Reception of South African Navy in MFSK-33 mode

 Good propagation conditions on short waves favour reception of distant utility stations located on other continents. One of these stations is using quite interesting multimode format which consists of 50 Bd RTTY with 170 Hz shift, 54.5 Bd FSK and 2200 Hz wide MFSK-33 parts. Such sequence is formed by Saab Grintek MHF-50 MFSK Modem used by utility stations of South African Navy.

Transmissions of the station with tentative call-sign ZSJ near Capetown can be currently heard in Kyiv at night time on the frequency of 6504 kHz. Here are results of reception on 25th of September 2020 at 23.11 UTC:

One can see alternating FSK and MFSK parts of the transmission. The signal mode cannot be confused with other ones due to its unique structure. The sequencies can also be heard on my audio sample of reception on 6504 kHz:

Results of TDoA analysis confirm South African origin of this signal:

RX: Airspy Mini SDR + SpyVerter

Ant: 10 m external random wire