SOLAS Chapter IV: Radio Communications and the GMDSS

Background

The pre-GMDSS regime

Before the GMDSS, ship-to-ship and ship-to-shore distress communication relied on a regime that traced its roots to the 1914 SOLAS Convention’s post-Titanic radio requirements. Key features:

• Continuous listening watches on 500 kHz (the international Morse distress frequency for medium-frequency radiotelegraphy), 2182 kHz (the medium-frequency radiotelephone distress frequency, mandatory from 1956), and 156.8 MHz VHF Channel 16 (the VHF radiotelephone distress frequency, mandatory from 1971).
• Trained radio officers on board (SOLAS-required for ships above defined thresholds), holding a Radio Officer’s Certificate, maintaining the listening watches and responsible for distress communications.
• Manual distress procedure: in the event of distress, the radio officer transmitted SOS in Morse on 500 kHz or MAYDAY on voice on 2182 kHz or Channel 16, with position, ship’s name, nature of distress, and number of persons on board. Coast stations and ships in the area copied the distress and coordinated response.
• Range limitations: VHF effective range was about 30 nautical miles; medium-frequency 200 to 400 nautical miles; high-frequency thousands of miles in suitable conditions but with significant geographical and atmospheric variability.

The pre-GMDSS regime had several limitations:

• Radio officer staffing burden: continuous listening watches required sufficient radio officer rotation to cover the 24-hour day.
• Variable coverage: a distressed ship might be hundreds of miles from any other ship listening; in remote oceans, distress alerts often went unheard.
• Manual response: each distress alert required manual processing and acknowledgement, with delays inherent in the procedure.
• Limited automation: radio equipment could not automatically alert the watchkeeper to incoming traffic; the watchkeeper had to be present and listening.

The GMDSS concept

The GMDSS, developed in the 1970s and 1980s and adopted in 1988, addressed these limitations through:

• Automated distress alerting: the ship transmits a digital distress alert that is automatically received and processed by coast stations, satellite-based search and rescue (SAR) systems, and other ships.
• Multi-system redundancy: distress alerts can be transmitted via VHF DSC, MF DSC, HF DSC, satellite (Inmarsat or Iridium), and 406 MHz EPIRB to Cospas-Sarsat, providing multiple paths to alert.
• Sea area-based equipment: ships are equipped to a level appropriate to their operating area, rather than uniform equipment for all ships.
• Watchkeeping by all bridge officers: the bridge OOW maintains the radio watch via DSC alarm, replacing the dedicated radio officer.
• Automated reception of safety information: NAVTEX and SafetyNET receive maritime safety information automatically, printing or displaying received messages without operator intervention.

Major milestones

• 1979 SAR Convention: adopted, providing the framework for international cooperation on search and rescue. Subsequent amendments aligned SAR with GMDSS development.
• 1988 GMDSS adoption (Resolution MSC.45(65)): GMDSS incorporated into SOLAS Chapter IV.
• 1992 GMDSS staged implementation begins: phased introduction starting with passenger ships.
• 1 February 1999: Full mandatory GMDSS implementation for all SOLAS ships, replacing the previous radio officer regime.
• 2002 amendments: tightening of GMDSS equipment carriage and adding specific provisions for high-speed craft.
• 2017 GMDSS Modernisation Plan (Resolution MSC.428(98)): comprehensive review of GMDSS technology with consideration of new satellite providers, updated equipment standards, and integration with e-navigation.
• 2020 Iridium recognition: Iridium recognised as a GMDSS satellite provider alongside Inmarsat, providing global coverage including polar regions.
• 2024 GMDSS Modernisation Phase 2 (under development): further updates including Inmarsat C and FleetBroadband phase-out planning, updated performance standards.

Relationship to SAR and IAMSAR

GMDSS provides the technical communication infrastructure; the SAR (Search and Rescue) Convention and the IAMSAR Manual provide the operational doctrine for distress response. A typical distress event involves:

• Distress alert transmitted via GMDSS (multiple paths).
• Reception by Rescue Coordination Centre (RCC) via the GMDSS infrastructure.
• SAR coordination by the RCC under SAR Convention provisions.
• On-scene communication between SAR units and the distressed ship using GMDSS channels.
• Recovery and aftermath documented through the GMDSS records and the SAR after-action report.

The IAMSAR Manual Volume III (Mobile Facilities) provides the detailed procedure for ship and aircraft on-scene SAR Mission Coordinators.

Application (Regulation 1)

Chapter IV applies to:

• All ships subject to SOLAS (passenger ships of any size, cargo ships of 500 GT and above on international voyages).
• High-speed craft under Chapter X with specific GMDSS provisions in the HSC Code Chapter 14.
• Industrial-personnel vessels under Chapter XV with provisions in the IP Code Chapter 11.

The Reg IV/3 application calculator returns the applicable provisions for a given ship and operating area.

Definitions and functional requirements (Regulations 2, 4)

Key definitions

Regulation 2 defines:

• Continuous watch: maintained without interruption.
• DSC (Digital Selective Calling): digital alerting system using ITU Recommendation 493 protocol on dedicated frequencies.
• MID (Maritime Identification Digits): the first three digits of the MMSI (Maritime Mobile Service Identity), identifying the country of registration.
• MMSI: nine-digit Maritime Mobile Service Identity uniquely identifying the ship in DSC and AIS communications.
• NAVTEX: NAVigational TEleX broadcast on 518 kHz (English) and 490 kHz (national language) for maritime safety information.
• NBDP (Narrow Band Direct Printing): digital teletype mode used in some pre-modern GMDSS configurations.

