[OSM-newbies] Consistently off tracks on second or third tracking

[Nick Black]

On Jan 20, 2008 6:36 PM, Lauri Hahne <lauri.hahne at gmail.com> wrote:
> On 20/01/2008, Lars Aronsson <lars at aronsson.se> wrote:
> > Harald Kirsch wrote:
> >
> > > Now, if the GPS would produce a Gaussian error, I would expect a
> > > track to zigzag left and right from an older track, but
> > > obviously this is not the case.
> >
> > The error is random (Gaussian or not), but not in every sample.
> > Satellites drift slightly out off course, and are corrected into
> > the right orbit once in a while.  You can get exactly the correct
> > position relative to the satellite, but the satellite can be 7
> > metres out of its planned orbit.  So your position might also be 7
> > metres off.  If you could know how much off each satellite is at
> > each moment, you might be able to calculate your correct position
> > with much higher accuracy.  There are several schemes for this,
> > some are military and secret, one is called "WAAS", another is
> > called EGNOS.  You can learn more by googling these terms.
> >
>
> Actually all GPS satellites broadcast correction parametres for their
> orbits. So that isn't a problem.

Yes it is - errors occur because of misjudged orbital paths.  The
signal broadcast by Navastar contains an estimate of the orbit, not
the exact orbit.

>
> WAAS and EGNOS (together known as SBAS) broadcasts information about
> ionospheric errors which amount to most error there is. This
> information is collected by ground stations who measure their position
> and compare that to their known location.
>
> A quick googling found the following page
> http://www.kowoma.de/en/gps/errors.htm It isn't a very accurate paper
> but it gives you the basics.
>
>
> --
> Lauri Hahne
>
>
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>



-- 
Nick Black
--------------------------------
http://www.blacksworld.net