Some interesting transmissions were noticed last weekend on 5074.20 KHz/USB, transmissions consisting of continuous blocks of different durations and sent using the standard MS-110A modem with a fixed data rate of 1200 bps/S. Judging by the intensity of the signals and the fading patterns shown in the FFT-Spectrum, the transmissions were one-sided, ie PtP or PtMP (like a broadcast style).
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| Fig. 1 |
Analyzing the resulting bitstream after the demodulation of the signals, surprisingly, it can be noted that one bit is replaced by 16 bits during the reversals section (Figure 2).
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| Fig. 2 - 16-bit stream |
However, looking more closely, it can be stated that the 1->8 replacement is adopted during the traffic period, i.e. each data bit is sent 8 times (Figure 3).
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| Fig. 3 - 1to8 bit replacement during traffic period |
In order to get some more information, I reshaped the bitstreams to a 8-bit format and then removed the 7 extra bit columns: as you can see in Figure 4, not all the single transfer sessions have a same period, indeed it may vary from 91 to 111 bit.
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| Fig. 4 |
Also, for some reason that I do not know, there are very few single bits of information, just pairs 11s or 00s; just for a try I arbitrarily replaced the pairs with single bits of the same value: the resulting stream, after the removal of the reversals sections, shows 60-bit patterns. I also tried the differential decoding but I didn't get any other interesting results about the nature of the data and the used transport protocol. My friend cryptomaster, who too heard and analyzed that transmissions, got the same results.
The working frequency (5074.20 KHz/usb) is not among those known or at least it is not reported on the UDXF logs, and given that the transmissions have not been repeated and the strange characteristic of the bitstreams, it could also be test transmissions.
What is certain is the geographic area where the Tx site is located: all the direction finding tries (TDoA method) point to the state of Lower Saxony, Germany (Figures 5,6).
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| Fig. 5 |
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| Fig. 6 |
I just want to report that a similar stream has been noted in some STANAG-4285 transmissions [1]: maybe these "expansions" are used to add redundancy and then increase the reliability of the channel, although HF protocols use their own FEC encoding.
https://disk.yandex.com/d/QgDFohyS8dqeVg
[1] http://i56578-swl.blogspot.com/2022/01/an-odd-16-times-expanded-5n1-framing-uk.html?m=0
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