[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Gerry Creager - NOAA Affiliate
Subject: Re: [gpsd-dev] [Patch Submission] $GPVTG w/magnetic course
Date: Wed, 13 Jun 2018 17:39:45 -0500

Yo, Gary,

On Wed, Jun 13, 2018 at 3:13 PM, Gary E. Miller <address@hidden> wrote:
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.

Agreed. It was but a matter of time. That said, a half-decent IMU should be able to at least temporarily substiture for a 2D fix and also discipline the GPS solution. But that depends on the vendors/manufacturers to have a clue about what they send out to people, doesn't it?
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?

Agreed. And I'll look at this and other docs.
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!

Old habits die hard. Trimble did this a LONG time ago until someone pointed out to them they were propagating erroneous information that theoretically could be affecting marine autopilots. I digress.
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

"competetive advantage". Argh. 

> 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.

I'm lookin'. It's a non-trivial university, and I might even get the Library to order one. For that matter, I might get the Lab to order me one... I can almost justify it. I'm well aware of the copyright protection. A friend sat on NMEA committees and couldn't even discuss some of the topics of discussion.

I started, prior to sounding off, by looking at some of the vendor docs. Noticed they're all still a bit opaque.ublox isn't too bad; Trimble isn't too useful, as examples.
> 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.

Recall the old saying about standards: Nice thing about 'em is there's so many to choose from. Or in this case, so many ways to misinterpret to your own advantage. Makes writing code hard.
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


Gerry Creager
“Big whorls have little whorls,
That feed on their velocity; 
And little whorls have lesser whorls, 
And so on to viscosity.” 
Lewis Fry Richardson (1881-1953)

reply via email to

[Prev in Thread] Current Thread [Next in Thread]