Long Range Communications at Sea - HF SSB, Iridium & GMDSS

Prerequisites

This lesson is part of the RYA/MCA Yachtmaster Ocean course. Before starting you should:

• Hold, or be working towards, the RYA/MCA Yachtmaster Offshore Certificate of Competence (or MCA OOW Yachts <3000 gt), the prerequisite for the Ocean exam.
• Hold the RYA/Ofcom Short Range Certificate (SRC) or equivalent and be fluent with VHF DSC distress, urgency and safety procedures (Mayday, Pan-Pan, Sécurité), MMSI numbers and the phonetic alphabet. The SRC is also required if you later seek a commercial endorsement.
• Understand GMDSS Sea Areas A1 and A2, NAVTEX and EPIRB basics from the Day Skipper and Coastal/Offshore syllabi.
• Know how to read a synoptic chart and a GRIB file (Offshore meteorology).

To operate an MF/HF radio legally you need the Long Range Certificate (LRC) or a GMDSS GOC, and the vessel's Ofcom Ship Radio Licence must list the equipment. The LRC is not a prerequisite for the Ocean exam itself, but an ocean skipper is expected to understand the systems in this lesson well enough to plan, use and troubleshoot them.

Learning Objectives

By the end of this lesson you will be able to do the following. The items map to the RYA Yachtmaster Ocean shorebased syllabus topics Long-Range Communications (HF/SSB operation, PACTOR email and GRIB reception, satellite systems, EPIRB and PLB) and GMDSS Area A3/A4 Operations (sea areas, DSC procedures, MF/HF operation for A4 waters, NAVTEX and SafetyNET, reserve power supply).

• RYA syllabus item: explain HF propagation: groundwave and skywave, the ionospheric layers, the skip zone, and day and night differences.
• RYA syllabus item: select an HF frequency band for a given time of day and distance, and use MUF, LUF and FOT in outline.
• RYA syllabus item: describe SSB operation and why marine HF uses upper sideband, and describe a typical HF installation and its common faults.
• RYA syllabus item: use HF and satellite systems for email, GRIB files and weather fax, and build a daily communications schedule within a power budget.
• RYA syllabus item: describe the four GMDSS Sea Areas and the equipment a vessel needs in A3 and A4.
• RYA syllabus item: explain how MF/HF DSC, NAVTEX, SafetyNET, EPIRBs and SARTs fit together in the distress and safety system, and carry out DSC distress, relay, acknowledgement and cancellation procedures correctly.
• RYA syllabus item: explain the difference between geostationary (Inmarsat) and low-orbit (Iridium, Cospas-Sarsat) satellite systems and what this means for coverage.
• RYA syllabus item: size reserve battery capacity for radio equipment.
• Plan layered redundancy and a failure plan for an ocean passage, and state the licensing needed to operate the equipment.

Why Long-Range Communications Matter

On a coastal passage VHF is your lifeline: a coastguard station is rarely more than 30 miles away, and a DSC button press reaches someone almost at once. Mid-ocean, VHF is limited to the horizon of the nearest ship, which may be 100 miles away and not listening. Everything you need from the outside world, such as weather, safety warnings, medical advice, routing updates and distress alerting, has to come by HF radio or satellite.

No single system is perfect. HF is free to use once installed and needs no subscription, but depends on the ionosphere and on operator skill. Satellite is simple and reliable but costs money, needs power and can fail with one antenna or one account problem. The ocean skipper's rule is layered redundancy: at least two independent ways to raise the alarm, at least two independent ways to get weather, and a written plan for when to use each.

HF Radio Propagation

Groundwave and skywave

An HF (3–30 MHz) or MF (300 kHz–3 MHz) signal leaves the antenna in two ways. The groundwave follows the curve of the Earth's surface. Over seawater it can reach 100–200 miles on MF (which is why 2182 kHz and 2187.5 kHz serve Sea Area A2), but it fades quickly at higher frequencies. The skywave travels upward, is bent back down by the ionosphere, and lands hundreds or thousands of miles away. It can bounce off the sea and up again (multi-hop) to go further still.

The diagram below shows both paths from transmitter to receiver.

Click to zoom

Between the point where the groundwave dies out and the point where the first skywave comes down there is a skip zone, where neither signal arrives. This is why a station 200 miles away can be impossible to work while one 2,000 miles away is loud and clear. If a coast station is silent, change frequency before you assume the radio is broken.

