SOLAS Chapter IV: Radio Communications and the GMDSS

SOLAS Chapter IV is the primary international instrument for shipboard radio communications. The nine functional requirements under Regulation 4 define what every GMDSS installation must be capable of doing, independent of which specific technologies achieve it. The 1 January 2024 amendments under MSC.496(105) completed the shift from a system defined around Inmarsat’s geostationary infrastructure to a technology-neutral framework that recognizes any IMO-endorsed Recognized Mobile Satellite Service.

Background

The pre-GMDSS regime

Before the GMDSS, ship-to-ship and ship-to-shore distress communication relied on a regime tracing 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), 2182 kHz (the MF radiotelephone distress frequency, mandatory from 1956), and 156.8 MHz VHF Channel 16 (mandatory from 1971).
• Trained radio officers on board, holding a Radio Officer’s Certificate and responsible for distress communications.
• Manual distress procedure: 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.
• Range limitations: VHF effective range about 30 nautical miles; MF 200 to 400 nautical miles; HF thousands of miles under suitable propagation but with significant atmospheric variability.

The pre-GMDSS regime had several limitations. Continuous watches required sufficient radio officer rotation to cover 24 hours. A distressed ship hundreds of miles from any other listening vessel might go unheard. Each alert required manual processing, with no automation to alert the watchkeeper to incoming traffic.

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 automatically received and processed by coast stations, satellite SAR systems, and other ships.
• Multi-system redundancy: alerts can go via VHF DSC, MF DSC, HF DSC, RMSS satellite, and 406 MHz EPIRB to Cospas-Sarsat, providing multiple independent paths.
• Sea area-based equipment: ships carry equipment appropriate to their operating area rather than a uniform standard.
• Watchkeeping by bridge officers: the OOW maintains radio watch via DSC alarm, replacing the dedicated radio officer.
• Automated MSI reception: NAVTEX and SafetyNET receive maritime safety information automatically, without operator intervention.

Major milestones

• 1979 SAR Convention: adopted, providing the framework for international SAR cooperation.
• 1988 GMDSS adoption (Resolution MSC.45(65)): GMDSS incorporated into SOLAS Chapter IV.
• 1 February 1999: full mandatory GMDSS implementation for all SOLAS ships, replacing the radio officer regime.
• 24 May 2018, Resolution MSC.451(99): IMO recognized Iridium Satellite LLC as a GMDSS mobile satellite service provider, ending Inmarsat’s 30-year exclusivity.
• January 2020: IMSO issued the Letter of Compliance confirming Iridium’s operational readiness for GMDSS service.
• MSC 105, Resolution MSC.496(105): GMDSS modernization amendments adopted, entering into force 1 January 2024.

Relationship to SAR and IAMSAR

GMDSS provides the technical communication infrastructure; the SAR Convention and the IAMSAR Manual provide the operational doctrine for distress response. A typical distress event involves: distress alert transmitted via GMDSS; reception by a Rescue Coordination Centre (RCC) via the GMDSS infrastructure; SAR coordination by the RCC; on-scene communication between SAR units and the distressed ship; and recovery documented through GMDSS records and the SAR after-action report. The IAMSAR Manual Volume III (Mobile Facilities) gives 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 and cargo ships of 500 GT and above on international voyages.
• High-speed craft under Chapter X with GMDSS provisions in HSC Code Chapter 14.
• Industrial-personnel vessels under Chapter XV with provisions in 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:

• Recognized Mobile Satellite Service (RMSS): a mobile satellite service recognized by the IMO as meeting the criteria for GMDSS provision. As of 2024, INMARSAT and IRIDIUM SATELLITE LLC are the two recognized providers.
• Continuous watch: maintained without interruption.
• DSC (Digital Selective Calling): digital alerting system using ITU-R Recommendation M.493 protocol on dedicated frequencies.
• MID (Maritime Identification Digits): the first three digits of the MMSI, 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).

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 to 10)

The 1 January 2024 MSC.496(105) amendments changed the Sea Area A3 definition. A3 now refers to any area covered by a ship’s installed RMSS earth station, rather than specifically to Inmarsat geostationary coverage. The table below reflects the post-2024 framework.

