Iridium constellation of 66 operational LEO satellites provides continuous pole-to-pole coverage at altitudes of approximately 780 km. Because the satellites are not geostationary, Iridium coverage is independent of latitude, making it inherently suitable for A4 polar sea areas where Inmarsat cannot guarantee service. The Iridium Short Burst Data service has been used for EPIRB and LRIT messaging for many years, but full integration of Iridium as a GMDSS provider required revision of the SOLAS performance standards and recognition by the Maritime Safety Committee.

In June 2021, MSC.526(106) - adopted at the 106th session of the Maritime Safety Committee - recognised Iridium Certus as a mobile satellite service provider for GMDSS purposes, effective from an implementation date that made Iridium-based alerting commercially available under the GMDSS framework from 2020 (the initial Iridium Certus maritime service launch) onwards. The recognition covers the Iridium Certus terminal’s ability to function as a ship earth station providing distress alerting, distress traffic, MSI reception, and general communications, satisfying the SOLAS A3/A4 obligations as an alternative to Inmarsat and HF DSC. Equipment must meet IMO Resolution MSC.431(98) (performance standards for Iridium GMDSS ship earth stations) and the associated IEC type-approval standards.

The practical significance is that ships trading in Arctic or Antarctic waters can now rely on Iridium Certus as their primary long-range alerting means rather than HF DSC, which requires skilled operation and is subject to ionospheric disruption. The shipboard LRIT coverage tool illustrates satellite coverage overlap in polar regions.

EPIRBs and locating signals

406 MHz EPIRB

The 406 MHz emergency position-indicating radio beacon is the primary individual vessel distress beacon under GMDSS. It transmits a coded digital burst message every 50 seconds (approximately) on 406 MHz with a duration of around 0.5 seconds, which is detected and decoded by COSPAS-SARSAT satellite payloads. The 15-digit beacon identification code encoded in the transmission uniquely identifies the vessel (through its MMSI or serial number) and the flag state. Position accuracy for 406 MHz beacons equipped with a GNSS receiver is typically better than 100 m when the internal GNSS fix is included in the transmission; without GNSS, the COSPAS-SARSAT Doppler location process achieves accuracy of approximately 5 km. The beacon simultaneously transmits a 121.5 MHz homing signal at low power to guide search aircraft and rescue craft to the immediate vicinity once the rough position has been established via satellite.

SOLAS Regulation III/7 requires EPIRBs to be capable of being released manually and to release automatically when the vessel sinks (float-free operation). The release mechanism is a hydrostatic release unit (HRU) that activates when the beacon is submerged to a depth of 1 to 4 m. The beacon is buoyant and is designed to float upright, transmitting from the water surface. Regulations require the EPIRB to be mounted in a bracket from which it is released either manually or automatically, positioned so that it can float clear of the sinking vessel. The float-free EPIRB system reference details the HRU and bracket specifications under SOLAS III/7 and the LSA Code.

Batteries must maintain the required transmission for at least 48 hours. The battery expiry date must be stencilled on the beacon and must not have expired; replacement is typically required every five years, though internal GNSS batteries may have shorter replacement intervals. Testing is carried out using the built-in self-test, which transmits a shortened test signal on 406 MHz at intervals specified by the beacon manufacturer, typically not more than once per month to avoid false alerts.

121.5 MHz homing signal

The 121.5 MHz signal transmitted by the EPIRB, and also by radar SARTs and AIS-SARTs, serves exclusively as a homing beacon for aircraft and surface craft equipped with direction-finding equipment. COSPAS-SARSAT discontinued the satellite processing of 121.5 MHz signals on 1 February 2009 because the high false-alert rate of 121.5 MHz beacons and the limited location accuracy made them unsuitable for primary alerting. Since that date, 121.5 MHz functions only as the short-range homing frequency for rescue forces already within approximately 10 to 15 nautical miles of the beacon. All maritime aircraft operated by rescue services maintain a watch on 121.5 MHz (the international aeronautical distress frequency), and many rescue vessels carry portable 121.5 MHz direction finders.

Radar SART (9 GHz)

The Search and Rescue Transponder (SART) is a portable device that, when activated, responds to interrogation by any X-band (9 GHz) ship or aircraft radar. The SART continuously monitors the 9 to 10 GHz band for radar pulses; on receiving a pulse, it sweeps across the 9 GHz band and radiates a series of pulses that appear on the interrogating radar as a line of 12 dots extending radially outward from the SART’s position, approximately 0.64 nautical miles in length. This distinctive pattern is easily distinguished from normal radar echoes and identifies the SART’s position to within approximately 200 m at a range of up to 10 nautical miles from a surface radar with a height-of-antenna of 15 m, and up to 40 nautical miles from a searching aircraft. SOLAS requires at least one SART per lifeboat and one per liferaft group not already served by a lifeboat. The radar SART system reference covers the performance standards under IMO Resolution MSC.246(83).

