HF CRY-2001 (Sailor-2001) uses basic rolling code inversion, the transmitted audio bandwidth is split, inverted and spaced separately at an audio frequency point which changes with each over. The short sync burst (100 Baud 170 Hz shift FSK-2) at the beginning and end of each transmission is what lets the receiving unit know at which audio frequency the split has occurred and its spacing so it can be reassembled. This transmission was heard on 6446.0 KHz on USB at 2058 UTC.
One would think that the split/spacing value is easily obtained by demodulating the FSK sync, but it's not so easy. Indeed, each CRY-2001 unit has a 6 digit key assigned to it, in normal operation a user will exchange keys with another approved operator and this key will be used to encrypt the FSK sync burst so without it anyone listening (including other CRY-2001 units), can't get the split/spacing data to reassemble it.
This system is frequently used for comunications between small fishing boats, images and docs are available on the web.
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