5.6 NAVTEX & Maritime Safety Information

5.6 NAVTEX & Maritime Safety Information

Quick Answer: NAVTEX broadcasts internationally on 518 kHz, exclusively. Messages arrive in categories indicated by a single letter. Unwanted broadcasts are suppressed by programming the receiver to reject them. Typical station range is 200–400 nautical miles (360–720 km), and range is greatest in the middle of the night. MSI reaches ships by NAVTEX, Inmarsat SafetyNET and HF NBDP.

Maritime Safety Information (MSI) is one of the nine GMDSS communications functions from section 3.5, and NAVTEX is its workhorse. Official sub-topic 1-D-24 (NAVTEX) closes Element 1's Other Equipment topic.

What NAVTEX is

NAVTEX is an automatic, unattended, narrow-band direct-printing service. A coast station transmits navigational warnings, meteorological warnings and forecasts, search-and-rescue information and other MSI on a schedule; the shipboard receiver listens continuously, decodes what is relevant, prints or displays it, and discards duplicates. No operator action is required, which is the entire point—an unattended bridge still gets its warnings.

518 kHz — the exclusive international frequency

Which frequency is used exclusively for NAVTEX broadcasts internationally? 518 kHz.

The word exclusively is doing real work in that answer. 518 kHz is set aside worldwide for international NAVTEX in English, so a vessel anywhere can leave one receiver on one frequency and be confident of getting the international service. Two companion frequencies exist and appear as distractors:

If a question says "exclusively" and "internationally," it wants 518 kHz.

Single-letter message categories

NAVTEX broadcasts are sent in categories of messages indicated by a single letter or identifier.

Each message carries a four-character header: a letter identifying the transmitting station, a letter identifying the message category, and two digits giving the serial number. The category letters that matter most:

The three that cannot be switched off—A, B and D—are the ones on which life may depend. That is the design safeguard behind the next item.

Rejecting unwanted broadcasts

What means are used to prevent the reception of unwanted broadcasts by vessels utilizing the NAVTEX system? Programming the receiver to reject unwanted broadcasts.

Note what this answer is not. It is not "switching the receiver off," not "retuning to another frequency," and not "asking the coast station to stop." It is receiver-side filtering: the operator programs which station letters and which category letters to accept. A vessel on a coastal passage sets its receiver to accept only nearby stations and only relevant categories, so the printer carries traffic that matters instead of forecasts for a sea area 600 miles away.

And the safeguard holds: categories A, B and D cannot be rejected regardless of how the receiver is programmed.

Range, and why it peaks at night

What is the transmitting range of most NAVTEX stations? Typically 200–400 nautical miles (360–720 km).

When do NAVTEX broadcasts typically achieve maximum transmitting range? Middle of the night.

518 kHz sits in the medium-frequency band, where daytime absorption in the lower ionosphere is strong and coverage is essentially ground wave. After dark that absorption falls away, sky-wave propagation supports the signal, and the same transmitter reaches substantially farther. This is why NAVTEX broadcast schedules are staggered—stations sharing 518 kHz transmit in assigned time slots so that the extended night-time footprints do not collide.

Two practical consequences:

1. Coverage plans are built around the daytime figure, so 200–400 nautical miles is the number to quote for planning.
2. At night you may receive stations well outside your area—which is exactly why receiver programming (above) matters.

Where else MSI comes from

MSI can be obtained by one (or more) of the following: all of the above. NAVTEX is one route among several, and GMDSS deliberately provides more than one because a single path is a single point of failure:

That table maps onto the sea areas from section 3.5: NAVTEX for coastal work, SafetyNET for A3, HF NBDP for the high latitudes of A4. When the pool asks how MSI can be obtained and offers several of these, the inclusive answer is correct.

Exam trap summary

• 518 kHz — exclusive, international. Not 490 kHz (national), not 2182 kHz (voice distress).
• Single letter identifies the category — not a numeric code, not a station callsign.
• Unwanted traffic is suppressed by programming the receiver, not by turning it off.
• 200–400 nautical miles typical range; maximum in the middle of the night.
• MSI routes: all of the above.