Module 7 — NAVTEX and maritime safety information

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:

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