GMDSS and DSC Communications

Prerequisites

• RYA Day Skipper level knowledge and experience; this lesson forms part of the Coastal Skipper / Yachtmaster Coastal syllabus.
• The RYA SRC (Short Range Certificate) or another GMDSS-compliant operator's certificate is mandatory before you can sit the Yachtmaster Coastal exam. This lesson revises and extends SRC content to the level a skipper is expected to apply under pressure; it does not replace the SRC course and assessment.
• Familiarity with your own VHF/DSC radio: how to read its MMSI, select channels, use dual watch and navigate its DSC menus.
• Sea time for the Coastal Skipper course: 15 days, 2 as skipper, 300 miles, 8 night hours.

Learning Objectives

This lesson revises and extends the RYA Short Range Certificate (SRC) syllabus to Coastal Skipper standard. The Yachtmaster Coastal exam requires a GMDSS-compliant operator's certificate (SRC or higher) and the practical examiner may ask you to demonstrate distress, urgency and routine calls. By the end you will be able to:

• Explain the purpose and structure of the Global Maritime Distress and Safety System (GMDSS), the compulsory-fit and voluntary-fit distinction, and define Sea Areas A1 to A4. (RYA syllabus: GMDSS and sea areas.)
• State the three legal requirements for a ship's radio installation and the licences, certificates and records involved. (RYA syllabus: licensing and regulations.)
• Describe the VHF band, simplex and duplex working, aerials and power, and use the channels correctly, including the roles of Ch 16, 70, 67, 0, 13, 80 and the inter-ship working channels. (RYA syllabus: VHF channels and frequencies, equipment.)
• Describe the MMSI, the four types of identity, and how DSC uses them for individual, group, all-ships and distress calls. (RYA syllabus: VHF DSC controller, identities.)
• Send a DSC distress alert correctly, follow it with a voice MAYDAY in the correct format, and handle acknowledgement, direction finding and the silence prowords. (RYA syllabus: distress procedures.)
• Respond correctly to a distress alert or Mayday you receive, relay a distress, and cancel a false alert from a radio or an EPIRB. (RYA syllabus: Mayday relay, false alerts.)
• Make urgency (PAN-PAN) and safety (SECURITE) calls, obtain medical advice, and make and receive routine DSC and voice calls using correct procedure words, the phonetic alphabet and the rules for unanswered and garbled calls. (RYA syllabus: urgency and safety traffic, voice procedure.)
• Understand the supporting equipment (EPIRB, PLB, SART, AIS-SART, NAVTEX) and how Maritime Safety Information is broadcast. (RYA syllabus: GMDSS equipment, MSI.)
• Apply the radio regulations as skipper: forbidden transmissions, secrecy of correspondence, control of communications, and the MCA guidance on VHF in collision avoidance.

What GMDSS Is and Why It Exists

Before GMDSS, distress relied on a human hearing a voice call or Morse signal. Morse operators were dropped in the 1970s as voice radio spread, watchkeeping officers were expected to listen instead, and ships continued to disappear without anyone hearing a distress call. GMDSS, introduced under SOLAS Chapter IV in 1992 and fully in force since 1999, automates alerting: radios send digital alerts that ring alarms at shore stations and on nearby ships, carrying the caller's identity and position, so that the alert reaches a Rescue Co-ordination Centre by two independent means without a specialist operator. Shore rescue coordination centres (in the UK, HM Coastguard) then coordinate the response.

GMDSS is mandatory for compulsory-fit vessels: cargo ships of 300 gross tonnage and above, passenger ships carrying 13 or more passengers, and fishing vessels. They must carry VHF DSC (plus MF or HF DSC outside Area A1), a 406 MHz EPIRB, a SART, radar, NAVTEX and waterproof handheld VHF sets. Pleasure yachts are voluntary fit: they are not obliged to carry any radio, but if they do they must follow GMDSS procedures, and nearly all modern VHF sets are DSC-capable. Every compulsory-fit ship and every Coastguard station keeps a DSC watch, so a yacht with DSC is fully part of the system. HM Coastguard keeps only a loudspeaker watch on Channel 16 and strongly encourages leisure craft to fit DSC. Small commercial vessels coded under the MCA codes have specific carriage requirements depending on their area category.

The GMDSS functions are: alerting (ship to shore, ship to ship, shore to ship), search and rescue coordination, on-scene communications, locating (homing), Maritime Safety Information, general radio communications, and bridge-to-bridge communications.

Sea Areas

The diagram below shows the A1 concept. A Coastal Skipper operates up to about 20 miles from a safe haven, so almost all of your sailing around the UK will be within A1. The whole UK coast is A1, with A2 cover from MF stations extending well out into the Western Approaches, the North Sea and the Irish Sea.

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The "20 to 30 miles" in the diagram is a typical figure for a yacht. Actual VHF range depends on antenna heights because VHF is essentially line of sight. A useful approximation is range in miles is about 2.2 multiplied by the sum of the square roots of the two antenna heights in metres. A yacht masthead antenna at 14 m (square root about 3.7) talking to a Coastguard aerial on a hill at 150 m (square root about 12.2) gives roughly 2.2 x 15.9, about 35 miles. A handheld at 2 m talking to another yacht's handheld might manage only 6 miles. That is why the masthead aerial and the fixed set are your primary distress equipment, and why a dismasting, which takes away your masthead antenna, is a reason to carry an emergency antenna.

