My understanding is that Flarm uses a pressure capsule anyway for
height information, not GPS height. The IGC version certainly uses the
pressure capsule for height verification, and it comes with a
calibration chart.

If you opt for the engine sensing function on the logger, I presume it
uses a microphone, though I don't know for sure.

If you can't get into the Flarm website, you could try accessing the
UK agents website at:

http://www.lxavionics.co.uk/traffic-monitor.htm

for PowerFlarm, and there is a link on that page to the other Flarm
options.

I should mention that I have no commercial interest in this at all,
except as a satisfied customer.

Chris N