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Re: Type 5 "to_bow"

From: Gary E. Miller
Subject: Re: Type 5 "to_bow"
Date: Mon, 23 Aug 2021 11:06:25 -0700

Yo Charles!

On Mon, 23 Aug 2021 06:43:36 -0700
Charles Douglass <> wrote:

> I'm curious why none of the class A transceivers seem to send 
> "to_bow"/"to_stern" info in their type 5 messages.  I'm in the
> Pacific Northwest so I see international freighter traffic as well as
> state passenger ferries and other commercial vessels that are class A.

Sadly you will find few AIS experts around here.

> Am I possibly configuring something wrong?

gpsd tries hard to avoid configuration options.  It has no AIS options.
The only processing gpsd does is to convert the incoming AIS native into
JSON.  It does no other interpolation, translation, etc.

There are of course, bugs.  

Looking at drivers_driver_ais.c, at the Type 5 decode, those 2 variables
are decoded:

        ais->type5.to_bow       = UBITS(240, 9);
        ais->type5.to_stern     = UBITS(249, 9);

And looking in libgps/, I see that to_bow and to_stern are

        "initname": "json_ais5",
        "headers": ("AIS_HEADER",),
        "structname": "ais->type5",
        "fieldmap": (
            # fieldname        type            default
            ('imo',           'uinteger',      '0'),
            ('ais_version',   'uinteger',      '0'),
            ('callsign',      'string',        None),
            ('shipname',      'string',        None),
            ('shiptype',      'uinteger',      '0'),
            ('shiptype_text', 'ignore',        None),
            ('to_bow',        'uinteger',      '0'),
            ('to_stern',      'uinteger',      '0'),

You have to look at your incoming AIS to see if to_bow and To-stern are
ever sent.

> The type 24 messages for class B correctly include "to_bow" and
> "to_stern".

I imagine this is like NMEA 0183, where different devices encode things

If you can send a binary log of a type 5 with to_bow, I could see
if is is properly handled.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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