Sea Area A1

Sea Area A1 covers coastal waters within VHF DSC alerting range of a designated coast station on Channel 70. Ships must carry: a VHF radio with DSC capability; a 406 MHz satellite EPIRB (VHF-EPIRBs are no longer accepted as A1 substitutes since 1 January 2024 per MSC.496(105)); a 9 GHz radar SART or AIS-SART; and three two-way VHF radiotelephones for survival craft. The Reg IV/8 sea area A1 calculator returns the applicable equipment.

Sea Area A2

Sea Area A2 covers waters within MF DSC coverage (continuous watch on 2187.5 kHz), excluding A1. Ships carry A1 equipment plus an MF radio installation with DSC on 2187.5 kHz and MF radiotelephony for distress and routine communication. The Reg IV/9 sea area A2 calculator returns the applicable equipment.

Sea Area A3

Sea Area A3 is now defined as the area, excluding A1 and A2, within the coverage of a Recognized Mobile Satellite Service Earth Station carried on board, with continuous alerting available. The pre-2024 Inmarsat-specific definition (between 70°N and 70°S) has been replaced by this RMSS-neutral language.

Ships in A3 carry A1 and A2 equipment plus an RMSS ship earth station. The former alternative route using only MF/HF radio (DSC + radiotelephony + NBDP, no satellite) is no longer valid after 1 January 2024; ships that used that approach must install an SES. The choice between Inmarsat and Iridium earth stations depends on route, flag state policy, and coverage needs. The Reg IV/10 sea area A3-A4 calculator returns the applicable equipment.

Sea Area A4

Sea Area A4 is defined as all sea areas outside A1, A2, and A3, covering the polar regions. Ships must carry all A1, A2, and A3 equipment, plus HF DSC (mandatory in A4 because Inmarsat geostationary satellites do not provide reliable coverage at high latitudes). Iridium’s LEO constellation provides continuous global coverage including the poles; many A4 operators now use Iridium SES as both the RMSS element and the primary distress satellite path, supplementing or replacing HF for MSI reception.

A4 ships operate under the Polar Code, which carries additional GMDSS provisions for polar operations.

Radio equipment in detail

VHF DSC

VHF DSC operates on Channel 70 (156.525 MHz) for digital distress alerts. The DSC controller transmits alerts with the ship’s MMSI, nature of distress, GPS-derived position and time, call type, and follow-up channel. Ships must maintain a continuous automated watch on Channel 70. Incorrect MMSI programming is a persistent false-alert cause; the MMSI must be programmed to the registered nine-digit number before the equipment enters service.

MF DSC and HF DSC

MF DSC operates on 2187.5 kHz with continuous watch on the dedicated DSC frequency. HF DSC operates on multiple frequencies in the 4, 6, 8, 12, and 16 MHz bands: 4207.5, 6312, 8414.5, 12577, and 16804.5 kHz are the HF DSC distress and safety frequencies. Frequency selection depends on time of day, propagation distance, and solar activity; higher bands (12, 16 MHz) generally favor daytime long-range; lower bands (4, 6 MHz) favor nighttime. A typical installation watches multiple frequencies simultaneously.

Inmarsat-C

Inmarsat-C remains the most widely deployed GMDSS satellite system for the majority of the global merchant fleet. It provides distress alerting via specialized distress messages, Enhanced Group Call (EGC) for SafetyNET reception of NAVAREA and METAREA warnings, store-and-forward telex/email, and position reporting for AMVER. Small omni-directional antennas make it cost-effective for general fleet use. Inmarsat has been operated by Viasat since 2023, but the GMDSS service obligations under IMO are unchanged.

Iridium

Iridium was formally recognized via IMO Resolution MSC.451(99), adopted 24 May 2018, with IMSO confirming operational compliance in January 2020. Its 66-satellite LEO constellation provides:

• Global coverage including polar regions, removing the A4/polar gap that Inmarsat geostationary satellites can’t fill.
• Voice and broadband data through Iridium Certus services.
• GMDSS-compliant distress alerting and MSI reception.
• Tracking through Iridium’s data network.

Under MSC.496(105), Iridium and Inmarsat are both named Recognized Mobile Satellite Services, and either satisfies the SES requirement for A3 compliance, subject to flag state acceptance of the specific equipment type.

NAVTEX

NAVTEX broadcasts navigational warnings, meteorological warnings, SAR information, and other Maritime Safety Information on two frequencies:

• 518 kHz: international service in English, range approximately 250 nm.
• 490 kHz: national language service.

