GMDSS basics, frequencies and sea areas

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.

6Exam keys & interpretation

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, 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?
1. SOLAS Chapter IV
2. STCW Regulation IV/2
3. The ITU Radio Regulations
4. IMO Model Course 1.25
Show the answer
C. The ITU Radio Regulations
Why: Spectrum, identities and priorities are the ITU Radio Regulations' layer. SOLAS IV is tempting, but it settles carriage and functions, not frequencies.
Under SOLAS IV as modernized from 1 January 2024, how is sea area A3 defined?
1. Coverage of an RMSS supported by the ship earth station on board
2. Coverage of the Inmarsat geostationary satellites, roughly 70°N to 70°S
3. Waters beyond about 100 NM from any MF coast station with DSC
4. Waters within HF range of a coast station keeping a DSC watch
Show the answer
A. Coverage of an RMSS supported by the ship earth station on board
Why: A3 is now defined by a recognized mobile satellite service supported by the terminal carried, with continuous alerting. A3 = Inmarsat is the old shortcut that now costs marks.
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?
1. MAYDAY (distress)
2. PAN-PAN (urgency)
3. SÉCURITÉ (safety)
4. Routine call to a coast station
Show the answer
B. PAN-PAN (urgency)
Why: Loss of propulsion with no immediate danger is the textbook urgency case. MAYDAY needs grave and imminent danger requiring immediate assistance, which is not met yet.
You are handling a medical PAN-PAN. The patient deteriorates and their life now depends on immediate assistance. What should you do?
1. Keep PAN-PAN, because medical cases are always urgency
2. Repeat the PAN-PAN on a satellite terminal to raise priority
3. Send a SÉCURITÉ so nearby ships know about the case
4. Reassess: distress may now apply; tell the station handling it
Show the answer
D. Reassess: distress may now apply; tell the station handling it
Why: Priority follows the danger, and a medical case can cross the distress threshold. Changing the medium never changes the priority, so the satellite option is a trap.
Which certificate is limited to operating GMDSS stations in sea area A1?
1. General Operator's Certificate
2. Restricted Operator's Certificate
3. Safety Radio Certificate
4. Ship station licence
Show the answer
B. Restricted Operator's Certificate
Why: The ROC is limited to A1; the GOC covers MF/HF and satellite in every area. The Safety Radio Certificate belongs to the ship, not to the operator.
Which actor validates a distress alert and coordinates the search and rescue response?
1. The RCC or MRCC
2. The coast station
3. The land earth station
4. The Cospas-Sarsat MCC
Show the answer
A. The RCC or MRCC
Why: The RCC/MRCC validates and coordinates. A coast station acknowledges and relays, and an LES or MCC only routes alerts towards the RCC.
The spoken opening MAYDAY, THIS IS followed by the ship's name, which identifies who is calling, is called the:
1. Alert
2. Message
3. Traffic
4. Call
Show the answer
D. Call
Why: The call identifies who is calling whom. The alert is the digital signal before it, and the message is the content that follows with position and nature.
A ship announces an urgency message through its satellite terminal instead of VHF DSC. What priority does the message have?
1. Distress, because satellite alerts go straight to an RCC
2. Safety, because satellite services mainly carry MSI
3. Urgency, because the medium never sets the priority
4. Routine, until an RCC confirms the urgency
Show the answer
C. Urgency, because the medium never sets the priority
Why: The priority is a judgement about danger, not about the equipment used. Routing to an RCC does not upgrade a PAN-PAN to distress.
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?
1. Distress, with a MAYDAY on Channel 16
2. Urgency, with a PAN-PAN announcement
3. Safety, with a SÉCURITÉ announcement
4. Routine, with a call to the coast station
Show the answer
B. Urgency, with a PAN-PAN announcement
Why: A steering failure with no immediate danger is very urgent for the safety of the vessel but below the distress threshold, so PAN-PAN fits. MAYDAY is tempting because she is disabled, but it needs grave and imminent danger.
Same scenario: an hour later water starts entering the engine room faster than the pumps can clear it, and the skipper fears MARINA may sink. What do you do about the priority?
1. Upgrade to distress and tell the station handling it
2. Keep urgency, as the case was opened as a PAN-PAN
3. Send a SÉCURITÉ so other ships keep clear of her
4. Repeat the PAN-PAN by satellite to raise its priority
Show the answer
A. Upgrade to distress and tell the station handling it
Why: Flooding that may sink her is grave and imminent danger, so the priority rises to distress and the station handling the PAN-PAN must be told. Keeping urgency because the case began that way is the trap: priority follows the danger, and the medium never changes it.

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

SOLAS IV/4, IV/12

Suggested study sequence: foundations, equipment, communication procedures and integrated simulator practice. This is the website’s learning structure, not an official course-duration, examination or qualification requirement.

Nine safety functions in reading order: ship-to-shore, shore-to-ship and ship-to-ship distress alerting; SAR coordination; on-scene communications; locating; maritime safety information; urgency and safety communications; bridge-to-bridge communications. General radiocommunications are a separate SOLAS IV requirement.

Illustrative equipment families, not a complete carriage checklist. SOLAS IV applies to passenger ships and cargo ships of 300 gross tonnage and upwards on international voyages; specific requirements depend on the applicable regulations and sea areas.