Mode C Altitude Reporting
A Mode C/S transponder reports pressure altitude referenced to 1013.25 hPa; dividing by 100 gives the flight level, independent of the local altimeter setting.
Formula
Variables
Example
Pressure altitude 35000 ft → FL350
Did You Know?
Every transponder transmits altitude at the standard 1013.25 hPa datum, so ATC always sees consistent levels even when different regions set wildly different altimeter pressures.
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