Transponder

The airborne radar beacon receiver/transmitter portion of the Air Traffic Control Beacon System that automatically receives radio signals from interrogators on the ground and selectively replaces with specific reply pulse-on-pulse group only those interrogations being received on the mode to which it is set to respond. Each aircraft transponder is capable of replying to 4,096 codes as selected by the pilot. Provides the air traffic controller positive location and, in some cases, altitude information. (FAA6)

A piece of equipment which, when set upon a specific radio frequency, emits a directional signal of its own and enables the receiver to track or monitor the truck's location. (BOC3)