Akula 250Bd/500 FSK version

A friend of mine, whom I'm grateful, sent me this interesting and quite rare example of the Akula 250Bd/500 FSK waveform. Transmission was recorded on 9202 KHz around 0800 UTC using a Japanese SDR: as you see, the values of the FSK parameters are the just the half of the usual ones (500Bd/1000) 

Fig. 1 - Akula 250Bd/500

The demodulated bitstream shows the normal Akula "stuff" (Figure 2):
- reversals
- sync group (6 code words followed by 6-bit "0"s separator)
- preamble group (7 code words with two different, but varying values arranged as 4 x 1st code word + 3 x 2nd code word)
- data block
- End-Of-Message group + EOT group (which never varies and consists of the five code words 010000 011101 011101 010000 100001)

Fig. 2 - Akula 250Bd/500 demodulated bitstream

It's worth noting in Figure 1,3 the continuous carrier in absence of messages, as already seen in other recordings [1]: probably this is due to the adopted ship-shore "paradigma". While in many of the western navies the shore stations are used to broadcast a list of available listening frequency (FABs/CARBs) (1), it could be that the Russian shore stations transmit a carrier on their known listening frequencies at scheduled times on behalf of subs which have something to comunicate to the shore station itself.  That's obviously my and my frield's guess.

Fig. 3

https://disk.yandex.com/d/kGyl5tQco6hKWg

(1) CARB is the acronym for Channel Availability and Receipt Broadcast, these transmissions radiate information on the frequencies available for ship-shore traffic and to pass control and receipt messages; sometimes also indicated as FAB or Frequency Availability Broadcast. These procedures are used to automatically perform a channel-link before a message could be sent [2]. 

[1] http://i56578-swl.blogspot.com/2022/04/akula-quite-unusual-session.html
[2] http://i56578-swl.blogspot.com/search/label/FAB