RYA SRC Part 7 — EPIRBs, SARTs, and NAVTEX
Syllabus coverage: CEPT/RYA sections A4 (NAVTEX) and C3 (alerting, communication, and locating signals — EPIRBs, SARTs, handheld VHF in locating use).
Three subsystems sit alongside the VHF DSC radio in the GMDSS toolkit for a Sea Area A1 vessel. None of them is operated by the SRC holder day-to-day — they’re either fit-and-forget (EPIRB, SART) or receive-only (NAVTEX) — but the exam expects you to know how each works, when each fires, and what they tell you (or the world about you).
NAVTEX — the shore-to-ship MSI broadcast
NAVTEX is the Maritime Safety Information system for Sea Area A1. It exists because GMDSS needs a way to push navigation warnings, weather forecasts, ice reports, and pirate-attack warnings to ships without a dedicated radio operator on watch. NAVTEX is receive-only, automatic, prints/displays on demand, and is part of GMDSS.
Frequencies and range
NAVTEX broadcasts use three frequencies:
- 518 kHz — the international service, English-language. The main one.
- 490 kHz — national-language service (in some countries; the UK uses 490 kHz for additional English broadcasts).
- 4209.5 kHz — tropical service, used in some equatorial regions.
NAVTEX uses MF (medium frequency), not VHF, and propagates via ground wave to roughly 250 to 400 NM from each transmitter. The international transmitter network is structured so a vessel anywhere along a coast is within range of at least one transmitter for each scheduled broadcast.
Message format
Every NAVTEX message begins with a 4-character header:
ZCZC B1B2B3B4- ZCZC is the start-of-message marker (the same marker used in older RTTY systems).
- B1 is a single letter identifying the transmitter (each station in the international network has a unique letter, e.g. “G” might be a specific Coastguard transmitter).
- B2 is a single letter identifying the message type (the syllabus table below).
- B3B4 are two digits — the message serial number, 00 to 99, incrementing with each new message of that type from that transmitter.
End of message is marked by NNNN.
Message types (B2 character) — and which cannot be rejected
The syllabus expects you to know the message type codes, particularly which ones cannot be filtered out at the receiver. A receiver-side filter lets the operator suppress message types they don’t want (e.g. “no Loran-C messages”) to save paper and attention — but some categories are mandatory and cannot be filtered out, no matter what the user sets.
| B2 | Message type | Rejectable? |
|---|---|---|
| A | Navigation warnings | NO |
| B | Meteorological warnings | NO |
| C | Ice reports | Yes |
| D | SAR information / piracy and armed robbery warnings | NO |
| E | Weather forecasts | Yes |
| F | Pilot service | Yes |
| G | AIS service messages | Yes |
| H | Loran-C messages (largely obsolete) | Yes |
| I | (not allocated) | — |
| J | SATNAV messages | Yes |
| K | Other electronic navaid messages | Yes |
| L | Other navigation warnings | Yes |
| M–V | Reserved / assignable | Varies |
| W–Y | National | Yes |
| Z | QRU — no message on hand | NO |
And independently of the type letter: any message with serial number 00 cannot be rejected — that’s reserved for urgent messages that must always be displayed.
Memorise: A, B, D, Z, and serial 00 cannot be rejected. (Navigation warnings, met warnings, SAR/piracy, “nothing to send” confirmations, and any zero-numbered urgent message.) The exam asks this directly.
Receiver behaviour
A NAVTEX receiver listens continuously on 518 kHz (and optionally 490 kHz). Scheduled broadcasts are slotted by transmitter (each transmitter has time slots so they don’t interfere). The receiver:
- Decodes incoming messages.
- Stores them in memory.
- Displays or prints based on user-set filters.
- Suppresses duplicates of the same B1/B2/B3B4 combination, so a message repeated for late-tuning receivers doesn’t get printed twice.
The user can configure which transmitters to receive (only the local ones, to suppress irrelevant traffic from distant stations) and which message types to display. They cannot suppress A/B/D/Z/00.
EPIRBs — 406 MHz Cospas-Sarsat
An EPIRB — Emergency Position-Indicating Radio Beacon — is a satellite distress beacon registered to a vessel. It exists to raise an alert when the vessel’s own VHF DSC may not have reached anyone (out of range, sunk, abandoned, on fire, capsized). The current generation operates on 406 MHz and is part of the Cospas-Sarsat international satellite system.
