(also XPDR, XPNDR, TPDR, TP)
A device used to receive electrical signals and emit different signals in response. The transponder receives an interrogator signal, converts the frequency, and responds to the signal in a different predetermined frequency. This allows both signals to be detected simultaneously.
Aviation. Pilots receive a signal and respond with their identification code, which includes their altitude. Transponders also help to amplify the aircraft’s signal and make it more visible on a radar.
Sonar devices. Transponders help calculate depth and distance underwater and mark and trace positions. Calculations are done by measuring the time that passes between sending and receiving signals.