1 instrument

[LAdamson 240]

so I certainly don't feel GPS altitude>is worthless!At least for us in the U.S. who have WAAS correction.L.AdamsonBTW--- The synthetic vision I spoke of, is something like your software, but it's a project that NASA and others are working on.

[Geofa 108]

http://www.flightprep.com/showmyturn.php?id=16http://mywebpages.comcast.net/geofa/pages/rxp-pilot.jpg

[Guest Adverse Yawn]

Maybe I've not caught up with the latest advanced in GPS. However, my understanding is that GPS altitude is derived from a mathematical approximation of the Earth that cannot account little warps in the elipsoid/geoid here and there. In other words, in many places it will be accurate enough, in other places it can several hundred feet out. I don't think WAAS will help in this regard. The geoid data is held on the receiver. WAAS merely corrects and relays a satellite signal. The receiver derives altitude from a minimum of four satellites, so the mathematical geoid approximation altitude remains a problem. Clearly this would be exacerbated when relying solely on uncorrected satellites, and you would need to have a minimum of four to play with at any one time. Also, for me, RAIM (Receiver Autonamous Integrity Monitoring) would also be a minimum requirement, I don't see it mentioned.

[Geofa 108]

People much more knowledgeable than me have debated this-but in a nutshell from what I can tell-the gps can actually be more accurate in giving a true altitude picture. Reason why-a pressure sensitive altimeter is being calibrated to a local station-and we all know that between stations things can change quite a bit-especially in the mountains where stations can be further spaced and conditions can change more dramatically. The gps altitude-while may be +-50 ft. off (and closer with waas correction), can actually be closer in giving what our real altitude is than an old altimeter setting in conditions that have changed.This was debated on my Anywhere Map forum where some of us were pushing to be able to "calibrate" the gps altitude to our altimeter setting so the 2 would read the same (like my kln94). It was decided by the experts that the gps altitude is for the most part probably closer, and more useful to have its' "real" altitude rather than a calibrated possibly more innacurate one. My argument was that atc however expects us to fly with this possibly more innacurate altimeter altitude-however the consenses was that in an emergency-you would actually be better off with the gps altitude which would be giving you a truer picture of reality.In any case, my kln94 which you do "calibrate" to local altimeter setting is usually spot on with the altimeter-and I have never seen my handheld more that 30-50' off-usually it runs only 10'-12'off my altimeter setting-probably about the same accuracy as an altimeter setting you got 5 minutes ago with conditions that have changed subtly.Of course I would not do an approach using this-but for enroute it is very useful. If in imc in the mountains, and the display is turning yellow or red-that is a good enough warning-even if for some reason it was even 100' off.I think of the recent crash of the business jet in the smokies last year and wonder if they had had this simple and relatively inexpensive unit I have, whether running into the mountain would have been averted. I tend to think so...For a backup and additional information-I can think of nothing more valuable in the cockpit.http://mywebpages.comcast.net/geofa/pages/rxp-pilot.jpg

[Guest Adverse Yawn]

You maybe correct, but those were the reasons given for GPS altitude not being permitted with GPS approaches. But the JAA still advises that GPS altitude are not used and that barometric altitude is. However, the JAA is known for being a bit behind the times and anti-GPS for political reasons, i.e. to promote Europes their Galileo solution.>>I think of the recent crash of the business jet in the smokies last >>year and wonder if they had had this simple and relatively inexpensive >>unit I have, whether running into the mountain would have been averted. >>I tend to think so...Yes. Probabaly. But certainly airspace busts still occur with the same monotonous regularity inspite of everybody haveing GPS moving maps. Replace a Class A airspace boundary with a mountain and you have the same category of error. For some reason intelligent and well trained people still get confused even with a GPS placing a little aeroplane right where they don't think they are. There seems to be other factors at play.

[Geofa 108]

Well-this is what I like it for.This summer I flew from Michigan to Asheville, Nc.-an airport that is notorious for people running into mountains. Being basically from flatland Michigan this always gets my hair up. The mea's are 7000-8000 ft. so when atc told me to descend to 5000 when these shots were taken-and in breaks in the clouds I saw what you see here-again the pucker factor goes up. I know atc persons are of the highest caliber-but still you start thinking...Now look at my unit in the same area on a simulated trip-I sure would have liked to have had it for this approach-even if one realizes it could be 100' off!(Not to mention to be able to see my aircraft flying on the actual approach plate) :-)http://mywebpages.comcast.net/geofa/pages/rxp-pilot.jpghttp://forums.avsim.net/user_files/141473.jpghttp://forums.avsim.net/user_files/141474.jpghttp://forums.avsim.net/user_files/141475.jpghttp://forums.avsim.net/user_files/141476.jpghttp://forums.avsim.net/user_files/141477.jpghttp://forums.avsim.net/user_files/141478.jpg

[Guest Adverse Yawn]

Thanks for that Geofa. I now see where you are comming from. An excellent example of real-life overriding theory and where a picture is woth a thousand words. Not something I've experienced nor considered fully :)My point about the airspace busts is real too. The UK CAA are sick of them. I am perplexed as to how they happen with all these moving map gizmos, but they do!