View source for Transponder

{{short description|Device that emits an identifying signal in response to a received signal}}
[[Image:407 Transponder.jpg|thumb|right|120px|An [[Ontario Highway 407]] toll transponder circa 2009]]

In [[telecommunications]], a '''transponder''' is a device that, upon receiving a signal, emits a different signal in response.<ref>{{Cite web|url=https://www.merriam-webster.com/dictionary/transponder|title=Definition of TRANSPONDER|date=22 July 2023 }}</ref> The term is a [[blend word|blend]] of ''transmitter'' and ''responder''.<ref>[http://dictionary.reference.com/browse/transponder transponder]</ref><ref>{{cite news |title=Kunkune |url=https://kunkune.co.uk/ |publisher=Tech Accessories}}</ref>

In [[air navigation]] or [[radio frequency identification]], a [[Transponder (aeronautics)|flight transponder]] is an automated [[transceiver]] in an aircraft that emits a coded identifying signal in response to an interrogating received signal.

In a [[communications satellite]], a [[Transponder (satellite communications)|satellite transponder]] receives signals over a range of uplink frequencies, usually from a [[satellite ground station]]; the transponder amplifies them, and re-transmits them on a different set of downlink frequencies to receivers on Earth, often without changing the content of the received signal or signals.

==Satellite/broadcast communications==
{{See also|Transponder (satellite communications)}}
A [[communications satellite]]’s [[Communication channel|channels]] are called transponders because each is a separate [[transceiver]] or [[repeater]]. With [[digital video]] [[data compression]] and [[multiplexing]], several [[video]] and [[sound reproduction|audio]] channels may travel through a single transponder on a single [[wideband]] [[carrier signal|carrier]]. Original [[analog video]] only has one channel per transponder, with [[subcarrier]]s for audio and automatic transmission identification service ([[Automatic Transmitter Identification System (television)|ATIS]]). Non-multiplexed [[radio station]]s 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 [[landline]]s to send it to an [[Earth station (communications)|earth station]] for multiplexing with other stations.

==Optical communications==
In [[fiber-optic communication]]s, a transponder is the element that sends and receives the optical signal from a [[optical fiber|fiber]]. A transponder is typically characterized by its data rate and the maximum distance the signal can travel.

The term "transponder" can apply to different items with important functional differences, mentioned across academic and commercial literature:
* according to one description,<ref>[https://books.google.com/books?id=xTIonQZI7TMC&q=%22MSA+transceivers+and+MSA+transponders%22&pg=PA353 Optical Fiber Telecommunications V: Systems and networks, Academic Press, 2008, page 353-354]</ref> a transponder and [[transceiver]] are both functionally similar devices that convert a [[full-duplex]] ''electrical'' signal into a full-duplex optical signal. The difference between the two is that transceivers interface electrically with the host system using a ''serial'' interface, whereas transponders use a ''parallel'' interface to do so. In this view, transponders provide easier-to-handle lower-rate parallel signals, but are bulkier and consume more power than transceivers.
* according to another description,<ref>{{Cite web |url=http://www.compeng.dit.ie/staff/tfreir/optical_2/Unit_4.ppt |title=WDM and DWDM Multiplexing Powerpoint presentation, School of Electronic and Communications Engineering, Dublin (Ireland), slide 61 |access-date=2011-02-16 |archive-date=2011-07-21 |archive-url=https://web.archive.org/web/20110721123432/http://www.compeng.dit.ie/staff/tfreir/optical_2/Unit_4.ppt |url-status=dead }}</ref> transceivers are limited to providing an ''electrical-optical'' function only (not differentiating between serial or parallel electrical interfaces), whereas transponders convert an optical signal at one [[wavelength]] to an optical signal at another wavelength (typically [[Frequency grid|ITU standardized]] for [[DWDM]] communication). As such, transponders can be considered as two transceivers placed back-to-back. This view also seems to be held by, for example, [[Fujitsu]].<ref>[http://www.fujitsu.com/downloads/TEL/fnc/pdfservices/DWDM_Tutorial.pdf Fujitsu DWDM Primer, 2004]</ref>

As a result, differences in transponder functionality also might influence the functional description of related optical modules like transceivers and [[muxponder]]s.

==Aviation==
<!-- This section is linked from [[SR-71 Blackbird]]. -->
{{Further|Transponder (aeronautics)}}
[[File:Transponder in Private Aircraft.jpg|thumb|A transponder in a private plane squawking 2000]]

Another type of transponder occurs in [[identification friend or foe]] (IFF) systems in [[military aviation]] and in [[air traffic control]] [[secondary surveillance radar]] (beacon radar) systems for [[general aviation]] and [[commercial aviation]].<ref>{{cite journal |last1=Dobkin |first1=Daniel M. |title=A Radio-Oriented Introduction to Radio Frequency Identification |journal=High Frequency Electronics |date=2005}}</ref>

[[Radar|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 always 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.{{citation needed|date=May 2024}}

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 between planes. Another mode called Mode S (Mode Select) is designed to help avoiding over-interrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are [[backward compatible]] with Modes A and C. Mode S is mandatory in [[controlled airspace]] in many countries. Some countries have also required, or are moving toward 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 [[balloon (aeronautics)|balloons]] and [[Glider (sailplane)|gliders]], the power requirements during long flights.{{citation needed|date=May 2024}}

