13 December 2016

unid FSK 50Bd/1000 (prob. CIS Navy occasional test)


Strong - and odd - signal copied on 12376.0 (cf) at 1423 UTC on 11 December. This is probably a totally occasional test since the used fequency and some other points as below. The signal exhibits numerous spurious in the AM detector output (Figs 1,2) and maybe the cause is the failure of the filter sitting just after the FSK modulator. This is why I filtered and re-filtered the signal in order to get something usable about baudrate and shift values.

Fig. 1
Fig. 2
The SA "scanraster method" shows 50 symbols/sec as the most probable value of the modulation speed, altough this frequency is not stable (Fig. 3). After filtering the signal to 50Bd value, it's possible to get a 1000Hz value for the frequency shift (Fig. 4).
Fig. 3
Fig. 4

Assumed 50Bd/1000Hz as the key values, I proceeded to demodulate the signal using the SA FSK demodulator and with the precious advices and comments from radioscanner.ru friends Karapuz and Cryptomaster.
It's worth noting the presence of 2 x 70 bit repeated sequences in the preamble, as in Fig. 5, which resembles the  CIS Navy T-600 modem (ACF = 70 bit in the preamble only) although the constant 50Bd speed and the shift of 1000 Hz is quite rare for such modem (typical shifts are: 200, 250 and 500 Hz). Also note as the End Of Transmission "000100" sequence contains at least 4 EOT characters and the 4:3 ratio alphabet code (Fig. 6).  It's worth noting that two identical sequences of 70 bits are also intended for installation of keys of a ciphering machine, Cryptomaster say.

Fig. 5
Fig. 6
 


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