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A Highway 407 toll transponder

In telecommunication, the term transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR or TPDR) has the following meanings:

• An automatic device that receives, amplifies, and retransmits a signal on a different frequency (see also broadcast translator).
• An automatic device that transmits a predetermined message in response to a predefined received signal.
• A receiver-transmitter that will generate a reply signal upon proper electronic interrogation.

For more details on this topic, see Transponder (Satellite communications).

A communications satellite’s channels are called transponders, because each is a separate transceiver or repeater. With digital video data compression and multiplexing, several video and audio channels may travel through a single transponder on a single wideband carrier. Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.

For more details on this topic, see Transponder (aviation).

Another type of transponder occurs in identification friend or foe systems in military aviation and in air traffic control secondary surveillance radar (beacon radar) systems for general aviation and commercial aviation. Primary radar works best with large all-metal aircraft, but not so well on small, composite aircraft. Its range is also limited by terrain and rain or snow and also detects unwanted objects such as automobiles, hills and trees. Furthermore it cannot estimate the altitude of an aircraft. Secondary radar overcomes these limitations but it depends on a transponder in the aircraft to respond to interrogations from the ground station to make the plane more visible.

Depending on the type of interrogation, the transponder sends back a transponder code (or "squawk code") (Mode A) or altitude information (Mode C) to help air traffic controllers to identify the aircraft and to maintain separation. Another mode called Mode S (Mode Select) is designed to help air traffic control in busy areas and allow automatic collision avoidance. Mode S tranponders are 'backwards compatible' with Modes A & C. Mode S is mandatory in controlled airspace in many countries. Some countries have also required, or are moving towards requiring, that all aircraft be equipped with Mode S, even in uncontrolled airspace. However in the field of general aviation, there have been objections to these moves, because of the cost, size, limited benefit to the users in uncontrolled airspace, and, in the case of balloons and gliders, the power requirements during long flights.

Navigational aids often have transponders called RACON designed to make them standout on a ships radar screen.

The E-ZPass system in the eastern United States is one of many systems for paying bridge and road tolls by a RFID transponder in the car.

Transponders are used in motorsport for lap timing purposes. A cable loop is dug into the race circuit near to the start/finish line. Each car has an active transponder with a unique id code. When the racing car passes the start/finish line the lap time and the racing position is shown on the score board.

Sonar transponders operate under water and are used to measure distance and form the basis of underwater location marking, position tracking and navigation.

• Racon
• Transceiver
• Secondary surveillance radar (Identification friend or foe)
• Transponding with DCC - Transponding in model railroading.
• RFID - low-cost transponder tags for asset tracking and telemetry.

Source: partly from Federal Standard 1037C and from MIL-STD-188 and from the Department of Defense Dictionary of Military and Associated Terms