On Nov 2, 12:52*pm, Grider Pirate wrote:
On Nov 2, 11:20*am, Darryl Ramm wrote:
[snip]

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.

James

Thanks for using a transponder!

You are asking a different question than I was talking about. flarm-
flarm collision avoidance radio links uses GPS altitude and does not
involve pressure. I did not say th PowerFLARM does not use pressure
information elsewhere.

PCAS systems today can suffer from altitide errors (esp if the PCAS
eqiupped glider does not have a local transponder to pick up an
accurate altitude). PCAS systems sniff the local transponder altitude
or use ambient cockpit pressure to sense their relative altitude to a
threat and I assume PowerFLARM will do similar. What exactly the PCAS
does I am not sure, then there are parts of the Zaon MRX that I don't
understand either. And I'm not losing any sleep over it given how
obviously the difference between encoder altitude and GPS altitude can
be (so pressure data is needed) and from conversation with these guys
over related issues where they clearly are smarter than your average
bear on this stuff.

I know you know this but the PCAS warning from a PowerFLARM or Zaon
MRX on the White Mountains to your Trig TT-21 will be a lot less
useful that the flarm-flarm link between two PowerFLARM units. I've
flown the whites with PCAS (and a Mode C) and on busy days you get too
many alerts with too little time to work out where the threat is. On
quiet days it is handy for letting you know somebody else is in the
area.

If you have a Trig TT21 in an experimental glider today you may want
to consider connecting a GPS and transmitting 1090ES data-out in
future. And before anybody asks - ADS-B data-out systems are required
to transmit GPS altitude _and_ pressure altitude.

Darryl