13 June 2024

async GFSK 300Bd/200 bursts (unid)

Some days ago my friend cryptomaster sent me an unid FSK transmission heard for several consecutive days at the frequency of 5114.0 kHz USB. Main waveform parameters are a keying speed of 300 Bd and a shift of 200 Hz: clearly a GFSK modulation (Figure 1).

Fig. 1 - GFSK modulation

The signal is not continuous as it consists of GFSK "bursts" lasting approximately 632ms separated by the transmission of only the higher frequency, the whole has a duration of approximately 1093 ms (Figure 2).

Fig. 2

Consequently, as can be seen from the bitmap in Figure 3, the signal has a period of 1093.83 ms (328 bits) which, in my opinion, is due to the particular formation of the signal and not to the structure of the transported data.

Fig. 3 - bitmap of the signal period

Each data segment is 8N1 framed and since its lasting of about 632ms, consists of 190 bits (speed is 300 Bd). Note that it seems that every data segment starts and ends with the same "sequences".

Fig. 4

Fig. 5

After removing the start/stop bits, the analysis of the code did not reveal anything structured or known.