Signal guide

NAVTEX (maritime safety text)

An international system that broadcasts navigation warnings, weather forecasts and safety messages to ships as text.

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NAVTEX sounds like a fast, choppy two-tone chatter, quicker and more clipped than ham RTTY, running continuously for a few minutes while each coast station takes its turn.

What does it sound like?

Plate 40FSK data

A rhythmic, chirpy two-tone chatter at a steady pace.

0 Hz1,0002,0003,000 Hz

4.5-second example. A clean example made by our own signal generator; signals on the air usually sound noisier.

Field marks518 kHz (international)Two tones, 170 Hz apart, 100 baudEach station sends in scheduled slots

Hear it, step by step

This computer-made NAVTEX-style test clip hops between two tones at 100 bits per second; its characters are random, so they spell no message.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.0 sTwo tones, 165 Hz apartThe clip is one signal hopping between two pitches, about 1,761 and 1,926 Hz, 165 Hz apart; the NAVTEX standard is 170 Hz. Nothing else sounds between 0 and 3,000 Hz.0.5 s100 bits per secondEvery stretch at one pitch lasts a whole number of bits, each about 10 milliseconds long. That is 100 bits per second, the NAVTEX rate.0.1 sSeven bits per characterThe bits come in groups of seven, one character every 70 milliseconds. Every one of the 63 groups here has four bits at one pitch and three at the other, the fixed ratio that lets a receiver spot a damaged character.

Who uses it?

Coast stations run by national maritime and coast guard authorities, received by ships at sea.

Where and when can I hear it?

518 kHz (international, in English), 490 kHz (local languages), and 4209.5 kHz on shortwave. Each station transmits in turn on a fixed schedule.

The science

NAVTEX rides on SITOR-B, a teleprinter code sent by frequency-shift keying (FSK): the transmitter hops between two tones 170 Hz apart at 100 baud, meaning 100 symbols per second. Each character is a 7-unit code from the CCIR 476 set, and every character is sent twice, 280 milliseconds apart, so a receiver can recover a letter lost to a static crash from its second copy; this is forward error correction without any back-and-forth with the station. Each message opens with ZCZC and a four-character header: a letter for the station, a letter for the subject (such as navigational warnings or weather), and a two-digit serial number, and it closes with NNNN.

Why use it?

NAVTEX is built for automatic reception: the receiver prints or displays messages with no operator and can be set to skip stations or subjects the crew does not want and to suppress repeats it has already received. The International Maritime Organization designates it as the primary means of delivering urgent coastal marine safety information to ships, so one dedicated receiver covers a ship's coastal warnings and forecasts.

Lore

The U.S. Coast Guard began operating NAVTEX from Boston in 1983. It now lists twelve U.S. stations, each identified by a single letter, including Boston (F), Miami (A) and Guam (V).

What do the decoded parts mean?

• Station letter: Each coast station has a letter (A to Z) and a fixed time slot.
• Message type: A letter showing the subject: A navigational warnings, B weather warnings, D search and rescue, E weather forecasts, and others.
• Message number: A serial number so ships can tell new messages from repeats.

Easily confused with

• Radioteletype (RTTY): ham RTTY is slower (45.45 baud) and comes in back-and-forth exchanges
• Digital Selective Calling (DSC): DSC bursts are much shorter

How to recognize it

NAVTEX uses two tones 170 Hz apart, like ham RTTY, but at 100 baud, more than twice as fast. It also sends every character twice, a short time apart, so a burst of noise that wipes out one copy can be covered by the other. The result is a rapid, busy chatter with a very even rhythm.

A broadcast opens with a steady phasing signal, then messages that begin with the letters ZCZC and a four-character header and end with NNNN. On 518 kHz, stations take turns in fixed ten-minute slots, so a station can be on for several minutes and then fall silent while another takes over.

Easily confused with:

• Ham RTTY: same tone spacing, but slower (45.45 baud) and usually on the ham bands.
• DSC (digital selective calling): also a maritime two-tone mode, but in bursts of a few seconds on distress and calling frequencies.
• SITOR-A: the same speed, but in short choppy blocks with pauses for replies.

Where and when to hear it

The international English-language service is on 518 kHz, with national-language services on 490 kHz and a shortwave service on 4209.5 kHz. Each coast station gets a ten-minute slot every four hours, identified by a letter. In the United States, the Coast Guard lists stations including Boston (F), Portsmouth (N), Charleston (E), Miami (A), Puerto Rico (R), New Orleans (G), San Francisco (C), Cambria (Q), Astoria (W), Kodiak (J) and Honolulu (O), plus Guam (V), which has sent only on 4209.5 kHz since July 2018.

Stations are designed for a range of about 400 nautical miles, and signals travel much farther at night. You need a receiver that tunes just below the AM broadcast band (518 kHz is in the medium-frequency range); many shortwave radios do.

A little history

NAVTEX was developed to send navigation and weather warnings to ships as printed text, so crews would no longer have to copy them by ear in Morse or voice. The service began in the 1980s; the U.S. Coast Guard began NAVTEX broadcasts from Boston in 1983. It became part of the Global Maritime Distress and Safety System, which reached full implementation on February 1, 1999.

Messages are coded by subject (A for navigation warnings, B for weather warnings, D for search and rescue, E for forecasts, and more) so a ship's receiver can print only what matters and skip repeats it already has.

Common questions

Sources

• USCG Navigation Center: NAVTEX broadcasts
• IMO: GMDSS introduction and history
• ITU-R Recommendation M.540-2: automated direct-printing telegraph system for warnings to ships (NAVTEX)
• Fldigi Users Manual: NAVTEX and SITOR-B
• ITU-R M.625-3: direct-printing telegraph equipment in the maritime mobile service (100 Bd, 170 Hz, 7-unit constant-ratio code)