International NAVTEX broadcasts on 518 kHz in English; national services may use 490 or 4209.5 kHz. Its header is ZCZC B1B2B3B4: B1 identifies the transmitter, B2 the subject and B3B4 the message number. A, B, D and L are non-rejectable categories. Piracy and tsunami information are included within D, not additional subject letters.
The receiver listens automatically; stations transmit according to schedules, with urgent broadcasts when required. Configure relevant transmitters and selectable subjects for the voyage, read warnings and pass operationally relevant information to the bridge. Arrange another appropriate MSI reception service before leaving coverage. A printed message alone does not demonstrate that its safety implications were assessed.
Reading a NAVTEX message
NAVTEX is an automated maritime safety information service. International NAVTEX uses 518 kHz in English; national services may publish 490 kHz or another arrangement. The USCG NAVTEX guide is a useful operational reference, but a learner must still check the service plan for the voyage.
A header such as ZCZC GB12 carries a compact identity: B1 identifies the transmitter, B2 identifies the subject, and B3B4 identifies the message number. NAVTEX guidance treats A, B, D and L as non-rejectable subjects. Piracy and tsunami information belong to the service’s published subject coding; they are not extra letters invented by the operator.
| Header part | Learner question |
|---|---|
| B1 | Which transmitter sent it? |
| B2 | What subject category is it? |
| B3B4 | Which message number is this? |
From printout to bridge decision
The receiver may listen automatically and stations may transmit on schedules or urgently when required. That does not make the process passive. Select transmitters and subjects appropriate to the voyage, read the complete warning, identify its geographic and time limits, and pass operationally relevant information to the bridge. Before leaving coverage, arrange another appropriate MSI source such as an applicable satellite broadcast or HF service.
The message number helps prevent duplicate handling, but it is not a substitute for checking a correction or cancellation. Treat the displayed text as a navigational input that must be correlated with the vessel’s present position, intended track and watch instructions.
A five-step MSI workflow
- 1Verify frequency, transmitter and message header.
- 2Read the warning’s subject, area, validity and any cancellation or correction.
- 3Decide whether it changes the route, watch, speed, communications plan or safety brief.
- 4Tell the bridge and record the decision or no-change assessment.
- 5Confirm an alternative MSI source before the receiver loses useful coverage.
Worked insight: filtering is not ignoring
Suppose a receiver is configured for transmitter G and subject C, then receives a subject B weather warning from G. A matching transmitter does not make subject B selectable, while non-rejectable subjects must still be presented according to the receiver’s rules. Filter behavior varies by equipment, so the operator must know the installed set and the published service procedure.
NAVTEX is not a DSC alert, an RCC conversation or a commercial weather guarantee. It is one source in the GMDSS information chain. Combine correct decoding with an explicit operational handover.
Official references · 2026-09-09
Apply what you learned
The receiver prints ZCZC GB12. The officer reads it but cannot say where it applies or whether the route changes.
What is missing from the handover?
Suggested answer: Decode the header, read area and validity, assess the route, and record the bridge decision.
Why this works: B1 identifies transmitter G, B2 identifies subject B and B3B4 identifies the message number. Those fields do not replace the message body or its geographic and time limits. The officer must evaluate the warning against the voyage, brief the bridge and record the resulting action or explicit no-change decision.
Continue with guided radio exercises.
Practise in the radio simulatorKey Takeaways
518 kHz is the international English NAVTEX service; national services can publish other frequencies.
B1 identifies transmitter, B2 subject and B3B4 message number.
A/B/D/L handling and receiver filters are service rules, not permission to ignore safety information.
The bridge must assess and record operational consequences of received MSI.
Common Mistakes
Treating a NAVTEX printout as a distress alert or RCC exchange.
Assuming every country uses only 518 kHz or the same schedule.
Filtering or printing a message without assessing its effect on the voyage.
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More GMDSS lessons
- Module 4 — MF/HF radiotelephony and DSCMF/HF DSC alerts and subsequent voice traffic use different frequencies.
- Module 5 — NBDP / radiotelexNBDP is narrow-band direct-printing telegraphy.
- Module 9 — SART, AIS-SART and survival VHFA radar SART responds to suitable X-band radar and creates twelve marks extending away from its approximate position; arcs or circles can appear at close range.
- Module 10 — GPS, antennas, batteries and reserve powerCheck GNSS position and UTC time, antenna connections, radio identities and power supplies.
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