The Legal Requirements

When fitting a radio the owner must satisfy three requirements, and the Yachtmaster examiner expects a skipper to know them:

1. Type approval. Equipment sold in the UK must conform to the Radio Equipment Directive, carry a conformity mark and have a Declaration of Conformity in the handbook. Second-hand sets, and sets bought abroad (especially American sets, whose channel plan differs), may lack the international channels and can be confiscated in an Ofcom inspection.
2. A Ship Radio Licence under the Wireless Telegraphy Act, issued by Ofcom. It is free online, must be revalidated every ten years and amended whenever equipment changes, and must be carried on board. It covers every transmitter declared on the application: MF, HF and VHF sets, DSC, handhelds used on the parent craft or its tender, radar and SARTs, EPIRBs, satellite terminals and low-power UHF on-board sets. With it come the vessel's international call sign, which stays with the vessel through changes of ownership and equipment, and the MMSI. A handheld intended for use on several craft needs a Ship Portable Radio Licence, licensed to a person, with a call sign ending in T and its own MMSI. Yacht clubs and marinas hold Coastal Station Radio licences for their base stations.
3. An operator's certificate. A maritime radio may be operated only by, or under the direct supervision of, the holder of the appropriate certificate. For VHF DSC that is the Short Range Certificate, examined by RYA training centres on behalf of the MCA. It has two parts: the Certificate of Competence and the Authority to Operate, the latter issued from age 16. For MF, HF and satellite equipment the Long Range Certificate (LRC) is required. Holders of the pre-2000 Restricted (VHF only) certificate must convert before operating DSC.

A distress alert is sent on the authority of the skipper. Crew may make routine calls under the certificate holder's supervision, and every crew member must know how to send a Mayday.

Records. Compulsory-fit vessels keep a radio log. A yacht's deck log should record distress, urgency and safety traffic heard, radio checks, MSI received and any call to the Coastguard, with times in UTC; the Coastguard may ask for it after an incident.

The VHF Band and the Equipment

Frequencies, simplex and duplex

The international maritime mobile band runs from 156.0 to 174.0 MHz and contains 59 international channels at 25 kHz spacing, plus nationally allocated private channels. Each channel is designated for one or more of eight purposes: distress, safety and calling; intership; bridge-to-bridge; port operations; ship movement; public correspondence; DSC; and, in the UK, small craft safety.

• Simplex channels use one frequency; you transmit or receive, never both, and the press-to-transmit switch changes over. All intership channels are simplex, so two ships cannot converse on a duplex channel.
• Duplex channels use two frequencies, one for the ship and one for the coast station (Channel 26: ship 157.300 MHz, shore 161.900 MHz), allowing simultaneous two-way speech with suitable equipment. A yacht's simplex set can use a duplex channel, but still one way at a time.
• Semi-duplex has simplex at one end and duplex at the other.

Aerials, cable and power

A high-gain aerial (about 2 m) concentrates the radiated power in a narrow horizontal beam and gives more range on a vessel that stays upright, such as a motor cruiser. A unity-gain aerial (about 1 m) has a wider beam and is recommended for sailing yachts because it keeps working when the boat heels. Use the best low-loss coaxial cable, keep connectors to a minimum and away from salt, route the feeder clear of instrument and autopilot cables (even low-loss cable radiates), and mount the aerial at the masthead where the rigging cannot mask it. Keep an emergency aerial for a dismasting. High pressure and humid air can lengthen range; a rolling aerial in a seaway causes "fluttering" and shortens it.

Maximum permitted output is 25 W, with a low-power setting of about 1 W. Many sets select 25 W at switch-on; except in an emergency select 1 W before transmitting to a nearby station. Marinas transmit on 1 W only, so a vessel calling on 25 W from miles away may not hear the reply. Channels 15, 17, 75 and 76 are restricted to 1 W and most sets switch automatically.

Installation and power supply

Mount the set securely in the cabin, clear of spray and heat, wired with correct polarity to a battery that is not used for engine starting; no earth is needed. A waterproof extension speaker at the helm lets the watch hear the radio without waking the off-watch. On receive the set draws little current; on transmit five amps or more. Consider what happens if the boat is sinking and the main batteries flood: a handheld with its own battery is the answer. Check monthly that the GPS position is reaching the DSC radio.

Controls

Volume and squelch (set so the hiss just stops; too much squelch hides weak distant signals), a Channel 16 override key, high/low power, dual watch (Ch 16 plus one other; triple watch on some sets), scanning (no priority for 16, so a distress call can be missed, and Ch 70 cannot be scanned), the protected red distress button, and the ENTER or CALL key used for every DSC call other than distress. Before sending a distress call, make a positive check that the set really is on Channel 16: dual watch lets you hear 16 while the set is actually on another channel.

The MMSI

Every DSC radio is programmed with a Maritime Mobile Service Identity, a nine-digit number that identifies your vessel (or, for a handheld, the radio itself). It is issued in the UK by Ofcom with your Ship Radio Licence, free of charge, and must be entered into the radio exactly once; most sets refuse a second change without a dealer reset.

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• A ship station MMSI begins with the three-digit Maritime Identification Digits (MID) for the flag state, followed by six digits. UK MIDs are 232, 233, 234 and 235. For example 235012345.
• A group MMSI begins with a single 0 (for example 023500001), used by fleets, sailing schools or clubs. It is programmable by the user, but the number must be allocated by Ofcom.
• A coast station MMSI begins with 00 (for example HM Coastguard stations are 002320xxx; Humber Coastguard is 002320007).
• A portable DSC set has a ship-style MMSI issued with its own licence.
• MMSIs beginning 970 are AIS-SARTs, 972 AIS MOB devices and 974 EPIRBs with AIS.

