Listening to Navtex
- Why listen to Navtex?
- What is Navtex?
- Tuning the FSK tones
- KAM receive modes
- Desktop software
- Other ways of decoding Navtex
- Spectra and samples
- USCG HF Navtex
- Oddities
- Document history
Why listen to Navtex?
Because it's there! I started when I remembered that a Kantronics KAM+ TNC which I hadn't used since the days of the amateur packet radio network some years ago was capable of decoding Navtex.
What is Navtex?
Navtex is a system of maritime weather and navigation bulletins which is sent using Sitor-B on MF and HF radio. Sitor-B uses FEC (forward error correction) which (in essence) is an FSK system with (usually) a 170Hz shift which sends each character twice. With receive-side processing this mode is far more immune from interference than RTTY, Sitor's most commonly-used predecessor.
The transmitting stations are designed and co-ordinated to cover an area of sea generally between 200 and 500 nautical miles (370 to 920km) radius from the station, while minimising the risk of interference to other stations. Each day is divided into six blocks of 24 ten-minute slots designated A to X. 518kHz is the primary frequency used; others are listed below. 518kHz and 490kHz are the busiest frequencies; at these frequencies distant stations are more likely to be heard at night, and where the signal path is over water rather than land.
Navtex frequencies
| kHz | Use |
|---|---|
| 424 | Stations in Japan. |
| 490 | Worldwide national bulletins in the local language. |
| 518 | Worldwide international bulletins, mostly in English though some are in the local language. |
| 4209.5 | Stations in Egypt, Taiwan, Turkey and Vietnam. (Further stations in Brazil, Mexico and the US are planned.) |
Further information, including content and schedule details, are available from a number of websites, though schedules can be varied to suit local needs. Some particularly informative websites are
The Frisnit site includes the soundcard software mentioned below, which can be used to upload received messages to a web-based database.
Tuning the FSK tones
The tones expected by the KAM in its 170Hz shift mode aren't always convenient. By using the KAM's SHIFT MODEM, MARK and SPACE commands, frequencies which may more closely match the filter characteristics of the receiver can be used instead. For example, a receiver with an offset of 800Hz in CW mode tuned to the listed frequency can be used by giving the KAM the commands
MARK 715
SPACE 885
SHIFT MODEM
KAM receive modes
The KAM has two modes suitable for decoding Navtex signals, NAVTEX and LAMTOR.
The NAVTEX mode applies additional error checking and filtering specific to NAvtex messages, including the ability to display only messages of specified types and from specified slots. This mode is best suited to maritime use where messages from far-distant stations are unlikely to be of interest. One must also use the NAVCLR command frequently enough to reset the message ID memory, as it is not possible to disable the rejection of of messages bearing an ID which has already been received. If a message contains too many errors, the KAM will stop processing the message and wait for the next one.
The LAMTOR mode is more suitable for Navtex DXing. Although it will output many more spurious characters in the absence of a valid Sitor signal, it will also decode often-significant parts of a weak-signal message which will fail the more stringent error-checking of the NAVTEX mode.
Desktop software
Software which can send commands to the KAM and accept the data it sends back is required. Some of the software designed for amateur RTTY, packet radio, etc, can be used, though the most readily-available on Windoze boxes (from Windoze 95 to XP, but not Vista) is Hyperterminal. This is a fairly robust and simple to use serial communication and telnet client, though after I left it running minimised for a few hours and tried to reactivate it, my PC reset (normally a rare occurrence). I now use PuTTY, which is primarily a SSH/telnet/rlogin client, though the latest version also has serial port capability. PuTTY is extremely well-behaved, with barely perceptible CPU use.
Another freeware alternative is RealTerm, which is a splendid hacker's tool with copious command line options and hooks for external software, but significantly higher CPU use. Locations for PuTTY and RealTerm are on my software list. (Similar software is readily available for Linux boxes.)
Other ways of decoding Navtex
A number of data communications programs can decode Navtex using a soundcard rather than a custom data processor like the KAM. Most are payware, but a freeware alternative exists which has a good reputation and is under active development, though it would not run on my PC (which worked comfortably with PuTTY and the KAM). All of these alternatives have a significantly higher CPU footprint as they have to do all of the processing done externally by the KAM.
An interesting possibility would be to use a KAM (or other hardware decoder) simultaneously with a soundcard software decoder and collate the two data streams "diversity decoding". I'm not aware of any software which does this.
Spectra and samples
This is a spectrogram of a Navtex transmission from Niton (about 150km distant over land) on 490kHz at 2007-12-26 0525, strong enough to be decoded without errors.
Navtex spectrogram