The rulebook behind GMDSS
No single document "is" GMDSS. Each layer answers a different question, and exam questions often test whether you know which layer to open.
| Layer | Document | What it settles |
|---|---|---|
| Carriage and functions | SOLAS Chapter IV (IMO) | What a SOLAS ship must be able to do, carry and keep watch on |
| Operator competence | STCW Regulation IV/2 and Section A-IV/2 | What a GOC or ROC holder must know and be able to do |
| Spectrum and procedures | ITU Radio Regulations | Frequencies, identities, priorities and protection of distress traffic |
| DSC technology | ITU-R M.493 | The structure of a DSC call (current version M.493-16) |
| DSC operation | ITU-R M.541 | How to send, receive, acknowledge, relay and cancel DSC calls (M.541-11, incorporated by reference in the Radio Regulations) |
| Practical integration | IMO GMDSS Manual | How the pieces fit together on board and ashore |
| Training | IMO Model Course 1.25 | The syllabus behind the General Operator's Certificate |
| National licensing | The flag Administration (in Spain, Real Decreto 269/2022) | Who issues the certificate and on what terms |
Check this section
Which document settles GMDSS frequencies, identities and the priorities that protect distress traffic?
What changed on 1 January 2024
The SOLAS Chapter IV modernization (resolution MSC.496(105)) entered into force on 1 January 2024, with matching changes to the Radio Regulations at WRC-23. Its aim was to remove obsolete requirements and to describe GMDSS by function rather than by brand.
- A3 is no longer "the Inmarsat area". It is defined by the coverage of a recognized mobile satellite service (RMSS) supported by the ship earth station carried on board, with continuous alerting available.
- NBDP (radiotelex) left the MF/HF distress and safety system. It survives in history, old exam banks and some non-distress uses.
- ACS (Automatic Connection System) uses DSC to set up MF/HF links automatically.
- NAVDAT entered the regulatory framework as a digital shore-to-ship MSI broadcast.
- The wording is technology-neutral: the rule says what must be achieved, not which product achieves it.
Check this section
Under SOLAS IV as modernized from 1 January 2024, how is sea area A3 defined?
GOC and ROC: the same system, different reach
The Restricted Operator's Certificate (ROC) is limited to operating GMDSS stations in sea area A1. The General Operator's Certificate (GOC) covers the full installation, MF/HF and satellite included, in every sea area. The competence standard for both is STCW A-IV/2; the syllabus depth for the GOC follows IMO Model Course 1.25.
Check this section
Which certificate is limited to operating GMDSS stations in sea area A1?
Who is who in a distress case
| Actor | Role in the chain |
|---|---|
| Ship station | The casualty, or a ship that hears and may help |
| Coast station | Keeps watch, acknowledges and relays alerts, broadcasts MSI, handles correspondence |
| RCC / MRCC | Rescue Coordination Centre: validates the alert and coordinates the search and rescue response |
| Satellite ground segment (LES/CES) | Connects ship earth stations to terrestrial networks and routes distress priority to an RCC |
| Cospas-Sarsat ground segment (LUT, MCC) | Processes 406 MHz beacon detections and passes them to the responsible RCC |
| SAR aircraft and units | Search, locate and rescue; may talk to ships on maritime channels |
| Survival craft station | Portable VHF, SART or AIS-SART used after abandonment |
Check this section
Which actor validates a distress alert and coordinates the search and rescue response?
Alert, call, message, traffic
These four words are not synonyms, and confusing them causes procedural mistakes.
- 1Alert: a structured digital signal (DSC, satellite, or beacon) that attracts attention and carries key data automatically.
- 2Call: the spoken opening that identifies who is calling whom, for example "MAYDAY ... THIS IS ...".
- 3Message: the content that explains the situation: identity, position, nature, assistance required and any other useful information, such as persons on board.
- 4Traffic: everything exchanged afterwards between the casualty, the coast station, the RCC and assisting units.
Check this section
The spoken opening MAYDAY, THIS IS followed by the ship's name, which identifies who is calling, is called the:
Priority is a judgement about danger
| Priority | Voice signal | Threshold | Typical examples |
|---|---|---|---|
| Distress | MAYDAY | Grave and imminent danger, immediate assistance required | Uncontrolled fire, sinking, abandoning ship, person in immediate danger |
| Urgency | PAN-PAN | Very urgent message about the safety of a ship or person, below the distress threshold | Loss of propulsion with no immediate danger, medical case depending on severity |
| Safety | SÉCURITÉ | Important navigational or meteorological safety information | Drifting container, light extinguished, gale warning |
The medium never sets the priority. A PAN-PAN announced by VHF DSC, MF/HF DSC or a satellite terminal is still urgency.
References: SOLAS IV as amended by MSC.496(105) · ITU-R M.541 · ITU-R M.493
Check this section
Your engine has failed and you are drifting slowly in open water. The weather is fair, nobody is hurt and there is no immediate danger. Which priority fits?
GOC case
You are the operator on the fishing vessel MARINA, 25 NM off the coast at 1400 UTC. The steering gear has failed, but the weather is moderate, nobody is hurt and the skipper can hold her clear of traffic with the engine. Which priority do you use to report it to the coast station?