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 _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