Coverage is regional; each coastal state operates transmitting stations assigned to one or more NAVAREAs under the Worldwide Navigational Warning Service (WWNWS). Ships carry a receiver that automatically prints or displays received messages identified by the ship’s set of requested NAVAREA and message category codes.

EPIRB and Cospas-Sarsat

406 MHz EPIRBs transmit to the Cospas-Sarsat satellite system on activation, providing automatic distress alert with ship identity and, on modern beacons, an integrated GPS position. The float-free hydrostatic-release bracket releases the EPIRB on submersion; the beacon floats free, self-activates, and transmits for at least 48 hours. Since 1 January 2024, VHF-EPIRBs are no longer accepted in place of satellite EPIRBs for Sea Area A1 compliance.

The Cospas-Sarsat system operates three satellite layers:

• GEOSAR (geostationary): instant detection of alerts but no independent location capability.
• LEOSAR (low-earth-orbit): doppler-derived location, with detection latency up to approximately 90 minutes in worst-case geometry.
• MEOSAR (medium-earth-orbit): SAR payloads on GPS, Galileo, GLONASS, and BeiDou navigation satellites providing near-instantaneous detection and multilateration location. Initial Operational Capability was declared by the Cospas-Sarsat Council in April 2023; Full Operational Capability is anticipated in 2024 to 2025.

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 searching vessel’s radar display at the SART’s location. Under MSC.496(105), SART carriage requirements were consolidated from SOLAS Chapter III/Reg.6 into SOLAS Chapter IV/Reg.7, alongside two-way VHF apparatus.

AIS-SARTs transmit on AIS frequencies (161.975 and 162.025 MHz) with location data, providing notification on any AIS receiver in the area. Radar SARTs are highly visible on radar at significant range; AIS-SARTs work well for vessels with AIS displays but are less effective for aircraft without AIS receivers.

Watches (Regulation 11)

GMDSS watchkeeping requirements

Every ship must maintain continuous automated watch on:

• VHF DSC Channel 70 at all times.
• MF DSC 2187.5 kHz where the ship carries MF equipment.
• HF DSC (multiple frequencies) in A4 or where HF is fitted.
• NAVTEX 518 kHz for receipt of maritime safety information.
• SafetyNET (RMSS EGC) where an SES is fitted.
• VHF Channel 16 for distress and on-scene communication in coastal waters; maintained manually or by scanning dual-watch.

The watchkeeping is automated; equipment alerts the bridge OOW on receipt of distress alerts. The OOW doesn’t need to actively monitor every frequency but must respond immediately to alerts. The Reg IV/11 watches calculator returns watchkeeping requirements for a given configuration.

False alerts and procedures

DSC false alerts are a recurring issue. The DSC distress button may be inadvertently pressed during maintenance or testing; incorrectly programmed MMSIs cause alerts attributed to the wrong vessel; drill alerts are sometimes transmitted as live alerts through procedural error.

Each false alert requires: immediate cancellation by voice on the same channel naming the ship, MMSI, and confirming the alert is false; documentation in the radio log; notification to the master; and where the alert may have been received, notification to the local RCC. The IMO has issued MSC.1/Circ.1422 and subsequent circulars on DSC false alert prevention. Repeated false alerts attract regulatory scrutiny.

Energy sources (Regulation 12)

GMDSS equipment must be supplied by main electrical power during normal operation, emergency electrical power during emergencies, and a reserve battery capable of supporting the radio installation for at least one hour of continuous transmission and reception, independent of any other source. The reserve battery is the last line of defense: if main and emergency power both fail, the radio installation can still alert distress. Battery capacity, charging arrangement, and voltage are specified in the regulation.

Performance standards and maintenance (Regulations 13 to 14)

Performance standards

Each piece of GMDSS equipment must be type-approved against 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 Reg IV/13 performance standards calculator covers compliance checks against these standards.

Maintenance

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

• Duplication of equipment: two complete sets, one operating and one as backup.
• Shore-based maintenance: operational checks at sea; major repair ashore.
• At-sea maintenance: trained personnel with spare parts on board.
• Combination: common on larger ships.

The radio surveyor verifies the maintenance arrangement during periodic surveys. The Reg IV/14 maintenance calculator covers the compliance checks.