Coast station MMSIs are listed in the almanac and the ITU keeps a searchable list of ship stations; if a vessel is not listed you must ask the owner. Your MMSI is part of your radio licence: if you sell the boat, the licence and MMSI must be updated with Ofcom, and if you buy a boat, check whether the previous owner's MMSI is still programmed in.

Channels You Must Know

Channel 70 is reserved for digital DSC messages. The radio itself transmits and receives the data bursts; you never speak on 70, which is why the diagram below shows the microphone crossed out.

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Marinas and yacht clubs are not licensed for Channel 16, so the initial call to them goes direct on their working channel. Ports monitor 16 and their own channels, but the call should be made on the port channel listed in the almanac, naming the channel you are using if the port monitors several. In UK harbours the radiotelephone may be used only for port operations and on licensed private channels; intership calls in port are permitted only on matters of safety.

Radio Fundamentals for the Skipper

The regulations

The ITU Radio Regulations list transmissions that are strictly forbidden: profane or obscene language; unnecessary or superfluous transmissions; transmissions without identification; broadcasting music; false or deceptive distress, safety or identification signals; transmissions not authorised by the skipper; operation by an unauthorised person; messages to an unlicensed shore station; personal names in place of the vessel's name; closing down before all operations arising from a distress are finished; and use of channels not covered by the licence. Everyone who hears radio traffic is bound by the secrecy of correspondence: the contents, and even the existence, of a conversation must not be divulged.

Control of communications. The station called controls the communication and should nominate the working channel, though when DSC is used the caller proposes the channel. A coast station always controls communication with a ship; obey its instructions, acknowledge a change of channel, and if told to stop transmitting, stay silent until released.

The MCA's guidance on the proper use of VHF at sea (MGN 324) adds: prepare before transmitting, listen first, avoid occupying a channel in poor conditions, use the lowest power that works, avoid needless repetition, and keep watch on Ch 16 and, where practicable, Ch 13 at sea. MGN 167 warns against using VHF to negotiate collision avoidance between approaching vessels: identification is uncertain, misunderstanding is easy, and the Collision Regulations, not a radio conversation, govern the manoeuvre. A judge in a 1995 collision case described VHF negotiation as a probable contributory cause of the collision.

The phonetic alphabet and procedure words

Spell out names, call signs and tricky words using the ITU phonetic alphabet: Alfa, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel, India, Juliett, Kilo, Lima, Mike, November, Oscar, Papa, Quebec, Romeo, Sierra, Tango, Uniform, Victor, Whiskey, X-ray, Yankee, Zulu. Say I SPELL before spelling and repeat the word afterwards if it is pronounceable. Numbers are said digit by digit (wun, too, tree, fow-er, fife, six, sev-en, ait, nin-er, zero); the word "decimal" marks a decimal point; multiples of thousands may be spoken as such.

Positions, bearings, distances, speed and time

The IMO Standard Marine Communication Phrases fix the form. Positions are given as latitude and longitude in degrees and minutes and decimals, spoken digit by digit, or as a true bearing and distance from a well-defined charted object to the vessel: "my position is one eight zero degrees from Portland Bill lighthouse, two decimal four miles." Bearings are in 360-degree notation from true north unless stated; say whether they are from the mark or from the vessel. Distances in nautical miles or cables (state metres or kilometres if used). Speed in knots, meaning speed through the water unless "ground speed" is said. Times in the 24-hour clock, stating UTC, local or zone time. Courses in 360-degree true notation unless otherwise stated.

Radio checks and testing

• A radio check should be asked of another vessel or a marina on a working channel, not of the Coastguard on Ch 16. Ask: "Marina, this is Sea Breeze, Sea Breeze, request radio check, channel 80, over." The reply gives readability on a scale of 1 to 5. A voice test call must not exceed 10 seconds and must carry your name.
• Never test the DSC distress alert. Use the radio's self-test routine, and a DSC test call (TEST category) to a coast station's MMSI: HM Coastguard acknowledges these automatically and prefers them to voice checks.
• Check that the radio's GPS feed is live and displays a position at every passage start; a DSC alert with no position is far less useful. If there is no GPS feed, enter the position manually at least every four hours; the set deletes a position more than about 24 hours old.

Dual watch and listening watch

At sea a vessel with a VHF should maintain a listening watch on Ch 16 (and the set keeps a DSC watch on Ch 70 automatically). Dual watch samples Ch 16 and another channel alternately; it is useful while working a channel such as Ch 67 or a port channel. Remember that while you are transmitting you cannot receive, which is why distress alerts are repeated. A skipper keeps a log of significant calls and notes the schedule for coastal MSI broadcasts.

Standard voice call procedure

To call another vessel on Ch 16, the full procedure is: name of the station called (twice, up to three times in bad conditions), THIS IS, your name (twice), a proposed working channel, and OVER. The called vessel replies and accepts or nominates a working channel; both switch and the caller makes the first call on the new channel. Keep it short; Ch 16 is shared.

SEASPRAY SEASPRAY, THIS IS SEA BREEZE SEA BREEZE, CHANNEL SEVEN TWO, OVER. SEA BREEZE, THIS IS SEASPRAY, CHANNEL SEVEN TWO.

Call a marina once on its working channel ("Mercury Marina, this is motor yacht Blue Dolphin, Blue Dolphin, over"). Where DSC is available, use a DSC individual call instead; it avoids a long call on Ch 16 and the other crew do not hear it.

