[time-nuts] DGPS at home
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:
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:
Not so bad.
Maybe a SIRF4 will even better?
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:
> Maybe I'll get a chance to collect some data outside in a reasonable location...
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