"GEG" wrote in message
...

Being a student pilot, I'm eating up all kinds of info.
So I go through various instrument info, and then get
to the "Mode C Transponder" that sends altitude info.

Is that alt info built in?

If so, is there no correction for barometric pressure changes?

I don't know that it would be severe, but it seems like the
window could be +/- 100 feet from day to day that the controllers
get from the transponder.

Anyone provide some more info on this issue?

Thanks!


There is no barometric adjustment for altitude encoders, they are set to
29.92. The correction for nonstandard pressure is made at the ATC end.

It's easy to understand why it's done this way. If the adjustment was made
by the pilot he could easily enter the wrong value in his encoder and his
altimeter. He'd then be at the wrong altitude and his encoder would provide
the same wrong altitude to ATC. By having the adjustment made at the ATC
end the radar displays the correct altitude and ATC can advise the pilot of
the error.