The ionospheric layers

The ionosphere is a region of the upper atmosphere ionised by solar radiation. It has several layers, each behaving differently:

The diagram below shows the daytime structure.

Click to zoom

Two rules follow from this:

1. The higher the frequency, the more ionisation is needed to bend it back down. Strong daytime ionisation can return 12–22 MHz signals; at night the weaker F2 layer lets them pass into space.
2. The lower the frequency, the more the D layer absorbs it. By day 4–8 MHz signals are soaked up by the D layer. At night the D layer vanishes and these frequencies travel well.

Frequency selection by day and night

The two diagrams below put these rules into practice.

Click to zoom

Click to zoom

A practical rule of thumb used by ocean sailors:

Distance also matters: the further the station, the higher the frequency you usually need, because a high frequency comes down further away. The sunrise and sunset periods along the path ("grey line") often give unusually good long-distance propagation. Solar activity matters too: near solar maximum higher frequencies open up; a solar flare can cause a sudden ionospheric disturbance that blacks out HF on the daylit side of the Earth for minutes to hours. When nothing works, log it, wait and try again. Do not drain the batteries transmitting into a dead band.

Worked example: choosing a frequency

You are at 25°N 40°W, about 1,800 miles from a shore email station in the eastern United States. It is 1400 local time (daylight at both ends). You want to download a GRIB file.

• Long path in full daylight: start high, around 16 MHz.
• If the station is weak, try 18 or 22 MHz; if there is nothing, drop to 12 MHz.
• At 0200 local the same link would need something like 8 MHz, perhaps 6 MHz.

Most HF email software shows a propagation forecast for each station and frequency; use it to choose, then confirm by listening for the station's signal before you transmit.

Predicting What Will Work: MUF, LUF and FOT

Frequency selection by rule of thumb gets you started. Propagation prediction software and published charts refine it using three terms the Ocean syllabus expects you to know.

• MUF (maximum usable frequency): the highest frequency that the ionosphere will return to Earth on a given path at a given time. Above the MUF the signal passes through the layer and into space.
• LUF (lowest usable frequency): the lowest frequency that will give a usable signal, below which D-layer absorption (by day) or noise swamps the signal.
• FOT (frequency of optimum traffic): roughly 85 per cent of the MUF. It is the best bet for a reliable link, because the MUF itself varies from moment to moment.

The usable window lies between the LUF and the MUF, and the best choice is just below the FOT. By day the window is wide and high (for example 9 to 20 MHz on a long path); at night it shifts down and narrows (for example 4 to 9 MHz).

Worked example: a propagation program forecasts a MUF of 21 MHz for your path at 1400 UTC. The FOT is about 0.85 times 21, so roughly 18 MHz. The nearest marine band to 18 MHz is the 16 MHz band, so you would try there first. At 0200 UTC the same program forecasts a MUF of 9 MHz, so the FOT is about 7.6 MHz and you would use the 6 or 8 MHz band, whichever the coast station is listening on.

Two solar indices help to decide whether HF is likely to be good:

• The solar flux index (SFI), measured at 10.7 cm, indicates how strongly the sun is ionising the upper atmosphere. A high SFI (above about 100) means higher MUFs and good conditions on the higher bands. A low SFI (about 70, near solar minimum) means the higher bands may be dead for much of the day.
• The K index (0 to 9) measures geomagnetic disturbance. A K index of 4 or more means a disturbed ionosphere and poor propagation, especially on high-latitude paths. Space weather services publish both indices and flare alerts.

The Solar Cycle

The sun's activity follows an eleven-year cycle. Near the maximum, the higher bands (18 to 28 MHz) are often open for long paths in daylight, and the lower bands give long-range paths at night. Near the minimum, 12 to 16 MHz is often the upper limit, and long daytime links may have to use the 8 to 12 MHz bands. In the tropics, thunderstorm static raises the noise level, particularly at night on the lower bands, and can make them unusable even when propagation is good.

Single Sideband (SSB) Operation

A normal AM signal sends a strong carrier plus two identical copies of the speech (the lower and upper sidebands). The carrier carries no information and uses most of the transmitter's power. Single sideband removes the carrier and one sideband, putting all the power into the one sideband that carries the voice.

The diagram below compares AM and SSB.