AIS-SART (channels 87B and 88B)

The AIS Search and Rescue Transmitter (AIS-SART) is an alternative to the radar SART, approved under IMO Resolution MSC.246(83) (2008) and permitted as a substitute for a radar SART since 2010. Rather than responding to radar interrogation, the AIS-SART broadcasts standard AIS Class A position reports on the two AIS simplex channels: 87B (161.975 MHz) and 88B (162.025 MHz). The position is derived from the internal GNSS receiver, and the MMSI of the transmission is a special SAR-transponder MMSI beginning with the digits 970. Any vessel or rescue craft within VHF range that has an AIS display - including ECDIS systems as covered in the AIS and ECDIS article - will see the AIS-SART as a target with the distinctive “man overboard” symbol.

AIS-SARTs have the operational advantage that the displayed position is derived from GNSS and is therefore accurate to within approximately 10 m, compared to the radar-bearing-and-range geometry of the radar SART. However, they share the limitations of VHF propagation (essentially line of sight), and they require the searching vessel to have AIS capability. Both types must operate for at least 96 hours on standby and at least 8 hours in active transmit mode, as specified in IEC 61097-17 for AIS-SARTs.

NAVTEX (Navigational Telex) is the primary means of broadcasting MSI to ships in coastal waters under the GMDSS framework, operating on dedicated narrow-band direct-printing frequencies. The international NAVTEX frequency is 518 kHz, used for English-language broadcasts in all NAVTEX service areas worldwide. A national frequency of 490 kHz allows broadcasts in languages other than English for ships trading in specific regional areas. A third frequency, 4209.5 kHz, provides a medium frequency NAVTEX service for areas with high interference on 518 kHz and for extended range in some regions, though 4209.5 kHz is less universally deployed.

The world’s coastlines are divided into approximately 21 NAVTEX service areas, each assigned a single letter code (A through X, excluding some letters). Within each service area, individual transmitters broadcasting on the same frequency are assigned single-letter station identifiers (B1). The message identifier consists of the station letter, a message type letter, and a two-digit sequence number. NAVTEX receivers are programmed to accept messages only from selected stations and selected message types, preventing message overload. The message categories under the current ITU-R M.540 standard include: navigational warnings (A), meteorological warnings (B), ice reports (C), search and rescue information (D), meteorological forecasts (E), pilot messages (F), AIS messages (G), Loran (H - now deprecated), SATNAV messages (I), other navigational aids (J), and amplifying navigational warnings (L). Types A and B are mandatory and cannot be rejected by the receiver.

The required range of NAVTEX transmitters is approximately 400 nautical miles, though propagation conditions on 518 kHz vary considerably. SOLAS Chapter IV, Regulation 14 requires ships fitted with NAVTEX receivers to monitor the service when operating in an area covered by NAVTEX.

Beyond the NAVTEX service areas, MSI is disseminated through the NAVAREA/METAREA system established under the International SafetyNET service and coordinated by the International Hydrographic Organization (IHO) and the World Meteorological Organization (WMO). The world’s oceans are divided into 21 NAVAREAs, each with a national coordinator responsible for collating navigational warnings from national hydrographic offices, coastal states, and the IMO, and broadcasting them via the Inmarsat SafetyNET service. Ships in A3 equipped with Inmarsat-C receive NAVAREA warnings automatically in their geographic area without needing to be within 400 nautical miles of a NAVTEX transmitter. In A4, where Inmarsat coverage is limited or unavailable, HF NAVTEX and HF Telex services operated by some national authorities provide partial MSI coverage, supplemented by Iridium-based safety broadcasts where available.

The IMO’s International SafetyNET manual and the ALRS (Admiralty List of Radio Signals), published by the UK Hydrographic Office, are the principal references for NAVAREA coordinator contact details, transmitter schedules, and frequency assignments.

