General Knowledge of Radio Communications and the GMDSS
What the GMDSS actually is
The Global Maritime Distress and Safety System is not a single piece of equipment - it is a worldwide framework of frequencies, procedures and carriage rules that lets any vessel raise a distress alert and lets shore authorities coordinate the response. The SRC syllabus tests the six GMDSS functions: alerting (DSC on VHF and HF, Inmarsat, 406 MHz EPIRB), search and rescue coordination by shore MRCCs, on-scene communications (Ch 16 and portable VHFs), locating (SART and 121.5 MHz homing), maritime safety information (NAVTEX, SafetyNET, coastguard broadcasts), and routine communications.
Sea areas and what they mean for you
Sea area A1 is the region within reliable VHF coverage of at least one coast station offering continuous DSC alerting, typically 20–30 nautical miles offshore. A2 is MF coverage, A3 is Inmarsat coverage, A4 is the polar regions. A small craft operating in A1 meets its GMDSS obligations with a VHF/DSC set plus whatever alerting and locating aids its flag state requires, commonly an EPIRB and a SART. Know which area you are in, because it drives what you must carry and what you can expect to reach.
Propagation: why VHF is short-ranged
VHF is FM and essentially line-of-sight. Range depends on antenna height at both ends rather than transmitter power: a 25 W set and a 1 W set reach similar distances, but the low-power option saves battery and reduces interference in a crowded marina. A masthead aerial on a 12 m yacht sees the sea horizon at roughly 5 nm; a coast station aerial on a cliff sees much further, which is why you can often hear the coastguard long before another yacht hears you.
Worked example
A 10 m yacht is 8 nm off a headland when a crew member collapses. Reasoning: the vessel is inside Sea Area A1, so a DSC distress alert on Ch 70 will reach a coast station directly and no relay is needed. The alert goes first because it carries the MMSI and GPS position automatically; the MAYDAY on Ch 16 follows to give the voice detail. The handheld in the grab bag provides on-scene communication with the lifeboat, and a SART would help the lifeboat home in.
Traps
- Treating GMDSS as distress-only; safety information and routine traffic are part of it.
- Assuming more transmitter power means more range.
- Forgetting that Ch 70 is data-only: the voice follow-up belongs on Ch 16.