unid PSK8 2400Bd burst waveform

Unid PSK8 2400 Baud burst waveform spotted on 10557.0 KHz (CF) by using OZ1AEF KiwiSDR (Skanderborg, Denmark). ACF value is 293.4 ms, which makes a period length of 704 tribit symbols or 2112-bit length frames (figs 1,2). The resulting bitstream after demodulation does not have a well defined structure formed by known/unknown data blocks,  it could be a Walsh modulation but it is only a guess.

Fig. 1

Fig. 2

As a test I tried to analyze the bursts using a STANAG-4538 demodulator and surprisingly the decoder reacts but only to the first 700ms of each burst by identifying a 256 symbols preamble and a subsequent block of data (960 symbols), even if not identifying any of the waveforms that it knows (BW1-BW7) and thus reporting the phase positions only:

6231017000770100000702070000001700000107010107000701070000000000
0000001001000070000010000000000107000000000000000010000000000000
0000000000000000000000000000000000000000000000000000000000070000
2357710003120162744325456425622470623725424155361132757073500120
 
0014030174251457266112142317764402235571057447051475133602436767
2703175471117746713161441063517300743117565244064305766450657067
3543135630554510775272301561043433561260120020160261523037545324
2056267511155732621360060321622731415212020343736426773247451624
1334736342650470251144547564221770112446074633674036402257132565
6167206436000663560205033075240623327644201057731763021170310231
2243202452744340766416127045073236601451345335341351405636207065
7530151204440026515461331365415522030410171723262531566213715260
5435401652143142551606034474235200464762472263162671465355421572
7246675441411506774573373574776501311646273035007266555622770517
4374175707126456760404241210370111043001250537550271063761077252
5455361471666153216322237212306173775100324261130557670052346734
3347363427504702511445475642217771124460746336740364022571325656
1672064360006635602050330752406233276442010577317630211703102312
2432024527443407664161270450732366014513453353423514056362070657

Although this may be a false positive (perhaps due to the modulation used?), the fact that the decoder somehow recognizes it as a modulation of the 4538 BW family is to be taken into consideration. The periods of the relative bitstreams are however in contrast to the value measured by the ACF (figure 3) but it could due to the short segment which is recognized and demodulated (700ms) and by now I still prefer to rely on SA.

Fig. 3

 As a final note, it's worth noting a 44ms segment which is transmitted before data (figure 4):

Fig. 4