Akula always reserves surprises...

A few days ago a friend of mine sent me some recordings of a serie of FSK bursts which had the same keying speed and shift (500Bd/1000Hz) as the "Akula" waveform but which differed due to the lack 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 exactly the same as Akula (see Figure 2).

Fig. 2 - some of the demodulated bitstreams

There and then I gave up and thought of a common EOM + EOT sequences perhaps also used in other 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 other "complete" bursts (1): say a kind of  Akula "data-only" burst (Figure 3). I had ever seen it before. 

Fig. 3 - a so-called Akula "data-only" burst

 

Could it be the same "physical" (possibly faulty) modem? Difficult to say. My friend's recordings date back to April 30th (three days ago) and made using a remote KiwiSDR in Azumino-city, Nagano Japan and therefore probably a vessel on-going in the Pacific Ocean; my recordings were made using an AirSpy server located in Tofta, Goland Is. Sweden and with an excellent SNR value: a clue of a vessel on-going in nearby waters. And yes, one could object that the propagation takes strange 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 quite unlikely given the time and the used frequency (Figure 4).

Fig. 4 - VOACAP chart

 

Among other things, the durations of the Akula transmissions recorded in Japan are unusual compared to those we are used to seeing, i.e. just short transmissions consisting of a few bursts likely to avoid triangulation by the "foe".

The question remains: faulty modems? a mode of Akula messaging that I don't know or have never met? or just mere coincidences or wrong receiver settings (ie AGC)?
Further successful registrations will (hopefully) help...

https://disk.yandex.com/d/ff_qZIkTWOwJyw

 

(1) As said, the other bursts of my recorded transmission have the Akula well-known format (1), ie:
- sync group (6 code words: 4x100101 + 2x110001) followed by 6-bit "0"s separator
- preamble group (7 code words arranged as: 4x1st code word + 3x2nd code word)
- data
- End-Of-Message group + EOT group (five code words: 101111 100010 100010 101111 011110)