What does Mode C under IFF report?

Mode C under Identification Friend or Foe (IFF) systems primarily reports altitude information. This mode is designed to transmit the aircraft's altitude in addition to the identification code sent by the transponder. This altitude data is crucial for situational awareness in air traffic management and military operations, as it helps differentiate aircraft that might be at similar vertical levels, thereby enhancing safety and operational effectiveness.

The ability to know an aircraft's altitude allows both air traffic controllers and other aircraft in the vicinity to have a clearer picture of the airspace, thereby reducing the likelihood of mid-air collisions. Mode C is automatically activated during flight and provides real-time updates of altitude, which is why it is a vital component of radar tracking systems used in both civilian and military contexts.