26 May 2017

FSK 100Bd/620, Polish Intel

update (26May17)

This morning I copied a transmission on 10211.6 KHz/USB with same ACF (96 bit) but with different frame structure that exhibits a sort of 24-bit length "preamble" followed  by 16-bit length data blocks (8+8 UK) and resembling, in some way, the Stanag/MIL-STD framing.



FSK 100Bd/620, Polish Intel (10Feb16)

FSK-2 modulation with 620 Hz shift and 100 Baud speed, in use by Polish Intel service.

pic. 1 - baudrate and shift
The signal exhibits clean ACF spikes at 96 bits (~960ms lenght). Looking more closely one can distinguish six periods of 16 bits, most likely they represent a five digits code and a separator between the groups as also shown in the bitstream (pic. 3).

pic. 2 - 96 bits ACF (6 x 16 bits digits)
pic. 3
Looking at the whole demodulated bitstream (pic. 4), although inclusive of the overhead bits, we can get an idea of the signal structure. First of all, note that the ACF is due to the final part "D" and then does not characterize the entire signal but only a part. While the parts A and E can be assumed as the 'start' and 'end' of transmission, it's difficult to fix the roles of the other groups: message, destination address, coded-instructions and so on. Talking with my friend KarapuZ, he says we need much more recordings for statistic comparisons since a previous recording of 2014 just exhibits that same characteristics. And if he says so...

pic. 4

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