On Fri, Sep 13, 2013 at 10:56 AM, Darren Addy <pixelsmi...@gmail.com> wrote:
> Good explanation, Matthew. I suppose that means that systems that can
> take advantage of the GROUND-BASED WAAS stations should perform a bit
> better on the elevation question because you have another point to
> calculate the triangulation/time measurements from. I have to believe
> that you would need a LOT of ground-based stations for that to work or
> the curvature of the earth would again get in the way of the
> ground-based signals (guessing here).
I don't think that's how WAAS works... the WAAS ground stations aren't
playing the same role as the satellites. Your GPS receiver doesn't
directly receive anything from the WAAS stations, and they don't add
another time delay measurement to the solution.
What they do is act as accurately surveyed points to serve as "ground
truth." The WAAS stations know exactly where they are (from surveys),
and use that information to constantly calculate corrections to the
GPS signals. ("GPS says I'm at X, but I know that's 1.4 meters off, so
we need to correct the GPS signal by Y so it puts me where I know I
am.") These WAAS corrections are then sent up to the GPS satellites,
and down to your GPS receiver, so your receiver can use them to
correct the GPS signals.
One of the biggest sources of error in GPS data is from the
ionosphere, which slows the signals down by a variable amount
(depending on how much ionized gas there is). Other sources of error
are slight deviations in the atomic clocks on the GPS satellites, and
imperfect knowledge of the satellites' orbits. Those are the kinds of
"unknowns" the WAAS stations are correcting.
I consider GPS to be one of the modern wonders of the world. We have a
couple of dozen atomic clocks flying through space, broadcasting their
time on a radio, and our cheap handheld gadgets measure the
differences in those signals to within a few nanoseconds, to tell us
where we are, anywhere on the earth, within a few feet. Science
fiction stuff.
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