Records (Regulation 15)

The radio log records all distress, urgency, and safety messages transmitted or received; daily, weekly, and monthly equipment tests; battery checks with voltage; DSC test calls; EPIRB tests; watchkeeper changes; and equipment failures with corrective action. The log is preserved typically for one to two years on board and is available to flag state and PSC inspectors. An incomplete or missing radio log is a common PSC deficiency.

Radio personnel (Regulations 16 to 17)

Radio operator certificates

GMDSS-equipped ships need certified operators under the STCW Convention framework:

• GOC (General Operator’s Certificate): required for unrestricted GMDSS operations covering all sea areas. STCW Section A-IV/2. Training covers all GMDSS equipment types including HF DSC, RMSS SES, distress procedures, and SAR coordination. Course duration is typically three to five weeks.
• ROC (Restricted Operator’s Certificate): sufficient for Sea Area A1 operations only. Shorter course, typically one to two weeks, covering VHF DSC with limited MF and satellite content.

Certificate validity is typically five years, with revalidation through continued sea service or refresher training and examination.

Personnel records

The ship maintains records of each operator’s certificate number, issuing flag state, date of issue, and expiry. PSC inspections include verification of operator certificates against the on-board complement. An expired operator certificate is a detainable GMDSS deficiency.

Position-updating (Regulation 18)

DSC, EPIRB, and other GMDSS equipment must support automatic position-updating from the ship’s GPS or other position source. The interface updates position at every GPS fix, ensuring distress alerts carry a current location rather than the position from initial equipment setup.

GMDSS modernization: MSC.496(105)

2017 Modernisation Plan

IMO Resolution MSC.428(98), adopted at MSC 98 in 2017, established a GMDSS Modernisation Plan covering: recognition of new satellite providers; updated equipment standards for modern technology; integration with e-navigation; and phase-out of obsolete technologies including NBDP. That plan resulted directly in the amendments adopted at MSC 105.

MSC.496(105): in force 1 January 2024

Resolution MSC.496(105) is the operative instrument. Its key changes:

1. RMSS terminology: all references to Inmarsat were replaced with “Recognized Mobile Satellite Service (RMSS)”. Both Inmarsat and Iridium Satellite LLC are recognized. Ships may select RMSS SES type based on coverage, subject to flag state policy.
2. Sea Area A3 redefined: now means any area, excluding A1 and A2, within the coverage of an RMSS-supported SES carried on board. The former definition tied A3 to Inmarsat geostationary coverage (approximately 70°N to 70°S).
3. NBDP removed: the Narrow Band Direct Printing function for MF/HF radio is no longer a GMDSS requirement. Ships using the MF/HF-only route for A3 compliance must install an RMSS SES.
4. VHF-EPIRB banned for A1: satellite EPIRB is now mandatory for A1; VHF-EPIRB no longer accepted as a substitute.
5. Consolidation into Chapter IV: two-way VHF radiotelephone apparatus and SART provisions were moved from SOLAS III/Reg.6 into SOLAS IV/Reg.7, centralizing all radio and radiocommunications-adjacent equipment in one chapter.

The implementation note MSC.1/Circ.1676 allows ships facing equipment supply delays to operate under the former standards until 1 January 2028.

Iridium recognition: MSC.451(99)

The foundation for RMSS pluralism was laid at MSC 99. Resolution MSC.451(99), adopted 24 May 2018, recognized Iridium Satellite LLC’s maritime mobile satellite services, including the Iridium Enhanced Group Calling service for MSI broadcast. IMSO monitored implementation under its oversight mandate and issued the Letter of Compliance in January 2020, confirming the Iridium service met all operational and technical GMDSS criteria. That 2018 resolution ended Inmarsat’s sole-provider status; the 2024 MSC.496(105) amendments embedded the multi-RMSS framework permanently into Chapter IV’s text.

Notable casualties and lessons

MV Sanchi (2018)

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

EPIRB false alerts

False EPIRB activations cost Cospas-Sarsat and SAR authorities significant processing resources each year. Causes include inadvertent activation during maintenance, hydrostatic release malfunction in normal operations, test signals interpreted as live alerts, and degraded batteries on old-stock beacons. IMO circulars on false alert prevention include documented testing requirements, careful stowage procedures, and mandatory replacement at end-of-battery-life.