Click to zoom

The results are half the bandwidth, much better range for the same power, and less interference. Marine MF/HF radiotelephony uses upper sideband (USB) on all maritime channels, including 2182 kHz and the HF distress frequencies. (Amateur radio uses LSB below 10 MHz, so set the mode carefully if you also use ham bands.) Because there is no carrier, the receiver has to insert one. If voices sound like "Donald Duck", adjust the clarifier (fine tune) slightly.

The installation

A typical yacht HF installation has:

• A 100–150 W transceiver (e.g. ICOM M803) with MF/HF DSC built in.
• An automatic antenna tuning unit (ATU) mounted close to the antenna feed point, because no single antenna is resonant across 2–22 MHz.
• An antenna: usually an insulated backstay or a dedicated whip of 7 m or more.
• A ground plane (counterpoise): copper foil to a keel bolt or sintered bronze ground plate, or a well-bonded hull area. A poor ground is the commonest cause of poor HF performance.
• A good DC supply: heavy cable direct to the battery, because transmit current reaches 20–30 A at full power on 12 V.

Watch the SWR (standing wave ratio) reading. A rising SWR means a problem in the antenna, insulator, tuner or ground; salt-crusted backstay insulators and corroded tuner connections are common faults on passage.

Common HF Installation Faults and Troubleshooting

Most HF failures on passage come from the installation, not from the radio. Work through the chain from the radio outwards.

Other precautions:

• Lightning: do not transmit during an electrical storm, and disconnect the antenna from the radio and tuner if lightning is nearby. A good ground and bonding of the mast and rigging help, but no installation is fully protected.
• Safety near the antenna: HF antennas carry high voltages and radio-frequency energy. Do not touch the backstay or the antenna during transmission, and keep people away from a whip antenna when transmitting at full power. An RF burn from an energised backstay is painful.
• Spares: carry a spare fuse, a spare antenna lead and coax connector, a spare modem cable, a small tube of contact grease for the connectors and a multimeter.

Voice procedure and nets

Ocean HF voice follows the same procedure as VHF: name the station called (up to three times), "this is", your name and call sign, the message, then "over". Keep a radio log of all contacts, frequencies and times. Many ocean crossings are supported by cruiser nets, informal scheduled gatherings on a known frequency where boats check in with position and conditions. They provide weather information, help with problems, and an informal safety check: if you miss two check-ins, someone will ask questions. A net is not a substitute for GMDSS. In a real emergency use DSC, an EPIRB and Mayday procedure.

Data Over HF: Email, GRIB and Weather Fax

HF email

Pairing the HF transceiver with a PACTOR modem (or using software modes such as VARA) allows email over HF via networks such as SailMail (subscription) or Winlink (free for licensed amateurs). Throughput is low, typically 3–5 kbps at best with PACTOR III/IV and often much less, so:

• Text only; strip attachments and signatures.
• Request GRIB files over a small area with only the parameters you need (wind and pressure, every 12 hours, 1° resolution) rather than a large high-resolution file.
• Compose offline, queue messages and send everything in one short connection.

Satellite email

The same email clients can work through a satellite terminal. The diagram below shows the chain from laptop to satellite to shore server.

Click to zoom

Iridium satellites orbit at about 780 km, so the terminal needs relatively little power and a small, non-directional antenna. Iridium GO! and similar devices give slow but usable data; Iridium Certus gives much faster broadband. Inmarsat FleetBroadband and the newer Fleet One use geostationary satellites with stabilised domes. Low-orbit broadband (e.g. Starlink maritime) now gives very high data rates and is excellent for routing and crew welfare, but it is not part of GMDSS and relies on a single commercial system, so it supplements rather than replaces recognised safety equipment.

Radio weather fax

HF weather fax (radiofacsimile) transmits analysis and forecast charts on published frequencies and times (listed in the Admiralty List of Radio Signals, ALRS Vol 3, and in NOAA's worldwide schedule). It is free and needs no transmitter: only a receiver in USB mode and decoding software. The diagram below shows the reception chain.

Click to zoom

Some PACTOR modems can decode fax directly; otherwise connect the receiver's audio output to a laptop sound card running fax software (or even hold a phone running a fax app next to the speaker). Tune 1.9 kHz below the published (carrier) frequency when using USB, and set the correct drum speed and IOC (normally 120 lpm, IOC 576). Useful products include surface analysis, 24/48/96-hour surface forecasts, wave height forecasts and 500 hPa upper air charts. Because fax is broadcast, it keeps working if your transmitter or satellite account fails, which makes it a valuable backup.