Rescue coordination structure

The 1979 SAR Convention

The International Convention on Maritime Search and Rescue was adopted in Hamburg on 27 April 1979 and entered into force on 22 June 1985. The 1979 SAR Convention established the requirement for states to maintain SAR services in their designated SAR regions and to coordinate operations across regional boundaries. The convention introduced the concept of maritime rescue coordination centres (MRCCs) and maritime rescue sub-centres (MRSCs) as the operational hubs of the SAR system. The world’s oceans are divided into SAR regions corresponding broadly to the areas of national responsibility; the boundaries are agreed bilaterally or multilaterally and deposited with the IMO.

The IAMSAR Manual (International Aeronautical and Maritime Search and Rescue Manual), published jointly by the IMO and the International Civil Aviation Organization (ICAO), is the three-volume operational reference covering SAR organisation (Volume I), mission coordination (Volume II), and mobile facilities (Volume III). Volume III is required to be carried on all SOLAS ships as an on-board reference, a requirement that interacts with GMDSS because the master of a ship receiving a distress relay must know how to transmit an appropriate DSC relay call and coordinate with the MRCC.

MRCC operation and GMDSS alerting chain

When a vessel in distress activates a DSC distress alert on VHF channel 70, the alert is received simultaneously by all VHF DSC-equipped vessels within range and by the nearest coast station. The coast station decodes the MMSI, the distress nature code, the position, and the time of the alert, and relays this information to the responsible MRCC. The MRCC then attempts to establish voice contact with the distressed vessel on VHF channel 16 or, if the distress is in A2/A3/A4, on the MF voice frequency 2182 kHz or the relevant HF voice frequency. The MRCC coordinates any nearby vessels designated as SAR participants, alerts national SAR aircraft, and, if the vessel cannot be raised, treats the alert as genuine and mobilises resources.

EPIRB alerts follow a different but complementary path: the 406 MHz signal is received by COSPAS-SARSAT satellites, downloaded to the nearest local user terminal, and forwarded to the responsible rescue coordination centre (RCC) via the mission control centre (MCC) network within approximately 90 minutes for LEO satellite processing, or in near-real-time if a medium-earth-orbit (MEO) or geostationary satellite (GEO) carries a 406 MHz payload. The MEO satellite network (using GPS, GLONASS, Galileo, and BeiDou satellites carrying COSPAS-SARSAT MEOSAR payloads) provides near-instantaneous global alerting with no orbital delay, and MEOSAR has been operational as the primary processing system since approximately 2020.

Radio licensing and certification

Ship station licence

Every vessel carrying radio equipment under SOLAS Chapter IV must hold a ship station licence issued by the flag state telecommunications authority (in many cases the maritime administration or a delegated body). The licence specifies the call sign, the MMSI, the frequencies authorised, the type of equipment, and the permitted operating area. GMDSS equipment fitted under the SOLAS obligation must be of an approved type and must bear the type-approval marking of the relevant national authority or a recognised mutual recognition arrangement. The licence must be kept on board and available for inspection during port state control examinations as described in the port state control article.

Call signs for SOLAS ships are allocated from the ITU’s maritime mobile service identities. The call sign is used for routine voice communication (including SAR coordination); the MMSI is used by DSC equipment. The MMSI is programmed into the DSC controller during installation and cannot be changed without reformatting the controller, a process that must be reported to the flag state authority to maintain database integrity.

STCW Regulation IV/2 - GOC and ROC

The 1995 and 2010 amendments to the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW) introduced radio communication certificates specific to GMDSS. Regulation IV/2 of STCW (and the associated tables in the STCW Code, Section A-IV/2) establishes two certificates:

The General Operator’s Certificate (GOC) is the full GMDSS radio certificate, required for any officer responsible for radio communications on ships in A2, A3, or A4 sea areas. The GOC curriculum covers all GMDSS sub-systems, DSC procedures, satellite communications, distress alerting procedures, NAVTEX, SAR coordination, and practical operation of all fitted equipment. GOC training is typically delivered over approximately 10 days of instruction, and the certificate is issued by the maritime administration.

The Restricted Operator’s Certificate (ROC) covers VHF only and is sufficient for ships operating exclusively in A1. The ROC is also the certificate required for operators of leisure craft fitted with VHF DSC radios in most European jurisdictions, where it is administered by national maritime authorities under ITU Radio Regulations Appendix 13.

The GOC and ROC replace the former Radiotelephone Operator’s Certificate and Radioelectronic Certificate. Since the mandatory date of 1 February 1999, no Morse code proficiency certificate is required for GMDSS operation, though flag states may still issue such certificates voluntarily. The requirement for a certificated radio officer who maintains a dedicated watch has been superseded; GMDSS distributes responsibility for radio communications among bridge watch officers holding GOC or ROC certificates.

