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Re: [gpsd-dev] [Patch Submission] $GPVTG w/magnetic course

From: Gary E. Miller
Subject: Re: [gpsd-dev] [Patch Submission] $GPVTG w/magnetic course
Date: Wed, 13 Jun 2018 13:13:42 -0700

Yo Gerry!

On Wed, 13 Jun 2018 11:27:19 -0500
Gerry Creager - NOAA Affiliate <address@hidden> wrote:

> VTG implies at least a 2D lock because it cannot determine velocity or
> track without at least the 2D data.

More and more modern GPS now include IMUs.

And for the sake of argument, if we assume that to be true, it is still
bad for the patch to be turning #D fixes into 2D fixes.  Agreed?

Back to the real world, some u-blox 8 explicitly allow for the
repeating of the last know GPVTG, no matter how stale it is!

See the section "7.5 Freezing he Course Over Ground" here:

> Some older receivers could
> extrapolate for some period of time (usually <10 sec) upon loss of
> lock, but I believe that bad habit was abandoned a long time ago.

The time a GPS is allowed to DR is often user programmable.  Newer
u-blox have some seriously good DR options.  Some even have wheel
sensors for automotive applications so they can still calculate
speed with no GPS signal.  Documentation on these new modes is

> Trying to find a current copy of the NMEA standards here in the
> library.

You will not find one.  NMEA standards are very expensive and they
defined their copyright vigorously.  Better to look at several
GPS vendors doc on how they implement GPVTG.

> That said, implementation and spec compliance are not all
> uniform. They are better than when I started playing with these
> receivers!

I'll agree with the first statement, but not the lateer one.  There
are now so many conflicting verions ov NMEA 183 that I can't even say
that one standard is uniform.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  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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