How the satellite system works
Cospas-Sarsat is a constellation of search-and-rescue satellite payloads carried on three types of platform:
- LEOSAR — Low Earth Orbit Search and Rescue. Polar-orbiting satellites at ~1000 km altitude. Each pass takes ~10–15 minutes to cover a region. Doppler-derived position fix. Traditional first segment.
- GEOSAR — Geostationary Search and Rescue. Equatorial satellites at ~36,000 km altitude. Instantaneous coverage between roughly 70°N and 70°S, but no Doppler — positions must come from the beacon itself (via embedded GNSS).
- MEOSAR — Medium Earth Orbit Search and Rescue. SAR payloads on GPS, Galileo, and Glonass satellites. Multiple satellites in view simultaneously, giving fast acquisition and trilateration-based position.
Together these give near-global coverage with detection times of minutes for a modern GNSS-equipped EPIRB.
What the beacon transmits
A 406 MHz EPIRB transmits a short burst every 50 seconds containing:
- The beacon’s HEX ID — a 15-character hexadecimal identifier (representing the 22 bits of unique beacon ID plus country code, beacon type, and a few flags) that ties the beacon to the registration record.
- The country of registration.
- Optionally, position from a built-in GNSS receiver (modern EPIRBs include one).
- Optionally, an AIS distress message (the newest EPIRBs add an AIS-SART layer for nearby SAR vessels).
- A 121.5 MHz homing beacon transmits continuously alongside the 406 burst, for direction-finding by close-in SAR aircraft.
Activation
Two ways to activate a fixed EPIRB:
- Manual — switch on the unit.
- Float-free / automatic — a hydrostatic release mechanism in the bracket releases the EPIRB at a depth of around 1–4 metres. The EPIRB floats free, water contact across the activation pins triggers it, and it transmits while floating.
A handheld PLB (Personal Locator Beacon) is manual only — no float-free, no hydrostatic release.
Registration
A 406 MHz beacon must be registered by law. In the UK, registration is via the MCA’s EPIRB Registry, free of charge, online. The registration links the beacon’s HEX ID to:
- Owner contact details.
- Vessel name, type, length, callsign, MMSI.
- Emergency contacts (next-of-kin, regular ports, sailing club).
- Vessel description for SAR identification.
When the beacon transmits, the RCC retrieves the registration in seconds and knows immediately what kind of vessel is in distress, how many people are typically on board, who to call ashore for context, and what the vessel looks like. An unregistered EPIRB is much less useful — SAR has to start from scratch. Update the registration when:
- Vessel ownership changes.
- Vessel is sold or scrapped.
- Emergency contacts change.
Inadvertent activation
If you activate the EPIRB by accident — knocking it loose, capsizing in a dinghy and triggering the water-contact pins, opening the bracket the wrong way — the satellite payload picks it up within minutes and the RCC starts dispatching resources.
What to do:
- Turn the EPIRB off.
- Call the MRCC immediately by VHF (DSC individual call to the Coastguard MMSI, or voice on CH16) or by phone, and report the false activation. Identify the EPIRB by its registration details.
- The RCC stands down the response.
Do NOT just turn the EPIRB off and hope no-one noticed. The alert has already gone out and there are people on the way.
Battery and service
EPIRBs use a long-life lithium primary battery (non-rechargeable) with a printed expiry date — typically 5 to 10 years from manufacture. The battery must be replaced by the manufacturer or an authorised service agent. Self-test (most units have a button) confirms the battery and electronics without satellite transmission and should be done monthly per manufacturer guidance.
EPIRB vs PLB — the exam comparison
| Feature | EPIRB | PLB |
|---|---|---|
| Registered to | A vessel | A person |
| Float-free | Yes (optional bracket) | No |
| Battery life when active | ≥48 hours | ≥24 hours |
| Flashing light | Yes | Usually no |
| Manual activation | Yes | Yes |
| Cost | Higher | Lower |
| GNSS | Increasingly common | Increasingly common |
A vessel typically carries one EPIRB on the deck/cockpit; individual crew may carry PLBs in their lifejackets for the case where they end up in the water separated from the boat.
