Transponders are typically used to support military or commercial aviation communication by broadcasting an aircraft's identity automatically. They also can be used as beacon systems that broadcast an aircraft's position in order to safely organize air traffic. In the latter case, and depending upon the program mode, transponders produce communications tokens or "squawks" that provide altitude information, allowing air traffic control centers to maintain aircraft separation.
Marine transponders are typically used by ship-borne radars to automatically establish position within a ship channel or as an aid to long-distance navigation. Essentially, marine systems query buoy-mounted transponders that, in turn, broadcast position squawks to the ship. Multiple transponders on either side of a channel triangulate, thereby allowing for accurate positioning.
Auto transponders are somewhat new and mostly used on toll roads with devices such as the Texas "TxTag," the state's electronic toll collection system. Transponders are also being tested to define the future of vehicular control. In this concept, a series of automatic transponders constantly monitor and center the car in a lane as it moves down the road.
Underwater transponders have been used by the military since the mid-60s. They operate the same way that surface ships do, by utilizing a series of ship-to-beacon devices to establish a submarine's physical position. They are primarily used for near-shore navigation.
Transponders in racing are primarily used for timing and scoring. Cable loops are embedded in the racetrack. A cars pass over the loops, the system identifies relevant information regarding the position of each car, then displays the information on scoring monitors.