GMDSS System Architecture
The functional concept
The GMDSS is built around functional requirements, not specific equipment. A vessel must be able to transmit ship-to-shore distress alerts, receive shore-to-ship alerts, transmit and receive distress and safety communications, transmit and receive MSI, and transmit and receive general radiocommunications. Understanding that the system is defined by functions helps answer questions about which equipment can substitute for which.
The subsystems
Know the main components and their roles: DSC on VHF, MF and HF for alerting and calling; radiotelephony on VHF and MF/HF for the actual distress and safety traffic; NBDP/telex on MF/HF for store-and-forward messaging; Inmarsat satellite services for voice, data and distress alerting; NAVTEX and EGC/SafetyNET for MSI; EPIRBs for alerting via satellite; SART and AIS-SART for on-scene location. A common trap is assuming DSC itself carries voice; it does not. Another is confusing the roles of EPIRB (alerting) and SART (homing).
Sea areas and coverage
The architecture is tied to sea areas A1, A2, A3 and A4, defined by the coverage of shore VHF, MF, Inmarsat geostationary satellites and polar regions outside those. Equipment carriage follows the sea area the vessel operates in, not the flag alone. Questions often ask which sea area requires which combination, and it is worth memorising that A1 needs VHF DSC plus a suitable alerting device, while A4 requires HF.
Worked example
A vessel operating only within VHF coast-station coverage is in Sea Area A1 and needs VHF DSC, a VHF radiotelephone, NAVTEX or another MSI receiver, an EPIRB and a SART. If the same vessel plans a passage beyond VHF coverage but within MF coverage, it moves into A2 and must add MF DSC or an approved alternative.
Rule of thumb
Match equipment to sea area, and match each function (alert, receive alert, communicate, receive MSI, general comms) to at least one item of equipment.