22 December 2020

An odd STANAG-4481F link

Odd STANAG-4481F transmissions consisting of (apparently) continuous KG-84/KIV-7 64-bit sync sequence, spotted on 11222.0 Khz (CF): this is the first stime I hear S-4481F on that frequency. These transmissions have been going on h24 for days and always keeping the same modality.

Fig. 1 - note the oscillations during the mark-space switch

As pointed out by my friend cryptomaster, although k500 decoder recognizes the KG-84 64-bit sync sequence, actually the stream consists of the 63-bit m-sequence generated by the polynomial x^6+x^5+1, or its counterpart x^6+x+1 (Figure 2); this way, the KG-84 sync sequence is obtained by adding one "1" bit. Otherwise, the KG-84 sync sequence may be obtained assuming (as the decoder does) the last bit "1" of the sequence n as the first bit of the sequence n+1, i.e. as if that bit were "in common" bewteen two consecutive sequences (Figure 3). In a few words, decoders are tricked by that 63-bit sequence.
In my opinion, the choice to send that m-sequence is not a "casual" one - they could have used any other test pattern - but raher it's a deliberate choice since its closeness to the KG-84 sync sequence (just one bit) and the fact that KG-84 is largely used in S-4481F links. Interestingly, the stream resulting after the removal of the scrambler consists of  bits all set to "1"; as above, they could have used any other scrambler polynomial.
Fig. 2
Fig. 3
Fig. 4 - a STANAG-4481F decoder working the 11222.0 KHz transmission

It's difficult to say what it is exactly: maybe tests in view of the setup of a new link,  a frequency marker or maybe some trials. Every attempt to find the Tx site by using TDoA method is different, almost surely it's somewhere in the North-East of US, most likely NSS/AFA Davidsonville (Figure 5).
I will update the post as soon as something new comes out. 
Fig. 5a - according to these DF attempts, TX seems located north of Baltimore (likely Davidsonville)
Fig. 5b - other TDoA attempt obtained by selecting receivers from east to west (...still Davidsonville)

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