23 June 2020

CIS Navy 50Bd/500 FSK 136 bit (T600-136)

Yet another 50Bd/500 FSK transmission, this time recorded monday morning on 14704.0 KHz (cf) around 1340Z and almost surely sourced by the CIS Navy T600 system (typical shifts: 200, 250, and 500 Hz): given the 136-bit length frames this waveform is also known as "T600-136".

Fig. 1 - FSK parameters
Note that the full transmission period is 544-bit length, i.e. 4 x 136-bit frames. Indeed, from a quick examination of the demodulated bitstream (Fig. 2), it is easy to see that it's composed of blocks of four repeated frames, probably to add redundancy to the system.

Fig. 2 - CIS-Ny 50Bd/500 bitstream (136-bit frames)
The same 136-bit framing is also used in the CIS-Ny 50Bd/250 FSK, still from a T600 system (Fig. 3): these two waveforms seem to be used to carry the same "type" of messages unlike the CIS-Ny 50Bd/200 FSK which shows a different structure of the frames (70-bit Message Indicator, 4:3 ratio,...) and it's mainly used for fleet broadcast.

Fig. 3 - CIS-Ny 50Bd/250 bitstream (136-bit frames)

Although the shift is a multiple of the manipulation speed, the two tones do not preserve their phase (Fig. 4).

Fig. 4 - CIS-Ny 50Bd/500 tones

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