On Monday 23 February 2004 14:16, Norman Vine wrote: > David Megginson writes: > > Norman Vine wrote: > > > This is because GPS positioning is *NOT* acurate with out a ground > > > based signal to augment it ! > > > > It's much better than it used to be before they turned off selective > > availability. > > Yes this makes a difference but .... DGPS or WAAS is *MUCH* better :-) > > > The big difference is that GPS accuracy remains roughly constant during > > any specific approach, while navaid accuracy increases as you approach > > the navaid and altimeter accuracy increases as you approach the station > > elevation. For example, a GPS without WAAS might have an error of 150 ft > > at any altitude; a pressure altimeter (with the proper altimeter setting) > > might have an error of over 1000 ft at 8000 ft AGL, diminishing to less > > than 25 ft at ground level. Ten miles back at 3000 ft AGL, even the > > non-WAAS GPS is probably going to be more accurate than the ILS and > > altimeter; a half mile back at 200 ft, the ILS and altimeter win > > hands-down. > > What I was pointing out is that elevation from GPS even DGPS is by it's > physical nature mathematically not as accurate as the horizontal position. > > So if one is to use GPS elevations one should understand their inherent > limitations > > AFAICT this is in essence what WAAS is all about in that it will provide > the 'local' elevation correction as well as the DGPS horizontal correction > in a verifiable way > > Cheers > > Norman
One of the things about GPS units is that the accuracy depends on how many sats the unit's tracking, and their disposition around the sky. My hand-held unit needs to find a minimum of four sats to get an elevation but if they're all in the same region of the sky the accuracy is pretty poor. The unit can use up to twelve sats, which increases accuracy, but it can still be good with just four if say, one's at the zenith and the other three are distributed around the horizon. Dunno if we want to get into adding nav sats to the celestial stuff so we know if they're above the horizon or not:) LeeE _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
