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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