This FSK signal has the same characteristic switch of the modulation speed (150Bd during reversals, 300Bd in traffic mode), but a different shift (~1000Hz instead of ~500Hz), compared to the one seen here (150-300Bd/500Hz FSK). The different speeds are well visible in the 93.33 ms raster in figure 2. However it must be said that I rounded the value of the shift to the nearest thousand Hz, given that - in some parts and without filtering - the signal can vary from 997 to 1000Hz. By the way, the same rounding was also applied to the measure of the shift of the similar 150-300Bd/500Hz signal.
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| Fig. 1 |
* 24-bit length period with the presence of a "phasing" bit (the column of "0s");
* initial sequence generated by the polynomial x^12+x^10+x^9+x^3+1;
* even parity-check on the differential decoded stream.
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| Fig. 2 |
Me and my friend cryptomaster also verified that the bitstream complies the same (8,24) check matrix generated with the polynomial x^7+x^3+x^2+x+1
1 0 1 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 0 0 0 0 0
0 1 0 1 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 0 0 0 0
0 0 1 0 1 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 0 0 0
0 0 0 1 0 1 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 0 0
0 0 1 1 1 0 0 1 1 1 0 1 0 1 1 1 0 0 0 0 1 0 0 0
1 0 1 0 1 1 1 0 0 0 0 1 0 0 1 1 0 0 0 0 0 1 0 0
0 1 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
that detects and fixes the polarity reversal of the ninth column of the bitstream and performs a H(24,16) coding (figure3).
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| Fig. 3 |
That said, we think that these two FSK waveforms belong to the same system although user and purposes are still unidentified. Monitoring is quite difficult since - apparently - a scheduling does not seem used (short transmissions have been heard "by chance" on 4535, 6511, and 8084 KHz).
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