Mode C Transponder
 

We've just had the transponder in our group SEP upgraded to provide Mode C.

How does that transponder get the altitude info? Is there some sort of connection to the altimeter that converts barometric pressure into a electronic signal :confused: Or is the altitude measured by ground-based means?

The most common source is a "blind" encoding altimeter. It is a little box behind the pannel which acts like an altimeter set to 1013hpa and provides the transponder with the required info.

In a typical unpressurised GA aircraft it is simply open to the cabin but in some aircraft it may be part of the general static system.

Regards,

DFC