Licensing and Certificates

Owning the equipment is not enough. To use it legally you need:

• Ship Radio Licence (Ofcom) for the vessel, listing every transmitting device (VHF, HF, EPIRB, PLB, AIS and satellite devices where required) with its call sign and MMSI numbers. Keep the licence aboard.
• Operator's certificate: the RYA/Ofcom Short Range Certificate (SRC) covers VHF and VHF DSC. The Long Range Certificate (LRC), or a GMDSS General Operator's Certificate, covers MF/HF DSC and radiotelephony and GMDSS-type satellite distress equipment. A yacht with an HF set needs an LRC holder to operate it.
• MMSI numbers: a nine-digit MMSI identifies the ship for DSC and is programmed into the VHF DSC and MF/HF DSC radios and AIS. The MMSI in the EPIRB registration, the radios and the licence must all agree. Handheld DSC radios and PLBs have their own identities and should also be registered.
• Foreign waters: many countries accept a UK licence and certificate, but some require a local licence for a prolonged stay, and some restrict HF use in harbour.
• Amateur radio is licensed separately. Winlink needs an amateur licence and amateur frequencies; SailMail and PACTOR on marine frequencies operate under the ship licence.

Planning a Communications Schedule

A communications plan is part of the passage plan. It answers: what information do I need, when is it available, which system will I use, how much power will it cost, and who ashore expects to hear from me?

The diagram below shows a typical daily schedule.

Click to zoom

The key planning rules:

1. Use UTC for everything. Nets, fax schedules and GRIB model runs are all published in UTC.
2. Time GRIB downloads to the model runs. GFS runs at 00, 06, 12 and 18 UTC and output is available about 4–5 hours later. Downloading at 0530 and 1730 UTC gets fresh data twice a day.
3. Match frequency to time of day at both ends of the path.
4. Batch email into a few short sessions.
5. Fix a daily position report time with a shore contact, with a clear agreed action if a report is missed (for example: if no contact for 48 hours, call the MRCC with the passage plan).
6. Log every contact.

Power budget

HF transmit draws 10–15 A on average at 12 V (peaks of 25 A or more). A 20-minute email session at an average 12 A uses about 4 Ah; two sessions a day plus a 15-minute net check-in add up to roughly 10 Ah per day. Satellite terminals use less on transmit but may be left on for long periods. Charge before the schedule, monitor the battery's amp-hours daily, and never let communications pull the house bank below the level needed for navigation lights and the autopilot overnight.

Choosing Satellite Systems: A Comparison

A satellite phone can also call a rescue coordination centre directly. An Iridium GMDSS terminal has a short code, 505, that connects to the nearest RCC. For a non-GMDSS satellite phone, the coastguard number must be known in advance and written beside the phone.

Medical and Weather Advice by Satellite

Satellite voice calls give access to telemedical advice from a doctor ashore. In the UK, call HM Coastguard, who will put you through to a Telemedical Maritime Assistance Service (TMAS). Before you call, write down the casualty's age, pulse, temperature, breathing, symptoms and history, and have the medical kit contents to hand.

For weather, a professional weather router can send email routing advice, and many yachts also use GFS and ECMWF GRIB data through services that compress it. A plain-text email service such as Saildocs accepts a short request message (for example, a GFS GRIB for a defined area at 2 degree resolution, 12-hourly, with wind and pressure only) and returns the file. Always keep the request small.

GMDSS Beyond Coastal Waters

The four sea areas

The Global Maritime Distress and Safety System divides the world's oceans by the type of shore station that can hear a distress alert.

The map below gives a simplified picture.

Click to zoom

Most ocean passages are in A3. Historically A3 meant "within Inmarsat geostationary coverage", roughly 76°N to 76°S. Since Iridium was recognised for GMDSS in 2020, a vessel with an Iridium GMDSS terminal effectively has A3-type alerting everywhere, including polar waters. For a vessel relying on Inmarsat, the polar seas remain A4, where only HF works.

SOLAS GMDSS carriage rules apply to ships of 300 gt and over on international voyages, not to pleasure yachts. Commercially coded yachts (MCA Category 0 and 1) have carriage requirements set by their code, and a well-found private ocean yacht should mirror the A3 standard: VHF DSC, a long-range alerting system (MF/HF DSC or a satellite system), a means of receiving MSI beyond NAVTEX range, a float-free 406 MHz EPIRB, a SART or AIS-SART, and a reserve power supply.

