On Nov 2, 1:05*pm, Greg Arnold wrote:
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.- Hide quoted text -

- Show quoted text -

My understanding from Urs's talk is that Flarm is using pressure
altitude, not GPS altitude. It does not make sense otherwise as other
pointed out. While GPS altitude is more accurate as an absolute
altitude (if you took a tape measure), the actual difference between
GPS altitude and pressure altitude can indeed be over 1000 feet, and
the accuracy of gps altitude relies heavily on reception and can
jitter a lot. Since all we need to know is relative altitude to
another aircraft carrying a flarm or transponder, pressure altitude
makes much more sense to me. Did I miss something?

Ramy