This is an ARQ ISB system heard on 6231.5 KHz/USB (Maritime Mobile band), 2800Hz offset from carrier, around 0630z on 12 October [1].
Most likely the two 3 KHz side bands are used as follows:
on USB: user data transfer using adaptive 2400 Baud PSK-2 and QPSK burst waveform, according to the channel conditions and the feasible data-rate;
on LSB: link and traffic management (ACKs and mode used for data transfer) using 1500 Baud PSK-2/QPSK waveform;
it is not clear if the adaptive mode is driven by the receive peer.
At least in this sample, data bursts last 2088 msec for PSK-2 and 3144 msec for QPSK, the initial bursts have a duration of 520msec; link management bursts last 310 msec.
I know that KLN Netwkors is testing a proprietary 3GHF system to support the ship traffic in Artic regions [2]. In that high latitudes satcomm links can't be easily used since geostationary satellites do not cover these areas, moreover due to the long dark periods the low portion of HF shall be used (lack of F layers). Perhaps may catches are related to their tests, but it's only my guess (I emailed them to ask a confirm and shed some ligth on this system).
Happy to read your comments
[1] signal download
[2] https://www.kyynel.net/news.html
