Furuno INMARSAT-C FELCOM 18 Operator's Manual page 171

SMB: IMO SafetyNet Message Body
$**SMB,xxx.xxx,x,x.x,c---c*hh<CR><LF>
1 2 3 4
1. Total number of sentences needed to transfer the message, 001 to 999
2. Sentence number, 001 to 999
3. Sequential message identifier, 0 to 9
4. Unique message number
5. Message body
SRP: System function ID resolution protocol
See SRP in input section.
Proprietary sentences
PFEC rmset, PFEC rminf, PFEC rmalr.
5
APPX. 6 DIGITAL INTERFACE
AP-37