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HF Mystery Signals / MOVED: 6660 LSB 17 SEPT 2023
« on: September 21, 2023, 1425 UTC »

Utility / Civilian aircraft on 11175 kHz USB, twice
« on: April 16, 2023, 0128 UTC »
Heard something on the HF-GCS network today that I don't think I have heard before. Two different regularly scheduled United Airlines flight, at different times, requested a radio check on 11175 kHz, and both times got a response from West Coast.

At about 2218 UTC on April 15, 2023, United 1948 requested a radio check from Mainsail on 11175 kHz. "Mainsail, Mainsail, United 1948 radio check on 11175 kHz". This was sent twice, two different transmissions. The response after the second transmission was "Last caller, last caller, this is West Coast, West Coast, please say again last, over". From United 1948 "I've got you loud and clear, I was just trying a radio check, I might try one more frequency also". From West Coast "Lima Charlie on triple one seven five, over". From United 1948 "thank you for that".

A few seconds later, from United 1948, "West Coast, West Coast, United Nineteen-Fortyeight on triple one seventy five". Form West Coast "Last caller, last caller, this is West Coast, have you Lima Charlie, over". From United 1948 "United 1948 has you Lima Charlie, thank you very much for the checks".

Checking flight schedules, at that time United 1948 was en route and a couple of hours out from Cancun to San Francisco.

Shortly after this set of transmissions I heard United 1948 working San Fran on 11282 kHz USB.

At about 0042 UTC on April 16, 2023, about 2 and a half hours later, United 519 requested a radio check. This time I really could not make out much of United 519's traffic, but I heard all of the HF-GCS (West Coast) response. From West Coast "Last caller, last caller, this is West Coast, please say again last, over". From West Coast "United 519 this is West Coast, I have you Lima Charlie, over". A bit later, from West Coast "Last caller, last caller, I have you Lima Charlie, over". From West Coast "Untied 519 this is West Coast, I have you Lima Charlie, over".

Again checking the flight schedules, at that time United 519 was en route from Washington and a couple of hours out of San Francisco.


KMUD on 6935 AM, with CW and voice ID, starting at about 0255 UTC, 17 April, 2022.

S8 here (edit, 10 over S9 on the right antenna).


(edit) at about 0346 UTC KMUD took a big dive, from  10+ over S9 to S4 or less in about 60 seconds.  At the same time 6930 USB came up in signal.  Looks like a major band condition change at that time.


I have been meaning to write up something on the MFSK Oddity that has been being reported by multiple people to this forum over the past year+.  Unfortunately, real life has gotten in the way and I have not had time to do it.

So I am going to pull all (well, all I can find) the threads together under this one thread and include a couple recordings I have made.

(edit)  Well, that did not work.  I will link the other threads to this one, not merge the threads.

For those not familiar, the MFSK Oddity is a signal that has been showing up since at least mid 2020, and I suspect it was around a bit before that.  Further, I think some of the phantom "1 PPS" signals before that time may be related.

Some listeners tie this signal to HFT (High Frequency Trading).  I do not think that has been proven, but geolocation via TDOA and HFDF, searches of related frequency authorizations for HFT license, and similar frequency use / habits of other HFT tied signals make the HFT association very possible, if not probable.  If not quite proven it is a best guess.  10Band LLC and related businesses are likely candidates.

The arguments against HFT are that the data rate is too slow to be useful for that service, and the data also appears static, unchanging, for very long periods of time.  If you are leveraging RF vs fiber to gain microseconds of advantage, why use a data format that sends bits in 10's or 100's of millisecond intervals?

I have measured bandwidths of about 2.6, 4, and 10 kHz, I have seen other bandwidths but they have been too weak for me to get a decent estimate of the occupied bandwidth.

I have seen pulse rates of between 200 Hz (5 msec pulse interval) to 1 Hz (1 second pulse interval).  I have seen repeating "bit patterns" (for lack of better term) of 8, 16, and 32 bits.

The signal appears to be MFSK like.  Since late 2020 it has generally appeared to be 4 channels of data (prior to that it was 3 channels, or less), each channel with 4 FSK like steps.  There may be some other modulation on each pulse, but if so I have not been able to confirm it.  Arbitrarily, for my notes, I have called the lowest frequency channel of data Ch1, and moving up in freq Ch2, Ch3, and Ch4.  Within each channel I have called the tones, again low freq to high, 1, 2, 3, and 4.

These signals move frequencies regularly, probably as needed for propagation.

Some videos below, sorry, they are not nicely annotated or explained, but at least they are recordings.  The frequencies listed are center frequency.

4 kHz bandwidth, 4 Channel, 16 bit, 1000 msec interval, 15910 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 100 msec interval, 14903 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 66.7 msec interval, 10160 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 50 msec interval, 10170 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 33.3 msec interval, 13909 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 20 msec interval, 13928 kHz

4 kHz bandwidth, 4 Channel, 16 bit, 10 msec interval, 10807 kHz

4 kHz bandwidth, 4 Channel, 32 bit, 100 msec interval, 15905 kHz

4 kHz bandwidth, 4 Channel, 32 bit, 5 msec interval, 14500 kHz

2.6 kHz bandwidth, 4 Channel, 3 step, 16 bit, 100 msec, 6993 and 6988 kHz

2.6 kHz bandwidth, 4 Channel, 3 step, 16 bit, 50 msec interval, 14480 kHz


Related threads:

UNID 9060 Digi

UNID 6940.0 kHz (CF) 00:00 UTC 01 FEBRUARY 2022

UNID 6985 (CF) 1310 UTC 09 JAN 2022

UNID 6988 USB 2223 UTC 09 DEC 2021

UNID 6965 (CF) 2200 UTC 09 DEC 2021

UNID 6970 2019 UTC 19 NOV 2021--see waterfall

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