Battery reserves and equipment maintenance

Power supply requirements

SOLAS Chapter IV, Regulation 13 specifies the power supply requirements for GMDSS equipment. All radio installations must have a primary power supply from the ship’s main switchboard, an emergency power supply from the emergency generator (where fitted), and an independent reserve power source capable of supplying the radio equipment for a minimum of one hour (for ships without an emergency generator) or at least one hour from the emergency switchboard. The reserve power source is typically a dedicated battery bank, which must be maintained in a fully charged state and kept at a suitable temperature. Battery capacity must be calculated to supply simultaneously the main radio installation and the emergency lighting, with an appropriate margin.

In practice, GMDSS radio equipment manufacturers publish the current consumption figures for each operating mode (standby, receive, transmit at rated power), and the installation engineer calculates the required battery ampere-hour capacity based on the longest expected period of main power loss and the proportion of time spent in each mode during that period.

Survey and maintenance

Under SOLAS Regulation IV/15, GMDSS radio equipment is subject to survey as part of the Harmonised System of Survey and Certification. The radio installation forms part of the Safety Radio Certificate issued to SOLAS cargo ships, with renewal survey required annually or at intervals not exceeding five years as specified in the SOLAS 1974 Consolidated Edition and the HSSC Annex. Class societies carry out radio surveys under their scope of activity as described in the classification society article. The two principal maintenance options recognised by SOLAS Regulation IV/15 are: shore-based maintenance, in which all radio equipment is maintained ashore by a qualified service organisation at each port or periodically; and at-sea maintenance, in which a ship’s GMDSS radio operator holds the authority to maintain and repair equipment and carries the necessary spare parts. A third option, duplication of equipment, avoids the need for individual component maintenance by ensuring that a complete backup system is available.

The maintenance requirements intersect with the ISM Code (ISM Code article), which requires the safety management system to include procedures for maintaining all safety-critical equipment in working order.

Notable incidents

MV Estonia, 1994

The Estonia was a ro-ro passenger ferry that sank in the Baltic Sea on 28 September 1994 with the loss of approximately 852 lives. The disaster occurred before GMDSS became mandatory for all ships (the mandatory date was still 1 February 1999, though Estonia was in the transition period and fitted with Inmarsat-C). The alerting sequence was complicated: the vessel transmitted a Mayday on VHF channel 16, which was received by another ferry and by the Finnish coast radio station, but the delay between the vessel taking on water and the effective alerting of rescue services contributed to the death toll. Subsequent analysis noted that the DSC watch on VHF channel 70 was not in continuous automatic operation, illustrating the importance of the mandatory automated watch that GMDSS requires. The Estonia disaster, alongside the introduction of the ISM Code and STCW amendments, shaped the regulatory response that produced the current integrated GMDSS framework.

Costa Concordia, 2012

The Costa Concordia ran aground off the island of Giglio on 13 January 2012, with the loss of 32 lives. The incident is significant in the context of GMDSS because of the documented delay in the captain’s decision to declare an emergency and the role of the Livorno MRCC in managing communications. The MRCC received an early report of difficulty from a vessel in the area, and subsequently received the formal Mayday call. The incident exposed deficiencies in the coordination between the ship’s master, the company (under the ISM Code), and the MRCC, as well as confusion over the use of different communication channels. The Italian Coast Guard’s handling of the incident - including the famous radio exchange between the MRCC duty officer and the master - became the subject of detailed scrutiny in the Italian judicial proceedings and subsequent IMO safety recommendations (MSC-MEPC.7/Circ.8, 2012).

Dali bridge allision, Baltimore, 26 March 2024

At 01:27 local time on 26 March 2024, the container ship Dali lost electrical power and propulsion while outbound in the Patapsco River and struck the Francis Scott Key Bridge in Baltimore, Maryland. The incident is relevant to GMDSS in the context of on-scene communications: the vessel’s bridge team transmitted a distress call on VHF channel 16 alerting the Maryland Transportation Authority Police and the Vessel Traffic Service, enabling the closure of the bridge to pedestrian traffic before the collision and saving the lives of approximately six people who would otherwise have been on the bridge. The timely VHF alert, transmitted approximately four minutes before impact, demonstrated the effectiveness of voice distress calling on channel 16 as a complement to the automated DSC architecture. The incident also highlighted the importance of maintaining the VHF watch on channel 16 by responding vessels and port authorities, as required under GMDSS and ITU Radio Regulations.