Geostationary versus polar-orbiting satellites

Inmarsat satellites sit in geostationary orbit 35,786 km above the equator, turning with the Earth so each one appears fixed in the sky. Three or four of them cover nearly all the world's oceans. Because they are over the equator, a vessel at high latitude sees them very low on the horizon; beyond about 76° they are below it.

Click to zoom

Cospas-Sarsat, the international satellite system for 406 MHz distress beacons, uses low-Earth-orbit polar satellites (LEOSAR) that pass over every part of the globe, including the poles, plus geostationary (GEOSAR) and, more recently, medium-orbit (MEOSAR) satellites carried on the GPS, Galileo and GLONASS constellations. MEOSAR gives near-instant detection and independent position calculation almost anywhere.

Click to zoom

The practical lessons: an EPIRB works anywhere on Earth; Inmarsat voice and SafetyNET do not work in the high polar regions; Iridium does.

MF/HF DSC

MF/HF DSC works like VHF DSC: a 9-digit MMSI identifies the vessel, and a red DISTRESS button sends an automated alert containing MMSI, position, time and (if selected) nature of distress. The difference is the frequencies. On HF the radio can scan all six DSC distress frequencies, and you choose the band most likely to reach a coast station given time of day and distance.

Click to zoom

8414.5 kHz is the band to try first, because it gives reasonable range day and night. Many radios can send a multi-frequency alert automatically. After the alert, set the radio to the matching voice frequency and send the Mayday by voice.

Receiving a distress alert. A yacht should not acknowledge an HF DSC distress alert by DSC: that is the coast station's job. Listen on the associated voice frequency. Do not relay a distant HF alert by DSC unless instructed by an RCC or it is clear no coast station has heard it; instead contact the nearest RCC by any means and pass the details. Note the MMSI, position and time in the radio log.

False alerts. If you send a DSC alert by mistake, cancel it immediately on the radio (most sets have a cancel function) and then make a voice broadcast on the associated voice frequency: "All stations, this is [name, call sign, MMSI], cancel my distress alert of [date, time UTC]."

Distress, Urgency and Safety Procedures at Long Range

The principles are the same as on VHF, with different frequencies and satellite equivalents.

Distress (Mayday): grave and imminent danger to a vessel or person that needs immediate assistance.

1. Send the alert: press the DISTRESS button on the MF/HF DSC radio for five seconds (select the nature of distress if there is time), or trigger the satellite distress button, and activate the EPIRB.
2. Wait for the DSC acknowledgment from a coast station. If none comes, repeat the alert on a different band. Many radios cycle through several bands automatically.
3. After the acknowledgment, tune to the associated voice frequency (for example 8291 kHz, USB) and send the Mayday message: Mayday three times, this is (vessel name) three times, call sign and MMSI, Mayday (name), position, nature of distress, assistance required, number of persons on board, other information. Over.
4. Keep the radio on and listen, and use the satellite phone as a second route to the RCC.
5. Prepare the liferaft, grab bag, SART and PLBs in case the situation worsens.

Urgency (Pan-Pan): a very urgent message about the safety of a vessel or person, but not grave and imminent. Send a DSC urgency call, then the Pan-Pan message by voice on the associated working frequency.

Safety (Securite): a navigational or meteorological warning. Send a DSC safety call, then the message by voice.

Receiving a distress alert on a yacht. A VHF or MF DSC alert normally comes from a vessel within a few tens of miles. If you are close enough to help:

• Listen and write down the MMSI, position and time.
• Do not acknowledge by DSC. Acknowledge by voice only, and only if no coast station has acknowledged within about five minutes.
• Tell the RCC what you have heard, and offer help if you can reach the casualty.

On HF, a yacht should never acknowledge a DSC distress alert by DSC. Only a coast station does that. Listen on the voice frequency, note the details and tell the nearest RCC by whichever route works (HF voice, satellite phone, email). If it is clear that nobody ashore has heard the alert, a distress relay to the RCC by voice (a Mayday Relay) may be needed, telling the RCC exactly what you heard. Follow the RCC's instructions.

Cancelling a false alert. Use the radio's cancel function if it has one, then make a voice broadcast on the associated frequency stating your name, call sign and MMSI and that you cancel the alert, with the date and time in UTC. Then call the RCC to confirm that no help is needed. A real Mayday that has been resolved is cancelled in the same way, but only with the RCC's agreement.

