Maritime Safety Information (MSI) and NAVTEX
NAVTEX questions are usually about reading the message header, selecting the right subject indicators, and knowing which frequency serves which purpose.
Frequencies and coverage
- 518 kHz: international NAVTEX, English, the primary GMDSS frequency.
- 490 kHz: national NAVTEX, usually the local language.
- 4209.5 kHz: tropical NAVTEX, for areas where 518 kHz performs poorly. Coverage is MF, typically 200–400 nm by day (more at night), so NAVTEX is an A1/A2 and coastal A3 aid, not a global service.
Message structure
A NAVTEX message begins ZCZC, then B1 (transmitter identity letter), B2 (subject indicator), B3 (serial number, 01–99, restarting at 01) and B4 (time of origin), followed by the text and NNNN. Example: ZCZC MB12 = transmitter M, subject B, message 12.
Subject indicators (B2)
A navigational warning; B meteorological warning; C ice report; D SAR information; E weather forecast; F pilot service; L additional navigational warning; V notice to fishermen; Z no message. Receivers must not be able to reject A, B, D and L, because these carry safety-critical traffic.
Worked example
A receiver log shows "ZCZC MB12 151200 NNNN" followed by a gale warning. Read it as: station M, meteorological warning, serial 12, originated 1200 UTC on the 15th. Action: enter it in the radio log, plot or note the affected area, and brief the bridge. If the same station later sends "ZCZC MZ00", it means no messages on hand.
Traps
- Expecting 518 kHz to be in the national language.
- Confusing B1 (station) with B2 (subject).
- Assuming NAVTEX gives global coverage; use SafetyNET outside range.
- Deselecting A, B, D or L and missing warnings.