GMDSS & Communications

The GMDSS carriage requirement is built around four sea areas, and each one exists because of what a particular piece of equipment can physically reach. A1 is defined by continuous VHF DSC coverage from a coast station — short range, but a clear, instant, line-of-sight link. A2 is defined by MF DSC coverage on 2187.5 kHz — a ground wave that reaches much further than VHF, at the cost of a noisier, less crisp signal. A3 is defined by the footprint of the Inmarsat geostationary satellites — near-global coverage between roughly 70°N and 70°S, wherever the satellite sits high enough above the horizon to be usable. A4 is everything the satellites cannot see: the polar caps, where HF DSC and radiotelex, riding the ionosphere rather than a satellite, become the only practical distress path.

The area a ship must be fitted for is not simply "wherever she happens to be right now" — it is set by the areas the intended voyage passes through, and fitting is cumulative: a ship that transits A1, A2 and A3 on a single passage carries the equipment for all three, not just the most remote one she reaches. That is why a coastal ferry can get away with VHF DSC alone while a deep-sea bulk carrier on the same coast is carrying MF, HF and Inmarsat equipment as well — her voyage does not stay in A1.

GMDSS gives three distinct levels of priority, and the exam leans hard on the difference between them because getting it wrong has real consequences at sea, not just on paper. Distress — mayday — means grave and imminent danger to a ship or a person, where immediate assistance is required: uncontrolled fire, flooding beyond the crew's ability to contain it, a person overboard whose recovery is in doubt, an order to abandon ship. Urgency — pan-pan — covers a serious situation that has not yet reached that point: a disabling mechanical breakdown in a busy shipping lane, a medical case that needs advice or evacuation but is not immediately life-threatening. Safety — securité — is purely informational: a navigational hazard, a severe weather warning, a broadcast nobody is expected to answer.

The classification is not a formality; it sets the response. A mayday obliges every ship within range to render assistance and puts a rescue coordination centre into gear. A pan-pan draws attention and cooperation without launching a full search-and-rescue response. A securité asks only that ships take note. Downgrading a genuine distress to a pan-pan because it feels embarrassing to declare a mayday wastes the time that response speed is meant to buy; inflating an urgency situation into a mayday draws resources away from somewhere they might be needed more, and repeated over-calling erodes trust in every alert a ship subsequently sends.

• Mayday — grave and imminent danger, immediate assistance required.
• Pan-pan — a serious situation, short of grave and imminent danger.
• Securité — navigational or meteorological information, no response expected.

Pressing and holding the distress button sends a short, formatted digital burst on the DSC channel — ch70 on VHF, 2187.5 kHz on MF, the equivalent HF channels — carrying the ship's MMSI, its position and the time that position was taken, and, if selected, a brief nature-of-distress code. The set repeats that burst automatically at intervals until it is acknowledged, because a lone digital burst competing with noise or a busy channel can easily be missed once.

What the alert deliberately does not carry is any real explanation. A coast station or nearby ship that receives it knows who is in trouble and roughly where, and almost nothing else — not the nature of the emergency, not how many people are involved, not what help is actually needed. That information has to follow as a voice distress call and message (or radiotelex message) on the associated working frequency — ch16, 2182 kHz, or the HF equivalent — giving ship's name, call sign and MMSI, position, the nature of the distress, assistance required, persons on board and immediate intentions. An alert without that follow-up leaves the rescue chain with a dot on a chart and nothing to plan around.

Once an alert is acknowledged — by DSC acknowledgement from a coast station or ship — the distress ship stops repeating it unless the situation changes. If a coast station does not acknowledge within the expected period, any ship that received the original alert should relay it, effectively vouching that a distress exists and has not yet reached help.