Tests. Never test the distress button by sending a real alert. Use the radio's self-test or a DSC test call, and log all tests.

Receiving Maritime Safety Information

NAVTEX is the coastal MSI service: automatic printed or displayed broadcasts on 518 kHz (international, in English) and 490 kHz (national language). Range is about 400 miles from each station, so it fails once you are well offshore. In tropical areas, where HF noise is heavy, some stations also broadcast tropical NAVTEX on 4209.5 kHz.

Click to zoom

Click to zoom

Each NAVTEX message carries a station identity letter (B1) and a subject letter (B2). Categories A (navigational warnings), B (meteorological warnings), D (SAR information and piracy alerts) and L (additional navigational warnings) cannot be switched off. Set the receiver to the stations covering your route.

Beyond NAVTEX range, MSI comes by SafetyNET, broadcast via Inmarsat Enhanced Group Call (EGC), or by the equivalent Iridium SafetyCast service, and by HF NBDP (narrow-band direct printing) from a few coast stations. SafetyNET broadcasts for each NAVAREA/METAREA, so your EGC receiver must be set to the right ocean region and area.

Click to zoom

For a yacht, an Inmarsat-C terminal (or an Iridium GMDSS terminal) is the standard way to receive SafetyNET. Some satellite phones and trackers do not receive EGC at all; check before you rely on them.

EPIRB, PLB and SART

A 406 MHz EPIRB sends a digital distress message, including its unique hex ID and (in GPS models) its position, to Cospas-Sarsat. The alert goes to the RCC responsible for the beacon's registration country and is passed to the RCC for the area. It also sends a 121.5 MHz homing signal, and many new beacons include AIS for final location.

Click to zoom

• A Category I float-free EPIRB in a hydrostatic release bracket deploys and activates if the yacht sinks, even if no one can reach it. This is the right choice for ocean passages.
• A Category II (manual) EPIRB must be switched on by a person, usually in the grab bag.
• Registration is mandatory (in the UK, free with the UK Beacon Registry). Update it with your passage plan and shore contacts before departure.
• Test monthly using the self-test (never a live activation). Check battery and hydrostatic release expiry dates.
• A PLB is personal, manually activated and lasts at least 24 hours; it does not replace the vessel's EPIRB.

A SART (search and rescue radar transponder) responds to a 9 GHz (X-band, 3 cm) radar. When interrogated, it transmits a sweep that appears on the searching vessel's radar as a line of 12 dots extending outward from the SART's position. As the searcher closes within about 1 mile the dots become arcs and finally concentric circles. An AIS-SART does the same job on AIS displays and chartplotters.

Click to zoom

Hold a SART as high as possible: at 1 m height its range to a ship's radar is about 5 miles, and much more for an aircraft.

Reserve power

GMDSS rules for ships require a reserve source of energy able to power the radio installation independently of main power. The diagram below summarises the SOLAS standard.

Click to zoom

A yacht has no emergency generator, so the six-hour standard applies. Worked example: VHF DSC (receive 0.5 A, transmit 6 A), MF/HF DSC (receive 2 A, average transmit 15 A) and a satellite terminal (1 A). Assume half the period on transmit for the two transmitters for the distress phase. Load is about 0.5 + 3 + 2 + 7.5 + 1 = 14 A for 6 hours = 84 Ah. Lead-acid batteries should not be discharged below 50%, so you need at least a 170 Ah dedicated reserve (or about 105 Ah of lithium at 80% usable). In practice many yachts use a separate radio battery with a charge isolator, a changeover switch that does not interrupt the supply, monthly charger checks and an annual capacity test, replacing the battery when it falls below 80% of rated capacity. A handheld VHF and the EPIRB's own battery are your last-ditch backup.

Click to zoom

Worked Example: Communications Plan for an Atlantic Crossing

Passage: Mindelo (Cape Verde) to Barbados, 2,100 miles, est. 16 days, crew of four. Equipment: VHF DSC with AIS, ICOM HF SSB with DSC and PACTOR modem (SailMail), Iridium GO!, float-free 406 MHz GPS EPIRB, 2 PLBs, AIS-SART, NAVTEX, handheld VHF in grab bag.

