GMDSS in plain words: sea areas, DSC, EPIRB and SART, false alerts and interview questions

The GMDSS is the reason a ship in trouble no longer depends on someone hearing a Morse key. One button on the bridge sends the ship's identity, position and the nature of the distress to every station in range, and a buoy that floats free when the ship sinks tells a satellite where it went down. Every SOLAS ship carries it, every deck officer holds a certificate to operate it, and it is one of the topics that come up at almost every interview. This page explains the sea areas, the equipment and the frequencies, how to send and receive a distress alert, what to do after a false alert, how EPIRB, SART and NAVTEX work in practice, the routine tests, the mistakes that cause trouble, and questions to test yourself.

What the GMDSS is

Before the GMDSS a ship in distress relied on a radio officer keeping watch in Morse and on other ships happening to hear. In 1988 IMO amended SOLAS to replace that with an automated system built on two ideas: shore authorities, not only nearby ships, are alerted at once, and the alert carries the ship's identity and position without anyone having to read them out. The system was phased in from 1992 and became fully mandatory on 1 February 1999, the day ships stopped keeping the Morse distress watch.

It applies to SOLAS ships: passenger ships and cargo ships of 300 GT and above on international voyages. Chapter IV was modernised from 1 January 2024 and now refers to any recognised mobile satellite service rather than a single provider: Inmarsat has been joined by Iridium, recognised by IMO in 2018.

The system has nine functions: distress alerting ship-to-shore, shore-to-ship and ship-to-ship; search and rescue coordinating communications; on-scene communications; locating signals; maritime safety information; general radiocommunications; and bridge-to-bridge communications.

Key numbers

The four sea areas

• A1 — within range of at least one VHF coast station with continuous DSC watch; typically 20–30 miles from the coast station.
• A2 — outside A1, within range of at least one MF coast station with continuous DSC watch; typically up to about 150 miles.
• A3 — outside A1 and A2, within coverage of a recognised mobile satellite service. Inmarsat's geostationary satellites cover roughly from 70° N to 70° S; Iridium covers the poles as well.
• A4 — everything outside A1, A2 and A3: in practice the polar regions, where only HF reaches.

The sea areas are declared by coastal states, so the same distance from land can be A1 off one coast and A2 off another. A ship's radio equipment is fitted for the areas it trades in and listed on its safety radio certificate.

Equipment on board

Every SOLAS ship, wherever it trades:

• a VHF radio with DSC on channel 70 and radiotelephony on channels 16, 13 and 6, keeping a continuous DSC watch on channel 70;
• a NAVTEX receiver for maritime safety information, and an enhanced group call receiver where NAVTEX does not reach;
• a float-free satellite EPIRB on 406 MHz;
• SARTs — radar or AIS search and rescue transmitters: at least one on each side on ships of 500 GT and above, one on smaller cargo ships;
• portable two-way VHF radiotelephones for survival craft: at least three on passenger ships and on cargo ships of 500 GT and above, two on cargo ships of 300–500 GT.

Added for the sea area:

• A2 — an MF radio installation with DSC on 2187.5 kHz and radiotelephony on 2182 kHz.
• A3 — either a ship earth station of a recognised satellite service together with the MF installation, or an MF/HF installation with DSC.
• A4 — an MF/HF installation with DSC on all distress and safety frequencies.

Ships in A1 and A2 keep the equipment working by one of three methods — duplication of equipment, shore-based maintenance or at-sea maintenance; ships in A3 and A4 by at least two of them. The radio installation runs from a reserve source of energy as well as the ship's mains.

The distress frequencies

• VHF: channel 70 for DSC; channel 16 (156.8 MHz) for distress, urgency and safety voice traffic; channel 6 for on-scene communications with aircraft; channel 13 for bridge-to-bridge navigation safety.
• MF: 2187.5 kHz DSC, 2182 kHz voice.
• HF DSC: 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz.
• HF voice: 4125, 6215, 8291, 12290 and 16420 kHz.
• EPIRB: 406 MHz to the satellites, with a 121.5 MHz homing signal for the rescuers.
• SART: 9 GHz — it answers the pulses of an X-band (3 cm) radar.

Every DSC set is programmed with the ship's MMSI — a nine-digit Maritime Mobile Service Identity whose first three digits identify the country. Coast station MMSIs begin with 00.

Sending a distress alert

The master decides that the ship is in grave and imminent danger and needs immediate help. Then:

1. Check the position on the DSC controller. With a working GNSS feed it is automatic; without one it must be entered by hand, or the alert goes out with an old position.
2. Select the nature of distress if there is time — fire or explosion, flooding, collision, grounding, listing, sinking, disabled and adrift, abandoning ship, piracy, person overboard. If there is no time, the alert goes as "undesignated".
3. Lift the cover and hold the DISTRESS button for several seconds — most sets want about five — until the set confirms the alert has gone.
4. Go to the voice channel — channel 16 on VHF, 2182 kHz on MF — and make the MAYDAY call and message: MAYDAY three times, "this is" and the ship's name three times, the call sign and MMSI; then MAYDAY, the name, call sign and MMSI, the position, the nature of the distress, the assistance needed, the number of persons on board and anything else that helps, and OVER.
5. Wait for the acknowledgement. Until a coast station acknowledges, the DSC set repeats the alert automatically every few minutes.

A ship can also send the alert through its satellite terminal, and in the last resort the EPIRB sends one by itself.

