23 May 2018

French Navy Broadcast, FSK 50Bd/850

Following the previous post, I decided to analyze the French FSK 50Bd/850 in order to replicate the results obtained by my friend Christoph Mayer in his blog: happy to say that the results coincide.

French-Ny FSK 50Bd/850 has a characteristic 21-bit period and, in a way similar to STANAG-5065, two/three sub-frames which are delimited by the bits of two LFSR markers M1 M2 and a logical "1" value bit (1-bit). The sequences for the two markers are generated by the LFSRs described by the polynomials x^6+x^5+1 and x^7+x^6+1.
Since the missing of an initial sync in the recording, the 1-bit can occur in any position within the 21-bit frame therefore markers' positions are reported as relative to the 1-bit. A possible more significative layout shows 2 possible sub-frames (5 and 13 bits length).

Obviously, the length of the markers' patterns match the length of the respective generator polynomials.

One of such transmissions  heard on 8516.8/usb (offset + 2000Hz) has DF'ed to "La Regine" (Saissac), callsign of the station is FUG

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