1. GMDSS area: after day one the route is entirely in A3. Distress alerting: EPIRB (primary), HF DSC on 8414.5 kHz, Iridium voice call to MRCC Falmouth or the RCC for the area.
2. MSI: NAVTEX until about 300 miles out, then no EGC receiver aboard. Mitigation: SafetyNET text for METAREA II and IV requested by email, plus NOAA high-seas forecast via HF fax from Boston/New Orleans at night.
3. Weather: GRIB (GFS wind and pressure, 12-hourly steps, 5 days) twice a day at 0530 and 1730 UTC via SailMail; Iridium GO! as backup.
4. Shore contact: position and status email at 1200 UTC daily. Shore contact holds the passage plan and EPIRB hex ID; if no report for 48 hours, the shore contact calls MRCC Falmouth.
5. Net: check in to a cruiser net at 1300 UTC on 8 MHz daily.
6. Power: roughly 12 Ah/day for comms within a 300 Ah/day budget, supported by solar and hydro-generator.
7. Failure plan: HF failure means Iridium for GRIB and email; Iridium failure means HF; both failed means fax reception, barometer and sky, and keep heading west in the trades.

A Medical Emergency at Mid-Ocean: Sequence of Events

You are 900 miles from the nearest land in the North Atlantic, on a yacht with an HF set, an Iridium GMDSS handset and a float-free EPIRB. A crew member suffers a serious head injury in a fall and is unconscious.

1. Assess: this is very urgent but there is not grave and imminent danger to the vessel. The skipper decides on an urgency call and medical advice, not a Mayday.
2. Call by satellite: the skipper dials the RCC (for example through the Iridium GMDSS short code, 505), gives the yacht name, MMSI and position, and states that the casualty needs medical advice. The RCC connects the yacht to a TMAS doctor.
3. Pass the information: the crew give the doctor the casualty's age, pulse, breathing rate, pupils, level of response and what happened, from notes written while the skipper made the call.
4. Follow the advice: the doctor recommends keeping the casualty lying with the head slightly raised, observations every 15 minutes, and a plan to divert. The RCC starts looking for a vessel that could help. If the casualty deteriorates, the situation may become a Mayday, with the EPIRB activated.
5. Back-up: if Iridium fails, the skipper sends a DSC urgency call on 8414.5 kHz and works through the voice frequency 8291 kHz to the coast station, using the RCC details recorded in the passage plan.
6. Log: every call, time (UTC), position and instruction goes in the log.

Common Mistakes

• Relying on one system. A single satellite device is one antenna, one battery and one account away from silence.
• Wrong frequency for the time of day. Calling on 4 MHz at noon or 22 MHz at midnight and concluding the radio has failed.
• Poor HF ground and corroded tuners. Most "weak radio" problems are installation problems.
• Huge GRIB requests over HF. Ask for the minimum area, resolution and parameters.
• No MSI beyond NAVTEX. Many yachts lose all official warnings once 400 miles offshore without realising it.
• Unregistered or out-of-date EPIRB registration. The RCC needs to know who you are, where you are going and whom to call.
• Acknowledging HF DSC distress alerts by DSC. Leave this to coast stations; listen and relay by voice to an RCC.
• No agreed action for missed reports. A shore contact who does not know what to do is no use.
• Transmitting until the batteries are flat. Communications must sit inside the power budget.
• Confusing MUF and FOT. Pick a frequency near the FOT, not at the MUF.
• Treating a satellite phone as GMDSS equipment. A handheld phone does not receive SafetyNET and is not recognised for GMDSS alerting.
• Operating HF without an LRC holder or a ship radio licence that lists the set.
• MMSI mismatch between the radio, the EPIRB registration and the licence, so the RCC cannot identify the boat.
• Testing a DSC distress button live. Use the test function only.

