What is a Transponder?

A transponder is an electronic device used to receive and transmit electrical signals wirelessly. Appropriately, its name is also derived from the words “transmitter” and “responder”. It was originally designed to be attached to an object that needed to be located, and some are still used that way today.

A transponder is a device that acts as both a transmitter and a responder and is used to receive and transmit electrical signals wirelessly.

This device works by receiving a signal, called an “interrogator” because it is effectively “asking” for information, and then automatically transmitting a radio wave at a predetermined frequency. To transmit a signal on a frequency other than the received one, a frequency converter is built-in. By receiving and transmitting on different frequencies, the two signals can be detected simultaneously.

A transponder key.

The first use of a transponder was on an aircraft during World War II, as part of the Identify Friend or Foe (IFF) system. By responding to secret interrogation frequencies, pilots could indicate to radar operators that they were friendly aircraft.

In aviation, transponders convey the position and identity of an aircraft.

These devices are still common today in military and commercial aviation. They receive a signal from the ground and automatically respond with an identification code for air traffic controllers, as well as altitude information. In aircraft applications, they are also configured to amplify the signal to make the plane more visible on radar.

Transponders on commercial aircraft transmit their carrier and flight identification numbers to air traffic control centers.

They are also used to measure distances by calculating the time elapsed between sending the interrogator signal and receiving the transponder signal. For example, sonar devices are used to mark underwater positions, calculate depth and track positions.

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It might seem like this is a technology that the average consumer never uses, but even if it used to be that way, it isn’t anymore. The modern commuter probably has at least one transmitter in their car, probably mounted on the windshield or dashboard. These are for roads that use electronic toll systems that calculate the toll amount to be paid and complete the transaction without requiring the driver to roll down the window. Some newer cars are also equipped with ones that operators can use to locate the vehicle in an emergency. Cell phones use a similar, albeit smaller, chip to send the phone’s location if it used to call an emergency number.

Even watching television occasionally involves using these devices. A network can uplink its terrestrial satellites to communications satellites orbiting the Earth, send multiple channels of digitally compressed video and audio to a single onboard transponder, and local stations can then take the program and rebroadcast it locally, targeting the appropriate soil-based dish.

Communications satellites carry transponders.

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