Nine GMDSS functions

Regulation 4 sets out the nine GMDSS functions that every ship must be equipped to perform:

1. Transmit ship-to-shore distress alerts.
2. Receive shore-to-ship distress relays.
3. Transmit and receive ship-to-ship distress alerts.
4. Transmit and receive search and rescue coordinating communications.
5. Transmit and receive on-scene communications.
6. Transmit and receive locating signals (SARTs and AIS-SARTs).
7. Transmit and receive maritime safety information (NAVAREA, METAREA, local warnings).
8. Transmit and receive general radiocommunications.
9. Transmit and receive bridge-to-bridge communications.

The Reg IV/4 calculator checks compliance against the nine functions for a given ship and equipment configuration.

GMDSS sea areas (Regulations 8-10)

Sea Area A1

Sea Area A1 is defined as the area within the radiotelephone coverage of at least one VHF coast station maintaining continuous DSC alerting on Channel 70. The area is typically:

• Within 30 to 40 nautical miles of the VHF coast station.
• Coastal waters of states with developed maritime infrastructure (most of Europe, North America, Japan, Australia, Singapore).

In Sea Area A1, ships must carry:

• VHF radio installation with DSC capability on Channel 70.
• 406 MHz EPIRB.
• 9 GHz radar SART (or AIS-SART).
• Two-way VHF radiotelephones for survival craft (3 minimum).

The Reg IV/8 sea area A1 calculator returns the applicable equipment.

Sea Area A2

Sea Area A2 is defined as the area within the radiotelephone coverage of at least one MF coast station maintaining continuous DSC alerting on 2187.5 kHz, but excluding A1. The area is typically:

• Within 100 to 200 nautical miles of the MF coast station.
• Continental shelf and offshore waters of states with developed maritime infrastructure.

In Sea Area A2, ships must carry the A1 equipment plus:

• MF radio installation with DSC capability on 2187.5 kHz.
• MF radiotelephone capability for distress and routine communication.

The Reg IV/9 sea area A2 calculator returns the applicable equipment.

Sea Area A3

Sea Area A3 is defined as the area within the coverage of an Inmarsat geostationary satellite, but excluding A1 and A2. The area is approximately:

• Between 70 degrees North and 70 degrees South latitude.
• Most of the world’s open ocean.

In Sea Area A3, ships must carry the A1 and A2 equipment plus:

• Either:
  • Inmarsat-C ship earth station with EGC (Enhanced Group Call) for SafetyNET reception, OR
  • HF radio installation with DSC capability on multiple frequencies (4, 6, 8, 12, 16 MHz bands), OR
  • Iridium ship earth station (recognised since 2020).

The choice between Inmarsat, Iridium and HF reflects operator preference, route coverage, and cost.

The Reg IV/10 sea area A3-A4 calculator returns the applicable equipment.

Sea Area A4

Sea Area A4 is defined as the area outside Sea Areas A1, A2 and A3, i.e. the polar regions above 70 degrees North or below 70 degrees South. In Sea Area A4, ships must carry:

• All A1, A2 and A3 equipment.
• HF radio installation with DSC capability (mandatory in A4 because Inmarsat geostationary satellites do not cover polar regions).
• Iridium (which has polar coverage through its low-earth-orbit constellation) is increasingly used as a more reliable alternative or supplement to HF for A4 operations.

A4 ships operate in Polar Code regions and have additional GMDSS provisions in the Polar Code itself.

GMDSS sea area determination

A ship’s GMDSS sea area is determined by:

• The voyage profile (the sea areas through which the ship will sail).
• The mandatory minimum carriage requirement (the highest sea area the ship will traverse).

The GMDSS sea areas calculator returns the sea area for a given route.

Radio equipment in detail

VHF DSC

VHF DSC operates on Channel 70 (156.525 MHz) for digital distress alerts. Equipment characteristics:

• DSC controller: dedicated digital controller transmitting DSC alerts, integrated with the VHF radio.
• VHF radio: typically 25 watts output, with continuous DSC watch on Channel 70 plus voice channels including Channel 16.
• MMSI programmed: the ship’s MMSI must be programmed before the equipment can transmit DSC alerts. Incorrect MMSI programming is a recurring source of false alerts.
• Position input: from the GPS receiver, providing automatic position-updating in DSC distress alerts.

The DSC distress format includes: ship’s MMSI, nature of distress (free text or category), position (from GPS), time (from GPS), call type (distress), and frequency to call back on.

MF DSC and HF DSC

MF DSC operates on 2187.5 kHz with continuous watch on the dedicated DSC frequency. Distress alerts are similar to VHF DSC.

HF DSC operates on multiple frequencies in the 4, 6, 8, 12, and 16 MHz bands. The choice of frequency depends on:

• Time of day: daytime favours higher frequencies (12, 16 MHz); nighttime favours lower (4, 6 MHz).
• Distance: greater distances generally favour higher frequencies for one-hop propagation, lower frequencies for multi-hop.
• Atmospheric conditions: solar activity affects HF propagation significantly.

