From: "John Barrett" <2thman@olympus.net>
To: "Lancair Mail List" <lancair.list@olsusa.com>,
	<hickham@webtv.net>
Subject: List #34"Wazzup with WAAS?
Date: Sun, 13 May 2001 22:54:07 -0700
Message-ID: <NEBBLABMMLJGBIFPPHBBOECCCCAA.2thman@olympus.net>
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Jack Hickam wrote:
>John Barrett,
>I am just back in town and catching up on the post.  Am I to believe
>that with the existing GPS system and with the SFS 2000 that flying
SNIP
>Jack Hickham

Jack,
Answer to your question is YES!  The SFS system uses all of the above plus
the topographical database stored in its computers to give you a reliable
altitude(height above ground)readout.  You have to assume that the database
is correct for the touchdown point of the runway in question.  This most
likely is not always the case.  I have flown into one VFR airport where the
entire runway is plotted about ½ mile away from its actual location.  In
this case, the altitude readout would not help no matter how accurate it is.
The point is that the accuracy of each approach has to be verified.
Therefore, for the system to work, the FAA would need to verify all
approaches on a repetitive basis just as they do now with the ILS system.
There may be some other way of verifying the accuracy of the database but I
wouldn’t know what that would be.
Hope this helps.....John Barrett


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