This is simply a fresh FSK signal for me that can be heard starting mid-morning, at least on that frequency, with a good SNR utilizing a KiwiSDR located in the UK [1]. Coming to the signal' main parameters, the FSK central frequency is 6938.55 KHz, 120 Hz shift and an apparently(!) keying velocity of 31 Baud (or bps, since the FSK mode): Figure 1 show these values.
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| Fig. 1 |
I wrote "apparent modulation speed" due to the fact that by demodulating the signal at a velocity of 31 Baud, duplicate bit values are obtained, as can be seen in Figure 2: a sign that the real velocity is half that detected with a superficial analysis.
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| Fig. 2 |
Indeed, the duration of a bit of information is about 64 ms, while the "raster" of 8 bits measures 511.4 ms (Figure 3): any very simple calculations indicate a modulation velocity of 15.6 Baud. As expected, in this case the obtained bitstream correctly shows the reversals "01"s series (Figure 4).
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| Fig. 3 |
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| Fig. 4 |
Direction uncovering attempts (TDoA algorithm) seem to point to an area in South West England as a possible transmitter site (Figure 5). hard to say something about the users and the purposes.
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| Fig. 5 |
As a final note, Figure 6 shows that the phases of the 2 frequencies are not constant and change after all switch: sign that a single oscillator, possibly a VCO, is utilized (if 2 distinct generators were utilized we would see no phase changes).
By the way, since Figure 6 shows the durations of 2 periods, it's possible to come back to the 2 tones frequency:
2: 0.003427 = 583.6 Hz
2: 0.004315 = 463.5 HZ
ie just 120 Hz shift.
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| Fig. 6 |
https://disk.yandex.com/d/lAp4MSgpbVRZ7g
