NAVTEX and maritime safety information (MSI) reception, including MSI broadcast schedules and message categories
This domain tests NAVTEX reception and maritime safety information (MSI): what NAVTEX receives, how the message header is read, which categories trigger attention, and how NAVTEX fits with other MSI services. The exam is practical: identify the service, the frequency and the category.
NAVTEX basics
NAVTEX is a receive-only MF service for coastal MSI. The international frequency is 518 kHz, with transmissions in English. Some administrations use 490 kHz for national-language traffic and 4209.5 kHz for tropical or extended service. A NAVTEX receiver displays or prints messages automatically when the station and category are selected. Coverage is typically a few hundred nautical miles by groundwave.
Message format and categories
A message begins with ZCZC, followed by the station identifier and a category letter, then a sequence number, and ends with NNNN. Common categories are A navigational warnings, B meteorological warnings, C ice reports, D search and rescue information, E meteorological forecasts, and F pilot service messages. Other letters cover AIS, GNSS and additional navigational information. The receiver should alarm for search and rescue and urgency-related traffic; routine forecasts do not need to interrupt watch.
Schedules and MSI
NAVTEX stations transmit on a fixed schedule, usually every four hours in ten-minute slots, so a receiver left on continuously collects all selected messages. NAVTEX is part of the Global Maritime Distress and Safety System MSI service. In A3, SafetyNET via Inmarsat provides similar MSI where NAVTEX is not available. The operator must read and act on messages, not simply leave them in the printer.
Worked example
A receiver shows ZCZC QA42 ... NNNN. The operator identifies the station, sees category A, and reads a navigational warning about a drifting buoy in the vessel's planned track. Because category A is selected and relevant, the operator plots the position, informs the bridge team and logs the message. A later message with category E is a routine forecast and is read at the next watch change rather than interrupting a distress watch.
Traps that cost marks
NAVTEX is not a transmitter and is not a VHF service. Do not assume every message alarms; the alarm is mainly for SAR and urgent traffic. Do not confuse 518 kHz international English service with 490 kHz national-language service. Do not treat an unread printout as received information.