GMDSS system overview and regulatory framework
Regulatory basis and sea areas
GMDSS is the SOLAS Chapter IV radio system for distress alerting, SAR coordination, locating, maritime safety information (MSI), and general communications. It applies to passenger ships and cargo ships of 300 gross tonnage and above on international voyages. Sea areas drive equipment: A1 is VHF coast-station coverage; A2 is MF coverage; A3 is Inmarsat geostationary coverage roughly between 70°N and 70°S, excluding A1 and A2; A4 is the polar remainder. FCC Part 80 governs U.S. station licensing, operator authority, and technical standards. A GMDSS Radio Maintainer license evidences competence to maintain GMDSS equipment, not authority to operate it.
Carriage logic
The minimum fit follows the sea area and the functional requirements. A typical A1 vessel needs VHF DSC on channel 70 plus radiotelephony on channel 16, a 406 MHz EPIRB, at least one SART, MSI reception by NAVTEX or EGC, and reserve power. An A2 vessel adds MF DSC and MF radiotelephony. An A3 vessel adds Inmarsat-C or approved HF, and an A4 vessel needs HF DSC, NBDP, and radiotelephony because satellite coverage is not reliable at the poles. Survival craft two-way VHF apparatus and reserve power are separate requirements.
Worked example
A 500 GT cargo ship trades worldwide, including polar routes, so it is an A4 ship. Minimum GMDSS fit: VHF DSC and channel 16; MF/HF DSC with NBDP and radiotelephony; 406 MHz EPIRB; two SARTs; NAVTEX or EGC for MSI; reserve energy; and survival craft VHF. Inmarsat-C alone is not sufficient for A4 because geostationary satellites do not cover the polar regions. The reasoning is functional: every sea area must support alerting, locating, MSI, and general communications, and A4 cannot rely on Inmarsat.
Traps
Do not confuse A3 with worldwide coverage; A4 remains outside Inmarsat. Do not treat a maintainer license as an operator certificate. Do not assume a NAVTEX receiver satisfies MSI if it is not installed or not tuned to 518 kHz. Do not substitute a non-GMDSS DSC radio for a required installation. Do not ignore reserve-power independence from the main and emergency switchboards. Read the ship's particular carriage document because flag interpretations and exemptions exist.
Key facts
Sea areas are defined by shore coverage, not by the ship's route name. GMDSS has four functional communications requirements. SOLAS IV applies to passenger ships and cargo ships of 300 GT and above. FCC Part 80 is the U.S. regulatory basis.