A typical HF DSC installation includes a controller with watch on multiple frequencies, with the operator selecting the frequency for the actual call.

Inmarsat-C

Inmarsat-C is the most-deployed GMDSS satellite system, providing:

• Distress alert via specialised distress messages.
• EGC (Enhanced Group Call): SafetyNET broadcast reception for NAVAREA warnings, METAREA warnings, and other Maritime Safety Information.
• Telex/email for ship-to-shore communication.
• Position reporting for AMVER and other ship reporting systems.

Inmarsat-C uses small, low-power omni-directional antennas, making it cost-effective for general fleet equipment. The Inmarsat-C network is operated by Inmarsat (subsidiary of Viasat since 2023).

Iridium

Iridium was recognised as a GMDSS satellite provider in 2020 alongside Inmarsat. It uses a low-earth-orbit (LEO) constellation of 66 satellites providing:

• Global coverage including polar regions (advantage over Inmarsat which has gaps above 70 degrees latitude).
• Voice and data through Iridium Open Port and other services.
• Distress alerting through dedicated GMDSS-recognised devices.
• Tracking and reporting through specialised data services.

Iridium adoption has grown rapidly since 2020, particularly for polar operations and for operators seeking redundancy or alternatives to Inmarsat.

NAVTEX

NAVTEX is the international automatic broadcast system for navigational warnings, weather warnings, search and rescue information, and other maritime safety information. NAVTEX uses two frequencies:

• 518 kHz: international service in English.
• 490 kHz: national service in the local language.

NAVTEX coverage is regional, with stations transmitting to a defined geographical area. Ships have a NAVTEX receiver that automatically prints (or displays) received messages.

EPIRB and Cospas-Sarsat

406 MHz EPIRBs transmit to the Cospas-Sarsat satellite system on activation, providing automatic distress alert with ship identity and (modern beacons) integrated GPS position. EPIRBs are:

• Float-free hydrostatic-release: mounted in a hydrostatic-release bracket that releases the EPIRB on submersion, with the EPIRB floating free and self-activating.
• Manually-deployable: secondary EPIRBs (on passenger ships) deployable by hand.
• Battery-powered: with battery life of 5 years typically (replacement cycle), and at least 48 hours of transmission once activated.

Cospas-Sarsat is the satellite system that receives and processes EPIRB transmissions, with three constellations:

• GEOSAR (geostationary satellites): instant detection without location.
• LEOSAR (low-earth-orbit): doppler-derived location, with up to 90-minute detection latency in worst case.
• MEOSAR (medium-earth-orbit, on GPS, Galileo, GLONASS payloads): real-time detection and location through multilateration, rolling out in the 2020s.

The EPIRB coverage calculator shows the coverage envelope for a given EPIRB type.

SARTs and AIS-SARTs

Search and Rescue Transponders (SARTs) activate on receipt of a 9 GHz radar interrogation, returning a series of 12 dots on the search-and-rescue ship’s radar display at the SART’s location. SARTs are typically deployed in liferafts or carried with personal LSA.

AIS-SARTs are an alternative or supplement, transmitting on AIS frequencies (channels 70 and 71 in the AIS spectrum) with location data, providing notification on AIS receivers in the area.

The choice between radar SART and AIS-SART depends on the rescue scenario: radar SARTs are highly visible on radar at significant distance; AIS-SARTs work well for vessels with AIS but are less effective for aircraft without AIS receivers.

Watches (Regulation 11)

GMDSS watchkeeping requirements

Every ship must maintain continuous watch:

• VHF DSC Channel 70 in Sea Area A1 (and at all times).
• MF DSC 2187.5 kHz in Sea Area A2 (and where the ship has the equipment).
• HF DSC in Sea Area A3-A4 (where applicable, with frequency selection appropriate to time of day).
• NAVTEX 518 kHz for receipt of maritime safety information.
• Inmarsat-C SafetyNET for receipt of maritime safety information.
• VHF Channel 16 for distress and on-scene communication where the ship is in coastal waters.

The watchkeeping is automated; the equipment alerts the bridge OOW on receipt of distress alerts or relevant traffic. The OOW does not need to actively listen to all frequencies but must respond to alerts.

False alerts and procedures

DSC false alerts are a recurring issue:

• Inadvertent button-pressing: the DSC distress button is sometimes inadvertently pressed during equipment maintenance or testing.
• MMSI errors: incorrectly programmed MMSI can cause alerts attributed to wrong ship.
• Drill alerts: drill alerts inadvertently transmitted as live alerts due to procedural errors.

Each false alert must be:

• Cancelled immediately by the originating ship via voice on the same channel.
• Documented in the radio log.
• Investigated by the master to identify and correct the root cause.

The IMO has issued multiple circulars (MSC/Circ.1124, MSC.1/Circ.1422 and successors) on DSC false alert prevention.

The Reg IV/11 watches calculator returns the watchkeeping requirements for a given configuration.

Energy sources (Regulation 12)

GMDSS equipment must be supplied by:

• Main electrical power during normal operation.
• Emergency electrical power during emergency conditions.
• Reserve battery for at least 1 hour of continuous transmission and reception, capable of supporting the radio installation independently of any other source.

The reserve battery requirement is a critical safety feature: in the event of complete electrical failure, the radio installation can still alert distress. Battery capacity, charging arrangement and voltage are specified in the regulation.

