What is the primary function of a Mode C transponder?

The primary function of a Mode C transponder is to automatically report an aircraft's pressure altitude to air traffic control (ATC) radar. A transponder is an electronic device on an aircraft that enhances its radar signature. It does this by receiving a radio signal from a radar source and then automatically transmitting a coded reply. 'Mode C' is a specific mode of operation for the transponder. In this mode, in addition to the basic identity code (Mode A) that the pilot can manually set, the transponder also encodes and transmits the aircraft's altitude derived from its pressure altimeter. The pressure altimeter measures altitude based on atmospheric pressure. ATC radar systems are designed to receive and interpret this altitude information, displaying it alongside the aircraft's radar position on the controller's screen. This allows ATC to maintain accurate vertical separation between aircraft, preventing collisions and ensuring safe and efficient traffic flow. Without Mode C, ATC would only have lateral position information from radar, requiring pilots to verbally report altitude changes, which is less efficient and potentially prone to errors. Therefore, Mode C provides automated, real-time altitude reporting, which is essential for modern air traffic control.