Receiving a distress alert

On VHF or MF:

1. Do not acknowledge the alert by DSC, and do not relay it by DSC, unless an RCC or a coast station tells you to.
2. Listen on channel 16 (or 2182 kHz) for the coast station's acknowledgement and the distress traffic — for about five minutes.
3. If no coast station has acknowledged and the ship in distress is close enough for you to help, acknowledge it by voice on channel 16 and inform the RCC or a coast station by any means.
4. Write everything in the radio log, tell the master and be ready to proceed.

On HF, ships do not acknowledge; the coast station does. If none has after about five minutes, inform the RCC.

The reason for the rule is simple: in the early years, ships acknowledging and relaying each other's alerts by DSC flooded the channel with automated traffic and hid the original alert.

A false alert: what to do

False alerts are one of the GMDSS's biggest problems — someone presses the button in a drill, while cleaning or out of curiosity, and the rescue system starts working. What matters is cancelling at once:

1. Do not switch the set off. The alert has already gone; switching off only hides the acknowledgements from you.
2. Stop the repetition — reset the controller, or use its cancel function if it has one.
3. Cancel by voice on the distress channel of the band used — channel 16 on VHF, 2182 kHz on MF: "ALL STATIONS, ALL STATIONS, ALL STATIONS, THIS IS (name, call sign, MMSI), POSITION (...), CANCEL MY DISTRESS ALERT OF (date and time) UTC — MASTER (name)."
4. Record it in the radio log and tell the company.

For an EPIRB that has gone off by mistake: switch it off and inform the nearest RCC or coast station straight away. A false alert cancelled promptly is a routine matter; one left uncancelled sends people and aircraft to sea for nothing.

EPIRB, SART and NAVTEX in practice

• EPIRB — stowed in a float-free bracket with a hydrostatic release unit (HRU). If the ship sinks, the HRU frees it at a depth of no more than four metres, it floats up, switches itself on and sends the ship's identity — and its position, if it has a GNSS receiver — to the COSPAS-SARSAT satellites. It can also be switched on by hand and taken into the liferaft. The lanyard must never be tied to the ship.
• SART — taken into the survival craft and switched on there. When a ship's or aircraft's X-band radar sweeps it, it shows on the screen as a line of 12 dots leading away from the SART's position; as the rescuer closes in, the dots turn into arcs and then concentric circles. Mounted at least a metre above the water it is seen from about five miles by a ship and much further by an aircraft. On standby its battery lasts 96 hours, then 8 hours of transmitting. An AIS-SART shows instead as a special target on AIS and on ECDIS.
• NAVTEX — prints navigational and meteorological warnings, search and rescue information and forecasts on 518 kHz in English (490 kHz carries national-language broadcasts), out to roughly 400 miles. Stations and message types can be selected, but navigational warnings (A), meteorological warnings (B), search and rescue information (D) and additional navigational warnings (L) cannot be switched off.

Tests and the radio log

• Daily: the DSC internal test without transmitting; the reserve batteries' charge; the NAVTEX printer and its paper.
• Weekly: a DSC test call to a coast station on MF or HF where possible.
• Monthly: the EPIRB self-test and its HRU and battery dates; the SART self-test; the survival craft VHF sets and their batteries — tested on a working channel, never on 16 or 70.
• Every 12 months: the EPIRB tested; and at intervals of no more than five years, maintenance at an approved shore facility.

Everything goes into the GMDSS radio log: distress, urgency and safety traffic received and sent, the tests and their results, the position at least once a day, and any fault and its repair.

Certificates: GOC and ROC

• GOC — General Operator's Certificate under STCW regulation IV/2: radio operator in all sea areas. Deck officers on SOLAS ships hold it.
• ROC — Restricted Operator's Certificate: sea area A1 only.
• SRC and LRC — short and long range certificates for non-SOLAS craft such as yachts. They do not replace the GOC.

Like other STCW certificates, the GOC is revalidated every five years. The ship's muster list names the officer with primary responsibility for radio communications during a distress — usually the master or one of the officers.

What they ask at the interview

• Masters and chief officers: the sea areas and the equipment for each; the methods of keeping the equipment available; who acknowledges a distress alert and when; false alerts; what the safety radio certificate covers.
• Second and third officers: the DSC distress procedure step by step; the MAYDAY message; the distress frequencies; the daily, weekly and monthly tests; NAVTEX settings; how a SART looks on radar.
• Ratings: where the EPIRB, SARTs and portable VHFs are and who takes them to the boats; how to switch a SART on and where to hold it; never tying the EPIRB's lanyard.

Common mistakes

1. Switching the set off after an accidental alert instead of cancelling it by voice.
2. Sending an alert with an old position because the GNSS feed failed and nobody looked.
3. Tying the EPIRB's lanyard to the bracket or the rail — the ship takes it down.
4. An expired HRU or battery on the EPIRB.
5. Testing the survival craft VHFs on channel 16.
6. Deselecting NAVTEX stations or messages "because the ECDIS shows the warnings anyway".
7. Holding the SART at the bottom of the liferaft — low down, the radar sees it from a fraction of the distance.

Test yourself

Who on board needs this

Every deck officer — the master, the chief officer, and the second and third officer, who usually keep the radio log and run the tests — holds a GOC. The ETO often looks after the equipment, and the able seaman and the deck cadet need to know where the EPIRB, SARTs and portable VHFs are and what to do with them when abandoning ship. The radio certificate itself is STCW IV/2 — see STCW in plain words; the drills and life-saving appliances that go with it are in SOLAS in plain words, and the words of the MAYDAY message in SMCP in plain words.

Put your GOC with its number and expiry date into your maritime CV.

Source: SOLAS chapter IV, the ITU Radio Regulations and IMO guidance on distress alerts and their cancellation. Your ship's radio installation, its manuals and the master's instructions prevail on board. This page is a study aid, not a legal text.