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Re: [gpsd-users] Communicating NMEA to gpsd as if my software was a GPS

From: Gary E. Miller
Subject: Re: [gpsd-users] Communicating NMEA to gpsd as if my software was a GPS device
Date: Fri, 6 Sep 2019 10:45:22 -0700

Yo Ellon!

On Fri, 6 Sep 2019 14:56:26 +0200
Ellon Paiva <address@hidden> wrote:

> >>> Field 10, HDOP.  Missing.  
> >> This was in purpose, as I am not really simulating localization
> >> from satellites.  
> > And yet you include accuracy estimates which are derived from the
> > DOPs, which are derived from the skyview.  
> My accuracy estimates come a covariance matrix from which I extract
> the error ellipsoid and standard deviations used to fill the fields
> of a GST sentence. I know that these fields are supposed to have
> information derived from skyview, but I think skyview doesn't apply
> here.

If you are confident in your values, then put them in your data output.
Just be on the watch any unexpected behavior in gpsd dues to their

> I think I could find a way to derive HDOP from the same covariance 
> matrix... but first I need to understand better the HDOP concept on

Easy "error = xDOP * Uere".

Where Uere is a variable broadcast by the GNSS system to tell the user
about how good the current signals are relative to optimal.

Current Uere is about 9.4.  So right now, on one GNSS, I see

7.80288 m Xerr = 0.8 XDOP * 9.4

But there is no need for you to provide the DOP's if your error
estimates are "good".  Sadly I never see "good" error estimates...

> By the way, I tried to understand the "226 degrees east" problem, and
> in fact I realized it's not a problem! In the sentence you mentioned
> $GPGGA,103642.879013,4850.432180742773,N,226.381659556439,E,7,08,,0,M,0,M,,*73
> the lon/lat are expressed in "Degrees Decimal Minutes" as I
> understood that's the way they should be described. So longitude 
> "226.381659556439,E" in fact means "2 degrees and 26.381659556439 
> minutes east". The same thing with the latitude:
> "4850.432180742773,N" means "48 degrees and 50.432180742773 minutes
> north".

Uh, oh.  My mistake, I read that too fast, you are correct.  226.38 is
indeed good.

> > And gpsd is not the gold standard, you likely want your psuedo NMEA
> > to work for most NMEA decoders.  
> In fact I would like it to work in gpsd because gpsd is already used
> in our project. Indeed having it working with most NMEA decoders
> would be a bonus, but we're not interested on that (as far as I know).

And to work with gpsd you just need to stay close to the spec and/or
common usage.  Even though you data is clearly uniquely created.

I'm sure with a bit of work we can make the two parts work together.

When you have another set of NMEA output, send it along and we'll
both take a closer look at it.

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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