21 April 2018

FSK-2 300Bd/200, 17-bit ACF 6-bit code

FSK-2 300Bd/200 transfer occupying ~500Hz bandwidth, each cycle lasts 6000ms (4640ms data block, 1360ms idling) and shows 17-bit length ACF. Each data block consists of 738 bits (data + framing), data seem to be sent using a 6-bit code. Very strong in JN52.

Fig. 1
Fig. 2
In Fig. 3, although contents are different, it's possible to see repeated sequences in the packets, expecially the last 12 bits of each packet. Same packets also exhibit a same sequence in their initial part. Figure 4 shows the result of the comparison between the first packet on the left in Fig. 3 and the following 10 packets.

Fig. 3
Fig. 4 - comparison between the first packet on the left in Fig. 3 and the following 10 packets
The transmission lasted for a long time, I followed it about two hours on Thursday evening  (Apr, 19) from 2030z - when I tuned it - and I still found it there the following morning until 0900z when it stopped. A friend from Northern Portugal reported this signal in UDXF list (18 Apr): "It has been on all morning on 3575.65kHz (USB dial freq)".
The same signal is reported in a post of 2012 (27 Jun) in radioscanner, user and purposes of that transmission/protocol are not yet known or verified.

Nice work indeed by 'cryptomaster': use Diff-FSK, remove extra columns, remove odd parity-bit column and then get the 6 bit code. Great!
http://www.radioscanner.ru/forum/topic36750-184.html#msg1372694 

Fig. 5
 


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