Re: RDAS Temperature Correction

Hi, Dave.

"David Schultz" <abuse@xxxxxxxxx> wrote in message
news:D9YOh.36$zC.11@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In the pressure/altitude conversion equations I have seen the
temperature decreases linearly with altitude. At least at some altitudes.

http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html

Yup. I have actually measured this on a takeoff
and landing of a jet liner. Standard rate is 6.5 Kelvins
per kilometer. I measured 6.38 over Beijing and 5.78
over Newark. Turns out such lapse rate differences
make for trivial errors in the formula.

FWIW, in the lower stratosphere, the lapse rate is zero.
It eventually reverses higher up, as ozone absorbs light
energy.



So I am not sure how temperature correction helps.
Altimeter formulas are based on two principles:
the first law of thermodynamics and hydrostatic
equilibrium.

Hydrostatic equilibrium says that at any altitude,
the the pressure on a given horizontal area of air
is balanced by the weight of the column of air
above it. That weight is affected by the temperature
of the air as the temperature lapses. It works out to

Const HC = 0.03418155
Const LapseRate = 0.0065

BaseTemp = BaseTempC + ZeroC

Ratio = PressRead / MeanGroundPressure


Altitude = (BaseTemp / LapseRate) * (1 - (Ratio ^ (LapseRate/HC))

Note that the whole thing is proportional to base temperature.

Besides, if I recall my density altitude stuff correctly the pressure stays
the same but the density decreases. I have yet to find any reasonably
authoritative work that describes how to use a base pressure and
temperature other than that in the standard atmosphere model to calculate
altitude.


Using a ratio of apogee pressure to pre-launch pressure in these
equations is an error in my opinion. Instead the two pressures should be
converted to altitude and the difference between the two altitudes used.

Well, this stuff isn't exactly controversial. :-)

I'm a little behind (OK. Some would say I'm a big behind...)
on this thread as I have production issues at work and have
to go in today, Saturday. Will reply to John D. and fill in more there.
The issue is involved.

Good to hear from you.
-Larry Curcio

and summarize to



.

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