"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