Transponder: Mode-C or Mode-S?

On Sep 12, 11:55*pm, John Smith wrote:
Just be aware that the TT21 is a class 2 tansponder, i.e. not certified
for use above 15,000 feet, and therefore not suited for gliders. Buy the
TT22 instead which is class 1.

To date I've had 392 flights in gliders. I flew above 15,000 ft on a
flight on 17 Jan 1995 (in fact to 19,000 or so).

That's the ONLY time.

Most flights here don't go much above 5000 or 6000 ft. Even when
there's wave it's unusual to be able to get higher than maybe 12,000.

Your conditions are not the same as everyone's.