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Re: [gpsd-dev] [gpsd] Altitude in TPV

From: Gary E. Miller
Subject: Re: [gpsd-dev] [gpsd] Altitude in TPV
Date: Mon, 29 Oct 2018 13:16:07 -0700

Yo Gerry!

On Mon, 29 Oct 2018 14:44:22 -0500
Gerry Creager - NOAA Affiliate <address@hidden> wrote:

> > I'm currently working on adding RINEX 3 support to gpsd.  At least
> > for the u-blox and GREIS GPS receivers.  That can give cm accuracy
> > when long term data is uploaded for post-processing.  
> Are you recovering carrier or solely code-phase for the three
> frequencies in your dataset?

The u-blox NEO-M8T reports psuedorange, carrierphase and doppler for
GPS, SBAS and GALILEO on the L1 band.  I can't get the M8T to output
anything for GLONASS.  That should be enough for RINEX 3.

The javad does the same on several bands.

> > Code exists. I'm partial to PAGES, originally by Mark Schenewerk
> > when he  
> was at the National Geodetic Survey HQ in Silver Spring. Most of the
> big surveying companies (Trimble, Astech, Javad, etc.) have only
> proprietary codes to perform the network adjustments. There is MATLAB
> code available to do it, and if we can relocate a copyof PAGES, that
> works, too.

I'll look at it, after I get RINEX working.  I hear RTKLIB is also
worth looking at.

Oh, Dr Mark Shenewerk.  His name is all over OPUS.  The US online
post processing system.  Sadly OPUS doe not work with single band
RINEX.  Some other online post processing does.

Here is a page on PAGES:

FORTRAN 77.  Wow.  Serious stuff.

> I believe the diurnal effects may actually be related to a spherical
> harmonic-induced perturbation to satellite ephemerises. Regardless of
> the start and stop times imposed, I found that observing times of
> 4-11 hours (plus/minus) gave the best results for getting height
> data,

Interesting.  Worth some testing.  Doing 4 hour data captures a lot
easier than 24 hour runs.

But height data is pretty problematic.  I can get many GPS to agree on
lat/lon, and yet report different heights by sometimes over 60 feet.
Same antenna, same measurement time, through a signal splitter.  My guess
is errors in the GPS receiver code for the geoid.

> However, if the
> observing period was in the range of ~4-11 hours, things were good.

Might be interesting to have gpsprof plot the lat/lon/alt errors
over time.  Take this out of anecdotal evidence to something more

> Simple code-phase solutions are temporally autocorrelated.
> Decorrelation by decimation of a long timeseries dataset, measured in
> days, with decorrelation to 45-60 sec, or even longer, will improve
> the long-term average results.

Wow, if it takes days of measurements to average then the post
processing of short data sets looks a lot better.

> In years past, I averaged a lot of L1
> code position report averages; the use of decimation was recommended
> to me by one of the NGS geodesists, rather than being independently
> derived. I did expand on the decimation parameters, to optimize the
> result, however.

I guess too much to ask for that code to be put in gpsd?

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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