Indian Ocean tsunami, 2004

The 26 December 2004 Indian Ocean tsunami exposed a gap: the GMDSS provided distress communication infrastructure for individual ships, but no regional early-warning system for tsunamis existed. The IMO subsequently coordinated with the Indian Ocean Tsunami Warning System to integrate tsunami warnings with the GMDSS Maritime Safety Information broadcast.

Port state control and GMDSS

PSC inspection of GMDSS

Port state control inspections of GMDSS examine:

• Equipment functionality: turning on each piece of 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 the ship’s MMSI is correctly programmed in DSC and AIS equipment.
• EPIRB self-test and battery replacement date.
• Operator certification: valid GOC or ROC certificates for designated radio personnel.
• Radio log: completeness, test entries, and any reported problems.
• Survey records: current Cargo Ship Safety Radio Certificate.

Common detainable deficiencies: battery below minimum; EPIRB beyond battery replacement date; SART or AIS-SART expired; expired operator certificate; radio log not maintained; equipment lacking type approval.

Detention patterns

PSC detentions for GMDSS deficiencies follow predictable patterns: operators with poor maintenance culture accumulating battery failures; EPIRBs retained beyond their battery replacement date; operator certificates not renewed before expiry. Each detention is recorded in the relevant PSC MOU database and raises the operator’s future inspection priority.

Survival craft radio equipment

Two-way VHF radiotelephones

Under MSC.496(105), two-way VHF radiotelephone requirements were moved from Chapter III into Chapter IV/Reg.7. Ships must carry at least three two-way VHF radiotelephones for survival craft (two on cargo ships of 300 to 500 GT). They must be waterproof, operable with gloved hands, carry battery capacity for at least eight hours of continuous use, and cover Channel 16 plus other working channels.

EPIRB carriage

The 406 MHz EPIRB requirements: float-free hydrostatic-release mounting; manual deployment capability; automatic water-immersion activation; self-test capability; and battery replacement at the manufacturer’s interval (typically five years). Most ships carry one EPIRB; passenger ships and certain other 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. Radar SARTs are visible on rescue vessel radar at ranges of several nautical miles; AIS-SARTs work well for surface vessel rescue but offer limited aircraft visibility. The choice depends on the rescue scenario and installed detection equipment at the rescue assets typically operating in the area.

GMDSS in special operations

Polar Code interaction

Polar Code operations introduce specific GMDSS considerations. A4 sea area coverage requires HF DSC and an RMSS SES; Iridium’s global LEO coverage is the standard choice for polar routes because Inmarsat geostationary satellites don’t cover high latitudes reliably. HF propagation in polar regions is affected by auroral activity and ionospheric disturbance. SAR response times are long due to remoteness; the GMDSS alert is the first step in a rescue chain that may take many hours to engage.

High-speed craft GMDSS

HSC GMDSS provisions are adapted to the short-route operational profile. HSC operating exclusively in coastal A1 waters typically need only the basic VHF DSC equipment plus the standard 406 MHz EPIRB. Shore-side AIS tracking provides additional position awareness.

Industrial-personnel ship GMDSS

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

Recent and emerging developments

NAVDAT

NAVDAT is a digital NAVTEX successor in development, providing higher message capacity, graphics support, and stronger interference rejection than the current 518 kHz analog service. Pilot implementations are operating in selected NAVAREAs. NAVDAT receivers will also receive legacy NAVTEX during transition.

VDES and e-navigation

VDES (VHF Data Exchange System) is an emerging ITU standard for two-way VHF data communication beyond AIS. It forms part of the broader IMO e-navigation framework alongside the Maritime Connectivity Platform, offering higher-bandwidth ship-to-shore data exchange than current GMDSS channels.

MEOSAR full deployment

MEOSAR is now the primary Cospas-Sarsat detection mode for most beacon activations. Initial Operational Capability was declared April 2023, with Full Operational Capability expected in 2024 to 2025 as the GPS III, Galileo, GLONASS-K, and BeiDou-3 satellite payloads complete deployment. MEOSAR detection eliminates the LEOSAR doppler-pass latency that could delay alert processing by up to 90 minutes under the legacy architecture.

