In modern warfare airplanes travel so fast that even pilots with good eyes cannot always tell whether an oncoming aircraft is a friend or foe until it is too late. For this reason, fighter planes were equipped with a device called a transponder during the Second World War.
Transponder” is a hybrid word formed by the fusion of the words “transmitter” and “responder.” Accordingly, a transponder is a device that receives a radio signal and automatically responds. Accordingly, when a Second World War fighter plane saw an unidentified aircraft approaching, it emitted a radio signal. If the oncoming plane had a transponder and it sent back a prearranged message at a prearranged frequency, the pilot could relax and refrain from combat preparations. But if the expected reply did not come, then the aircraft might be approaching with hostile intentions. Of course, the pilot could not be certain. Sometimes a transponder did not work properly.
In airports, transponders determine the position of airplanes in the vicinity. Each plane is assigned a squawk (a picturesque name for the reply that the transponder will make). The airport’s air traffic control center emits radio signals, and the transponder will squawk its reply, thus revealing where the plane is located. If two transponder squawks are dangerously close to each other, the air traffic control center will direct one or both of the planes to move to a different location. This helps to avert collisions.
Transponders also help airplanes avoid collisions while flying to their destinations. The atmosphere is a big place, but two planes can and do collide at times. For this reason, most commercial aircrafts employ transponder technology so that they can warn one another of their presence.
Motor vehicles employ transponder technology. In many automobiles, the car keys come equipped with little transponder chip. These cars have an engine control unit (ECU), an electronic device that performs many different functions. One of these functions is sending a signal to the transponder in the key when the key turns the ignition. If the transponder sends the correct signal back to the ECU, the ECU allows the car to start.
In some cases, transponders enable a motor vehicle to pay a toll automatically. As the car approaches the place where it must pay the toll, it receives a radio signal. The vehicle’s transponder sends a reply. The transponder’s reply reveals the type of vehicle it is, and the toll is assessed and paid automatically. As a result, the vehicle does not have to stop to pay the toll. This system is called E-ZPass, according to Wikipedia.
Communications satellites are equipped with transponders. Television networks send their programs up to a satellite, and the transponder sends it back down to earth, where a television set can receive it. Spaceships also can be equipped with transponders, and NASA puts them to good use.
References:
The Free Dictionary by Farlex: Transponder
http://www.thefreedictionary.com/transponder
Wikipedia: Transponder
http://en.wikipedia.org/wiki/Transponder
AV Web: Transponder Basics
http://www.avweb.com/news/avionics/183231-1.html
Wikipedia: Transponder Car Key
http://en.wikipedia.org/wiki/Transponder_car_key
Wikipedia: E-ZPass
http://en.wikipedia.org/wiki/E-ZPass
