Akula zawsze rezerwuje niespodzianki...

A fewer days ago a friend of mine sent me any recordings of a serie of FSK bursts which had the same keying velocity and shift (500Bd/1000Hz) as the "Akula" waveform but which differed due to the deficiency of the sync and preamble sequences as well as the IVs, as shown in Figure 1.

Fig. 1

The demodulation of these bursts, however, reserved a surprise: although the sync and preamble groups were missing, the EOM + EOT groups (101111 100010 100010 101111 011110) were precisely the same as Akula (see Figure 2).

Fig. 2 - any of the demodulated bitstreams

There and then I gave up and thought of a common EOM + EOT sequences possibly besides utilized in another CIS waveforms, until this morning I accidentally came across an Akula traffic on 8284.0 KHz (cf)... and I found a burst with the same characteristics, i.e. without the usual sync-preamble-IVs groups, among another "complete" bursts (1): say a kind of Akula "data-only" burst (Figure 3). I had always seen it before.

Fig. 3 - a alleged Akula "data-only" burst

Could it be the same "physical" (possibly faulty) modem? hard to say. My friend's recordings date back to April 30th (three days ago) and made utilizing a distant KiwiSDR in Azumino-city, Nagano Japan and so most likely a vessel on-going in the Pacific Ocean; my recordings were made utilizing an AirSpy server located in Tofta, Goland Is. Sweden and with an excellent SNR value: a clue of a vessel on-going in close waters. And yes, 1 could object that the propagation takes unusual paths, and that's ok, but assuming the same area of origin of the signal (and thus the same vessel/modem), my listening would be rather improbable given the time and the utilized frequency (Figure 4).

Fig. 4 - VOACAP illustration