8 April 2021

CIS-75 FSK 75Bd/200 (4)

During the last week I have been monitoring the 75Bd/200 FSK transmissions (T-208 equipment?) on 9044.0 Khz (CF): transmissions are on-air during daytime only, are encrypted (likely linear encryption) and appear look like  "fleet broadcast" in the way of T-600 50Bd/200 FSK or NATO S-4285 (ie continuous broadcast). 

Fig. 1

 All TDoA results point to Smolensk area, a Russian military communications center (Figure 2).

Fig. 2

T-208 equipment is announced as QYT9 in CW op-chats, for example:
RCB de RJF94 QYT9 QSX 8573 K.      
RJF94 de RCB OK QYT9 QWH 8573 K.

(RJF94 and RCB negotiate T-208 mode on 8573 Khz)


A similar transmission (75Bd/200 FSK) was heard on 11 Jan 2018 on 4540.0 KHz. In that case, after differential decoding, the stream showed up a clear 365-bit period (Fig. 3) due to the sequence of the scrambler polynomial x^8+x^6+1. The descrambled stream is shown in Figure 4 (thanks to cryptomaster).



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