Hi all,
I talked with Mike Borgelt re altitude source for final glide computing. He
told me he only uses GPS
altitude for his final glide computers. Mike knows a thing or two about such
matters!
Dave L
> Date: Sat, 2 Oct 2010 02:13:42 +0200
> From: henrik.bie...@gmx.de
> To: xcsoar-user@lists.sourceforge.net
> Subject: Re: [Xcsoar-user] Barometric Alt vs. GPS Alt
>
>
> Exactly. That's the point. Wouldn't it be desireable to have xcsoar
> handle the final glide (and all glide calculations) with the physically
> presice GPS-Altitude and at the same time (!) handle Airspaces with
> pressure altitude?
> I don't see a technical reason, why it has to be either GPS or pressure
> Altitude for both.
> What do you think?
>
> Henrik
>
>
>
> Am 02.10.2010 00:48, schrieb Rob Dunning:
> > US contests use pressure altitude as the primary measurement for
> > altitude. From US FAI Contest Rules:
> >
> > 6.7.3.3 If a Flight Recorder records both calculated and pressure
> > altitude, pressure altitude will be the primary data source and
> > calculated altitude will be the backup data source for flight evaluation.
> >
> > However, depending on the pilot and the day, either GPS or pressure
> > altitude may be preferable for the glide computer (either final glide
> > or for airspace violations).
> >
> > XCSoar has an option in its configuration to use pressure altitude,
> > but defaults to GPS altitude.
> >
> > Rob
> >
> > At 05:36 AM 9/30/2010, Henrik Bieler wrote:
> >> Hey Ian,
> >>
> >> I am not really an expert in this field but I disagree. Provided there
> >> are enough satellites in view I would prefer GPS Altitude for final
> >> glide calculations over QNH corrected Pressure Alt.
> >> If I recall some of my Flight Mechanics and Aerodynamics classes
> >> correctly, the glide ratio L/D is completely independent of Air density,
> >> temerature, etc. Its a function of the Wing-Profile (in our case the
> >> whole Glider) dependent of the AOA.
> >> Therefore, lets say for example at best L/D, with a given Altitude the
> >> glider can travel a given distance. If you know the Altitude precisely,
> >> you can calculate the distance. You can also think of it in terms of
> >> pontential energy...
> >> The GPS-Altitude is pretty precise in my opinion. The pressure altitude
> >> (corrected by QNH) is only precise at the ground of the airport, where
> >> the QNH was measured.
> >> If you fly cross country you get an error, but I think more important
> >> you get a temperature error if you fly and the atmosphere is not at ISA
> >> condition (which it never is).
> >> Lets say for example you do a wave flight in the spring. the temperature
> >> is 15° less than ISA and your flying at 2500m MSL true (!) altitude.
> >> Your QNH corrected Pressure Altimeter will show approx. 2650 m ( 4% per
> >> 10K ISA Difference)
> >> In Summer with Temperature at 15° above ISA, you're still at 2500m but
> >> your Altimeter will show 2350m.
> >> Remember the old saying: "In Winter the mountains are higher" ;-)
> >> With GPS-Altitude, you don't get this error.
> >>
> >> However for vertical airspace distances, I think QNH-Alt should be the
> >> Master, since Airspaces are defined that way. The "true" vertical
> >> boundarys just "fluctuate" a little bit with temperature.
> >>
> >> Anybody know how the usual competion scoring software calculates if you
> >> vertically violated an Airspace? The same algorythm in XCS would be the
> >> right thing to have ;-)
> >>
> >> Since I don't know the XCS internal computations it would be nice to get
> >> Max or Tobias comment on these issues. Thanks :-)
> >>
> >> My personal preference for the use of Altitude sources in XCS would be:
> >>
> >> Final glide calculations: GPS-Alt primary, fallback to QNH-Alt.
> >> Airspace vertical distance warnings: primary QNH-Alt, secondary GPS
> >>
> >> Just my 2 cents...
> >>
> >> Henrik
> >>
> >>
> >> Am 28.09.2010 10:02, schrieb Ian:
> >> >
> >> > I would think for final glide calcs, the best source of altitude
> >> data is
> >> > (in order of accuracy):
> >> >
> >> > - Barometric altitude with QNH correction. (Can this be done
> >> > automatically before take off?).
> >> >
> >> > - GPS altitude.
> >> >
> >> > - Barometric altitude without QNH correction. (Often out by 100's
> >> of feet).
> >> >
> >> > - Barometric "flight levels" rounded off to 100' as per the B500.
> >> >
> >> > I know the B500 uses barometric pressure and outside air
> >> temperature to
> >> > provide corrections to TAS and vario readings. The "flight levels"
> >> data
> >> > would be good enough for this purpose. I am not sure which altitude
> >> > source it uses for its internal final glide calcs.
> >> >
> >> > I will have to find something else to display in that box with the
> >> "---"
> >> > indication ;-)
> >> >
> >> > Thanks
> >> >
> >> > Ian
> >> >
> >> >
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