This is a new FSK signal for me that can be heard starting mid-morning, at least on that frequency, with a good SNR using 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 speed 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" because by demodulating the signal at a speed of 31 Baud, duplicate bit values are obtained, as can be seen in Figure 2: a sign that the real speed 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): some very simple calculations indicate a modulation speed of 15.6 Baud. As expected, in this case the obtained bitstream correctly shows the reversals "01"s sequence (Figure 4).
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| Fig. 3 |
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| Fig. 4 |
Direction Finding attempts (TDoA algorithm) seem to point to an area in South West England as a possible transmitter site (Figure 5). Difficult 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 two frequencies are not constant and change after every switch: sign that a single oscillator, maybe a VCO, is used (if two distinct generators were used we would see no phase changes).
By the way, since Figure 6 shows the durations of two periods, it's possible to come back to the two 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
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