Unanswered calls. Check your own set first: power, volume, squelch, channel, high power if the station is distant, and that the transmit switch is not jammed. Then wait at least two minutes before calling again; three calls at two-minute intervals, then a three-minute wait. Two unanswered calls to another boat usually mean it is not listening. Garbled calls: if you cannot tell who a call is for, do not reply until it is repeated; if it is for you but the caller is unknown, reply "Station calling Barbican, this is Barbican, say again, over."

DSC Call Categories

DSC calls have a priority and an addressing type:

A DSC call is like ringing a telephone: it alerts the receiving station and tells it which channel to go to for the voice conversation. It does not replace the voice message. The set asks for confirmation before an all-ships call because every set in range will be switched to Channel 16 by it. Incoming calls are stored in the set's log.

Sending a Distress Alert

A MAYDAY is used only when a vessel, aircraft, vehicle or person is in grave and imminent danger and requires immediate assistance: sinking, fire out of control, man overboard, a crew member with a life-threatening condition, a vessel dragging towards rocks in a gale. The distress transmission has three parts: the DSC alert, the voice distress call and the voice distress message. A set without DSC uses the last two.

The sequence is shown below.

Step by step:

1. Check the battery switch and the set are on. Lift the cover on the red DISTRESS button and press it once.
2. If time allows, select the nature of distress from the menu: fire/explosion, flooding, collision, grounding, listing/in danger of capsize, sinking, disabled and adrift, abandoning ship, piracy/armed attack, man overboard, or undesignated. A designated alert gives rescuers far more information, but never delay an alert to scroll menus if the situation is critical.
3. Press and hold the button for five seconds, through the countdown, until the display confirms the alert is sent.
4. The radio transmits your MMSI, position and time from the connected GPS (or a manually entered position), and the nature of distress, as a data burst on Ch 70. It then switches to Ch 16 and repeats the alert automatically about every four minutes (randomised between 3.5 and 4.5 minutes) until acknowledged by a coast station or cancelled. The whole process takes about 15 seconds.
5. Wait about 15 seconds for a DSC acknowledgement, then send the voice MAYDAY on Ch 16 at high power, whether or not an acknowledgement has arrived.

The voice MAYDAY

MAYDAY MAYDAY MAYDAY THIS IS yacht SEA BREEZE SEA BREEZE SEA BREEZE, MMSI 235012345 (call sign MABC1 if no DSC) MAYDAY SEA BREEZE MY POSITION IS 50 degrees 31 minutes North, 002 degrees 46 minutes West (or 270 degrees from Portland Bill 12 miles) Taking on water, flooding in the engine room I require immediate assistance Four persons on board Liferaft ready, all crew wearing lifejackets OVER

The memory aid is MIPDANIO: Mayday, Identify, Position, Distress nature, Assistance required, Number of persons, Information, Over. Speak slowly, give position as latitude and longitude or as a true bearing and distance from a charted landmark, and use the phonetic alphabet for letters. Give the total number of persons; it decides the rescue method.

The worked example diagram below walks through the complete activation for a yacht flooding off Portland Bill.

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Keep a procedure card by the set, in the RYA format, with the vessel's name, MMSI and call sign filled in, so that any crew member can send the alert and message. Make two versions if you carry a non-DSC handheld.

After the MAYDAY

• The Coastguard will acknowledge by DSC (which stops your set repeating) and by voice ("MAYDAY, SEA BREEZE SEA BREEZE SEA BREEZE, MMSI..., this is Solent Coastguard x3, RECEIVED MAYDAY, launching lifeboat, stand by"). Answer their questions concisely.
• Keep listening on Ch 16. If you must abandon to the liferaft, take a handheld VHF (ideally DSC with GPS), the EPIRB and the SART.
• A lifeboat or helicopter may ask for a direction-finding transmission: "for direction finding purposes request you hold your press to transmit button closed for ten seconds followed by your name, repeat three times." Reply with MAYDAY, their name, THIS IS your name, then transmit for ten seconds, say your name, and repeat the sequence three times.
• Control of the channel: the coordinating station or the casualty may impose silence with SEELONCE MAYDAY. Another station close to the incident may say SEELONCE DISTRESS. When restricted working may resume the Coastguard broadcasts PRUDONCE, and when the distress is over, SEELONCE FEENEE, each in the form MAYDAY, ALL STATIONS x3, THIS IS (station x3), MAYDAY (casualty), proword, time, OUT. No station aware of the distress may transmit on the channel until then.
• Do not switch off the radio until the Coastguard stands you down.

If you receive a distress alert

Your radio will sound an alarm, switch to Ch 16 and display the casualty's MMSI, position and nature of distress. In GMDSS, coast stations acknowledge distress alerts by DSC, not ships. A yacht's Class D set cannot send a DSC acknowledgement, and a vessel must not try to: a DSC acknowledgement stops the casualty's radio repeating its alert and may lead the casualty to believe help is organised when the Coastguard has not heard.

Instead:

1. Note the time, MMSI, position and nature of distress in the log, plot the position and inform the skipper.
2. Listen on Ch 16 for the voice MAYDAY and the Coastguard's response.
3. If the Coastguard acknowledges, keep listening; if you are close and can help, inform the Coastguard of your position and ETA when asked.
4. If the casualty's MAYDAY is not acknowledged by a coast station after about four minutes (the alert will repeat), send a Mayday Relay. If that is impossible and still nobody has answered, acknowledge the casualty by voice ("Mayday, Sea Spray, this is Wanderer, received Mayday") and proceed to assist, while continuing to try to raise the Coastguard.

