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The Global Maritime Distress and Safety System (GMDSS) is the IMO scheme that replaced the manual Morse watch on 500 kHz and 2182 kHz with an automatic, multi-redundant system based on satellite and digital terrestrial radio. It entered into force in stages between 1992 and 1999. Under SOLAS Chapter IV, every passenger ship and every cargo ship of 300 GT and over on international voyages must carry GMDSS equipment matching the sea area in which it operates.
The system has nine basic functions: transmit ship-to-shore distress alerts, receive shore-to-ship alerts, transmit and receive ship-to-ship alerts, transmit and receive SAR coordinating communications, transmit and receive on-scene communications, transmit and receive locating signals, transmit and receive Maritime Safety Information, transmit and receive general radiocommunications with shore-based systems, and transmit and receive bridge-to-bridge communications.
Within VHF DSC range of at least one shore station providing continuous DSC alerting (typically 20–30 nm).
Primary alerting: VHF channel 70 (DSC) and channel 16 (radiotelephony).
Minimum equipment: VHF radio with DSC, 406 MHz EPIRB, SART (or AIS-SART), 9 GHz radar transponder, NAVTEX.
Within MF DSC range (≈ 100–150 nm), excluding A1.
Primary alerting: MF DSC 2187.5 kHz and radiotelephony 2182 kHz.
Minimum equipment: A1 equipment plus MF radio with DSC and watch on 2187.5 kHz and 2182 kHz.
Within Inmarsat geostationary satellite coverage (approximately 76°N to 76°S), excluding A1 and A2.
Primary alerting: Inmarsat-C SafetyNET (or equivalent), or HF DSC.
Minimum equipment: A1 + A2 equipment plus either Inmarsat-C or HF DSC and radiotelephony.
All polar sea areas outside Inmarsat geostationary coverage.
Primary alerting: HF DSC on the six distress frequencies, plus Iridium GMDSS (recognised since 2020).
Minimum equipment: A1 + A2 + A3 equipment, HF DSC mandatory, Iridium GMDSS optional (recognised under MSC.1/Circ.1645).
Sends a pre-formatted distress alert containing identity (MMSI), nature of distress, position and time. On VHF channel 70, MF 2187.5 kHz, HF 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz.
Channel 16 (156.800 MHz) for distress, urgency, safety, calling. Continuous listening watch required by SOLAS Ch IV/12.
2182 kHz MF, plus 4125 / 6215 / 8291 / 12290 / 16420 kHz HF. Two 3-minute silence periods per hour on 2182 kHz at H+00 and H+30 to allow weak distress traffic to be heard.
Automatic receive-only print/display of MSI (Maritime Safety Information) — navigational warnings, weather, distress relays — on 518 kHz international (English), 490 kHz national, and 4209.5 kHz tropical.
Geostationary satellite delivery of MSI and SAR alerts to A3 area ships. Two-way text and forms.
Recognised by IMO in 2018 (MSC.99) as a second satellite provider for GMDSS, including A4 polar coverage. Must comply with the Performance Standards in MSC.434(98).
Float-free emergency beacon. Transmits MMSI/identity and GNSS position via LEOSAR/GEOSAR/MEOSAR satellites; 121.5 MHz homing for SAR aircraft.
When triggered, replies to any X-band (9 GHz) radar with a line of 12 dots, helping a SAR vessel locate the survival craft on its radar plan.
Transmits the survival craft's position via AIS for plotting on shipboard ECDIS or AIS displays. Replaces the radar-SART on some ships and is mandatory on others.
Under STCW Chapter IV (Regulation IV/2), every GMDSS ship must carry at least one person qualified to perform GMDSS radio duties. Qualification is the General Operator's Certificate (GOC) for ships in any sea area, or the Restricted Operator's Certificate (ROC) for ships operating only in Sea Area A1. Both certificates are issued by the flag state on completion of an approved course and examination. See the dedicated distress frequencies and distress procedures pages.
The IMO concluded the GMDSS Modernisation Plan in 2024. Key changes: recognition of Iridium as a second mobile-satellite provider (since 2020); update of the Performance Standards for VHF, MF/HF, satellite, NAVTEX, EPIRB; phasing out of dedicated voice-only HF requirements where modern data services provide equivalent function; introduction of the IMO NAVDAT (HF data system) as an alternative to NAVTEX in some areas. SOLAS Ch IV amendments giving effect to the modernisation entered into force on 1 January 2024.