Performance standards and maintenance (Regulations 13-14)

Performance standards

Each piece of GMDSS equipment must be type-approved against the IMO performance standards:

• VHF DSC: Resolution A.803(19).
• MF DSC: Resolution A.804(19).
• HF DSC: Resolution A.806(19).
• Inmarsat-C ship earth station: Resolution A.807(19) and successors.
• Iridium ship earth station: Resolution MSC.434(98) (2017) and updates.
• EPIRB 406 MHz: Resolution A.810(19).
• NAVTEX: Resolution A.525(13).
• 9 GHz SART: Resolution A.802(19).
• AIS-SART: Resolution MSC.246(83).

The performance standards specify functional requirements, environmental tests (temperature, humidity, vibration, EMC), and minimum performance metrics.

Maintenance

Regulation 14 requires that GMDSS equipment be maintained throughout the ship’s life. The maintenance regime can be:

• Duplication of equipment: two complete sets of GMDSS equipment on board, with one operating and one as backup.
• Shore-based maintenance: at-sea maintenance limited to operational checks; major repair conducted ashore.
• At-sea maintenance: repair capability on board with trained personnel and spare parts.
• Combination: typical larger ships use duplication plus shore-based maintenance.

The radio surveyor verifies the maintenance arrangement during periodic surveys.

The Reg IV/13 performance standards calculator and the Reg IV/14 maintenance calculator cover compliance.

Records (Regulation 15)

The radio log is the GMDSS equivalent of the deck logbook. It records:

• All distress, urgency and safety messages transmitted or received.
• Daily, weekly and monthly tests of equipment.
• Battery checks with voltage and condition.
• DSC test calls to coast stations and other ships.
• EPIRB tests.
• Watchkeeper changes and any disruptions to watch.
• Any failures of equipment with corrective action.

The radio log is preserved for a defined period (typically 1 to 2 years on board, longer ashore) and is available to flag state and PSC inspectors.

Radio personnel (Regulations 16-17)

Radio operator certificates

Every GMDSS-equipped ship must have personnel certified to operate the equipment. The STCW Convention provides the certification framework:

• GOC (General Operator’s Certificate): required for unrestricted GMDSS operations, covering all sea areas. STCW Section A-IV/2.
• ROC (Restricted Operator’s Certificate): required for Sea Area A1 operations only. STCW Section A-IV/2.

The certificates are issued after training course completion and examination, with currency requirements (operator must demonstrate continued familiarity through practice or refresher training).

Personnel records

The ship maintains records of:

• Each operator’s certificate (number, issuing flag state, date of issue, date of expiry).
• Operator training and currency demonstrations.
• Designation of operator(s) responsible for GMDSS during voyages.

PSC inspections often include verification of operator certificates against the actual on-board complement.

Position-updating (Regulation 18)

DSC, EPIRB and other GMDSS equipment must be capable of automatic position-updating from the ship’s GPS or other position source. The requirement ensures that distress alerts include current position rather than position from initial setup.

Position-updating is achieved through interface between the GPS receiver and the GMDSS equipment, with the position automatically updated typically every minute or every position fix.

GMDSS modernisation

2017 Modernisation Plan

The IMO 2017 GMDSS Modernisation Plan (Resolution MSC.428(98)) addressed:

• Recognition of new satellite providers: leading to Iridium recognition in 2020.
• Update of equipment standards to reflect modern technology.
• Integration with e-navigation: connecting GMDSS with broader navigation and communication systems.
• Phase-out of older technologies: NBDP and certain Inmarsat services scheduled for phase-out.
• Improved Maritime Safety Information: streamlining of NAVTEX and SafetyNET.

2020 Iridium recognition

The 2020 amendments recognised Iridium as a GMDSS satellite provider, providing:

• Global coverage including polar regions (a major advantage for A4 operators).
• Alternative to Inmarsat for redundancy and competition.
• Modern data services through Iridium Open Port and other offerings.

Future developments

Ongoing development includes:

• NextGen Cospas-Sarsat: rolling out MEOSAR through GPS, Galileo and GLONASS satellite payloads, providing real-time detection and location.
• Inmarsat-C phase-out: planned for the late 2020s as Inmarsat moves to next-generation services.
• Maritime Connectivity Platform (MCP): e-navigation framework integrating GMDSS with broader maritime communication.
• Cybersecurity of GMDSS equipment, addressing vulnerabilities in modern digital systems.

Notable casualties and lessons

Indian Ocean tsunami, 2004

The 26 December 2004 Indian Ocean tsunami exposed limitations in early-warning communication. While the GMDSS provided the technical infrastructure for distress alerts from individual ships, the regional warning system for tsunami was not yet developed. The IMO subsequently coordinated with the Indian Ocean Tsunami Warning System (IOTWS) and similar regional bodies to integrate tsunami warnings with the GMDSS Maritime Safety Information broadcast.

MV Sanchi (2018)

The Iranian-flagged tanker MV Sanchi collided with the bulk carrier CF Crystal in the East China Sea on 6 January 2018, with 32 dead. The post-incident review found that GMDSS distress alerts were transmitted promptly but that the on-scene communication was complicated by language differences, equipment issues, and the rapid escalation of the casualty. The casualty drove attention to GMDSS operator training in multilingual emergency response.

