Generic transponder can emulate KT-70 props. These emulated props
will likely die in the future, based on feedback from aircraft devs.
See (upcoming) Wiki docs on the transponder for more info.
I've made an encoder and a transponder. The transponder gets C-Mode altitude
information from the encoder. These two might not be very usefull right now,
but I think they might come in handy when the ATC network gets going.