1The lesson in plain words
In one sentence…
GMDSS is a chain: alerting, coordination and help continue through information, watchkeeping and decisions.
What is GMDSS?
GMDSS combines ship equipment, shore services and procedures. An alert attracts attention; the whole message continues with explanation, acknowledgement and information.
What are the nine functions?
They are jobs the system must accomplish: alerting, coordinating, on-scene communication, locating and receiving safety information. They are not nine radios or nine buttons.
How do you order traffic?
Danger and assistance required decide it: distress, urgency, safety and routine. A routine call must yield to distress traffic.
Who does what?
Someone on board identifies the problem; a station organizes help; nearby units exchange instructions. The chain divides the tasks and continues with a watch.
2Step by step
GMDSS communication chain
- 1
Identify the problem and gather identity, position, time, facts and people involved.
The chain starts with an identified situation and concrete information.
- 2
Send the alert by an available means that can reach someone able to respond.
The alert attracts attention from a station able to organize help.
- 3
Exchange the information needed to coordinate the response with that station.
Coordination establishes who controls the response and what information is needed.
- 4
Communicate with nearby units and follow their instructions on scene.
On-scene communication lets units exchange instructions.
- 5
Provide data that helps locate the vessel or survivors.
Locating helps rescuers find those needing assistance.
- 6
Keep watch and receive instructions and safety information.
Communication continues while you receive guidance and voyage warnings.
3Key data
- GMDSS functions
- Nine GMDSS functions
- Distress alerting
- Ship-to-shore, shore-to-ship, ship-to-ship
- Traffic priority
- Distress → urgency → safety → routine
- MAYDAY
- Grave and imminent danger
- PAN-PAN
- Very urgent safety message
- SÉCURITÉ
- Safety information
- Ship-to-shore means
- Two separate, independent means; different radiocommunication services
- Channel 16 watch
- When practicable
- MF/HF DSC watch
- 2187.5 + 8414.5 + one of 4207.5/6312/12577/16804.5 kHz
- A3 and HF
- HF carriage is not universal
4Key points
- 1.
Alerting → coordination → on-scene → locating → safety.
- 2.
The nine functions are system jobs, not nine radios.
- 3.
Priority: distress → urgency → safety → routine.
- 4.
MAYDAY, PAN-PAN and SÉCURITÉ name different messages.
5Frequent mistakes
Wrong: "The nine functions require nine radios."
Right: They are functions the system must accomplish, not nine buttons or nine mandatory radios.
Wrong: "The alert ends the operator's communication task."
Right: Afterwards, explain, listen for the acknowledgement and continue communicating.
Wrong: "A routine call keeps priority once it has started."
Right: Routine traffic must yield to distress traffic.
6Practice case & exam keys
Situation
During a drill on an A3 ship fitted with MF/HF DSC and an Inmarsat C terminal, the MF/HF transceiver fails. A cadet points to a second MF/HF control screen on the bridge wing and says the independent backup is ready.
What is the real second means, and why is the cadet wrong?
Show the answer →
The Inmarsat C terminal is the remaining independent means; the second screen controls the same failed transceiver.
Why: Independence means a different radiocommunication service. The bridge-wing screen depends on the same transceiver and antenna, so one failure has silenced both. The satellite terminal uses a different service, so ship-to-shore alerting now depends on it, and the failure must be reported and logged.
Exam keys
If you see "nine buttons", remember: the functions are not nine radios.
If you see "A3 = universal HF", reject it: A3 does not make HF carriage universal.
If you read "grave and imminent danger", choose MAYDAY; "very urgent safety message" points to PAN-PAN.
How to read it
Think of a relay: each part receives, adds or directs information until help can act.
Do not confuse a powered device with a completed function: the chain, watch and human decision matter.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
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 |
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.
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.
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 |
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.
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
GOC exam check
Which document settles GMDSS frequencies, identities and the priorities that protect distress traffic?
8Full lesson & sources
Open the full lesson (diagrams and official references)
GMDSS combines ship equipment, shore services and procedures. SOLAS IV/4 distinguishes nine GMDSS functions from general radiocommunications. Ship-to-shore distress alerting needs two separate, independent means using different services. Distress, urgency, safety and routine traffic have decreasing priority. MAYDAY concerns grave and imminent danger to a vessel or a person requiring immediate assistance; PAN-PAN concerns a very urgent safety message; SÉCURITÉ introduces safety information.
