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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: Thu, 5 Sep 2019 10:48:17 -0700

Yo Ellon!

On Thu, 5 Sep 2019 14:25:48 +0200
Ellon Paiva <address@hidden> wrote:

> > NMEA only allows 2 digits past the decimal point.  Some GPS use 3
> > or 4.  
> OK, thanks for the info about the number of digits past the decimal
> point.

On re-reading the NMEA 0138 doc, I see I made a mistake.  The spec
allows as many digits as you want.  But it still looks like way more
digits than possible given GNSS accuracy.

> > 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.

> Should I put a value, like one of this table here ? 

A good rule of NMEA, if you don't know the value, leave the field blank.

> > Field 11, altitude: 0
> > Missing decimal point.  
> Thanks!

And on re-reading the spec, I see it is not required to have the decimal
point, just not normal.

> > Field 13, geoid separation: 0
> > Zero?  Really???  
> Yeah, Really. :-) I based myself in the formula h = H + N, and that 
> altitude means H and geoid separation means N. Since I only have z 
> height ("h" in the formula), I thought it was doable to leave geoid 
> separation as zero and set altitude to my z height. It seems I got
> the right value on the gpsd json output.

There are very few places on the planet where the geoid separation is
zero.  If you don't know the correct value, then do not make it up.
gpsd will supply the correct value for you.

> At the end I'm not sure these would disturb gpsd.

I found some of these by looking at the gpsd output.  Some, like the
228 E latitude clearly confused gpsd.  Others, like the variable, or
missing, decimal points, are handled by gpsd.

And gpsd is not the gold standard, you likely want your psuedo NMEA to
work for most NMEA decoders.

I also found other illegal items that I did not mention.  Spend some
time with the spec.

> About the missing fields, it seems that they 
> are just not informed to gpsd clients,

gpsd can pass on the NMEA to clients for further processing.  

Always best to follow Postel's Law.

> which would be a problem if
> the clients expect them, but I think that's not my case...

There are a ton of NMEA clients, very hard to know what they expect, best
to do things by the spec.

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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