EPIRB false alerts

False EPIRB activations are a persistent issue in the GMDSS, with thousands of false alerts processed by Cospas-Sarsat each year. The causes include:

• Inadvertent activation during equipment maintenance.
• Hydrostatic release malfunction during normal operations.
• Test signals misinterpreted as live alerts.
• Old-stock beacons with degraded batteries that activate unexpectedly.

Each false alert costs Cospas-Sarsat and SAR resources processing time. The IMO has issued multiple circulars on EPIRB false alert prevention, with operator-side procedures including documented testing, careful stowage, and beacon replacement at end-of-life.

Port state control and GMDSS

PSC inspection of GMDSS

Port state control inspections of GMDSS focus on:

• Equipment functionality: turning on each piece of GMDSS equipment to verify operation.
• Battery condition: voltage measurement and condition assessment.
• Antenna integrity: visual inspection of VHF, MF, HF, satellite and EPIRB antennas.
• MMSI verification: confirming that the ship’s MMSI is correctly programmed in DSC and AIS equipment.
• EPIRB self-test: verifying that the EPIRB self-test passes.
• Operator certification: verifying that on-board personnel hold valid GOC or ROC certificates.
• Radio log: reviewing log entries for completeness, daily/weekly/monthly tests, and any reported problems.
• Survey records: verifying that the GMDSS has had its required periodic surveys.

A serious GMDSS deficiency can result in detention until rectified. Common deficiencies include:

• Battery condition below minimum.
• EPIRB beyond battery replacement date.
• SART or AIS-SART expired or non-functional.
• Operator certificate expired.
• Radio log not maintained.
• Equipment not type-approved or with expired type approval.

Detention case studies

Multiple PSC detentions per year involve GMDSS deficiencies. Common patterns include:

• Operators with poor maintenance culture failing to maintain battery condition.
• Operators with expired EPIRB battery cycles.
• Operators not ensuring operator certification renewal.
• Operators using outdated equipment beyond type-approval expiry.

Each detention is recorded in the PSC database and contributes to the operator’s risk profile, with cumulative detentions affecting future inspection priority and contributing to operator-level enforcement actions by the flag state and by the broader regulatory community responsible for maritime safety oversight.

Survival craft radio equipment

Two-way radiotelephones

Each ship must carry at least three two-way VHF radiotelephones for survival craft (or two on cargo ships of 300 to 500 GT). Characteristics:

• Waterproof construction rated for use in survival craft environment.
• Battery capacity sufficient for at least 8 hours of continuous transmission and reception.
• Channel 16 plus other channels for distress and on-scene communication.
• Operable with gloved hands in survival conditions.
• Daylight visibility controls and display.

The two-way radiotelephones are typically stowed at the embarkation deck near the survival craft, ready for deployment with the survival craft.

EPIRB carriage

The 406 MHz EPIRB requirements:

• Float-free hydrostatic-release mounting at a location where the EPIRB will float free if the ship sinks.
• Manual deployment capability for crew to take the EPIRB into the survival craft.
• Automatic activation on water immersion or by manual switch.
• Self-test capability for periodic verification.
• Battery replacement at the manufacturer’s interval (typically 5 years).

Most ships carry one EPIRB; passenger ships and certain other ship types carry two.

SART and AIS-SART

Each survival craft carries either a 9 GHz SART or an AIS-SART, deployed when distress is declared. The SART or AIS-SART is stowed in the survival craft’s emergency equipment locker, ready for deployment.

Equipment maintenance and replacement

Survival craft radio equipment is subject to the same survey regime as the main GMDSS equipment, with battery replacement, hydrostatic seal verification, and operational testing at defined intervals.

GMDSS in special operations

Polar Code interaction

Polar Code operations introduce specific GMDSS considerations:

• A4 sea area for polar regions with HF DSC and Iridium as primary distress paths.
• Reduced satellite coverage: Inmarsat geostationary coverage degrades above 70 degrees latitude; Iridium provides full polar coverage.
• HF propagation in the polar regions is affected by auroral activity and ionospheric disturbance.
• Long response times for SAR resources due to remoteness; the GMDSS distress alert provides only the first link in a long rescue chain.
• Cold-environment equipment: GMDSS equipment must operate in extreme cold; manufacturers provide cold-weather specifications and the Polar Water Operational Manual documents the operational limits.

The Polar Code Chapter on radio communications provides additional guidance for polar operations.

High-speed craft GMDSS

HSC GMDSS provisions are adapted to the short-route operational profile:

• HSC operating in coastal Sea Area A1 typically need only the basic VHF DSC equipment.
• The shore-side coordination with the operator’s control room is supplemented by GMDSS infrastructure.
• Continuous shore-side AIS tracking provides additional position and traffic awareness.

Industrial-personnel ship GMDSS

Industrial-personnel ships typically operate in Sea Area A1 or A2 and have GMDSS provisions appropriate to short coastal operations. The integration with the destination industrial site’s communication system extends the shore-side coordination beyond the GMDSS infrastructure.

Recent and emerging developments

Inmarsat-C end-of-life

Inmarsat has announced phase-out of Inmarsat-C, with end-of-life expected in the late 2020s. Operators are progressively migrating to:

• Inmarsat FleetBroadband for higher-bandwidth services.
• Iridium Open Port for global coverage including polar.
• Iridium Certus for higher-bandwidth Iridium service.

