GMDSS System Architecture and Sea Area Requirements
What this domain tests
This domain tests the four sea areas, the functional requirements GMDSS satisfies, and the minimum equipment a given ship must carry for the area it sails.
The four sea areas
- A1 - within radiotelephone coverage of at least one VHF coast station offering continuous DSC alerting (normally about 20–30 nm, further where a coast station extends it).
- A2 - beyond A1, within radiotelephone coverage of at least one MF coast station offering continuous DSC alerting (roughly 100–150 nm).
- A3 - beyond A1 and A2, within coverage of a recognized mobile satellite service (Inmarsat, about 70°N to 70°S).
- A4 - the remaining areas, essentially the polar regions outside satellite coverage.
Functional requirements
Alerting (DSC, 406 MHz EPIRB, satellite), SAR coordination, locating (9 GHz SART, 121.5 MHz homing on the EPIRB), maritime safety information (NAVTEX, EGC/SafetyNET, HF NBDP), general communications, and bridge-to-bridge on VHF Ch 13.
Minimum carriage
A1: VHF DSC on Ch 70 plus VHF radiotelephony with Ch 6, 13 and 16; float-free 406 MHz EPIRB; SART (one for 300–500 GT, two at 500 GT and above); two-way VHF for survival craft (two for 300–500 GT, three at 500 GT and above); NAVTEX receiver where the service exists; reserve source of energy. A2 adds MF DSC on 2187.5 kHz and 2182 kHz radiotelephony, or satellite equipment. A3 requires a ship earth station or HF. A4 forces HF.
Reserve energy must carry the GMDSS installation for at least 1 hour where a compliant emergency source of electrical power exists, otherwise 6 hours.
Worked example
A 400 GT cargo ship runs Seattle to San Diego, always inside VHF DSC coverage, and never needs MF or satellite. Reasoning: the route is A1 only, so the fit is VHF DSC plus VHF radiotelephony, a float-free EPIRB, one SART (300–500 GT band), two survival-craft VHF sets, a NAVTEX receiver (US NAVTEX coverage exists) and a reserve battery sized to the 1-hour or 6-hour rule. No MF/HF, no Inmarsat, no NBDP terminal.
Traps
Treating A2 as simply "beyond VHF" without checking MF coverage; placing A4 inside the Inmarsat footprint; getting the SART and survival-craft VHF counts wrong by tonnage; forgetting that NAVTEX is usable only where a service is provided; assuming an EPIRB removes the need for DSC alerting; ignoring the reserve-energy hour rule.