French Ny FSK Fleet Broadcast

Last update: 29th March, 2021

French-Ny FSK 50Bd/850 has a characteristic 21-bit period (say a kind of "domestic" framing) and, in a way similar to STANAG-5065, two (or three) sub-frames which are delimited by the bits of two markers M1 M2 and a logical "1" bit (Figure 1). 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, the "1" bit can occur in any position within the 21-bit frame therefore markers' positions are reported as relative to this bit. The possible layout in Figure 2 shows two possible sub-frames of  5 and 13 bits length.
All these transmissions, FSK and STANAG-4285, are sourced from the French Navy complex locate in La Regine (Saissac) whic uses the callsign FUG.

Fig. 1

Fig. 2

 Frequencies/modes active (at present)

Freq. (cf) Service Modulation Framing Notes
2065.80nighttimeFSK21-bit
6431.80nighttimeFSK,S-4285KW-46(*)
8566.80nighttimeFSK21-bit
6367.30daytimeFSK21-bit
8518.80daytimeFSK,S-4285KW-46(*)
13419.80daytimeFSK21-bit

Although the 21-bit period is a distinctive sign of such transmissions, it's worth noting that there is always one transmission atleast that uses the NATO standard framing with KW-46 encryption and that is active either in daytime (8518.80 KHz Figure 3) and nighttime (6431.80 KHz Figure 4), probably it's due to compatibility requirements with NATO traffic (1). The two frequencies shown above can use both the FSK and STANAG-4285 600bps/L waveforms indifferently: perhaps a long-term monitoring could reveal more details about their scheduling. Notice that the STANAG-4285 tuning frequencies are are exactly the FSK center frequency minus 1800Hz

Fig. 3

Fig. 4

1) A similar behavior, ie the use of the 21-bit & NATO framings, was also observed in some MSK subComm  transmissions on 65.8 KHz from French Navy Kerlouan (FUE).  As shown in Fig. 5, X1 and X2 channels use the same 21-bit format of 50Bd/850 FSK, the other two channels Y1 and Y2 use the 14-bit frames with KW-46 encryption.

Fig. 5

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