On Fri, 22 Feb 2008 09:39:06 -0800, Marc Ramsey wrote:

GPS altitude and pressure altitude measure two distinct concepts which
happen to use the same units. Conversion between the two is an
approximation requiring both local sounding data and agreement on use of
a specific set of formulas. The IGC has been using pressure altitude
almost exclusively for quite some time, and will continue to do so for a
variety of reasons. These ease of comparability with past flight
performances and records, the need to detect incursions into airspace
defined by pressure altitude limits, collection of redundant altitude
data for security and reliability, and general bureaucratic inertia.
While there is provision in the Sporting Code for optical and radar
altitude measurements (which are more closely related to GPS altitude),
other requirements pretty much eliminate the possibility of actual use,
and if someone managed to find a legitimate way, the conversion would
have to be handled on a case specific basis.

None of this has provides any good reason why a badge for silver or gold
height gain should not be awarded on the basis of height gain measured by
means of GPS altitude instead of barometric pressure altitude.

Both GPS and pressure altitude are subject to errors and pressure
altitude is not necessarily the more accurate. Two performances by
different pilots on different days in different weather, both providing
equal height gain measured by pressure altitude does not imply that the
two gliders actually achieved the same height gain if could be measured
with a tape measure. Just the same if one performance had been measured
with GPS altitude. But both performances would be an excellent
demonstration of pilot skills and worthy of recognition with the
appropriate badge!

This may not be 100% "fair". But a distance performance navigated with a
compass and map and measured with turn point photographs is a more
difficult achievement than the same task navigated with GPS and measured
with a Flight Recorder. You have to fly further to get a decent photo and
the cockpit workload is significantly higher. Yet we award the same badge
for both performances - thus some "unfairness" already exists in the code.

Tying the COTS GPS rules to the use of GPS altitude would almost
certainly result in failure to pass the proposal. Requiring use of a
pressure altitude recording device (i.e. barograph) increases the
probability that a majority of the IGC delegates will vote in favor of
the proposal. That's politics for you...

This political argument sounds far more plausible than any technical one.
I would like to see a list of IGC delegates (identified by country) who
are apposed to the use of GPS altitude to measure height gain. Then we
could explain to them that their attitude is quietly killing our sport...

To put it another way, if we accept COTS GPS together with COTS altitude,
then overnight there would be 10000 Flarm equipped gliders which would
each have their own dedicated Flight Recorder at no additional cost at
all... Now that would be a powerful boost for our sport.


Ian