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Re: [gpsd-dev] Warning messages re: AIS type 24 and type 21 message leng

From: Eric S. Raymond
Subject: Re: [gpsd-dev] Warning messages re: AIS type 24 and type 21 message length
Date: Wed, 20 Aug 2014 08:15:26 -0400
User-agent: Mutt/1.5.21 (2010-09-15)

Roy Barkas <address@hidden>:
> Ok, then how do I reconcile the following case where first gpsd complains
> that 160 is less than 168, then a few seconds later
> complains that 168 is > 160?  Based on your explanation, the first is not
> decoded and the second is decoded, but gpsd seems confused about the
> correct size to look for.  Is this a type 24 part A vs part B issue?
> Aug 20 02:57:39 sop gpsd[3465]: gpsd:ERROR: AIVDM message type 24 size <
> 168 bits (160).
> Aug 20 02:58:12 sop gpsd[3465]: gpsd:WARN: AIVDM message type 24 size > 160
> bits (168).

It's a part A thing.  Here's text O just added to the AIVDM decoding doc:

According to the standard, both the A and B parts are supposed to
be 168 bits. However, in the wild, A parts are often transmitted
with only 160 bits, omitting the 'spare' 7 bits at the end.
Implementers should be permissive about this.
                <a href="";>Eric S. Raymond</a>

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