New discovery: Source of Selcall signals at 7.1 and 8.0 MHz in Region 3

edited June 3 in Signals Received

While debugging the new DSC decoder (in NAVTEX extension) I happened to notice some DSC-like signals around 7.1 and 8.0 MHz using Kiwis in Region 3 (East Asia, Oceania). I used the Kiwi's FSK extension in "framing" mode to study them. They didn't start to decode until mark/space inversion was used. Even then what I saw wasn't quite the DSC protocol, although there were similarities. The signals appeared in a dozen different frequency channels on both bands. In many channels transmissions overlapped in time making decoding difficult.

It finally dawned on me that this was the old HF Selcall format pioneered by Codan and others in Australia and refined for amateur use by Bonnie, KQ6XA: HFLINK Selcall

Selcall is not to be confused with HF SELCAL which is the two-burst twin-tone signaling system used by aviation.

Some Googling quickly found that the venerable Tony has seen this also: I56578-swl blog

But the story gets confusing from here. The original Australian Selcall format using 4 and 6-digit calling is proprietary. Later, the UN apparently published an open source version using only 6-digit calling: CCIR 493-4/UN And commercial equipment brochures mention supporting both the proprietary and open standards. Confusingly, the ITU's DSC spec also contains the "493" designation: ITU-R M.493 (note however the "M.")

Some careful on-air monitoring was the only way to figure out the protocol differences. A Selcall decoder was added to the Kiwi's NAVTEX extension. And by software version v1.537 the format of the 7.1/8.0 MHz signals was mostly understood and decoded. Fortunately, these messages are format type "automatic calling" containing position information. So each one could be plotted on a map as is done for aircraft positions with the HFDL extension. These signals originate in the South China Sea mostly off the coast of Vietnam. So the presumption is they are associated with fishing such as drift net marker buoys.

Below is the Selcall decoder in action. Note the new "Selcall HF" menu on the right of the extension control panel. On the map decodes from the 7.1 MHz band are in magenta, 8.0 MHz in blue. The decoded messages show each protocol word, changing color to separate command words (>= 100) from BCD digits (00-99). 4-digit calling is preceeded by "__" to match the spacing of 6-digit calling. Links to a map at are generated from the decoded lat/lon. It is unknown if the message lat/lon is specified as decimal degrees (DD) or degrees and minutes (DMM). There is some evidence it is DMM (links for both interpretations are given). The map marker will be red if the minute value is more than 59. Please let me know if you see this.

It isn't clear to me if operation at 7.1 MHz constitutes an intrusion into the amateur band. ITU regulation 5.151B allows primary operation of the fixed/mobile service (7.1 - 7.2 MHz) in Vietnam, and many other countries, as an "additional allocation". I don't know what that means for marine use in their coastal waters.

I'd appreciate comments from anyone with additional information about this topic.


  • jksjks
    edited June 3

    If anyone knows of reliable frequencies I can monitor for Selcall signals (or where I can ask) please let me know. Either here or at Information about CCIR 493-4(/UN) nets seems to be somewhat scarce. Below is one I happened to catch, next to the frequency I was actually monitoring. 1931 calling 0239, individual call (120), routine (100). Possibly West Africa given where/when I was looking.


  • edited June 4

    Hi John,

    It may also be worthwhile considering the Australian Outback HF networks which tend to use Codan and Barrett equipment, which I think is compatible with the UN standard.

    There are at least two semi private KiWis that are associated with one of more of these networks. Some of the channel DX tags have notes with 4 digit selcall codes for the relevant base stations.

    This company seem to provide outboard encoders and decoders for use on these networks.



  • edited June 4

    Hi John ,

    Very interesting find and development of this new selcall decoder. Just like Martin I also wonder about the Codan and Barett selcall signals some of which probably also carry position data although likely scrambled.

    Looking at UDXF reports it seems the 8 Mhz frequencies are used by the CN Island Tracking Network with net control at Sansha. Astounding developments have taken place over the last 15 years in the South China sea where an enormous infrastructure is built on what used to be partly submerged reefs.

    The above picture comes from a study found at:

    Just a quick scan of some of the UDXF frequencies reported recently

    8006 8009 8018 8027 8030 8058 8066 8069 8072 8078 8084 8480 8539 8580 8706 9054

    73 Ben

    PS : Currently unfortunately unable to run V1.537 and monitor from Jakarta, because after upgrade from V1.486 to V1.537 Kiwi power up shows an immediate "server has closed connection" message. After several tries I am back to version 486 and hopefully upgrade to a newer version will be successful.

  • Hi Ben,

    Yes the Chinese activity in the South China sea is very disturbing, for all sorts of reasons.

    This website offers more clues about who is doing what and where in that region.



  • Thanks. I'll check for activity on the AU outback networks.

    I found a few things that discount the fishing buoy theory. Some buoy manufacturer info mentioned how they transmit only on demand after receiving a wakeup signal to conserve power, which makes sense. Whereas these signals seem to transmit repeatedly/continuously. But there seem to be too many of them, and spread out over too great an area, to be non-fishing related. The number of fishing vessels alone reported by AIS signals as seen on marine in that area is staggering.

    An interesting technical description of buoy development is here, although no frequencies used are mentioned:

    Another point is that these position signals only report to the nearest minute of lat/lon (i.e. not like CCIR 493-4/UN which has full GPS resolution). A minute of lat/lon at the equator is about 1.8 km. So you'd still have to DF the signal to find your drift net buoy (or use binoculars to see the buoy itself).

  • @benson If you set a temporary admin password and email it to I can take a look and see why after an update admin logins are failing. This assumes you still have a way to get back to a working version if I can't make any progress using only an admin connection. It would be better to also have Linux ssh access to debug an issue like this.

  • Ben, that PDF file from Johns-Hopkins is fascinating (and disturbing). That's a lot of interesting analysis just based on open source info.

  • edited June 6

    @jks Kiwi working again after upgrade to version 1.538. I encountered a problem when adding band bars and have sent an email to support.

    Extracting a bunch of frequencies mainly from UDXF reports and running an automated scan for a couple of hours I found quite a lot active (dial) frequencies with these HF selcall signals and these are shown in the attached txt file. Please disregard the frequencies I mentioned in an earlier post.

    Looking at the reported positions I guess that the majority of these stations are probably related to CN exploration drilling or production of oil, gas or methane hydrate. From the air some really large production platform complexes are visible in the SCS. I haven't checked but years ago there were talks about joint exploration with Vietnam and that could explain many of these stations showing up in their EEZ. Then again when approaching the area off the coast of Da Nang at night the hundreds of lighted fishing vessels make it look like a metropolis at sea.

    To find out about the nature of these stations if someone has access to current AIS data via or marine then a look for position matches might reveal more.

    Driftnets : it seems that the position reporting types have moved to 162 Mhz AIS.

    @johnk5mo Indeed an interesting analysis based on publically available information. Would be good for further discussion elsewhere, probably not in the KiwiSDR forum. I mentioned that pdf because it also describes the HF steerable array antennas used for OTH purposes.

    73 Ben

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