Typically either an encoding altimeter or a "blind" encoder provides
altitude info to the transponder. The encoding altimeter is by far the best
method but expensive hence most light aircraft use the encoder. Problems
with encoders are very common.

Encoders usually have a heater in them which keeps the pressure sensor at a
near constant temperature and in most cases, disables the altitude signal
until that temperature is reached. On a really cold day, this may take a
long time. If ATC report no mode C immediately after takeoff, you may just
need to wait for a while.

As an aside, in 30 years of flying "in the system", I have never once
observed a "recycle transponder" to be effective. I have have formed the
impression it is contoller-speak for "Turn it on, dummy!"

Steve


"log" wrote in message
...
On a recent trip, ATC said my transponder was now showing my altitude. Is
it
the altimeter or the transponder or the encoder?
BED