The migration is being managed through SOLAS amendments to ensure continuity of GMDSS coverage.

Maritime Connectivity Platform (MCP)

The MCP is an e-navigation framework integrating GMDSS with broader maritime communication including:

• Maritime Service Portfolios: defined service offerings from coastal authorities.
• VDES (VHF Data Exchange System): emerging standard for VHF data communication beyond AIS.
• Common Maritime Data Structure: standardised maritime data exchange.

The MCP is in early-stage development with pilot implementations in selected regions.

NAVDAT replacement of NAVTEX

NAVDAT is a digital NAVTEX successor in development, providing:

• Higher capacity: longer messages and graphics support.
• Improved interference rejection: more robust signalling than analog NAVTEX.
• Backward compatibility: NAVDAT receivers can also receive NAVTEX during transition.

NAVDAT pilots are operating in selected NAVAREAs as of 2024-2026.

MEOSAR full deployment

MEOSAR is progressively becoming the dominant Cospas-Sarsat detection mode:

• Real-time detection rather than the up-to-90-minute LEOSAR latency.
• Real-time location without doppler-pass requirement.
• Improved alert validation through multi-satellite reception.

Full MEOSAR deployment is expected by the late 2020s as the GPS, Galileo and GLONASS satellite payloads complete deployment.

Coast stations and the GMDSS infrastructure

Coast stations

The GMDSS depends on a network of coast stations providing continuous DSC watch and voice support. Major coast station networks include:

• UK Coastguard: comprehensive UK coverage with Maritime Rescue Coordination Centres (MRCCs) at strategic locations.
• US Coast Guard: USCG operates coastal communications stations and distress monitoring with NMN (Norfolk), NMC (Chesapeake) and other call signs.
• Australian Maritime Safety Authority (AMSA): AMSA operates coast stations covering Australian and adjacent waters.
• JCG (Japan Coast Guard): JCG coverage of Japanese waters and beyond.
• Norwegian Telegraph Service: through historic Bodø Radio and other stations covering Norwegian waters and the Arctic.
• Multiple national authorities in Europe, Asia, Latin America, and Africa.

Each coast station maintains:

• Continuous DSC watch on the relevant frequencies.
• Voice working channels for follow-up communication.
• Maritime Safety Information broadcast for the coastal area.
• SAR coordination as the local point of contact.

The geographic distribution of coast stations defines the GMDSS Sea Area A1 and A2 boundaries.

Inmarsat ground segment

Inmarsat operates a ground segment supporting the GMDSS satellite services:

• Land Earth Stations (LES): 12 to 15 LES locations globally providing the gateway between Inmarsat satellites and the terrestrial network.
• Network Coordination Stations: managing the satellite resource allocation.
• Maritime Operations Centre: providing 24/7 coordination of distress traffic.
• SafetyNET broadcast: from designated LES with content from NAVAREA coordinators.

The Inmarsat infrastructure is operated under contract to the IMO with mandatory service guarantees for GMDSS operations.

Iridium ground segment

Iridium operates a different ground architecture using its LEO constellation:

• Gateway stations: at fixed locations connecting the Iridium satellites to terrestrial networks.
• Network Operations Center: managing the constellation and traffic.
• Distress monitoring: 24/7 monitoring of GMDSS distress traffic.

Iridium’s LEO architecture provides global coverage including polar regions and is increasingly the choice for operators with polar service.

Cospas-Sarsat ground segment

Cospas-Sarsat operates a multinational ground segment:

• Local User Terminals (LUTs): ground stations receiving GEOSAR, LEOSAR and MEOSAR satellite downlinks.
• Mission Control Centers (MCCs): 31 MCCs distributed globally, processing alerts and forwarding to RCCs.
• Beacon registries: national databases of registered EPIRB MMSIs and ship identification.

The Cospas-Sarsat system handles approximately 1 million alerts per year (mostly false alerts that are quickly resolved); about 20,000 to 30,000 of these are actual distress events leading to SAR response.

Specific operational scenarios

Distress alerting procedure

When the master decides to declare distress:

1. Activate the DSC distress button on VHF, MF or HF DSC (or all three for redundancy).
2. Select nature of distress if the equipment supports (e.g. fire, flooding, collision, abandoning ship, piracy/armed attack).
3. Confirm transmission with the equipment’s verification.
4. Listen for acknowledgement from coast stations or other ships.
5. Follow up by voice on the appropriate channel (Channel 16 VHF, 2182 kHz MF, or HF voice frequency).
6. Activate EPIRB by manual deployment if appropriate (the float-free EPIRB will activate automatically on submersion).
7. Maintain on-scene communication with rescue resources.

The procedure is documented in the GMDSS Master Plan and in operator-specific procedures.

False alert correction procedure

If a false DSC alert is transmitted:

1. Cancel the alert immediately on the same frequency by voice transmission, including ship’s name, MMSI, location, and confirmation that the alert was false.
2. Document in the radio log the time, frequency, and circumstances of the false alert.
3. Notify the master for review.
4. Notify the local RCC if the alert may have been received and is being processed.
5. Identify and correct the root cause to prevent recurrence.

Repeated false alerts may attract regulatory attention to the operator.

