From: shauns@vice.ICO.TEK.COM (Shaun Simpkins)
Newsgroups: rec.aviation.homebuilt
Subject: Re: How accurate can I make a homebrew altimeter?
Date: 7 Nov 92 21:11:38 GMT
Organization: Tektronix Inc., Beaverton, Or.


I looked into designing a IFR-grade electronic air data computer
(airspeed/vertical speed/temperature sensing, TAS, IAS, etc. outputs) a
while back.  Take a look at the FAA requirements for air data computer
accuracy - they are amazingly tight and stand in testament to the
remarkable accuracy of the kollsman-type mechanical altimeter.
These specs are off the top of my head, so please excuse any errors,
netters.  If you want the exact stuff, let me know and I'll dig them up.

In general, the accuracy of the altimeter must be better than +-25' up to
about 5,000 ft, 50' up to about 15,000',
and degrades to about +-300' at 40,000 ft.  The display
must resolve better than 10' increments throughout the range.  The test
temperatures are -40 to 70C, and the unit must show no more than around
60' variation at sea level over that band.  There are other bake and
freeze cycles that the instrument must endure before testing.
Translating this into percentages, for light aircraft a 20,000' altimeter
is more than sufficient. We get 0.25%-0.5% accuracy and linearity below
15K, and we must resolve to 1 part in 2000 full scale. 
Temperature stability is also in the 0.5% of full scale range.

The relationship between pressure and altitude is nonlinear - not
vastly so, but enough that it must be accounted for if the above accuracy
requirements are to be met.

This requires at least 12 bits of digitization to meet the resolution
requirements.  The nonlinearity of the pressure/altitude curve requires
another bit or so, so about 14 bits is right.

The linearity spec cannot be met with current pressure sensors. I was
working with a Motorola part, and the best I could do was 0.5% at room
temperature.  Thus, you will have to determine the pressure/voltage
curve for your chosen sensor and compensate for it in some fashion.
Semiconductor pressure sensors also have a strong temperature coefficient
which has to be reduced to near zero to pass FAA regs.

I eventually decided to use a Voltage/Frequency converter and "sync" its
output to the sensor's bias to null out conversion temperature errors.
The temperature compensation circuitry was hideous, and the back-end
processing required to linearize the sensor definitely non-trivial.
I stopped when I realized I would have to gain access to an FAA-certified
tester to both calibrate and get approval for the instrument.

About this time, Rocky Mountain Instruments came out with a product similar
to the one I was working on, with a display, that fit into a 3 1/4" hole.
It used the same sensors I was contemplating, and was only $900 in kit form.
You sent it back to them for calibration when you were done.  Frankly,
$900 is cheap considering the work that has to go into one of these things.

Viva Kollsman!  It probably would be better and cheaper for you to devise
a way to pick off pointer position from an el cheapo mechanical altimeter
than to start from scratch on your own.

ss
:
converter's bias port to the bias port of
-- 
				Shaun Simpkins
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