What altitude information is transmitted by MODE C of the transponder?

MODE C of the transponder transmits the aircraft's altitude information as it is sensed by the aircraft's altimeter, but this data is not adjusted for changes in barometric pressure. This means that the altitude reported is essentially an indication of the aircraft's height above a standard reference, without accounting for variations in atmospheric pressure that could alter the actual altitude above sea level.

In practical terms, this uncorrected altitude can provide information for air traffic control and collision avoidance, allowing for vertical separation of aircraft in controlled airspace. It’s important to understand that while MODE C provides altitude information, it relies on the altimeter settings in the aircraft, which might not reflect the sea-level pressure at all locations at all times, particularly if the aircraft is flying in varying weather conditions.

The other options may suggest that the altitude reported includes adjustments or pertains only to specific forms of altitude, which is not the case with MODE C. It strictly presents the altitude as derived from the aircraft’s altimeter without accounting for barometric pressure adjustments.