Which IFF mode is used for altitude?

Mode C is specifically designed to transmit altitude information in addition to identifying an aircraft. This mode uses a 4096-code transmission, where it provides both identity and altitude data by sending pressure altitude information that corresponds to the aircraft's flight level. The altitude is expressed in feet relative to a standard datum plane, usually based on the International Standard Atmosphere. This capability is essential for air traffic control and situational awareness, allowing for effective vertical separation between aircraft.

Other modes, while serving different purposes, do not provide altitude information in the same manner. For instance, Mode A transmits only an identification code without altitude data, and Mode B is not commonly used in contemporary air traffic management. Mode D is an enhancement of Mode C but primarily focuses on specific applications such as enhancing target identification rather than merely providing altitude. Thus, Mode C is the definitive choice for altitude reporting among the options.