On-scene communication

When responding to a distress, the on-scene SAR Mission Coordinator (typically the most-suitable ship or aircraft on scene) coordinates:

• VHF Channel 16 for distress voice traffic.
• VHF working channels for routine on-scene coordination.
• Hand-held VHF between rescue boats and the parent ship.
• Aircraft VHF for ship-aircraft coordination.
• Status updates to RCC at regular intervals.

Effective on-scene communication is critical to rescue success and is a focus of GMDSS operator training.

Cybersecurity of GMDSS

The 2017 IMO MSC.428(98) resolution requires cyber risk management in the ISM Code, with implementation through the regulatory framework that includes GMDSS. Specific cybersecurity considerations for GMDSS:

• DSC controller security: DSC controllers are typically dedicated single-function devices with limited connectivity, providing some inherent security.
• Inmarsat and Iridium terminals: increasingly integrated with general ship IT systems, exposing them to broader cyber threats.
• NAVTEX receiver tampering: theoretically possible through corrupted broadcasts.
• GPS spoofing: false GPS positions could corrupt DSC distress alerts.
• EPIRB MMSI spoofing: theoretically possible but operationally difficult.

Operator-side cyber risk management includes:

• Network segregation between bridge equipment and general ship IT.
• Access controls on equipment configuration.
• Software update procedures.
• Incident response procedures.
• Regular security assessment as part of the SMS.

Frequency allocation and ITU coordination

Maritime mobile service frequencies

The ITU (International Telecommunication Union) Radio Regulations allocate specific frequency bands to the maritime mobile service. Key allocations include:

• VHF Maritime Mobile Band: 156.025 to 162.025 MHz, with 57 channels including:

  • Channel 16 (156.800 MHz): distress, urgency, safety, calling, and on-scene communication. Continuous bridge watch maintained.
  • Channel 70 (156.525 MHz): VHF DSC channel. Continuous DSC watch.
  • AIS channels (87B, 88B at 161.975 and 162.025 MHz): AIS Class A and Class B operation.
  • Public correspondence channels (24, 25, 26, 27, 28): voice communication with coast stations.
  • Inter-ship channels (06, 08, 09, 10, 13, 67, 69, 71, 72, 73, 77): bridge-to-bridge communication.
  • Port operations channels (varies by port): coordination with VTS, harbour master, pilots.

• MF (Medium Frequency) Maritime Mobile Band: 1605 to 4000 kHz, with key frequencies:

  • 2182 kHz: international distress, urgency and safety frequency for radiotelephone.
  • 2187.5 kHz: MF DSC distress and safety.
  • 2174.5 kHz: NBDP distress and safety (legacy).

• HF (High Frequency) Maritime Mobile Band: covers 4 to 25 MHz with multiple sub-bands, key DSC frequencies:

  • 4207.5 kHz, 6312 kHz, 8414.5 kHz, 12577 kHz, 16804.5 kHz: HF DSC distress and safety.
  • Voice working frequencies in 4, 6, 8, 12, 16, 22 MHz bands.

Channel 16 watchkeeping

Despite the introduction of GMDSS DSC, Channel 16 continues as the primary VHF voice distress and on-scene communication frequency. Bridge VHF watchkeeping on Channel 16 is mandatory for all SOLAS ships when in coastal waters and is good practice in open ocean.

The IMO has periodically considered phasing out Channel 16 watch in favour of DSC-only, but the value of voice communication for on-scene SAR coordination has kept Channel 16 in continuous use.

MMSI assignment

MMSI (Maritime Mobile Service Identity) is the nine-digit identifier used in DSC and AIS communications. The structure:

• First three digits (Maritime Identification Digits, MID): identify the country of registration. Each country has assigned MIDs (e.g. 232/233/234 for UK, 273 for Russian Federation, 311 for Bahamas, 366 for USA, 477 for Hong Kong).
• Last six digits: assigned by the national maritime authority to uniquely identify each ship.

A correctly programmed MMSI is essential for DSC operation. The IMO maintains the MID assignment list through ITU coordination.

Maritime Safety Information broadcast

MSI categories

Maritime Safety Information (MSI) consists of:

• Navigational warnings: hazards to navigation including derelicts, light failures, channel obstructions, exercise areas.
• Meteorological warnings: tropical storm warnings, gale warnings, ice warnings.
• Meteorological forecasts: regular weather forecasts for shipping areas.
• Ice information: ice extent, drift, and routing recommendations.
• SAR information: information related to search and rescue operations.
• Pilot information: pilot station status and procedures.
• AIS information: AIS-related notices.

Broadcast media

MSI is broadcast through multiple media:

• NAVTEX (518 kHz international, 490 kHz national): regional coverage out to about 250 nautical miles from the transmitting station.
• Inmarsat SafetyNET (Inmarsat-C): broadcast through Inmarsat geostationary satellites, providing global coverage between 70 degrees North and South.
• Iridium Maritime Safety Information: through Iridium’s LEO constellation, providing global coverage including polar regions.
• HF NBDP: legacy broadcast (being phased out).
• VHF voice broadcast: local coastal warnings at scheduled times.

