Out of curiosity, how do you know it's off, whenever I think about this sort of thing I have a really hard time working out what might be a good 'source of truth'.
cheers On Jan 21, 2008 7:48 AM, Nick Black <[EMAIL PROTECTED]> wrote: > On Jan 20, 2008 6:36 PM, Lauri Hahne <[EMAIL PROTECTED]> wrote: > > On 20/01/2008, Lars Aronsson <[EMAIL PROTECTED]> 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 > > > > > > _______________________________________________ > > newbies mailing list > > [email protected] > > http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/newbies > > > > > > -- > Nick Black > -------------------------------- > http://www.blacksworld.net > > _______________________________________________ > newbies mailing list > [email protected] > http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/newbies > -- Franc
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