Which altitude is transmitted by a Mode C transponder?

Mode C transmits **pressure altitude**, referenced to the standard pressure setting, rather than an altitude corrected to local QNH or QFE. Air traffic systems can use this standardized pressure-altitude report for vertical surveillance and apply the appropriate settings or procedures.\n\n**Key takeaway:** Mode C reports pressure altitude.\n\n**Glossary:** **Pressure altitude**: altitude indicated with the standard pressure setting; **QNH/QFE**: altimeter pressure references.