decoding maritime fsk
 

There are numerous hf fsk signal that appear to be
commercial traffic to ships at sea.
(6.486 MHz 6.960 7.591 8.969 9.958 13.529
16.956 etc.)
The shift is about 1 kHz and they are about 70 Baud.
I've demodulated down to a bit stream but the bits
seem to be random.
An autocorrelation was flat.
Anybody got any ideas on how the information might
be coded or encrypted?
Differentially coded?
Convolutionally coded?
DES encrypted?
Other?

--
rb