Transponders are used on some military aircraft to ensure ground personnel can verify the functionality of a missile's [[flight termination system]] prior to launch. Such radar-enhancing transponders are needed as the enclosed weapon bays on [[Fifth-generation jet fighter|modern aircraft]] interfere with prelaunch, flight termination system verification performed by [[range safety]] personnel during training test launches. The transponders re-radiate the signals allowing for much longer communication distances.<ref>{{Cite web|url=https://www.curtisswrightds.com/infocenter/white-papers/telemetry-re-radiation-system.html|title=TELEMETRY RE-RADIATION SYSTEM|last=Cook|first=Paul|date=2008|website=Curtiss-Wright Desfense Solutions}}</ref>

===Marine===
The [[International Maritime Organization]]'s [[International Convention for the Safety of Life at Sea]] (SOLAS) requires the [[Automatic Identification System]] (AIS) to be fitted aboard international voyaging ships with {{GT|300 or more|disp=long}}, and all passenger ships regardless of size.<ref>{{cite web
 |work        = [[International Convention for the Safety of Life at Sea|SOLAS]]'1974
 |url         = http://www.imo.org/KnowledgeCentre/ReferencesAndArchives/HistoryofSOLAS/Documents/SOLAS%201974%20-%20Brief%20History%20-%20List%20of%20amendments%20to%20date%20and%20how%20to%20find%20them.html#43
 |title       = December 2000 amendments
 |author      = International Maritime Organization
 |url-status     = dead
 |archive-url  = http://arquivo.pt/wayback/20140725015425/http://www.imo.org/KnowledgeCentre/ReferencesAndArchives/HistoryofSOLAS/Documents/SOLAS%201974%20-%20Brief%20History%20-%20List%20of%20amendments%20to%20date%20and%20how%20to%20find%20them.html#43
 |archive-date = 2014-07-25
}}</ref> AIS transmitters/receivers are generally called ''transponders'', but they generally transmit autonomously, although [[Coast radio station|coast stations]] can interrogate [[Automatic identification system#Detailed description: Class B units|class B transponders]] on smaller vessels for additional information.{{citation needed|date=May 2024}} In addition, [[navigational aid]]s often have transponders called [[Racon|RACON]] (radar beacons) designed to make them stand out on a ship's radar screen.{{citation needed|date=May 2024}}

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

==Other applications==

===Electronic toll collection===
[[Electronic toll collection]] systems such as [[E-ZPass]] in the eastern United States use [[RFID]] transponders to identify vehicles.<ref>{{Cite web |title=Understanding E-ZPass: The Ultimate Guide - Bestpass by Fleetworthy |url=https://bestpass.com/resources/blog/understanding-e-zpass |access-date=2025-03-08 |website=Understanding E-ZPass: The Ultimate Guide {{!}} Bestpass.com |language=English}}</ref>

===Lap timing===
Transponders are used in races for lap timing. A cable loop is dug into the race circuit near to the start/finish line. Each individual runner or car has an active transponder with a unique ID code. When the individual passes the start/finish line, the lap time and the racing position is shown on the score board.{{citation needed|date=May 2024}}

Passive and active RFID systems are used in [[motor sports]], and off-road events such as [[Enduro]] and [[Hare and Hound (Motorcycle Race)|Hare and Hounds]] racing, the riders have a transponder on their person, normally on their arm. When they complete a lap they swipe or touch the receiver which is connected to a computer and log their lap time.{{citation needed|date=May 2024}}

[[NASCAR]] uses transponders and cable loops placed at numerous points around the track to determine the lineup during a caution period. This system replaced a dangerous [[Racing back to the caution|race back to the start-finish line]].{{citation needed|date=May 2024}}

===Car keys===
Many modern automobiles have keys with transponders hidden inside the plastic head of the key. The user of the car may not even be aware that the transponder is there, because there are no buttons to press. When a key is inserted into the ignition lock cylinder and turned, the car's computer sends a signal to the transponder. Unless the transponder replies with a valid code, the computer will not allow the engine to be started. Transponder keys have no battery; they are energized by the signal itself.<ref>{{Cite web |url=http://carlocksmith.co/7-how-does-transponder-car-key-work.html |title=HOW DOES TRANSPONDER CAR KEY WORK? |date=19 February 2015 |access-date=2 May 2019 |archive-date=2 May 2019 |archive-url=https://web.archive.org/web/20190502135641/http://carlocksmith.co/7-how-does-transponder-car-key-work.html |url-status=dead }}</ref><ref>{{Cite web | url=http://www.rfid-handbook.de/downloads/E2E_chapter03-rfid-handbook.pdf | title=Fundamental Operating Principles | website=www.rfid-handbook.de | access-date=2024-07-26}}</ref>

===Gated communities===
Transponders may also be used by residents to enter their [[gated community|gated communities]].{{citation needed|date=May 2024}} 
However, having more than one transponder causes problems. If a resident's car with simple transponder is parked in the vicinity, any vehicle can come up to the automated gate, triggering the [[Identification friend or foe|gate interrogation signal]], which may get an acceptable response from the resident's car. Such units properly installed might involve [[beamforming]], unique transponders for each vehicle, or simply obliging vehicles to be stored away from the gate.{{citation needed|date=May 2024}}

==See also==
* [[Acronyms and abbreviations in avionics]]
* [[Transponder car key]]
* [[Transceiver]]
* [[Muxponder]]
* [[Rebecca/Eureka transponding radar]]

== References ==
{{reflist}}
* {{FS1037C MS188}}
* {{DODDIC}}

{{Commons category|Transponders}}

==External links==
* [http://www.dccwiki.com/Transponding Transponding with DCC] - Transponding in model railroading

{{Authority control}}

[[Category:Communication circuits]]
[[Category:Radio electronics]]
[[Category:Radar]]
[[Category:Motorsport terminology]]
[[Category:Radio-frequency identification]]
[[Category:Wireless]]

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