Summary

• HF reaches beyond the horizon by skywave. The ionosphere changes between day and night: use higher frequencies (12–22 MHz) by day and lower (4–8 MHz) by night, and higher frequencies for longer distances.
• Marine MF/HF uses single sideband, upper sideband, which concentrates power in the useful signal.
• HF and satellite both carry email and GRIB files; weather fax is a free, receive-only backup. Plan a daily communications schedule in UTC and keep it within the power budget.
• GMDSS Sea Areas: A1 VHF, A2 MF, A3 satellite (Inmarsat 76°N–76°S, or Iridium globally), A4 polar HF only. Most ocean passages are in A3.
• Distress layers: float-free 406 MHz EPIRB (Cospas-Sarsat, worldwide), MF/HF DSC (8414.5 kHz first), satellite voice, then SART/AIS-SART for final location.
• MSI: NAVTEX near the coast (about 400 miles), SafetyNET/SafetyCast or HF NBDP and fax offshore.
• Reserve power for radio: at least six hours without an emergency generator.
• Use the FOT (about 85 per cent of the MUF) for a reliable link; the solar flux index and K index tell you whether HF is likely to be good.
• A yacht never acknowledges an HF DSC distress alert by DSC: it listens, notes details and tells an RCC. Cancel false alerts promptly.
• To operate HF legally you need a ship radio licence that lists the set, and an LRC holder.

Check Your Understanding

1. Why can a yacht 200 miles from an HF coast station sometimes be unable to contact it while a yacht 2,000 miles away can?

Answer: The first yacht is in the skip zone, beyond the groundwave's range but closer than the point where the skywave returns to Earth. Changing frequency moves the skip distance and may establish contact.

1. It is 0300 local time and you want to work a station 1,200 miles away. Which band do you try first, and why?

Answer: Around 6–8 MHz. At night the D layer has gone, so lower frequencies are not absorbed, and the weaker F2 layer will not reliably return 12–22 MHz signals, which pass into space.

1. What is the main advantage of SSB over AM, and which sideband does marine HF use?

Answer: SSB suppresses the carrier and one sideband, so all transmitter power goes into the signal carrying the voice, giving greater range for the same power and half the bandwidth. Marine MF/HF uses upper sideband.

1. Define GMDSS Sea Area A3 and explain why the definition of A4 depends on the satellite system fitted.

Answer: A3 is the area outside A1 and A2 within coverage of a recognised mobile satellite service. Inmarsat geostationary satellites only cover about 76°N–76°S, so for an Inmarsat vessel the polar seas are A4 (HF only). Iridium's low-orbit constellation covers the poles, so an Iridium GMDSS vessel has satellite alerting there too.

1. Which HF DSC distress frequency would you try first in most conditions, and what do you do immediately after sending the alert?

Answer: 8414.5 kHz. After the alert, go to the associated voice frequency (8291 kHz USB) and transmit a Mayday call and message.

1. You are 900 miles from land. Your NAVTEX has been silent for days. Why, and how should you get MSI?

Answer: NAVTEX range is only about 400 miles. Offshore, MSI comes via SafetyNET (Inmarsat EGC) or Iridium SafetyCast for the correct NAVAREA/METAREA, via HF NBDP, or via HF radio fax and voice broadcasts.

1. What does a SART look like on a ship's radar, and what happens as the ship gets close?

Answer: A line of 12 dots extending outwards from the SART's position along its bearing. Within about a mile the dots become arcs, and very close they become concentric circles.

1. Your radio load in a distress situation is 12 A. What is the minimum lead-acid reserve battery capacity to meet the six-hour standard?

Answer: 12 A × 6 h = 72 Ah usable. At 50% maximum discharge for lead-acid, the battery needs at least 144 Ah rated capacity (in practice about 150 Ah or more).

1. Why should you time GRIB downloads around 0530 and 1730 UTC rather than at random?

Answer: Global models such as GFS run at 00, 06, 12 and 18 UTC and output is available a few hours later. Downloading just after new data is available gives the freshest forecast for the transmission time and power spent.

1. A propagation program shows a MUF of 20 MHz on your path at midday. Which band would you try, and why?

Answer: The frequency of optimum traffic is about 85 per cent of the MUF, so about 17 MHz, in the 16 MHz band. Using the MUF itself is unreliable because it fluctuates, and anything above it passes through the ionosphere.

1. You hear an HF DSC distress alert from a vessel you cannot see, and no coast station acknowledges it. What do you do?

Answer: Do not acknowledge by DSC. Write down the MMSI, position and time, listen on the associated voice frequency, and contact the nearest RCC by any route that works (HF voice, satellite phone) to report what you heard. Relay the alert by voice if the RCC asks, or if it is clear nobody ashore has heard it.

1. What licence and certificate are needed to use an HF set legally on a UK-registered yacht?

Answer: A Ship Radio Licence (Ofcom) that lists the HF installation with its call sign and MMSI, and an operator holding the Long Range Certificate (LRC) or a GMDSS General Operator's Certificate. The SRC alone covers VHF only.