GMDSS general principles
What the GMDSS is
The Global Maritime Distress and Safety System is a SOLAS Chapter IV system built around functional requirements, not a single radio. It provides six functions: distress alerting, SAR coordination, locating, maritime safety information, general radiocommunications, and bridge-to-bridge communications. A GOC operator is authorised to operate the ship's GMDSS equipment and to handle distress, urgency and safety traffic.
Sea areas drive the fit
The four sea areas are A1 (VHF coast-station coverage), A2 (MF), A3 (Inmarsat geostationary coverage, roughly 70°N–70°S), and A4 (outside A1–A3, polar). Equipment carriage follows the area, so a ship may need VHF DSC, MF/HF DSC, Inmarsat, or a combination. Distress alerting must be possible from the bridge, and the alert must identify the ship, position and distress.
Worked example
A ship certified for A1–A3 is ordered to an A4 area. The master asks whether the Inmarsat-C terminal is enough. Reasoning: Inmarsat geostationary coverage does not reach A4, so the terminal cannot meet the distress alerting function there. The ship must have HF DSC and HF radiotelephony, with the associated watchkeeping, or it cannot comply for A4. The correct answer is not 'add a second Inmarsat terminal'; it is to use the equipment the area requires.
Traps
Do not confuse GMDSS with a single box or with satellite only. Do not assume a GOC certificate authorises equipment not fitted to the ship. Do not press the distress button to 'test' it; use the test facility. A false alert must be cancelled immediately by DSC or voice and reported to the coast station.