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^)

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.

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.

Marc