What does a Mode C transponder transmit?

A Mode C transponder transmits the aircraft's altitude information, which is encoded in the transmission when the aircraft is flying above 18,000 feet. This altitude data is crucial for air traffic control as it allows them to maintain separation between aircraft and ensure safe and efficient management of airspace. The transponder works by responding to radar interrogations, sending back a signal that includes the altitude in thousands of feet, thereby providing ground controllers with a real-time 3D position of the aircraft.

This altitude reporting is essential for the functioning of radar systems, contributing to situational awareness for both pilots and air traffic controllers. Other parameters such as ground speed, aircraft heading, and precise position data are not transmitted by the Mode C transponder; it focuses specifically on altitude information.