Tutorial NAVTEX

Tutorial: NAVTEX

What it is. NAVTEX is the maritime safety information service: short text bulletins with navigational warnings, weather forecasts, ice reports and search-and-rescue information. Each coastal station has its own schedule and a one-letter identity so ships can filter what they want. Ships print the bulletins; with OnAir you read them on screen. It is a one-way broadcast.

NAVTEX, DSC and weather fax share one mode panel (group Maritime).

What you need

• Radio. A radio that tunes 0.1 to 5 MHz or so: an HF-capable SDR, or a dongle with an upconverter or direct-sampling mode. The mode asks for 2 Msps; the lowest rate that works is 250 ksps. See Supported-radios.
• Antenna. A long wire or an active loop, away from noise. NAVTEX is MF ground wave: it works best near the coast and at night is often longer range. 518 kHz is also where lightning noise and mains hash show up.
• Frequencies from the repository's frequency database. 518 kHz (international, English), 490 kHz (national language), 424 kHz (some Baltic and Turkish stations) and 4209.5 kHz (HF, tropical zone). The mode starts at 0.518 MHz. The signal is SITOR-B, 100 baud FSK.

Before you start

• Tune the dial exactly: the tones sit 85 Hz either side of it. Use the Tuning view to check.
• Gain: low enough to keep the radio linear at night when broadcast stations overload MF. If a nearby AM broadcast is strong, add a notch or high-pass filter.
• Keep the signal away from the radio's DC spike.

Step by step

1. Mode list, group Maritime, entry Ship warnings, 518 kHz.
2. Service: leave on auto (frequencies 490, 518 and 4209.5 kHz are recognised as NAVTEX) or select NAVTEX.
3. Press Start.
4. In the sub-views along the top (NAVTEX, DSC, Fax, Tuning, Fax stations) select NAVTEX.
5. Check Tuning shows two tones 170 Hz apart centred on the dial.
6. Wait. A station sends only at its scheduled times, in turn with other stations on the same channel, so the first bulletin can take a long time. The panel says "no NAVTEX message yet (one takes a minute or more)".

What you should see

A message list grouped by station (newest first within each station), with the station letter, number and heading; click one to read the full text (the text of the selected message shows in the pane, and copy text copies it). Bulletins start with ZCZC and end with NNNN in the raw text. Each message shows the station letter, message number, number of characters, how many were unreadable (with a percentage), how often it was heard and the last time.

Reading the screen

• Header. ZCZC plus station letter, subject letter and a two-digit number. The subject names the type (navigational warning, weather, ice, SAR and so on).
• Unreadable characters. Errors are marked in the text; a message that came in cut short is marked as incomplete. NAVTEX repeats are useful: the count "heard n x" shows how many times OnAir has seen the same message.
• Tuning. FSK tones and shift, baud and signal-to-noise over the noise.

Options and settings

• Save messages writes each NAVTEX message to a text file as soon as it has been received (also one cut short, marked as such), one file per message in a folder per day. Show folder opens the folder (navtex in the data folder).
• Listen plays the audio.
• clear empties the list.

Troubleshooting

• No FSK signal found. Retune; the message says NAVTEX and HF DSC sit on the dial frequency, 85 Hz either side.
• Garbled text. Fading or noise. Characters are marked unreadable; wait for repeats.
• Nothing on 518 kHz in the daytime. MF range is short by day. Try at night or on 490 kHz.
• Strong AM station breakthrough. Lower the gain; add a filter.
• Hundreds of messages of the same station. Normal: bulletins are repeated.

Tips

The station letter tells you the coastal station; compare with the schedule of your area. Save messages to build a log.

Legal and privacy notes

NAVTEX is a public safety broadcast. Receiving is legal in most places. See Legal.

Under the hood

SITOR-B (NBDP forward error correction), 100 baud, 170 Hz shift FSK, each character sent twice with a time offset; defined in ITU-R M.540 and M.625; the service is described in the IMO NAVTEX manual.

Related

Mode-NAVTEX, Tutorial-DSC, Tutorial-Weather-fax, Tutorial-AIS, Troubleshooting, Legal