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Re: Getting PDOP, VDOP and HDOP in C

From: Gary E. Miller
Subject: Re: Getting PDOP, VDOP and HDOP in C
Date: Wed, 30 Mar 2022 10:59:56 -0700

Yo Greg!

On Wed, 30 Mar 2022 07:28:31 -0400
Greg Troxel <> wrote:

I'll skip the parts I agree with.


> I dimly recall some notion of UERE, User Expected Range Error,
> describing the error in a single pseudorange, and some notion like
>   eph = uere * hdop

Some people, and some code, including some gpsd code, assume that UERE
is a constant.  Instead you use URE from the ephemeris of each satellite.
Except URE is not exlicitly sent, and bunch of quadratic parameters
have to be evaluated to compute it.

See IS-GPS-200L section 6.2.1

Then you have to look at the URA to see how accurate the URE is, and
at the HOW to see if the URE is valid at all.

> If that's really in the ICD, then surely UERE has a definition, and
> the above could be adjusted into those terms, and then it would be a
> bug in a driver not to translate, and a bug in a device to report it
> wrong in NMEA, perhaps fixed by a quirk in the code.

The problem is most GNSS receivers do not give the user the ephemeris.
For the few that do, gpsd does not decode them.  But gpsd can pass
on the subframes to gpssubfram for you to look at.

If it were easy, one of the GNNS vendors would ahve already done it.

Anyone wanting to know the annoying details can look at gpssubframe.

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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