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

[Lauri Hahne]

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.

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