Saab Grintek MHF-50 "preamble" variant

Saab Grintek MHF-50 variant recorded on 8603.0 KHz/USB using the KiwiSDR located at TWR Kempton Park, South Africa: notice that 8603.0 Khz is believed to be one of working channels used by South African Navy.

The signal has a kind of "preamble" (Figs. 1,2) which is followed by the usual multimode waveform. This preamble consists of a short 75Bd/170 FSK

010100101001010010100101001010010100101001010010100101001010010100101

followed by a 120 sec long 1622 Hz tone which exhibits interesting markers each 12 & 1 seconds maybe to serve sync purposes.

Fig.1

Fig.2

As said, the preamble is followed by the well known multimode waveform (Figs. 3,4) consisting of 54.4Bd/390 FSK and 54.4Bd/65 MFSK-33 with the characteristic 3 tones signaling the EOM.

Fig. 3

Fig. 4

6 June 2020 update

As noted by my friend KarapuZ, the 75Bd/170 FSK segment 

010100101001010010100101001010010100101001010010100101001010010100101

can be successfully descrambled using the polynomial x^8 + x^6 + x + 1; maybe it serves sync purposes for the following MFSK decoder. It's interesting to note that the same polynomial is used in CIS-75 waveform.

It's supposed that some frequencies (4346, 6504, 8580, 12982, ...) are either channel markers, propagation markers and/or FAB channels and some other (4245, 6407, 6493, 8603, ...) are traffic channels; anyway, it seems that they carry different patterns.

 

https://yadi.sk/d/B3M6yXgKxU_81Q