GMDSS modernisation 2020-2028

IMO modernisation programme

Following a comprehensive review of GMDSS initiated at the 94th session of the Maritime Safety Committee (MSC 94, 2014), the IMO has been implementing a phased modernisation of the GMDSS framework. The review was prompted by advances in satellite technology, the availability of LEO providers, the maturation of AIS infrastructure, the emergence of the VHF Data Exchange System (VDES), and recognised weaknesses in the existing alert priority hierarchy.

SOLAS Chapter IV was amended in 2019 (Resolution MSC.436(99)) to allow the acceptance of new mobile satellite service providers such as Iridium, pending the development of performance standards. Further amendments (MSC.496(105), 2021) updated the definitions of ship earth stations and performance requirements. The overall modernisation programme envisages completion of major SOLAS Chapter IV revisions by approximately 2028.

VDES integration

The VHF Data Exchange System (VDES) is a next-generation extension of the AIS VHF infrastructure, standardised in ITU-R M.2092 (2015, revised 2019). VDES encompasses three components: AIS (legacy, channels 87B/88B), an Application Specific Messaging (ASM) channel for broadcast data exchange, and a Satellite VDES component (S-VDES) for pole-to-pole coverage. VDES is designed to provide a high-throughput, low-latency digital data link on the VHF maritime mobile band, capable of supporting MSI delivery, two-way messaging, and position reporting at much higher data rates than legacy AIS. Integration of VDES into GMDSS as a recognised alerting and MSI delivery medium is anticipated under the 2020-2028 roadmap, subject to ITU Radio Regulation coordination for S-VDES satellite spectrum. The connection to AIS navigation is covered in the AIS and ECDIS article.

Second-generation 406 MHz EPIRBs

The COSPAS-SARSAT Second Generation Beacon (SGB) specification defines a new class of 406 MHz beacons that include an AIS transmitter (on channels 87B and 88B) and a Near Field Communication (NFC) interface for inspection and registration update by a smartphone. The AIS transmitter allows nearby vessels with AIS displays to see the EPIRB as a target, complementing the satellite alerting chain with a short-range visible beacon. The NFC interface enables port state control officers, surveyors, and service technicians to verify beacon registration data and battery status without activating the 406 MHz transmitter. IMO MSC has endorsed the SGB concept, and carriage of SGB-compliant EPIRBs is expected to become mandatory for new-build ships within the modernisation timeframe, with a phase-in period for existing vessels.

Alert priority simplification

The existing GMDSS defines five communication priorities: distress, urgency, safety, routine maritime communications, and public correspondence. Operational experience has shown that the distinction between urgency (PAN PAN) and safety (SECURITE) communications is poorly understood by many bridge watchkeepers and that the procedural complexity of selecting the correct DSC priority code reduces the reliability of the alerting chain. The modernisation programme proposes a simplified hierarchy: distress, safety (encompassing current urgency and safety categories), and routine. Revised ITU Radio Regulation procedures and associated IMO performance standards for DSC controllers implementing the simplified hierarchy are under development.

Integration with polar operations

The recognition of Iridium Certus and the anticipated VDES S-VDES component address the long-standing gap in GMDSS coverage at high latitudes. The Polar Code (in force since 1 January 2017) requires ships operating in polar waters to demonstrate that their communications equipment provides adequate coverage for the intended voyage area; in A4 waters, this requires either HF DSC or an approved LEO satellite terminal. The modernisation programme’s acceptance of Iridium Certus closes the gap that existed when only Inmarsat geostationary satellites were recognised, and the Polar Code’s additional communication equipment requirements in Part I-A, Regulation 6 will be updated to reflect the expanded provider list as SOLAS Chapter IV amendments are finalised.

Interaction with other regulatory instruments

GMDSS does not operate in isolation. SOLAS Chapter V (Safety of Navigation) requires carriage of AIS, a voyage data recorder (VDR), and a bridge navigational watch alarm system (BNWAS) - all of which interact with GMDSS equipment in an integrated bridge system. The VDR records radio communications on the monitored channels as part of its mandatory data set, providing post-incident evidence of the DSC alerts, NAVTEX messages, and Inmarsat-C traffic that occurred before and during an accident. The AIS transponder’s MMSI is the same as the GMDSS MMSI, linking the position-reporting and distress-alerting functions.

The ISPS Code (ISPS Code article) introduced the Ship Security Alert System (SSAS), a covert distress alert mechanism separate from GMDSS that transmits a silent alert to the ship’s company and the flag state authority in the event of a security threat. The SSAS uses Inmarsat-C or equivalent satellite infrastructure and must not be confused with the GMDSS Distress Priority call, which is an overt alert intended to summon rescue resources.

