[time-nuts] DGPS at home

ehydra ehydra at arcor.de
Mon Nov 28 13:02:18 EST 2011


Hi Hal -

Thanks for your efforts!

I just settled down my GPS for the car on my desk (under a brick roof) 
and left it over night alone. Between evening and morning I wrote 4 
locations on paper and later dumped it into Google. So this is a test 
case for ONE unit. It is a TomTom equipped with a SIRF3 chipset. Yes I 
know this is not a statistical proven example ;-)

It shows 3 locations within 7 meters. One jumped away.

Here it is fully interactive:
http://ehydra.dyndns.info/NG/time-nuts/DGPS/GoogleMaps-Route.html

Download the file and open it LOCALLY in your Web-Browser. As far as I 
figure out how Google-Maps work, then you don't need a site-key from 
Google. Otherwise you can get a new key at Google.

The same as static picture:
http://ehydra.dyndns.info/NG/time-nuts/DGPS/Google-Route.jpg

Not so bad.

Maybe a SIRF4 will even better?


- Henry




Hal Murray schrieb:
>> 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...
> 
> 

-- 
ehydra.dyndns.info




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