[time-nuts] DGPS at home

Hal Murray hmurray at megapathdsl.net
Sat Nov 26 04:20:46 EST 2011

> 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...

These are my opinions, not necessarily my employer's.  I hate spam.

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