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 a well defined 100Hz keying at transitions edge (Figure 1):

Fig. 1 - 10Hz FM: a) after phase detector, b) SA scan-raster method
although the raster shows 50 symbols/sec as the value of the modulation speed: maybe the 100Hz keying si due to a filtering failure. After filtering the signal to 50Bd value, it's possible to get a 1000Hz value for the frequency shift (Fig. 2).

Fig. 2

Some notes about the demodulated stream, thanks to  radioscanner friends Karapuz and Cryptomaster for their advices and comments.
It's worth noting the presence of 2 x 70 bit repeated sequences in the preamble (likely the Message Indicator), as in Fig. 3, 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. 4). 


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