[time-nuts] DGPS at home

Steve . iteration69 at gmail.com
Sat Nov 26 05:35:48 EST 2011


Hal,

Those sure look like GNU plot graphs :)

Sorry, i didn't mean to change subjects but i do like gnu plot.

Steve
On Sat, Nov 26, 2011 at 4:20 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

> > If you do a test, let us know your findings.
>
> I think the answer will depend upon how good the location is.  If the
> limitation is ionosphere delays, two units near each other should have
> similar errors.  If the limitation is multipath, being near each other
> probably won't help much.
>
> -------
>
> This was a good excuse to make some graphs.
>
> I have lots of data.  Most of it is from units that are indoors and barely
> work.  Not surprisingly, the location data is far from good.
>
> Conveniently, I had a pair of units next to each other and grabbed all the
> NMEA data for over a month.  I took a random day.  One of the units had
> 74777
> valid samples, the other had 32439.  There were 28651 seconds that had good
> data from both units.  I wrote some hack software to merge the data on the
> seconds that overlapped then plotted the difference in lat/lon.
>
> I've seen samples off by miles. Yes, not many samples but a few.  :)  Data
> collection may need another filter: don't treat a sample as "good" unless
> the
> previous few samples were good.  Remember, this is a crappy location.
>
> Quick summary for those who don't like graphs, if you ignore anything off
> by
> more than 20 feet in either lat or lon, it's a random number generator.
>
> Here is the same data with different vertical scales:
>  http://www.megapathdsl.net/~hmurray/robot/diff.png
>  http://www.megapathdsl.net/~hmurray/robot/diff-1000.png
>  http://www.megapathdsl.net/~hmurray/robot/diff-20.png
>
> Maybe I'll get a chance to collect some data outside in a reasonable
> location...
>
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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