bugGPSD - Bugs: bug #40298, AIS type 27 too strict size check

 
 

bug #40298: AIS type 27 too strict size check

Submitted by:  Marius Kintel <kintel>
Submitted on:  Fri 18 Oct 2013 06:51:35 AM UTC  
 
Category: NoneSeverity: 3 - Normal
Item Group: NoneStatus: Fixed
Privacy: PublicAssigned to: None
Open/Closed: Closed

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Fri 18 Oct 2013 03:42:19 PM UTC, comment #3:

That at least matches my decode of the sentence.

Eric S. Raymond <esr>
Project Administrator
Fri 18 Oct 2013 03:02:12 PM UTC, comment #2:

This is the output from another decoder for the mentioned type 27 message.
(I wrote that other decoder and haven't done vigorous testing, so don't trust the results blindly).

gnss: 0,
course: 167.0,
speed: 57.0,
raim: 0,
status: 2,
accuracy: 0,
repeat: 1,
lon: 137.02333333333334,
lat: 4.84,
mmsi: 206914217

Marius Kintel <kintel>
Fri 18 Oct 2013 02:56:37 PM UTC, comment #1:

I'm pretty sure this is a simple implementation error. The underlying cellular-radio prodocol uses packets woth 168-bit payloads. Probably the implementor just botched a length.

I've done as you suggested - issued a warning and then decoded the first 96 bits.

>I can try to get hold of more type 27 messages if you're interested in test data.


A 96-bit one would be nice. Even better would be a type 27 together with a decoding from a different AIS decoder- I have no test pairs for this type.

Eric S. Raymond <esr>
Project Administrator
Fri 18 Oct 2013 06:51:35 AM UTC, original submission:

AIS type 27 should, according to the AIVDM document, be 96 bits long.
In the wild, however, I see type 27 messages of 168 bits in length, e.g.

!AIVDM,1,1,,B,KC5E2b@U19PFdLbMuc5=ROv62<7m,0*16

I'm not sure what this means in practice, and if the first 96 bits are good in this case.
One suggestion would be to issue a warning, but to keep decoding if the length is at least 96 bits.

I can try to get hold of more type 27 messages if you're interested in test data.

Marius Kintel <kintel>

 

(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

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  • -unavailable- added by kintel (Submitted the item)
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    Follow 2 latest changes.

    Date Changed By Updated Field Previous Value => Replaced By
    Fri 22 Nov 2013 01:44:54 PM UTCesrOpen/ClosedOpen=>Closed
    Fri 18 Oct 2013 02:56:37 PM UTCesrStatusNone=>Fixed

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