SARTs — locating signals for the rescue stage
A SART — Search and Rescue Transponder/Transmitter — is a locating signal used IN distress, once SAR is already on the way, to help the searching units find the casualty. Two distinct types now exist, with very different operating principles.
Radar-SART
The classic SART. It is a transponder, not an active transmitter:
- Listens on the 9 GHz (X-band) radar frequency continuously.
- When struck by a marine radar pulse from a searching vessel, responds with a sequence of 12 echoes spaced at fixed intervals along the same radial as the SART.
- On the searching vessel’s radar screen, this appears as 12 dots in a line radiating outward from the SART’s position — unmistakable.
- As the searching vessel closes to within about 1 NM, the dots expand into 12 short arcs and finally into 12 wider arcs as you close further. This change is the close-range homing signal.
- Operation height: get it as high as possible. In a liferaft, the SART is typically mounted on a short pole holding it 1–2 m above the deck — that’s a few NM of radar range; mast-mounted on a yacht, much further.
- Range: ~5 NM in a liferaft, more from a mast.
AIS-SART
The newer SART. Active transmitter, AIS-based:
- Transmits its position on the AIS channels (AIS1 and AIS2) every minute for at least 96 hours.
- Identifies itself with an MMSI prefixed 970 (covered in Part 4).
- Visible on any vessel’s AIS receiver — chartplotter, dedicated AIS display, or AIS-enabled VHF.
- Often includes a built-in GNSS for accurate position.
- Battery: ≥96 hours active.
AIS-SARTs are now the more common type on small craft, because every vessel with a chartplotter already has an AIS display. Radar-SARTs remain the standard on SOLAS vessels.
The exam asks you to compare the two:
- Radar-SART = passive transponder, responds to X-band radar.
- AIS-SART = active transmitter, broadcasts position via AIS every minute.
- Both appear on the searching unit’s display; the radar-SART as 12 dots/arcs on radar, the AIS-SART as a position-tagged target on AIS.
MOB beacons and DSC-MOB
Various commercial AIS-MOB and DSC-MOB beacons exist — small personal units worn in a life jacket that activate on immersion and broadcast either:
- An AIS distress message (MMSI prefixed 972), making the casualty visible on the parent vessel’s AIS, OR
- A DSC individual call to the parent vessel’s MMSI, alerting the helm to a man overboard.
These are not part of GMDSS but are commonly fitted on small craft and feature on the SRC syllabus as awareness items. Know they exist, know what they do, know they’re not part of the international SAR system.
Handheld VHF as a locating signal
The CEPT syllabus includes handheld VHF under “alerting, communication and locating signals” (C3.3) because:
- A handheld in a liferaft is a primary survival comm.
- Its battery life is finite — having a charged spare battery matters.
- Range from a liferaft at sea level is short (refer to the range formula in Part 2) — even calling a Coast Station 50 m up only gets you a few NM if your antenna is at deck level.
- Hold the handheld up high when transmitting from a small craft — the few extra metres of height help.
The exam may ask you to calculate the approximate range between a handheld VHF in a liferaft and a SAR helicopter at 1000 ft. (Helicopter altitude ~300 m, handheld ~1 m: 1.23 × √300 + 1.23 × √1 ≈ 21 + 1.23 ≈ 22 NM.) That’s why airborne SAR units pick up handheld VHF traffic from a long way out.
What to take into the next post
You should leave Part 7 able to:
- Describe the NAVTEX message format and recite which message types and serial numbers cannot be rejected (A, B, D, Z, 00).
- State the EPIRB frequencies (406 MHz primary, 121.5 MHz homing) and the Cospas-Sarsat satellite segments (LEOSAR, GEOSAR, MEOSAR).
- Explain how to register an EPIRB in the UK and why it matters.
- Describe what to do if you activate an EPIRB by mistake.
- Compare a Radar-SART (12 radar dots) with an AIS-SART (AIS broadcast every minute, MMSI 970*).
- Recognise that AIS-MOB and DSC-MOB beacons exist but are not part of GMDSS.
Part 8 covers the rules that keep the distress frequencies usable — false alerts, testing protocols, guard bands, and the cancellation procedure when you press the wrong button.