Cybersecurity of GMDSS

IMO Resolution MSC.428(98) requires cyber risk management through the ISM Code. Specific GMDSS exposure points: Inmarsat and Iridium terminals increasingly integrated with general ship IT networks; GPS spoofing that could corrupt DSC distress position data; and NAVTEX receiver tampering through broadcast injection. Operator-side controls include network segregation of bridge equipment from general ship IT, access controls on equipment configuration, and formal incident-response procedures.

Frequency allocation and ITU coordination

Maritime mobile service frequencies

Key ITU allocations for the GMDSS:

• VHF Maritime Mobile Band (156.025 to 162.025 MHz):

  • Channel 16 (156.800 MHz): distress, urgency, safety, calling, and on-scene voice communication.
  • Channel 70 (156.525 MHz): VHF DSC. Continuous watch mandatory.
  • AIS channels 87B/88B (161.975 and 162.025 MHz): Class A and B AIS operation.

• MF Maritime Mobile Band (1605 to 4000 kHz):

  • 2182 kHz: international distress, urgency, and safety radiotelephone frequency.
  • 2187.5 kHz: MF DSC distress and safety.

• HF Maritime Mobile Band (4 to 25 MHz): multiple sub-bands with DSC distress frequencies at 4207.5, 6312, 8414.5, 12577, and 16804.5 kHz.

Channel 16 watchkeeping

Despite full DSC automation, Channel 16 continues as the primary VHF voice distress and on-scene communication frequency. Bridge watchkeeping on Channel 16 is mandatory for all SOLAS ships in coastal waters and standard practice at sea. The IMO has periodically considered phasing out Channel 16 watch in favor of DSC-only, but its value for on-scene SAR voice coordination has kept it in continuous use.

MMSI assignment

MMSI is the nine-digit identifier in DSC and AIS. The first three digits are the Maritime Identification Digits (MID), identifying the country of registration: for example, 232/233/234 for the UK, 273 for the Russian Federation, 366 for the USA, 477 for Hong Kong. The last six digits are assigned nationally to uniquely identify each ship. Correct MMSI programming is essential for DSC; a mismatch between the MMSI in the equipment and the registration database complicates SAR response to a live distress.

Maritime Safety Information broadcast

MSI categories

Maritime Safety Information consists of: navigational warnings (derelicts, light failures, channel obstructions, exercise areas); meteorological warnings (tropical storms, gales, ice); regular weather forecasts; ice extent and routing information; SAR information; and pilot station notices. Reception of MSI is one of the nine Regulation 4 functional requirements.

Broadcast media

MSI reaches ships through:

• NAVTEX (518 kHz international, 490 kHz national): regional coverage approximately 250 nm from the transmitting station.
• SafetyNET (RMSS EGC): broadcast through Inmarsat satellites covering 70°N to 70°S; Iridium’s global EGC service extends coverage to polar regions.
• VHF voice broadcast: local coastal warnings at scheduled times.

HF NBDP broadcasts are being discontinued following the 2024 NBDP removal from GMDSS requirements.

Worldwide Navigational Warning Service (WWNWS)

The WWNWS coordinates navigational warning broadcast across 21 NAVAREAs, each with a designated state coordinator: NAVAREA I (UK, northwest Europe), NAVAREA II (France), NAVAREA III (Spain), NAVAREA IV (USA), NAVAREA V (Brazil), NAVAREA VIII (India), NAVAREA XI (Japan), NAVAREA XII (USA), and NAVAREAs XIII to XXI covering the Pacific, Indian Ocean, Arctic, and Antarctic. Each coordinator collects warnings from contributing states, formats them, and broadcasts via SafetyNET and other media.

Crew certification under STCW

General Operator’s Certificate (GOC)

The GOC is the full GMDSS operator certificate for unrestricted operations. The training course covers theory of GMDSS; hands-on equipment operation including VHF DSC, MF DSC, HF DSC, Inmarsat-C, Iridium SES, NAVTEX, and EPIRB; distress and false-alert procedures; on-scene SAR coordination; and GMDSS-specific maritime English. Practical examination is simulator-based. Course duration is typically three to five weeks; the certificate is flag-state endorsed and recognized internationally under STCW.

Restricted Operator’s Certificate (ROC)

The ROC covers Sea Area A1 only, with a shorter course (typically one to two weeks) covering VHF DSC with limited MF and satellite content. Sufficient for ships operating exclusively in coastal waters.