The obligation to accept distress messages is absolute, but acknowledging commits you to go and help: a yacht making 4 knots is little use to a ferry 15 miles away, and if every yacht in the Solent acknowledged, Channel 16 would be blocked.

Mayday Relay

You transmit a MAYDAY RELAY when:

• you have heard a distress message that has not been acknowledged by a coast station after a reasonable time, or
• you know of a vessel in distress that cannot transmit a message itself (for example you see red flares, a vessel on fire, or a person on deck raising and lowering outstretched arms), or
• the master of your vessel decides further help is needed even though the distress has been acknowledged.

A Class D set has no Mayday Relay alert. Send a DSC urgency alert first: it wakes every set in range and switches it to Ch 16 without transmitting a position that could confuse direction finding. Newer sets may offer an individual distress relay call addressed to the Coastguard's MMSI; use that if you have it. Then by voice:

MAYDAY RELAY MAYDAY RELAY MAYDAY RELAY THIS IS yacht WANDERER WANDERER WANDERER, MMSI 235098765 MAYDAY fishing vessel SEA SPRAY, MMSI 232001234 Position 50 degrees 46 minutes North, 001 degrees 17 minutes West On fire, require immediate assistance, three persons on board Time 1415 UTC OVER

Make it quite clear that you are not the casualty, otherwise a direction-finding bearing may be taken on you and help sent to the wrong place. Other vessels should not acknowledge a Mayday Relay unless they can assist. Never relay a distress that the Coastguard has already acknowledged unless you have new, important information. A light aircraft seen to crash near you, or a capsized dinghy with someone waving, is therefore handled with a DSC urgency alert and a voice Mayday Relay, not with your own red button.

Urgency and Safety Calls

PAN-PAN (urgency)

Used when there is an urgent message about the safety of a vessel or person but the danger is not grave and imminent: a boat taking on water but not yet sinking, engine failure drifting towards a lee shore in moderate conditions, a crew member needing medical advice, a dismasting without immediate risk to life. Anything less than distress may justify urgency, and urgency can always be upgraded.

1. Send a DSC urgency alert (all ships, or individual to the Coastguard) from the menu with ENTER, confirming when asked. The alert carries your MMSI and the time but no position.
2. After 15 seconds, on Ch 16:

PAN-PAN PAN-PAN PAN-PAN ALL STATIONS ALL STATIONS ALL STATIONS (or Solent Coastguard x3) THIS IS yacht SEA BREEZE SEA BREEZE SEA BREEZE, MMSI 235012345 MY POSITION IS ... Engine failure, drifting towards ... at 1 knot, require a tow Four persons on board Hull white, blue sprayhood OVER

The nearest coast station acknowledges by voice if in range, may repeat the Pan-Pan on 16, and takes control of further traffic. For medical advice, the Coastguard connects you to a doctor through the Telemedical Advice Service (TMAS); this is often called a PAN-PAN MEDICO, though in UK practice you simply describe it as a medical urgency. Be ready with the casualty's age, sex, history, symptoms, pulse and breathing.

If the situation is resolved, cancel the PAN-PAN by a voice "all stations" message on Ch 16.

SECURITE (safety)

Used for navigational or weather warnings: a semi-submerged container, a buoy off station, an unlit wreck, a gale warning. In practice yachts send these rarely; most SECURITE broadcasts come from the Coastguard (MSI). If you must:

1. DSC safety call to all ships from the menu, confirming when asked.
2. On Ch 16: "SECURITE x3, ALL STATIONS x3, THIS IS ... x3, for urgent navigational warning listen channel six seven, out", then transmit the full warning on the working channel with the position and time of origin.

All stations hearing the safety call should switch to the working channel and listen until satisfied the message does not concern them, without transmitting anything that could interfere.

Routine DSC Calls

DSC lets you call another vessel or a marina directly, without crowding Ch 16, as shown in the procedure diagram.

1. Select Individual call and enter the other station's MMSI (or pick it from the radio's directory).
2. Select the category ROUTINE and an appropriate inter-ship working channel (for example 72 or 77). For a coast station, the station will usually nominate the channel.
3. Press ENTER. The call goes on Ch 70 automatically.
4. The other radio alarms and displays your MMSI. When its operator accepts, an acknowledgement is returned and, on most sets, both radios switch to the working channel.
5. Begin the voice call on the working channel ("SEASPRAY, this is SEA BREEZE, over").

If there is no acknowledgement, wait before retrying; repeated calls clog Ch 70. Group calls work the same way with the group MMSI. Never use the distress button to "test" a link to a friend, and never select Ch 16 as the working channel for a routine call.

Cancelling a False Alert

False alerts waste SAR resources but are not an offence if cancelled promptly and honestly. If you send a distress alert by mistake:

1. Let the alert complete once. If your radio has a cancel function, use it (it sends a DSC cancellation); otherwise switch the DSC off briefly to stop the repeats, then back on and select Ch 16.
2. On Ch 16 transmit:

ALL STATIONS ALL STATIONS ALL STATIONS THIS IS yacht SEA BREEZE SEA BREEZE SEA BREEZE, MMSI 235012345 POSITION ... Distress alert sent in error. CANCEL my distress alert of [time UTC] OUT

1. Confirm that the nearest Coastguard has received the cancellation, respond to any follow-up, and log it.

False EPIRB alert. Do NOT switch the 406 MHz beacon off. Report the false alert to the nearest Coastguard, relaying through another station if necessary, giving your position and the beacon identity, and switch it off only when instructed; a beacon that goes silent before the RCC knows it was a mistake can keep a search going for hours. For a SART activated by mistake, switch it off and inform the Coastguard in case a ship has seen the echo and begun a search.

