Originally Posted by Meindert Sprang

"Larry Tonnade" JUNK wrote in message
...

Hello all,
I'm trying to understand AIS messages that are passed around between
boats.
My sotware job is requiring I write or reuse a parser.
I've read some documentation by Eric S Raymond but he's not all that
clear for a beginner like me.
Now to begin with the messages are typically in a NMEA0183 format, which
I wont explain here. In the case of AIS they begin with
AIVDM or AIVDO, then there are three fields:
1) a number that indicates the number of NMEA messages that are used to
send one AIS message

No, it is the number of NMEA *sentences* to send one message

2) a number that indicates the message number (must be less or equal to
the previous)

Again, the *sentence* number

3) a sequence number for multi-sentence messages.

This is indeed the *message* number.

An AIS message can be spread over more than one sentence, this is taken case
of with the first to fields and an AIS message can also have a sequence
number, which is represented by the third field.

From the top down:
You can have message 0 to 9 (rolling over) and each message can have up to 9
sentences.

Meindert