On Sep 22, 8:19*pm, Larry wrote:
So I've got a mode-C xponder in my current glider. *Since I fly near
Reno airspace, when I get another glider I'll be installing a xponder
if it doesn't already have one.

Question- is there a reason why I would want a mode-S rather than a
mode-C?

Thanks,
Larry

Not really, in Europe Mode S is important. In the USA not so. Until
the TT21 my advice would have been to just by the cheaper Mode C like
the Becker 175W.

With Mode S you get a permanent aircraft ID so ATC can always tell who
you are. Some folks might not like that compared to Mode C. The Trig
will allow ADS-B data-out tricks in future but that is about a decade
from needing this. The main thing with the Trig TT21 is that it is
easier to install, smaller, more power efficient and around the same
price as competing Mode C transponders and it is Mode S with "extended
squitter" support aka 1090ES/ADS-B data out for future possible use.
As I've said before the TT21 is a game changer and if it performs as
good in practice as it looks on paper it will do well. I currently fly
with a Becker Mode-C 175W (my second in two gliders I've owned). If I
was buying a new transponder today it would be the TT-21.

Darryl