Certificate currency and revalidation

GMDSS operator certificates are valid typically five years, with revalidation through continued sea service, refresher training, or examination demonstrating continued competence.

Survey regime for GMDSS

Periodic surveys

GMDSS equipment is subject to:

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

The radio surveyor issues the survey report and the Cargo Ship Safety Radio Certificate.

Daily, weekly, monthly tests

Onboard testing requirements:

• Daily: VHF and MF radio operational check, antenna check.
• Weekly: MF and HF radiotelephony operational check, DSC test call to coast station or another ship.
• 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 Chapter IV ship carries on board: the Cargo Ship Safety Radio Certificate (cargo ships) or Passenger Ship Safety Certificate (passenger ships); current radio log; GOC or ROC certificates for radio personnel; equipment manuals; MMSI registration documents; EPIRB registration with the national database; battery records; and performance standard documentation for each GMDSS equipment item.

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 networks: UK Coastguard (MRCCs at strategic locations); US Coast Guard (coastal communications stations NMN, NMC, and others); Australian Maritime Safety Authority; Japan Coast Guard; and national authorities across Europe, Asia, Latin America, and Africa. Each station maintains continuous DSC watch on relevant frequencies, voice working channels, MSI broadcast, and SAR coordination for the local area. The geographic distribution of coast stations defines the A1 and A2 boundaries.

Cospas-Sarsat ground segment

Cospas-Sarsat operates: Local User Terminals (LUTs) receiving GEOSAR, LEOSAR, and MEOSAR downlinks; 31 Mission Control Centers distributed globally, processing alerts and forwarding to RCCs; and national beacon registries of registered EPIRB MMSIs and ship identification. The system processes roughly one million beacon activations per year, most of which are quickly resolved as false alerts; approximately 20,000 to 30,000 events per year lead 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.
2. Select nature of distress if equipment supports it (fire, flooding, collision, abandoning ship, piracy).
3. Confirm transmission via the equipment’s verification.
4. Listen for acknowledgement from coast stations or other ships.
5. Follow up by voice on Channel 16 VHF, 2182 kHz MF, or the HF voice frequency.
6. Activate EPIRB manually if appropriate; the float-free EPIRB activates automatically on submersion.
7. Maintain on-scene communication with rescue resources.

False alert correction procedure

If a false DSC alert is transmitted: cancel immediately by voice on the same frequency with ship’s name, MMSI, location, and confirmation the alert is false; document in the radio log; notify the master; notify the local RCC if the alert may have been received; identify and correct the root cause.

On-scene communication

When responding to a distress, the on-scene SAR Mission Coordinator uses VHF Channel 16 for distress voice traffic, VHF working channels for routine coordination, hand-held VHF between rescue boats and the parent ship, aircraft VHF for ship-aircraft coordination, and regular status updates to the RCC. Effective on-scene communication is a focus of GOC training.

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
• ISM Code

Additional calculators:

• Radio/EPIRB Battery - 6 h Autonomy
• Polar Comms - Iridium Backup
• System - EPIRB: 406 MHz float-free

References

• IMO Resolution MSC.496(105) (MSC 105): Amendments to SOLAS Chapters II-1, III, IV, V, in force 1 January 2024.
• IMO Resolution MSC.451(99) (24 May 2018): Statement of recognition of maritime mobile satellite services provided by Iridium Satellite LLC.
• IMO Resolution MSC.428(98) (2017): GMDSS Modernisation Plan.
• IMO Resolution MSC.434(98) (2017): Performance standards for Iridium ship earth station.
• IMO MSC.1/Circ.1613/Rev.2 (5 July 2023): Iridium Global Satellite EGC System Manual.
• IMO Resolutions A.803(19), A.804(19), A.806(19), A.810(19), A.802(19), A.807(19): performance standards for VHF DSC, MF DSC, HF DSC, EPIRB, SART, and Inmarsat-C equipment.
• IMO Resolution MSC.246(83): performance standards for AIS-SART.
• International Cospas-Sarsat Programme: system documentation and MEOSAR IOC declaration (April 2023).
• ITU Radio Regulations, frequency allocations for the maritime mobile service.
• IAMSAR Manual Volume III, Mobile Facilities, current edition.
• ICS Bridge Procedures Guide, current edition.