Module 9 - GMDSS & Safety Equipment
NAVTEX Message Identity
What you will learn
Decode B1, B2 and B3B4, preserve mandatory categories and recognise the exceptional 00 message number.
A NAVTEX identity is an operational filter: B1 identifies the transmitter, B2 the subject, and B3B4 the serial message number.
B1 identifies the NAVTEX transmitter and therefore helps define the service area selected. B2 identifies the subject category. B3B4 is the NAVTEX serial number used by the receiver to suppress repeats; it is distinct from any navigational-warning number printed in the message body.
Subject categories A, B, D and L cover navigational warnings, meteorological warnings, search-and-rescue information or piracy warnings, and additional navigational warnings. They are mandatory selections in current guidance, with D causing an alarm. Older receivers may technically allow L to be deselected, but the current NAVTEX Manual strongly advises retaining it.
B3B4 equal to 00 overrides normal transmitter and subject selections and repeat rejection, so the message cannot be rejected. That exceptional identifier is tightly controlled and must not be used for routine or service messages. Its power is exactly why an operator must not treat every 00 as ordinary traffic.
Worked example
A receiver shows identity KD00. The operator has selected another transmitter and normally suppresses this subject category.
- 1Read K as the B1 transmitter identity and D as search-and-rescue or piracy-warning subject information.
- 2Read 00 as the exceptional serial that overrides normal selection and repeat rejection.
- 3Open and assess the message immediately rather than dismissing it as a duplicate or out-of-area routine broadcast.
Sense check: The 00 is the NAVTEX serial field, not automatically the navigational-warning number printed inside the message.
| Character | Meaning | Operator use |
|---|---|---|
| B1 | Transmitter identity | Select relevant service areas without excluding a needed station |
| B2 | Subject indicator | Retain mandatory A, B, D and L categories |
| B3B4 | NAVTEX serial message number | Recognise repeats; do not confuse with warning number in text |
| 00 | Exceptional override serial | Cannot be rejected; never routine/service traffic |
Configure for relevance without filtering safety
A crew deselects several subjects to reduce printouts and accidentally removes a mandatory warning category.
1. Restore the safety core
Retain A, B, D and L in accordance with current guidance and receiver capability.
2. Select transmitters by route
Include the stations whose service areas cover the planned passage and margins.
3. Review identities and bodies
Use B1/B2/B3B4 to triage, then read the content and applicability.
Sense check: A quiet receiver is not evidence of a safe sea; overly narrow selection can create the silence.
Common mistake or limitation
- Calling B3B4 the navigational-warning number in the message body.
- Deselecting mandatory A, B, D or L categories to reduce output.
- Treating 00 as an ordinary serial that normal filters can reject.
Recap
- B1 is transmitter, B2 is subject and B3B4 is NAVTEX serial number.
- A, B, D and L form the mandatory safety-category core in current guidance.
- 00 overrides selection and repeat rejection and is never for routine/service messages.
Optional quick check
Section 9 of 10
What is special about NAVTEX B3B4 equal to 00?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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