On Thu, 20 Nov 2003 06:20:57 GMT, Marc Ramsey wrote:

Mike Borgelt wrote:
So you feed a pressure sensor to the computer controlling the GPS
pseudolites and it roughly matches with GPS altitudes with appropriate
corrections for reasonable guesses as to the mean temperature in the
atmosphere. Dead easy! And a trivial enhancement to your pseudolite
system. Given that some IFR aviation GPS systems already use pressure
altitude for GPS aiding it would not surprise me if test equipment
that can do all this isn't available off the shelf.

When is the demo going to be ready? 8^)

Easier to just break in to the logger and install the switchable IR
link. Trivial. Mickey mouse microswitches just don't do it. The
professionals use thermite.


Why limit the change to geometric altitude to above 32K feet? Most
loggers are on cockpit static (an original adamantly insisted on
requirement by GFAC now changed I believe - why?). That is good for 50
to 100 feet of error, you get sea level pressure changes and huge
errors due to temperature in the atmosphere, let alone running the
pressure sensors at maybe -20 C or colder. The fully approved
Volkslogger only claims +/- 2hPa over temperature which is another
+/-100 feet at around 20,000. You are already over any reasonable GPS
error budget.

The IGC works in mysterious ways. It seems eminently sensible to me to
switch completely over to GPS measured geometric altitude, but I don't
get to make the rules. In any case, a number of people with expertise
in the area have argued rather convincingly that the relationship
between pressure altitudes measured above 32,000 feet or so and actual
elevation above the ground is tenuous, at best.

I believe that was calculated before SA was turned off. As I pointed
out above I doubt very much that any cockpit static can be better
than 50 to 100 feet.Static ports on gliders are sometimes pretty
terrible too so may not be any better. Try a good side slip and see
what happens also.
Add in the other error sources and you are worse than GPS altitude at
any altitude much above 1000 feet AGL.


ISA day sea level 1000 feet pressure altitude, geometric altitude 1000
feet
Try sea level 45 degrees C with DALR and a pressure altitude of 1000
feet. Mean temperature of layer is 43.5 geometric altitude 1102 feet

102 feet error! GPS is at least as good as this most of the time.


The reason for the change allowing panel mounted flight recorders to use
aircraft static as an alternative to cockpit static is very simple. An
instrument manufacturer requested the change, and persuaded us that the
original reasoning behind the requirement for cockpit static was no
longer relevant.

Then again neither the original reasoning nor the persuasion seems to
have seen the light of day.

Do you realise that the original requirement drove some serious system
architecture considerations for manufacturers? As I said the GFAC were
originally adamant about no static connections - what changed their
minds?
How does anyone trust the rules when they may change next week?
Nothing I've seen written here convinces me that anyone on GFAC has a
clue.

Mike Borgelt