Mode C transponder:


The aviation transponder was originally developed during World War II by the British and American military as an "Identification friend or foe" (IFF) system to differentiate friendly from enemy aircraft on radar.

Basic Navigation principles:

Mode C is a part of the ATC system. The transponder uses two frequencies, one for Tx and one for Rx mode. 1030 MHZ interrogation and 1090 MHZ reply. Azimuth is augmented by pressure altitude. The altitude is determined on the standard altimeter setting of 29.92 inches of mercury, regardless of what the aircraft altimeter is set to. The altitude reported is, therefore, the pressure altitude of the aircraft, not the actual altitude.

Now you might ask, "if my altimeter is set at 30.23 and the Mode C is putting out altitude referenced to 29.92, won't the controller see my altitude incorrectly?" No, because ATC's ground equipment automatically adjusts your Mode C readout for the local altimeter setting. Pressure altitude is obtained from an altitude encoder passed to the transponder using a modified form of the Gray code; this coded data is in 100 ft increments. Aircraft with air data computers will send altitude to the transponder via serial data form, usually ARINC 429.

Aircraft Equipment

The main piece of aircraft equipment would be the altitude encoder or encoding altimeter. The transponder depends on an external altitude encoder to provide the digitized pressure altitude that the transponder needs for its Mode C replies. The encoder is connected the same static air line as the altimeter and is wired electrically to the transponder.

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Altitude Encoder.


Aircraft communications and Navigation systems, Mike Tooley and David Wyatt. Pg 254 : ATC transponder codes