Which IFF mode is automatically derived from the aircraft barometric altimeter?

Mode C is the IFF (Identification Friend or Foe) mode that is automatically derived from the aircraft's barometric altimeter. This mode provides altitude information in response to interrogations from Air Traffic Control radar and is crucial for ensuring safe separation between aircraft at different altitudes.

Mode C transmits the aircraft's altitude in feet to facilitate the identification of the aircraft's vertical position in the airspace. This automatic derivation from the barometric altimeter is key, as it ensures that the altitude data is accurate and reflects the current flight conditions. This mode enhances situational awareness for both pilots and air traffic controllers, especially in congested airspace.

Other modes, such as Mode 1 and Mode 2, are designed for different purposes, such as military identification or responding to specific identification tasks, and do not involve altitude reporting derived from the barometric altimeter. Mode D also does not have the same focus on altitude accuracy and is used in different contexts, making Mode C the clear choice for altitude data derived from the barometric altimeter.