IALACOLREG
Academy
Create a free account to save your progressSave your progress
My modules›M3 GMDSS›5. NAVTEX and the 406 MHz EPIRB
See units

NAVTEX and the 406 MHz EPIRB

Lessons 7–8
No notes
GMDSS≈ 7 min read

Module 7 — NAVTEX and maritime safety information

Illustrative NAVTEX header: ZCZC starts the message; G is transmitter B1, B is the meteorological-warning subject B2, and 12 is serial number B3B4. NNNN ends the message.

1The lesson in plain words

In one sentence

NAVTEX gives you safety information for the voyage: read the warning, identify its limits, and pass what matters to the bridge.

What do you receive?

NAVTEX is an automated maritime safety information service; the receiver listens automatically to station broadcasts.

Which frequencies matter?

The international service uses 518 kHz in English; national services may use 490 or 4209.5 kHz.

How do you read the header?

In ZCZC B1B2B3B4, B1 identifies the transmitter, B2 the subject, and B3B4 the message number.

What do you do with it?

Select voyage-relevant transmitters and subjects, read area and validity, and pass relevant information to the bridge before coverage ends.

2Step by step

Operational NAVTEX reading

  1. 1

    Configure transmitters and subjects relevant to the voyage.

    The receiver must get information relevant to the voyage.

  2. 2

    Verify the frequency, transmitter and message header.

    The header identifies transmitter, subject and number.

  3. 3

    Read the full warning, its area, validity, and corrections or cancellations.

    The number does not replace checking message changes.

  4. 4

    Assess whether it changes route, watch, speed, communications or safety.

    A printout shows reception, not navigational assessment.

  5. 5

    Tell the bridge and record the decision, including “no change”.

    The handover makes the warning's consequence clear.

  6. 6

    Arrange another MSI source before leaving coverage.

    NAVTEX is one source in the GMDSS information chain.

3Key data

International NAVTEX
518 kHz · English
National services
490 or 4209.5 kHz
Header
ZCZC B1B2B3B4
B1
Identifies the transmitter
B2
Identifies the subject
B3B4
Message number
Non-rejectable subjects
A, B, D and L

4Key points

  1. 1

    518 kHz = international English NAVTEX; national services may use other frequencies.

  2. 2

    B1 identifies transmitter · B2 subject · B3B4 message number.

  3. 3

    A, B, D and L cannot be rejected under the service rules.

  4. 4

    The bridge assesses and records the operational consequence of MSI.

5Frequent mistakes

  • Wrong: "A printed message has already been assessed."

    Right: A printout proves reception; assess its effect and pass it to the bridge.

  • Wrong: "Every country uses 518 kHz and the same schedule."

    Right: National services may use other frequencies and different schedules.

  • Wrong: "Piracy and tsunami information are extra subject letters."

    Right: Both types of information are included within D, not new letters.

6Exam keys & interpretation

Exam keys

  • If you see «B1», choose transmitter; B2 is subject and B3B4 is number.
  • If «A, B, D or L» appears, remember: they are non-rejectable subjects.
  • If asked about a printout, look for «assess and pass on»: reception is not assessment.

How to read it

  • NAVTEX gives you information; the bridge decides what changes for the voyage.
  • Think of the header as a label: identify origin, subject and number before interpreting the warning.

7Go deeper: GOC exam level

The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.

Illustrative NAVTEX header: ZCZC starts the message; G is transmitter B1, B is the meteorological-warning subject B2, and 12 is serial number B3B4. NNNN ends the message.

What counts as MSI

Maritime Safety Information covers navigational warnings, meteorological warnings and forecasts, shore-to-ship distress alert relays, SAR-related information and other urgent safety information. The worldwide service delivers it by terrestrial and satellite means according to where the ship is.

NAVTEX: frequencies and format

NAVTEX is an automatic narrow-band direct-printing service for coastal MSI.

FrequencyUse
518 kHzThe international service, in English
490 kHzNational services, often in the local language
4209.5 kHzNational/regional NAVTEX-type broadcasts where coordinated

Every message has a fixed frame:

ElementMeaning
ZCZCStart of message
B1Transmitter identity, a letter A–X, linked to the station's time slot
B2Subject indicator: the type of message
B3B4Serial number 01–99
TextArea, hazard, position, validity, instructions
NNNNEnd of message

Serial number 00 is reserved for messages that must always be printed or shown, so it is never used for routine traffic.

Time slots: when each station transmits

All NAVTEX stations in a region share 518 kHz, so each B1 letter has its own 10-minute slot, repeated every 4 hours (UTC). Knowing the slot tells you when to expect a station's broadcast, and why an urgent warning may be sent at once outside it.

