On Sun, 2007-02-11 at 13:42 -0500, John Denker wrote:
>On 02/11/2007 10:02 AM, Dave Perry wrote:
>
> > So the altimeter setting is the same thing as the mean-sea-level
> > barometric pressure.
>
> Huh? That does not follow at all. The quoted passage does not
> say that at all.
>
Your own paper implies this statement as follows.
Using equation (13) in
http://www.av8n.com/physics/altimetry.htm,
simplifies to (canceling the Pb^N')
hi = (1/K')*[S^N' - P^N'] (13')
where K' and N' are the constants in your paper and
S = Altimeter Setting and P = static pressure at hi.
At what pressure P will hi = 0 MSL?
Note that hi = O if and only if P = S. So your own paper implies that
S = static pressure at 0 MSL.
Why talk down to anyone on the list that corrects you or disagrees with
you on a point? We all have been known to have to retract assertions.
There are many on this list with advanced technical degrees and several
with PhDs. We can read your paper. This comment is not intended to
imply that degrees differentiate value of contributions to FlightGear,
but is only an appeal for courteous dialog.
> A patch to make the altimeter conform to FAA standards can be
> found at
> http://www.av8n.com/fly/fgfs/altimeter.diff
>
> Right now it uses transcendental functions, which may or may not
> be somewhat inefficient. It would not be hard to build suitable
> interpolation tables, but that's not my top priority at the moment.
>
> There are lots of broken things that take precedence over merely
> slightly inefficient things.
>
By the way, I agree that the current algorithm in altimeter.cxx is
wrong. This evening, I had time to look at your posted patch and I
think it would give the right hi. It does require two calls to ln() and
exp() (i.e. the pow function). Have you thought of just using the C
term in equation (15) and PA = pressure altitude which is already being
computed. That would only require one call to pow(). Or since
C = ( hi offset from PA)
only varies with S, compute a replacement table for delta h in terms of
S and interpolate that table. Then current hi = PA + interp(S).
Thanks for your enthusiasm and contributions in a short span of time.
Regards,
Dave
--
Dave Perry
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