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

[Eric S. Raymond]

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="http://www.catb.org/~esr/";>Eric S. Raymond</a>