A 406 MHz EPIRB, whether float-free or manually released, transmits a digitally coded message through the Cospas-Sarsat satellite system. Because the code is unique to the registered vessel, a rescue coordination centre can pull up the ship's identity, owner and emergency contacts within moments of the first satellite pass — long before anyone speaks to the ship. A GNSS-equipped EPIRB includes a position in the message and is refined further by Doppler processing and near-instant geostationary relay, so the position quality keeps improving after activation. The unit is built to keep transmitting for roughly two days on scene, on the reasoning that survivors may need that long before a search platform arrives. Crucially, an EPIRB works globally and needs nobody nearby — but it gives no return whatsoever on a ship's radar.

A SART does the opposite job. It is silent until interrogated: a 9 GHz X-band radar sweep from a searching ship triggers it, and it answers with a distinctive line of twelve dots spreading outward from the searcher's own position along the bearing, spaced at roughly one-mile intervals on the display, changing to a series of arcs once the range has closed to about a mile — a signature no other target produces. It carries no identity and reaches no satellite; its whole purpose is to be unmistakable on a radar screen that is already close enough to matter.

Put together, the EPIRB gets a search started and gets it to roughly the right area by satellite; the SART, together with visual aids like a torch, flares and a VHF handheld, gets the last mile covered once a searching unit is close. Expecting either device to do the other's job — an EPIRB return on radar, or a SART signal reaching a satellite — is a genuine misunderstanding of how the two systems are built, and the exam tests for it directly.

Maritime safety information — navigational warnings, weather warnings and forecasts, and other urgent notices to shipping — is broadcast automatically to every ship in range, without any ship having to ask for it. Two systems between them cover the whole GMDSS picture. NAVTEX, on 518 kHz internationally in English (with a national-language service commonly carried on 490 kHz), prints automatically with no watchkeeping burden and reaches out to roughly the edge of coastal waters — broadly the A1/A2 range. Each message is tagged with a station identity letter and a message-type letter, so a receiver can be set to print only the categories and stations that are actually relevant to the current passage and reject the rest.

Beyond NAVTEX range, and throughout A3 and A4, the same class of information is carried by SafetyNET, broadcast through the Inmarsat Enhanced Group Call (EGC) service. Because EGC messages are addressed to ocean regions or specific sea areas rather than broadcast blind, a ship well offshore still receives only the warnings that apply to where she actually is.

The pattern mirrors the sea-area logic from earlier in this chapter: which system a ship needs to be running depends on where she trades, and carriage follows from that. The officer's obligation does not stop at having the equipment switched on — warnings of a new wreck, ice, an ongoing search, or deteriorating weather are only useful if the traffic is actually read and folded into the passage plan, not left to print unread.

Accidental activations happen constantly — a distress button pressed during testing, an EPIRB knocked from its bracket, a DSC menu misread under pressure — and how they are handled matters as much as how a genuine distress is handled. The alert must be cancelled at once, on the same medium it was sent on: a DSC cancellation or all-ships correction message on the same channel the alert used, followed, if any voice or telex traffic was already transmitted, by a plain cancellation on the relevant working frequency. Where more than one frequency or channel carried the false alert, the cancellation has to go out on all of them, not just the first one that comes to mind.

Beyond the airwaves, the nearest coast station or rescue coordination centre should be contacted directly — by telephone, or by working channel — to confirm the alert was accidental, because by the time anyone notices the mistake the alert may already have triggered part of the response chain through the satellite or coast-station network.

Simply switching the set off is not a correction; it is the worst possible signature. The alert has already gone out and, particularly for an EPIRB via Cospas-Sarsat, has likely already reached a rescue coordination centre. A ship that then goes silent looks exactly like a ship that has stopped transmitting because things got worse, and the natural response is to escalate the search rather than stand it down. Most administrations treat an uncancelled false alert as a reportable, sometimes punishable, failure — not because the original mistake is unusual, but because the follow-up is what actually protects the credibility of the whole DSC network.

The three examples below move from estimating a coverage boundary, to sizing the reserve source of energy that keeps the set transmitting, to turning a SART detection range into a practical time on the approach — the kind of two- and three-step reasoning the exam expects, not a single formula substitution.