Hi Gang
I had a Trig mode S transponder installed in my new Phoenix motor
glider. It appears to work fine as a mode C transponder. After
questioning NorCal while soaring they confirmed they could detect it
with their sqwark code, it would IDENT and so on - all the mode C
stuff. However most of the US including northern California/Nevada is
not yet set up for mode S so except for the future there is no reason
to have mode S in the US. Of course there are other reasons to have
mode S, probably the most important would be if you wanted to sell a
glider in Europe where mode S is becoming mandatory. For that market
having a mode S transponder will save the buyer about $2,500 - the
cost of replacing a mode C with a mode S transponder.
Dave