Supporting GMDSS Equipment

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EPIRB and PLB

An Emergency Position Indicating Radio Beacon transmits on 406 MHz to the Cospas-Sarsat satellite system, which locates it and passes the alert, with the registered owner's details, to the relevant rescue centre, giving worldwide coverage. Modern EPIRBs include GNSS, giving a position within about 100 m in minutes (older beacons rely on Doppler positioning from the satellites, good to about two miles, and take longer), plus a 121.5 MHz homing signal (and often AIS) for rescuers on scene. Category I EPIRBs float free and activate automatically from a hydrostatic release; Category II are manual. A Personal Locator Beacon (PLB) is the same principle for a person, manually activated and held clear of the water, with shorter battery life (at least 24 hours versus 48 for an EPIRB).

All UK-coded 406 MHz beacons must be registered (free) with the UK Beacon Registry at Falmouth; it is a legal requirement and lets the Coastguard call your shore contact and identify the vessel instantly. Check battery and hydrostatic release expiry dates and perform the self-test as the manufacturer directs. Mount the beacon where it is protected from accidental release and inquisitive fingers.

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Image: Reise Reise, CC BY-SA 4.0, via Wikimedia Commons

A typical 406 MHz EPIRB, like the one shown, is a sealed unit with a lanyard, a test button and an arming switch. It is designed to be mounted in a bracket with a hydrostatic release so that it floats free and activates automatically if the vessel sinks; it can also be released and activated by hand. Key practical points:

• The beacon transmits a unique ID coded with your country and registration details.
• A 121.5 MHz homing signal lets rescue craft and aircraft locate the beacon in the final approach.
• Operating life is at least 48 hours; the battery must be replaced by the date on the label (usually every 5 years).
• Test only with the self-test function. Never activate in a "test" or leave an expired beacon aboard as a safe option.
• A PLB is a smaller beacon carried on the person, usually without a float-free bracket; many sailors carry one on the lifejacket as a MOB aid.

SART and AIS-SART

A radar SART (Search and Rescue Transponder) responds to a 9 GHz (X-band, 3 cm) radar and shows as a line of 12 dots on the searching vessel's display, pointing from the SART outwards. An AIS-SART transmits its GPS position as an AIS target, visible on any AIS receiver and chartplotter. Both are used for on-scene location, usually from a liferaft; mount them as high as possible, at least a metre above the sea. A SART in a liferaft is detected by a ship's radar at about 5 miles and by an aircraft at 3,000 feet at up to 30 miles; its battery lasts about 96 hours. A radar reflector near the SART can reduce its effectiveness.

NAVTEX and MSI

NAVTEX is an automated printer or display receiving navigational warnings, weather forecasts, gale warnings and SAR information on 518 kHz (international, in English) and 490 kHz (national; in the UK the inshore waters forecast and outlook). Each station has an identity letter and a time slot so that stations do not interfere; you select stations and message types (A navigational warnings, B gale warnings and D SAR information cannot be deselected). The aerial is mounted low, since height gives no advantage at medium frequency. It reaches about 250 to 400 miles, so it covers coastal passages well, and it is receive-only.

HM Coastguard also broadcasts MSI by voice on VHF: inshore waters forecasts, gale warnings and navigational warnings every three hours, with a new forecast every six hours, announced on Ch 16 and broadcast on a nominated working channel (commonly 62, 63, 64 or 10). New gale warnings are announced on receipt with a Securite prefix. A skipper's passage plan should list the broadcast times and channels, and the almanac gives the different channels used by rescue centres abroad.

Battery and installation

A distress radio is useless if the battery is flat or the antenna cable is corroded. Good practice: radio wired to the domestic or a dedicated battery that is not used for engine starting, a handheld DSC VHF with GPS as backup, an emergency antenna, and a monthly check that the GPS position is reaching the DSC radio (look for a live position, not "no position", on the radio's display). Do not send live test distress alerts.

Coastguard liaison and mobile phones

The Coastguard encourages skippers on longer passages to pass passage information on Ch 67 (who you are and where you are going), but has no system for tracking yachts and will not start a search unless your shore contact reports you overdue; so nominate a shore contact and give them your plan and a latest arrival time. Several Coastguard stations have VHF direction finding and can sometimes give a yacht a bearing, but as a bonus rather than an entitlement. A mobile phone can call 999 or 112 inshore, but it cannot keep a listening watch, nearby vessels cannot hear it, and rescuers cannot direction-find on it; it is a backup only.

Worked Example: Urgency Call for a Tow

At 1530 your 11 m yacht is 5 miles south-east of Start Point with a fouled propeller and a falling wind. The tide is pushing you towards the shore at 1.5 knots; the nearest hazard is 3 miles away so you have about two hours. No one is hurt.

