Automatic Altitude Reporting

Automatic altitude reporting is the transponder function that continuously transmits the aircraft’s current pressure altitude to ATC radar systems. When the transponder is set to Mode C or Mode S with altitude reporting, the altitude readout appears next to the aircraft’s radar return on the controller’s screen.

Mode C Requirement in Canada

Mode C altitude reporting is required in Class C and Class B airspace and above 12,500 ft ASL. Aircraft without a functioning transponder in these areas require an ATC authorization. Mode C must also be enabled when transiting the ADIZ.

How Altitude Reports Are Processed

Mode C transmits pressure altitude (29.92 inHg reference). ATC computers apply the current altimeter setting to convert the readout to indicated altitude. A readout differing from expected values by more than 300 ft may trigger an altitude check request from ATC.

Related Terms

See also: Transponder, Transponder Codes, Pressure Altitude.