First developed as a way to identify friendly aircraft from the enemy during World War II, the aircraft transponder helps air traffic control keep us safely away from each other in the sky. Today’s Mode C transponders allow ATC to determine the position of every equipped airplane, including altitude. Unique ATC-assigned codes further identify aircraft on the controller’s radar scope and help the controller keep track of specific airplanes.
When the radar antenna on the ground sweeps, it sends out signals that “request” data from all the transponders it can reach. The transponder in the airplane then responds to the interrogation with the aircraft’s position and altitude.
Each unit does this a little differently. Older units have dedicated altitude encoders that are hooked directly into the pitot-static system, while newer solid-state units transmit most information digitally. Either way, the altitude tolerance usually is within 100 feet. For this to work the pilot must select “Alt” on the face of the transponder. Selecting “On” only provides position information, and no altitude. “Stby” warms up the transponder prior to takeoff.
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