As a bit of a radio enthusiast, I’ve been into trying to receive distant LF/MW/HF services for many years. While reception at home has steadily become more difficult, the world of KiwiSDRs and WebSDRs have injected new life into the hobby, allowing me to chase signals effectively around the world from the comfort of my bedroom.

Of all the services on the air, I often like to monitor utility stations. These stations are set-up to serve a specific purpose, often transmitting at scheduled times on a fixed set of frequency for many, many years. They act like a “constant” on the air, except for when they get turned off for good. Having recently preserved the present state of radiofax services by attempting to collect faxes from all presently operating stations, I thought it would be good to explore other services which may be at risk of becoming extinct in the near future.

A service that has often evaded my consideration is NAVTEX, as Australia is in a NAVAREA with no NAVTEX service since I started this hobby. But now that I am not restricted to reception at home, I decided it would be good to give NAVTEX DXing a try.

What is NAVTEX?

NAVTEX is short for Navigational Telex, which is a standardised international broadcast service to mariners which transmits text-based bulletins over medium-wave frequencies. It is sometimes known as a narrow-band direct printing service as many older NAVTEX receivers would print the bulletins onto paper allowing them to be reviewed later. Common frequencies include 518kHz which is used for international use, 490kHz which is used for national use and various other frequencies used for local use (e.g. 423kHz for Japan). There is also NAVTEX on 4MHz at 4209.5kHz. Transmissions occur using the SITOR-B mode at 100 baud with a frequency shift of 170Hz which uses the CCIR-476 character set with forward error correction (FEC) by transmitting characters twice. This makes the mode relatively slow, with the simple and unsophisticated FEC scheme providing only a basic level of redundancy. As the transmissions share the same frequencies, a 10-minute time-slot system which repeats every four hours and careful control of broadcast power is used to minimise the chance of interference.

Schedules of NAVTEX transmitters can be found on Wikipedia and on DX Info Centre (by William Hepburn), although there are some inconsistencies in the data which is fairly common as users rarely take the time to update these resources.

This service complements other utility services to mariners, such as radiofax, SSB weather voice broadcasts and HF-SITOR to name a few. Owing to the variability of propagation, slow transmission rates and cost of operating transmitters, some NAVAREAs have already moved to eliminate NAVTEX service, instead only providing services via satellite services. Other casual mariners have instead opted to do without, instead relying on mobile data when close to shore.

Message Format

NAVTEX messages start with a 10-second phasing signal which has an easy to identify cadence. This is followed by the message header “ZCZC”, a space, and a four-character string which includes the ID of the sending station, the message type and the serial number of the message. This is followed by CR, LF, the message and “NNNN” to end the message. This is followed by a CR and two LFs and by further messages as necessary after a minimum amount of phasing signal or an idle signal.

When tuned with a centre frequency of 1kHz, NAVTEX transmissions sound like this (with Japanese NAVTEX transmissions sounding slightly different). With the audio can be decoded into the messages using various software packages – I prefer fldigi as it seems most sensitive, but NAVTEX-J transmissions only seem to decode correctly with Frisnit NAVTEX Decoder.

As the message itself is not standardised in format, there are some differences in the format between stations which proved to be worth the effort to examine in case it helps others identify NAVTEX transmissions they hear on air. There are also some interesting messages and slight deviations which are interesting.

For instance, some stations send BRK between messages when there isn’t a message to follow for a few seconds. Others send ZZZZ prior to going off air. Some send NNNN on a separate line while others keep them at the end of the message, while some transmitters send an extra LF between messages. Some transmitters are easy to identify as the station name is sent with almost every message, while others are “seemingly anonymous” to improve sending efficiency by reducing wasted air-time and printer paper.

The NAVTEX Message Gallery

Rather than try to monitor NAVTEX messages manually, I decided to take a more brute-force approach. Over a number of days when I had some free time in August and September, I selected a few receivers around the world to record the NAVTEX frequencies for 24 hours, saving the audio for spectral analysis and decoding just the transmissions later without regard to original transmission timeslot. These receivers included publicly available KiwiSDRs and the WebSDR located at the University of Twente. After a few days, I would select different receivers and repeat the process until I felt satisfied I had collected a representative sample of NAVTEX messages.

The transmissions would be decoded by replaying the audio segments through a virtual audio cable to fldigi or Frisnit NAVTEX Decoder in the case of NAVTEX-J transmissions. The collected messages would then be analysed, sorted by the likely transmitter and NAVAREA, with duplicate messages removed. The best received sample of any message is retained, although not all messages are received without error and are still included as they may be significant in indicating a particular station is still active (although perhaps obscured by another station in the same timeslot).

While it may seem a simple affair to try and DX NAVTEX transmissions via KiwiSDR, there were a number of technical challenges including time limits, the use of MW filters to avoid broadcast overload and the low sensitivity of many of the antennas used in the MW band. With careful use of receivers, it was possible to receive a good fraction of the known NAVTEX stations, although due to a lack of good receivers in Russia, South-East Asia and South America, the results are perhaps a little lacking in some NAVAREAS.

As the number of unique collected messages is so numerous, rather than clogging up this webpage with the individual messages, the NAVTEX Message Gallery is instead presented as a 296 page PDF file with bookmarks for each station sorted by frequency and NAVAREA. I hope you enjoy looking at this collection – while not exhaustive, is still comprehensive.

You can download/view the PDF file here:

NAVTEX Messages Collected Aug-Sep2019 (3660KiB)

Note that the messages above are an archive of messages received during August to September 2019 and are not updated. They are not to be used for navigation and are not free of errors.

Note that the message content is presented as received – namely between ZCZC to NNNN, but the spacing between messages have been altered to ensure consistent presentation. While I have tried to clean it up as best as possible, there’s likely to be a few “runt” messages or sheared messages due to the nature of recording/replay decoding.

Selected NAVTEX Messages

I decided to select a few interesting NAVTEX messages to present here.

ZCZC EA16
WZ 796
ENGLISH CHANNEL.  TSS IN THE STRAIT OF DOVER-WEST, EAST BOUND LANE.
1.  UNIDENTIFIED BLACK OBJECT REPORTED IN 50-24.6N 000-27.5E.
2.  CANCEL THIS MESSAGE 301600 UTC AUG 19.
NNNN

From Niton, their transmissions seem to have no clear identification, but there is a “WZ” header to the messages from UK stations. Most NAVTEX messages are either weather or related t