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. http://pro.magellangps.com/en/products/aboutgps/dgps.asp http://pro.magellangps.com/en/products/aboutgps/rtk.asp http://en.wikipedia.org/wiki/Real_Time_Kinematic 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" http://www.leica-geosystems.com/uk/en/lgs_8276.htm In the UK the Ordnance Survey is making its data available for commercial RTK networks. http://www.ordnancesurvey.co.uk/oswebsite/gps/commercialservices/index.html _______________________________________________ OpenMoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community