Re: Unclear about dgps precision?

[Gary E. Miller]

Yo M!

On Thu, 28 Mar 2024 17:36:31 +0000 (UTC)
M de Luis <gimme_the_giffs@yahoo.com> wrote:

> When I observe NMEA sentences from a Ublox GPS in u-center,

You don't need u-center to read NMEA 0183!  Why would you ever
run a u-blox in NMEA mode??

> I see
> 7-digits of precision in both latitude and longitude.

Depends very much on your u-blox model, and its configuration settings.

And note, those 7-digits may not be encoded the way you think they are.

Check this out:

https://gpsd.gitlab.io/gpsd/gpsd-numbers-matter.html#_latitude_and_longitudeo

Assuming that you have decimal degrees, not a good assumption, is that a
total 7 digits?  Or seven digits after the decimal point?

If it is a total 7 digits, that is about 1.1 cm.  No way your GPS is
stable at 1.1 cm!

> These numbers
> change by a small amount with each position report.

Mormal.

> When I observe
> JSON emitted from gpsd, I see 8-digits of precision in both latitude
> and longitude.

You have said nothing about you u-blox model, how you configured it,
what gpsd version, and how you start gpsd.  These all affect your
lat/lon output.

> These numbers are static, and I haven't seen them
> change yet, though my receiver is stationary.

There could be many reasons for this.

> What I want, is for the
> position reports from gpsd, to reflect the exact positions as
> reported by the GPS.

And luckily that is what gpsd does!  But don't expecct the positions
to be exact!

> I figure then, that with 2 GPS units, I can
> cobble my own differentially corrected position reports.

That is called RTK, most u-blox models do not support RTK.

> I'm not
> understanding why the dgps output is static, and does not reflect the
> very dynamic GPS output?

How do you know the GPS output is dynamic?  The way u-center controls
your GPS is different than how gpsd controls your GPS.  And gpsd likely
forced your receiver into UBX mode, and you can not easily copmare the
UBX output to NMEA output.

> Clues on what is going on here, anyone?

Since my mind reading is weak today, and you provided no details, how
about you run gpsddebuginfo, as root, and post the output here?

Latest version here:

    https://gpsd.io/gpsdebuginfo




RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  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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