Worldwide Navigational Warning Service (WWNWS)

The WWNWS is the global coordinated framework for navigational warning broadcast, divided into 21 NAVAREAs. Each NAVAREA has a designated coordinating state:

• NAVAREA I: UK (covering northwest Europe).
• NAVAREA II: France.
• NAVAREA III: Spain.
• NAVAREA IV: USA.
• NAVAREA V: Brazil.
• NAVAREA VIII: India.
• NAVAREA XI: Japan.
• NAVAREA XII: USA.
• NAVAREA XIII to XXI covering Pacific, Indian Ocean, Arctic, Antarctic.

Each NAVAREA coordinator collects warnings from contributing states, formats them, and broadcasts them via SafetyNET and other media.

Crew certification under STCW

General Operator’s Certificate (GOC)

The GOC is the comprehensive GMDSS operator certificate, required for unrestricted operations. The training course covers:

• Theory of GMDSS: regulatory framework, equipment, procedures.
• Equipment operation: hands-on training with VHF DSC, MF DSC, HF DSC, Inmarsat-C, Iridium, NAVTEX, EPIRB.
• Procedures: distress alerting, distress acknowledgement, on-scene SAR coordination, false alert correction.
• English language proficiency: GMDSS-specific maritime English.
• Practical examination: simulator-based scenario testing.

Course duration is typically 3 to 5 weeks. The certificate is endorsed by the flag state and is recognised internationally under STCW.

Restricted Operator’s Certificate (ROC)

The ROC covers Sea Area A1 only. The training course is shorter (typically 1 to 2 weeks) and covers VHF DSC operations with limited MF and satellite content. The certificate is sufficient for ships operating exclusively in coastal waters.

Certificate currency and revalidation

GMDSS operator certificates have validity periods (typically 5 years), with revalidation through:

• Continued service at sea using the GMDSS equipment.
• Refresher training at an approved training centre.
• Examination demonstrating continued competence.

The currency requirement ensures that operators maintain familiarity with the equipment and procedures.

Survey regime for GMDSS

Periodic surveys

GMDSS equipment is subject to periodic survey:

• Annual survey: visual inspection, equipment functional check, battery condition, log review.
• Periodical survey (every 30 months): detailed examination including measurement of transmitter output, receiver sensitivity, antenna VSWR (voltage standing wave ratio).
• Renewal survey (every 5 years): comprehensive examination with full equipment performance verification, battery testing, software/firmware update verification.

The radio surveyor (typically employed by a Recognised Organization or by the flag state’s radio inspectorate) issues the survey report and the Cargo Ship Safety Radio Certificate (or equivalent).

Daily, weekly, monthly tests

Onboard testing requirements include:

• Daily: VHF and MF radio operational check, DSC test (where supported), antenna check.
• Weekly: MF and HF radiotelephone operational check, DSC test call to a coast station or to another ship with DSC.
• Monthly: EPIRB self-test (without satellite transmission), SART self-test, battery condition check, full radio installation check.

Each test is recorded in the radio log with the result.

Documentation

Every ship covered by Chapter IV carries on board:

• Cargo Ship Safety Radio Certificate (cargo ships) or Passenger Ship Safety Certificate (passenger ships, integrated radio coverage), evidence of compliance with Chapter IV.
• Radio log with current entries.
• Operator certificates (GOC or ROC) for personnel responsible for GMDSS.
• Equipment manuals for all GMDSS equipment.
• MMSI registration documents showing the ship’s MMSI.
• EPIRB registration with the national EPIRB database.
• Battery records with voltage and condition.
• Performance standard documentation for each piece of GMDSS equipment.
• MARPOL Annex VI documentation related to bridge equipment power consumption (where applicable).
• STCW certification records for radio personnel.

Related Calculators

• SOLAS IV/3, Exemptions radio Calculator
• SOLAS IV/4, Functional requirements radio Calculator
• SOLAS IV/5, Provision of radio services Calculator
• SOLAS IV/6, Radio installations Calculator
• SOLAS IV/7, Radio equipment general Calculator
• SOLAS IV/8, Sea area A1 Calculator
• SOLAS IV/9, Sea area A2 Calculator
• SOLAS IV/10, Sea area A3 Calculator
• SOLAS IV/11, Sea area A4 Calculator
• SOLAS IV/13, Sources of energy (radio) Calculator
• SOLAS IV/14, Performance standards Calculator
• GMDSS, Sea Area Coverage Check Calculator
• EPIRB, Coverage Check Calculator

See also

• SOLAS Convention parent article
• SOLAS Chapter V: Safety of Navigation
• GMDSS Overview
• AIS and ECDIS
• STCW Convention
• SOLAS Chapter X: Safety Measures for High-Speed Craft
• SOLAS Chapter XV: Ships Carrying Industrial Personnel
• Polar Code

References

• IMO, International Convention for the Safety of Life at Sea (SOLAS), 1974, as amended, Chapter IV.
• IMO Resolution MSC.45(65) (1995), GMDSS adoption (subsequent updates).
• IMO Resolution MSC.428(98) (2017), GMDSS Modernisation Plan.
• IMO Resolution MSC.434(98) (2017), Iridium GMDSS recognition.
• IMO Resolution A.803(19), A.804(19), A.806(19), A.810(19) and others on equipment performance standards.
• ITU Radio Regulations covering frequency allocation.
• Cospas-Sarsat System Documentation.
• IAMSAR Manual Volume III, Mobile Facilities.
• ICS Bridge Procedures Guide, current edition.