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.

Having never flown with SeeYou Mobile and Flarm I'll admit to not
being sure exactly what is shows for altitude on the Flarm radar
screen. But that does not stop some reverse engineering...

Some other Flarm 3rd party display products only show relative height.
But lets assume SeeYou Mobile shows absolute altitude not differences.
But that does *not* automatically mean it is showing pressure altitude
(corrected for QNH) or GPS altitude of those other gliders?

The altitude data presented to SeeYou Mobile or any other application
that connects to a Flarm is only ever *relative* altitude between the
remote gliders flarm GPS and your flarm's GPS. The dataport interface
document is on Flarm's web site and is great quick bedtime read.
SeeYou Mobile would have to add your GPS altitude to the difference to
get the absolute GPS altitude of the other glider. Now if SeeYou
Mobile knows your pressure altitude I would hope it works with that
and all the other Flarm targets are show with pressure altitudes.
That would be consistent with the general handling of altitudes in
SeeYou Moble but I'll let Andrej or others confirm what they do.

BTW anybody else got an idea that Flarm folks working on this know
what they are doing? :-)

And to be clear to others in case they are worried about the absolute
altitude description - when you get a pop-up Flarm alert in SeeYou
Mobile the altitude displayed is always relative. The Flarm "radar"
display also has user settable color bands to show it the other
aircraft are near, above or below your altitude.

You know for a large map/radar display I'd personally rather see
absolute altitudes - especially as we eventually forward to ADS-B and
may see people over hundreds of miles. Ultimately having a choice of
relative/absolute would be best. Now somebody tell us that SeeYou
Mobile shows relative altitudes now and we can all go back to
sleep :-)


Darryl