[ntp:questions] Re: Oncore driver in 4.2.0

Wolfgang S. Rupprecht wolfgang+gnus20031114T105058 at dailyplanet.dontspam.wsrcc.com
Fri Nov 14 19:15:10 UTC 2003


hmurray at suespammers.org (Hal Murray) writes:
>>In an urban canyon you may never get a low DOP... but what you describe
>>is a half-canyon and may not be too bad.
>
> A half canyon is probably better than a whole canyon.
>
> I was interested in systematic errors.  Suppose I collect a bunch of
> data and filter out the ones with poor DOP.  Maybe the position
> reported will be biased because the remaining data is mostly when
> the satellites are to the west.

The way I see it, short surveys of under 24hrs should also have
systematic offsets due to the sun angle.  The ionosphere and
troposphere should be thicker whereever the sun is beating on it and
that in turn should change the average speed of light by enough to
notice a few meters offset.

When I graph position vs. time-of-day I don't see a glaring 24hr
pattern, so the rough iono/tropo corrections transmitted in the gps
data stream are taking out the bulk of this effect.

Here is what I get when I plot the offsets in meters vs. time.  t=0 is
UTC midnight, which is 4pm here (PST).

     http://www.wsrcc.com/wolfgang/gps/gps-wander.png

The largest potential systematic offset folks always overlook is the
old software bug. ;-) Nothing says that the gps firmware writer got
the math 100% right.  In fact, when I do long-term averages off-of the
same roof-top antenna all my gps's converge to slightly different
numbers.

-wolfgang
-- 
Wolfgang S. Rupprecht 		     http://www.wsrcc.com/wolfgang/
       The above "From:" address is valid.  Don't mess with it.



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