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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 15:17:09 -0700

Yo Gerry!

On Mon, 29 Oct 2018 16:50:00 -0500
Gerry Creager - NOAA Affiliate <address@hidden> wrote:

> > 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.
> >  
> Is pseudorange rate in there, as well? If we've got range and phase,
> we can approximate rate and that'd give us a good shot ad solid 3d
> postprocessing...

I'm unfamiliar with 'pseudorange rate'.  Is that the same as doppler?
RINEX just wants pseudorange, carrierphase and doppler.  Doppler is the
speed that each satellite is approaching or receding.

> Single-freq data didn't provide sufficient information in carrier
> phase to resolve either horizontal or vertical, save for RTK, and RTK
> never quite got vertical right although it was better than autonomous
> code phase L1 only. L5 was added to allow a civil code and carrier
> capability. L1C/L2C has mitigated that to a great extent. A lot of
> people have tried to make survey-quality results work with L1/code
> only, and have reported errors unsupported by repeats of their
> studies in an objective manner. You can get close, but it's not
> geodesy at that point.

I'm looking forward to the u-blox 9 series with its multiple frequency
support.  That is why I'm trying to update gpsd for what that needs.

> >
> > FORTRAN 77.  Wow.  Serious stuff.
> Easy enough even a scientist can learn it.

Something to look at after RINEX support for gpsd.

> > Interesting.  Worth some testing.  Doing 4 hour data captures a lot
> > easier than 24 hour runs.
> It's actually in the makeup of the constellation. The geoid and
> ellipsoid are straightforward. There's plenty of computational
> horsepower and memory for those datasets and calculations in most
> receivers. I can't speak to the dongles, but even when I started
> playing with "low cost" board level GPS, the processors were
> hard-core and had lots of memory for the time.

Since we can't presently change the GPS receiver firmware, it would
have to be done in gpsd, or a gpsd client.  Easy to do, if gpsd
has the needed data and algorithms.

> I'll have to find the paper I wrote. Gotta have it somewhere. It
> was  enough to get NGC to create NGS-58 at the time.

Wow, 2 cm accuracy 95% of the time, in 1997.

> > Wow, if it takes days of measurements to average then the post
> > processing of short data sets looks a lot better.
> Over time, sure. That said, 10 or so hours decimated will give you a
> good first guess, and you can augment it with another dataset, or 3,
> later improving the estimate. Just remember that your error is a
> least-squared error instead of simple standard deviation.

That is why gpsprof and ntpviz compute skewness, kurtosis, and much
more.  To clearly show that the data is not even close to a standard
distribution.  That has been misleading people for decades.

> > I guess too much to ask for that code to be put in gpsd?
> Depends on how we want to go about it. The decimation approach is a
> bit memory expensive but not too bad. Or write it to ramdisk or a
> temp file and process from there in a 2-step fashion.

Storage is cheap now.  I just looked at 512 GB SSD for $80.  Run time
not an issue since the raw data collection time already many hours long.

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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