1. Decide the category. The danger is real but not immediate: PAN-PAN, not MAYDAY.
2. DSC. Select urgency, all ships (or individual to Brixham Coastguard's MMSI), and send with ENTER, confirming when asked.
3. Voice. After 15 seconds on Ch 16: PAN-PAN x3, ALL STATIONS x3 (or Brixham Coastguard x3), THIS IS SEA BREEZE x3, MMSI, position as a bearing and distance from Start Point lighthouse (for example 120 degrees from Start Point 5 miles), nature of urgency "fouled propeller, drifting towards the shore at 1.5 knots, require a tow", number of persons, any other information (vessel description, colour and length, sails), OVER.
4. Response. Brixham Coastguard replies and asks for further details; they may broadcast an urgency message to all ships and contact the local lifeboat or a commercial towage operator.
5. Keep updating. Report any change in position, drift, weather or crew condition. If the situation worsens (say the anchor drags and the shore is 1 mile away) upgrade to MAYDAY.
6. Cancel. When help arrives or you clear the fouling, cancel the PAN-PAN on Ch 16: "ALL STATIONS x3, this is Sea Breeze x3, MMSI..., my Pan-Pan of 1530 UTC is cancelled, out."

Exam and Practical Tips

• Rehearse the DSC distress sequence for your own boat's radio until you can do it in the dark. Examiners often ask you to demonstrate it on the actual set (without transmitting) and to explain what each step does.
• Be able to give your position three ways: latitude and longitude, bearing and distance from a charted object, and in plain words ("two miles south of the Needles").
• Know the difference between a MAYDAY, MAYDAY RELAY, PAN-PAN and SECURITE and be able to pick the right one from a short scenario, including which are sent with the red button and which with ENTER.
• Know who acknowledges what: coast stations acknowledge DSC distress; ships acknowledge by voice only if no coast station does within a few minutes, and then they must go and help.
• The SRC written test includes questions on channels, MMSIs, phonetics, controls, forbidden transmissions, EPIRB, PLB, SART, AIS-SART and NAVTEX; state what each does and how you test or maintain it. The practical assessment includes entering an MMSI, a routine DSC call, a test call, manual position entry and the full distress and cancellation drills.
• As skipper, be able to explain the licensing position of the boat you are on: licence, call sign, MMSI, who holds the SRC, and where the procedure card is.

Common Mistakes

The diagram below collects the most frequent errors seen on SRC courses and in Coastguard incident reports.

Click to zoom

• Acknowledging a distress alert from a yacht by DSC. Only coast stations acknowledge by DSC. Listen on Ch 16 instead.
• Thinking the DSC alert is enough. The alert contains no detail about the problem or number of people. Always follow with the voice MAYDAY.
• No GPS position in the alert. If the GPS link has failed, the alert goes with no position or an old one. Check the radio shows a live position before every passage, and enter it by hand if it does not.
• Too short a press. A quick tap does nothing; hold the button until the radio confirms.
• Pressing the red button for an urgency or a relay. Only your own distress uses the red button.
• Giving a position "from" yourself. Positions are given as a bearing and distance from a charted object to you, not from you to the object.
• Using Ch 70 or Ch 16 for routine chat. Ch 70 is digital only; Ch 16 is for distress, urgency, safety and brief calling.
• Calling on 25 W from alongside a marina and never hearing its 1 W reply.
• Relaying unnecessarily. Duplicate relays block the channel.
• Switching off a false EPIRB before the Coastguard has been told.
• Not knowing the boat's MMSI and call sign. Write them, with the vessel name and a Mayday script, on a card next to the radio so any crew member can send a distress call.

Worked Example: Fire at Night off the Isle of Wight

At 0215 your 10 m yacht is 8 miles south of St Catherine's Point. A crew member reports smoke from the engine compartment; within a minute flames are visible and the extinguisher does not control it.

1. Decision: fire not under control at sea is grave and imminent danger. This is a MAYDAY.
2. Skipper allocates tasks: one crew to fight the fire from outside the compartment and shut off fuel, one to prepare the liferaft and grab bag, everyone into lifejackets.
3. Skipper lifts the DSC cover, presses once, selects "Fire/Explosion", holds the red button for 5 seconds. The display confirms the alert, with a live GPS position of 50 degrees 26 minutes N, 001 degree 18 minutes W.
4. About 15 seconds later, with no acknowledgement yet, the skipper transmits the voice MAYDAY on Ch 16 at 25 W, giving position as both latitude and longitude and "180 degrees from St Catherine's Point 8 miles", nature (engine fire, not under control), assistance required, four persons on board, preparing to abandon to liferaft.
5. Solent Coastguard acknowledges by DSC and voice, tasks the lifeboat and a helicopter, imposes SEELONCE MAYDAY on Ch 16, and asks for a description of the vessel and liferaft.
6. Fire spreads; the crew abandons into the liferaft taking the handheld DSC VHF and the EPIRB, which is activated. The AIS-SART is deployed on its pole.
7. The lifeboat homes on the AIS-SART and 121.5 MHz signal and recovers all crew. The Coastguard later broadcasts SEELONCE FEENEE.

The DSC alert got the Coastguard's attention with an accurate position in seconds; the voice MAYDAY gave them what they needed to send the right assets; the EPIRB and SART kept the casualty findable after the main radio was lost.

