AGPS - protocol specs?
openmoko at mazikeen.demon.co.uk
Fri Mar 9 13:46:19 CET 2007
On Friday 09 March 2007 11:43, Bartlomiej Zdanowski AutoGuard Ltd. wrote:
> Ian Stirling napisał(a):
> > Bartlomiej Zdanowski AutoGuard Ltd. wrote:
> >> Hello.
> >> Nils Faerber napisał(a):
> >>>> There is all sorts of wacky stuff - for example, peer-peer DGPS
> >>>> that can
> >>>> be done, where all stationary neos on charge with a GPS signal and a
> >>>> free internet connection contribute to a global ionospheric model.
> >>>> Then any Neo can connect to this model, download 200 bytes or so, and
> >>>> get +-0.3m (or better) position for a short while.
> >>> Exactly.
> >>> Also relative positioning can be made much more precise using the raw
> >>> data (AFAIK in the range of cm not m).
> >> I asked my colleagues who are GPS devices specialists and they said
> >> that this is all wrong. Military devices can do magic but civil not.
> >> They said that with civil GPS receiver you can get accuracy up to
> >> about 5 - 10 meters. For such precision it is required to be seen at
> >> least 8 satellites, a clear sky and good magnetic and ionosphere
> >> conditions (also solar magnetic field's change is important).
> > It's a little more complex than that, and not quite as bad.
> > http://www.mauve.demon.co.uk/gps-average.gif is some data I took a few
> > years back with a garmin GPS12.
> > The circles show radiuses inside which the stated number of points
> > fall, in a 10 second average.
> > For example, 99.99% of points fall within 13.8m.
> > 10s averages are red dots,
> > green 100 second, magenta circles 1 hour, cyan squares 6 hours, and
> > black 24h.
> Try it while walking on streets between buildings :(
> Another problem can be when we put Neo out of pocket. I don't know if
> it's GPC received would need some extra time to compute pos.
> > If a number of factors in the hardware all come together (that details
> > are not public on) position under 10cm may be possible after
> > downloading the few hundred bytes of correction information. (It may
> > be possible to optimise this down a lot)
> So *military* devices does it. But not civil.
This is for *civil* devices.
Here's a civil device using RTK that "determines the position to centimeter
accuracy within a few seconds at ranges up to 50 km from a reference station"
In the UK the Ordnance Survey is making its data available for commercial RTK
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