B1First slot (UTC)B1First slot (UTC)B1First slot (UTC)
A0000I0120Q0240
B0010J0130R0250
C0020K0140S0300
D0030L0150T0310
E0040M0200U0320
F0050N0210V0330
G0100O0220W0340
H0110P0230X0350

Each slot repeats every 4 hours: station G, for example, also transmits at 0500, 0900, 1300, 1700 and 2100 UTC.

The subject indicators (B2)

B2Message type
ANavigational warnings
BMeteorological warnings
CIce reports
DSAR information, piracy warnings, tsunamis and other natural phenomena; triggers the receiver's alarm
EMeteorological forecasts
FPilot and VTS service messages
GAIS service messages (not AtoN)
HLORAN messages
ICurrently not used
JGNSS messages
KOther electronic navigational aid system messages
LNavigational warnings additional to A
ZNo message on hand

Paper or screen, the work is the same

On a printing receiver, the paper is part of the bridge workflow: read it, mark it, apply it. On a screen receiver you manage selected stations (B1), categories (B2), read/unread status and alarms. Check after every voyage change that the right stations for the route are selected.

NAVAREA and METAREA

The oceans are divided into coordination areas for navigational warnings (NAVAREAs) and meteorological information (METAREAs). Their coordinators produce and distribute information that reaches the ship by NAVTEX near the coast and by satellite EGC on the ocean.

SafetyNET and SafetyCast

Both broadcast MSI and SAR information by satellite to geographic areas. The terminal's position and area settings decide what the ship receives, so a wrong position or configuration means wrong reception.

NAVDAT: higher-capacity digital MSI

WRC-23 integrated NAVDAT into the GMDSS regulatory framework: a digital shore-to-ship system with more capacity than NAVTEX, designed for MSI and related data, with frequencies identified in MF and HF. Deployment depends on Administrations and coast networks.

References: SOLAS IV as amended by MSC.496(105) · IMO GMDSS circulars

GOC exam check

  1. On which frequency is the international NAVTEX service broadcast in English?

    1. 490 kHz
    2. 518 kHz
    3. 4209.5 kHz
    4. 2187.5 kHz
    Show the answer

    B. 518 kHz

    Why: 518 kHz is the international English service. 490 kHz carries national services, often in the local language.

  2. A NAVTEX message has subject indicator B2 = D. What is it about?

    1. Meteorological forecasts for the area
    2. Ice reports and ice limits
    3. Pilot and VTS service messages
    4. SAR information, piracy or tsunami warnings
    Show the answer

    D. SAR information, piracy or tsunami warnings

    Why: D covers SAR, piracy, tsunamis and other natural phenomena, and it sounds the alarm. Forecasts are E and ice reports are C.

  3. Which NAVTEX message categories cannot be deselected on the receiver?

    1. A, B, D and L
    2. A, B, D and E
    3. A, B, C and D
    4. B, D, E and L
    Show the answer

    A. A, B, D and L

    Why: Navigational warnings (A and L), meteorological warnings (B) and SAR information (D) must always be received. Forecasts (E) and ice (C) can be filtered.

  4. What does serial number 00 in a NAVTEX header mean?

    1. A test message that can be ignored
    2. The first routine message of the day
    3. A message that must always be printed
    4. A message cancelling the previous one
    Show the answer

    C. A message that must always be printed

    Why: Serial 00 is reserved for messages that must always be printed or shown, so it is never used for routine traffic.

  5. In a NAVTEX header, what does B1 identify?

    1. The type of message
    2. The transmitter and its time slot
    3. The serial number of the message
    4. The NAVAREA that issued it
    Show the answer

    B. The transmitter and its time slot

    Why: B1 is a letter A–X for the transmitter, linked to its time slot. The message type is B2 and the serial number is B3B4.

  6. Your NAVTEX receiver has printed nothing for two days on a busy coastal route. What should you assume?

    1. There has been nothing to report in the area
    2. The station only transmits when a warning is due
    3. Silence is normal outside forecast times
    4. Check stations, categories and antenna first
    Show the answer

    D. Check stations, categories and antenna first

    Why: A quiet printer is not a quiet sea. Check the selected B1 stations, deselected categories and the antenna before trusting the silence.

  7. Planning a passage, you need the NAVTEX stations and NAVAREA/METAREA services for each leg. Where do you look?

    1. ADMIRALTY List of Radio Signals Volume 1
    2. ADMIRALTY List of Radio Signals Volume 6
    3. ADMIRALTY List of Radio Signals Volume 3
    4. ITU List V of ship stations
    Show the answer

    C. ADMIRALTY List of Radio Signals Volume 3

    Why: ALRS Volume 3 covers Maritime Safety Information, NAVTEX and EGC broadcasts. Volume 1 lists radio stations and Volume 6 VTS and port services.

