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.
--
*Bartlomiej Zdanowski*
Programmer
Product Research Department
AutoGuard & Insurance Ltd.
Omulewska 27 street
04-128 Warsaw
Poland
phone +48 22 611 69 23
www.autoguard.pl <http://www.autoguard.pl>
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