On 11/2/2010 12:52 PM, Grider Pirate wrote:
I don't have FLARM, but I have a TT21 transponder. I WILL be talking
on 123.5 on the Whites!
Let me get this straight. FLARM uses GPS altitude. My transponder
uses Pressure altitude.
FLARM will see my transponder (assuming I'm interrogated) much as a
PCAS.
FLARM will look at its GPS altitude, and maybe not worry about me
because it thinks we have 1000' vertical separation. Too bad for us
if it's a smokin' day, and the tranponder pressure altitude show
16,800 when the GPS altitude shows 18,000!

B2016373615714N11540384WA 05179 05561 000076000000
B2016413615718N11540296WA 05202 05585 000072000000
(just an example of a 1,256 difference between pressure and GPS alt,
spaces added for clarity)

The difference between cabin and static is trivial compared to the
difference between pressure and GPS.


Also, we just learned that the altitude of other gliders displayed by
SeeYou for Flarm purposes is absolute, not relative. So we will be
comparing the GPS altitude of other gliders with the pressure altitude
on our altimeter. This seems like a problem.