  8. Your ship leaves coastal NAVTEX range for an ocean crossing. How will it receive MSI?

    1. By satellite EGC for its NAVAREA and METAREA
    2. By NAVTEX on 4209.5 kHz across the ocean
    3. By NAVDAT, which replaced NAVTEX offshore
    4. By AIS safety messages from passing ships
    Show the answer

    A. By satellite EGC for its NAVAREA and METAREA

    Why: Away from the coast MSI arrives by satellite EGC (SafetyNET or SafetyCast) for the area. 4209.5 kHz is a national or regional service, not an ocean one.

  9. Your ship sails from Lisbon towards Gibraltar along the coast. At 0800 UTC the NAVTEX still shows only messages from stations you passed yesterday and nothing from the stations ahead. What is the most likely cause?

    1. 518 kHz signals do not carry in daylight
    2. Category D was deselected on the receiver
    3. The stations ahead broadcast only on 490 kHz
    4. The new leg's B1 stations are not selected
    Show the answer

    D. The new leg's B1 stations are not selected

    Why: NAVTEX prints only the stations selected by B1, so a receiver set up for yesterday's leg ignores the stations ahead. Category D cannot be deselected, and a missing category would not explain why whole stations are silent.

  10. Same scenario: you select the stations for the new leg. One of them has B1 = G. After 0800 UTC, when do you expect its next scheduled broadcast?

    1. 0810 UTC
    2. 0900 UTC
    3. 1000 UTC
    4. 1300 UTC
    Show the answer

    B. 0900 UTC

    Why: Station G's slot starts at 0100 UTC and repeats every 4 hours: 0100, 0500, 0900, 1300. The next one after 0800 is 0900 UTC; 1300 is also a G slot, but not the next one.

8Full lesson & sources

Open the full lesson (diagrams and official references)

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 the coastal MSI service: shore stations broadcast navigational warnings, weather warnings and SAR information on 518 kHz in English, and a dedicated receiver prints or stores them automatically. Many countries also run a national service on 490 kHz in their own language. A station's range is typically a few hundred miles, so NAVTEX is what you rely on near the coast. Offshore, satellite EGC takes over. The USCG NAVTEX guide is a good operational reference, but always check the stations on your route.

Every message starts with a four-character header after ZCZC. Take ZCZC GB12:

CharacterValueMeaning
B1GTransmitter G: look up which station holds that letter in your area
B2BSubject: meteorological warning
B3B412Serial number 12 from that station for that subject

B1 identifies the transmitter. Stations share 518 kHz by taking turns: each B1 letter from A to X has a 10-minute slot every 4 hours (A at 0000, 0400, 0800 UTC and so on, B ten minutes later). Stations close enough to interfere never share a letter. The receiver also uses the full header to avoid reprinting a message it has already received cleanly, so the same warning repeated in later slots does not flood the bridge.

From printout to bridge decision

A printed message is evidence of reception, not evidence that anyone has thought about it. Work through it like this. Message GA37 reads: "LIGHT 43-12.4N 008-31.0W EXTINGUISHED." Plot it. Your track passes 12 NM off at night, and you do not use that light for position fixing. Decision: no change to the passage plan. Note it on the chart and in the log, and tell the officer of the watch. Message GB13 reads: "GALE WARNING, NW 8 TO 9 EXPECTED WITHIN 6 HOURS" for the sea area you enter in 4 hours. That changes the plan: secure cargo and the deck, review speed and routing, and brief the master.

A five-step MSI workflow

  1. 1Before sailing, select the B1 stations that cover each leg of the route; ALRS Volume 3 lists them.
  2. 2Read each message in full: subject, area, validity, and whether it cancels or corrects an earlier one.
  3. 3Plot it and decide whether it changes the route, speed, watch or safety briefing.
  4. 4Tell the officer of the watch and record the decision, including "no change".
  5. 5Before you leave NAVTEX range, confirm that satellite MSI (SafetyNET or SafetyCast) is set up for the next area.

Worked insight: filtering is not ignoring

Suppose the receiver has stations G and S selected and subject E (forecasts) deselected. A subject B gale warning from G still prints: B cannot be refused. A forecast from G does not print, which is your choice. A navigational warning from station K, which you did not select, does not print at all, even if it concerns your route, because the receiver only accepts the stations you selected. That is the real risk: a quiet receiver may mean only that the wrong stations are selected. When you change areas, change the B1 selection.

NAVTEX is an MSI input; the bridge team supplies the judgement. It cannot send an alert, talk to an RCC, or replace a full weather routing service. Its value lies in decoding the message correctly and making a clear decision from it.

Official references · 2026-09-09

USCG NAVTEX

Finish this unit

When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.

My notes for this unit

No notes