Summary

• GMDSS automates distress alerting by two independent means; compulsory-fit ships must carry it, yachts are voluntary fit but must use its procedures. Area A1 is within VHF DSC coast station range, which covers all Coastal Skipper operations.
• Three legal requirements: type-approved equipment, a Ship Radio Licence (Ofcom, free, ten-year revalidation, carried aboard) and an operator with the SRC or under supervision. Handhelds used on several boats need a Ship Portable Radio Licence.
• Your MMSI (nine digits; UK MIDs 232 to 235; coast stations 00; groups 0) is your radio identity and must be correctly programmed and licensed.
• 16 for distress, safety and calling; 70 DSC only; 67 UK small craft safety; 6, 8, 72, 77 intership; 13 bridge-to-bridge; 80 marinas; port channels per the almanac. 25 W maximum, 1 W whenever it will do.
• Distress: hold the red button, wait 15 seconds, then voice MAYDAY on Ch 16 using Mayday, Identify, Position, Distress, Assistance, Number, Information, Over.
• Ships do not acknowledge DSC distress alerts; coast stations do. If a MAYDAY goes unanswered by the Coastguard, relay it (urgency alert, then voice Mayday Relay) and acknowledge by voice only if you will help.
• PAN-PAN for urgency (no position in the DSC alert, so give it by voice; medical advice via TMAS); SECURITE for navigation and weather safety, announced on 16 and passed on a working channel.
• Routine DSC calls go on Ch 70 to an individual or group MMSI and nominate a working channel. Wait two minutes before repeating an unanswered voice call.
• Cancel false DSC alerts promptly by voice on Ch 16; never switch off a false EPIRB until told to. Register EPIRBs and PLBs, test equipment correctly, and keep the GPS-to-DSC link working.

Check Your Understanding

1. Define GMDSS Sea Area A1, and say which vessels must comply with GMDSS.

Answer: The area within radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available, typically 30 to 50 miles from the station depending on antenna heights. GMDSS is compulsory for cargo ships of 300 GT and over, passenger ships carrying 13 or more passengers and fishing vessels; yachts are voluntary fit.

1. What are the three legal requirements before a VHF is used on a UK yacht, and what licence does a handheld used on several boats need?

Answer: Type-approved equipment, a Ship Radio Licence from Ofcom (carried on board, revalidated every ten years), and an operator holding the SRC or working under their direct supervision. A handheld used on various craft needs a Ship Portable Radio Licence, with its own call sign ending in T and its own MMSI.

1. What does a DSC distress alert automatically transmit, and how often is it repeated?

Answer: The vessel's MMSI, its position and the time (from the connected GPS or manual entry), and the nature of distress if one was selected. It repeats about every four minutes until acknowledged by a coast station or cancelled.

1. Your radio alarms with a DSC distress alert from another yacht. What should you do?

Answer: Do not acknowledge by DSC. Log the details and plot the position, inform the skipper, listen on Ch 16 for the voice MAYDAY and the Coastguard response. If no coast station acknowledges after about four minutes and the alert repeats, send an urgency alert and a voice Mayday Relay; if that is impossible and you can help, acknowledge by voice and proceed.

1. What is the purpose of Ch 70, why must you never speak on it, and why can it not be scanned?

Answer: It is reserved for DSC digital calls, which the set monitors continuously whatever voice channel is selected. Voice transmissions would block the digital alerts, including distress alerts. Scanning gives no priority and would interrupt the DSC watch.

1. List the information in a voice MAYDAY in order.

Answer: MAYDAY x3; THIS IS vessel name x3, MMSI (or call sign); MAYDAY, vessel name; position; nature of distress; assistance required; number of persons on board; other useful information; OVER.

1. Give two situations that justify a MAYDAY RELAY, and say how a Class D set alerts other stations before it.

Answer: A distress message not acknowledged by a coast station after a reasonable time; or a vessel known to be in distress that cannot send its own message (red flares, a vessel on fire, a person raising and lowering their arms). A Class D set has no relay alert, so a DSC urgency alert is sent first (with ENTER), then the voice Mayday Relay on Ch 16, making clear that you are not the casualty.

1. A crew member has severe abdominal pain but is conscious and stable, 15 miles from port. Which category of call, how is it sent, and who will you be connected to?

Answer: An urgency call: a DSC urgency alert from the menu (not the red button) followed after 15 seconds by a PAN-PAN voice call on Ch 16 giving the position, which the DSC alert does not include. The Coastguard will connect you to a doctor through the Telemedical Advice Service.

1. You press the distress button by accident. What do you do? And if an EPIRB is activated by accident?

Answer: Let the alert complete once, use the cancel function or switch the DSC off briefly to stop repeats, then on Ch 16 broadcast to ALL STATIONS x3, give your name x3, MMSI and position, say "distress alert sent in error, cancel my distress alert of (time) UTC", and confirm the Coastguard has received it. For an EPIRB: do not switch it off; report to the Coastguard with the position and beacon identity and switch off only when instructed.

1. What is the difference between a SART and an EPIRB, and where should a SART be mounted?

Answer: An EPIRB alerts the rescue authorities via 406 MHz satellite with its identity and position. A SART is a locating device used on scene: a radar SART shows as 12 dots on a searching X-band radar, and an AIS-SART appears as an AIS target. Mount it as high as possible, at least a metre above the sea, away from a radar reflector.

1. Which NAVTEX message categories cannot be deselected, and on what frequencies does NAVTEX work?

Answer: A (navigational warnings), B (meteorological warnings such as gale warnings) and D (search and rescue information). 518 kHz for the international English service and 490 kHz for national or local-language services.

1. You call a marina on Channel 80 at 25 W from two miles out and hear no reply. What is the likely cause, and how long must you wait before calling again?

Answer: Marinas transmit on 1 W only, so you may be out of range of their reply even though they heard you; close the distance and use low power. In any case wait at least two minutes before repeating a call, and check your own set first.

1. What is the Coastguard's position on VHF for collision avoidance, and what guidance note covers it?

Answer: MGN 167 warns that using VHF to agree how to pass is dangerous: identification is uncertain, messages are misunderstood, and time on the radio is time not spent applying the Collision Regulations, which must be complied with regardless of any radio conversation.