Under the ISM Code (ISM Code article), the ship’s safety management system must include procedures for operating and testing all GMDSS equipment, for reporting deficiencies to the company designated person ashore (DPA), and for ensuring that bridge watch officers hold current GOC or ROC certificates. Deficiencies in GMDSS equipment are a common finding in port state control inspections, as documented in the annual reports of the Paris MOU and Tokyo MOU as described in the port state control article.

The Long-Range Identification and Tracking (LRIT) system, mandated under SOLAS Regulation V/19-1 from 31 December 2008, uses Inmarsat-C or Iridium infrastructure to transmit ship position reports to flag states, port states, and coastal states at intervals of six hours (or more frequently on request). While LRIT is a safety and security tool rather than a distress alerting system, it uses the same satellite infrastructure as GMDSS and the same terminal in many installations. The LRIT terminal reference covers combined GMDSS/LRIT terminal configurations.

See also

References

  1. International Maritime Organization. International Convention for the Safety of Life at Sea (SOLAS), 1974, as amended. Consolidated Edition, 2020. IMO Publishing, London.
  2. IMO Resolution MSC.436(99), Amendments to the International Convention for the Safety of Life at Sea, 1974 (SOLAS Chapter IV), adopted 24 May 2018.
  3. IMO Resolution MSC.526(106), Recognition of Iridium as a GMDSS mobile satellite service provider, adopted June 2021.
  4. IMO Resolution MSC.431(98), Performance standards for Iridium GMDSS ship earth stations, adopted 2017.
  5. IMO Resolution MSC.246(83), Adoption of performance standards for survival craft AIS search and rescue transmitters (AIS-SART) for use in search and rescue operations, adopted 2007.
  6. IMO. IAMSAR Manual, Volume III: Mobile Facilities. 2022 Edition. IMO Publishing, London.
  7. International Telecommunication Union. Radio Regulations, 2020 Edition. ITU, Geneva. Appendix 13 (distress and safety procedures); Appendix 15 (MMSI); Resolution 331 (GMDSS).
  8. ITU-R Recommendation M.493-15. Digital selective-calling system for use in the maritime mobile service. ITU-R, Geneva, 2019.
  9. ITU-R Recommendation M.540-3. Operational and technical characteristics and requirements for narrow-band direct-printing telegraph equipment in the maritime mobile service (NAVTEX). ITU-R, Geneva, 2015.
  10. ITU-R Recommendation M.2092-1. Technical characteristics for a VHF data exchange system in the VHF maritime mobile band. ITU-R, Geneva, 2019.
  11. COSPAS-SARSAT Secretariat. COSPAS-SARSAT System Overview, Document C/S G.003. Montreal, 2023.
  12. COSPAS-SARSAT. Second Generation Beacons (SGB) - System Definition Document, C/S T.018. Montreal, 2022.
  13. Joint Investigation Commission. Report on the Capsizing on 28 September 1994 in the Baltic Sea of the ro-ro passenger vessel MV Estonia. Government of Estonia, Finland, and Sweden, 1997.
  14. Italian Ministry of Infrastructure and Transport. Investigation Report on the sinking of the passenger ship Costa Concordia, 2013.
  15. National Transportation Safety Board. Preliminary Report MRI-24-05: Allision of Container Ship Dali with Francis Scott Key Bridge, Baltimore, Maryland, 26 March 2024. NTSB, Washington DC, 2024.
  16. IMO. International SafetyNET Manual, 2nd Edition. IMO Publishing, London, 2011.
  17. UK Hydrographic Office. Admiralty List of Radio Signals (ALRS), Volumes 1-6. Current edition. UKHO, Taunton.
  18. IMO Resolution MSC-MEPC.7/Circ.8. Principles of minimum safe manning, 2012. (Cited in context of Costa Concordia recommendations.)

Further reading

  • Bole, A., Wall, A., and Norris, A. Radar and ARPA Manual, 3rd edition. Butterworth-Heinemann, Oxford, 2014. (Chapter on GMDSS radar interfaces.)
  • Eurocae. ED-227: Minimum Operational Performance Standards for 406 MHz Emergency Position Indicating Radio Beacons, 2018.
  • Featherstone, C. and Sherwood, S. GMDSS: A User’s Handbook, 5th edition. Witherby Seamanship International, Edinburgh, 2011.
  • IMO. Model Course 1.25: GMDSS General Operator’s Certificate, revised edition. IMO Publishing, London, 2019.