GMDSS System Architecture
What the exam tests here
The GMDSS is a ship-to-shore system: the ship alerts a shore authority, and that authority (an RCC) organises the rescue. Questions in this domain ask you to name the functional requirements, to place a ship in the correct sea area, and to say which equipment performs which function.
Functional requirements (SOLAS IV/4, ITU RR Article 33)
The system must provide: distress alerting ship-to-shore (and shore-to-ship alerting of ships); SAR coordinating communications; on-scene communications (ship-to-ship, including 121.5 MHz homing); locating signals (SART, AIS-SART, EPIRB); promulgation of MSI; general radiocommunications (public correspondence); and bridge-to-bridge communications. Candidates often lose marks by treating the GMDSS as distress-only.
Sea areas
- A1 - within continuous DSC alerting coverage of at least one VHF coast station (typically 20-30 nm).
- A2 - within MF coverage, excluding A1 (roughly 100-150 nm, DSC on 2187.5 kHz).
- A3 - within Inmarsat geostationary coverage, 70°N to 70°S, excluding A1 and A2.
- A4 - outside A1, A2 and A3 (the polar regions).
A1 is defined by alerting coverage, not by whether you happen to hear voice traffic on Ch16.
Worked example
A vessel on passage 60 nm offshore can no longer receive a VHF DSC coast station on Ch70 but does receive MF DSC on 2187.5 kHz. The vessel is in Sea Area A2: it keeps VHF DSC watch on Ch70 and MF DSC watch on 2187.5 kHz, carries a float-free 406 MHz EPIRB, SART and NAVTEX, and maintains a radiotelephony watch on 2182 kHz alongside the DSC watch.
Traps that cost marks
- Describing A3 as "everywhere Inmarsat reaches" - coverage stops at about 70° latitude.
- Forgetting that MSI and general communications are GMDSS functions, not extras.
- Confusing A1 alerting coverage with simple VHF voice range.