Keep the automatic watches required by the installed equipment and voyage, plus a channel 16 listening watch when practicable. For MF/HF installations, required DSC watch includes 2187.5 and 8414.5 kHz plus at least one suitable additional HF frequency. A3 does not make HF carriage universal.
Think in functions, not boxes on a console
When an examiner asks "what does this ship need?", the weak answer is a shopping list. The strong answer starts from jobs. SOLAS IV/4 says every SOLAS ship must be able to do nine things:
- 1Send ship-to-shore distress alerts by at least two separate and independent means.
- 2Receive shore-to-ship distress alert relays.
- 3Send and receive ship-to-ship distress alerts.
- 4Send and receive search-and-rescue coordinating communications.
- 5Send and receive on-scene communications.
- 6Send and receive locating signals.
- 7Receive maritime safety information (MSI).
- 8Send and receive urgency and safety communications.
- 9Send and receive bridge-to-bridge communications.
General radiocommunications (crew calls, company email) sit outside that list. Once you think in jobs, two things become obvious: one device usually covers several functions, and one function often needs two devices.
| Device | Functions it really covers | What it cannot do for you |
|---|---|---|
| VHF DSC set (Ch 70, 16, 13, 6) | Ship-to-ship alerts, on-scene, bridge-to-bridge, urgency and safety calls | Reach the shore once you are beyond a VHF coast station |
| NAVTEX receiver on 518 kHz | MSI near the coast | Send anything at all |
| 406 MHz EPIRB | Alert into Cospas-Sarsat, homing on 121.5 MHz | Hold a conversation or tell anyone what is wrong |
| Radar SART or AIS-SART | Locating survivors at close range | Alert the shore |
The two-means rule, worked out
For ship-to-shore alerting, SOLAS asks for two separate independent means, each using a different radiocommunication service. "Different service" is the part people skip. Take an A3 ship fitted with MF/HF DSC and an Inmarsat C terminal: one path is terrestrial HF radio, the other is a satellite network. A cyclone that ruins HF propagation does not touch the satellite path, and an outage at the land earth station does not touch HF. That is real independence.
Now test some false answers. Two VHF sets are one service: both die the moment you leave VHF coast-station range. Two Inmarsat C terminals are one service: a network outage takes both. A second display mirroring the main MF/HF transceiver is not even a second radio.
Priority reflects the danger
Priority reflects the danger and the time left before it becomes unsurvivable, not how frightened anyone feels. Work it with numbers. Your engine fails 10 NM off a lee shore and you drift towards the rocks at 1 knot. You have about 10 hours and nobody is in immediate danger: that is urgency, PAN-PAN, and you ask for a tow. Six hours later the anchor drags, you are 1 NM off and still drifting at 1 knot: one hour to grounding in heavy weather is grave and imminent danger, and you upgrade to MAYDAY. The facts changed, so the priority changed, and you tell the station handling your traffic that it has.
Whatever the priority, the listener needs the same core facts: who you are, where you are and when that position was valid, what has happened, what help you want, and how many people are involved. "Fire in the engine room, out of control, 18 on board, preparing to abandon" lets the RCC send the right assets; "we have a big problem" makes them call you back.
A watch is something you do
A receiver with a green light is not a watch. A watch means the right receiver is on the right frequency, its alarm is audible on the bridge, someone notices the alarm, and the information reaches the officer of the watch. Check each link: is the DSC receiver really on Channel 70 rather than scanning? Is the alarm volume up? Who answers it at 0300?
Before you sail, run through five checks:
- 1Which sea areas the voyage crosses, and therefore which equipment and watches apply.
- 2That the automatic DSC watches are running and their alarms are audible.
- 3That the radios show the ship's own MMSI and a current position.
- 4Where the portable VHF, SART and EPIRB are, and who takes them if you abandon ship.
- 5Who operates the radio in an emergency and who must be told when an alert arrives.
Official references · 2026-09-09
9Test your knowledge
What best describes GMDSS?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: