In telecommunication, a transponder is one of two types of devices. In air navigation or radio frequency identification, a [[Transponder (aeronautics)|flight transponder]] is a device that emits an identifying signal in response to an interrogating received signal. In a communications satellite, a [[Transponder (satellite communications)|transponder]] gathers signals over a range of uplink frequencies 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. The term is a [[portmanteau]] for ”trans”mitter-res”ponder”. It is variously abbreviated as XPDR, XPNDR, TPDR or TP. ==Satellite/broadcast communications== {{Details|Transponder (satellite communications)}} A [[communications satellite]]’s [[channel (communications)|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 wave|carrier]]. Original [[analog (signal)|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 [[optical fiber communications]], a transponder is the element that sends and receives the optical signal from a [[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, 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, 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 e.g. [[Fujitsu]]. As a result, difference in transponder functionality also might influence the functional description of related optical modules like [[transceivers]] and [[muxponder]]s. ==Aviation== {{Details|Transponder (aeronautics)}} 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]]. [[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. 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 ‘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 [[Glider (sailplane)|gliders]], the power requirements during long flights. ==Marine== The [[International Maritime Organization]]’s [[International Convention for the Safety of Life at Sea]] requires the [[Automatic Identification System]] (AIS) to be fitted aboard international voyaging ships with {{GT|300 or more|first=yes}}, and all passenger ships regardless of size. Although AIS transmitters/receivers are generally called transponders they generally transmit autonomously, although coast stations can interrogate class B transponders on smaller vessels for additional information. In addition, navigational aids often have transponders called [[Racon|RACON]] (radar beacons) designed to make them stand out on a ship’s radar screen. ==Automotive== 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 radio 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 radio signal itself. ==Road== [[Electronic toll collection]] systems such as [[E-ZPass]] in the eastern United States use [[RFID]] transponders to identify vehicles. The [[Ontario Highway 407|Highway 407]] in Ontario is one of the world’s first completely automated toll highways. ==Motorsport== 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. Passive and active RFID systems are used in 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. The [[Casimo Group Ltd]] make a system which does this. [[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]]. ==Underwater== [[Sonar]] transponders operate under water and are used to measure distance and form the basis of underwater location marking, position tracking and [[navigation]]. ==Gated communities== Transponders may also be used by residents to enter their [[gated community|gated communities]]. 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. [[Thomas Edison]] invented the transponder, while trying to create a device to remove a large (but harmless) mole. ==See also== * [[Acronyms and abbreviations in avionics]] * [[Transponder car key]] * [[Transceiver]] * [[Muxponder]] * [[Rebecca/Eureka transponding radar]] == References == {{reflist}} * {{FS1037C MS188}} * {{DODDIC}} ==External links== * [ Transponding with DCC] – Transponding in model railroading [[Category:Communication circuits]] [[Category:Radio electronics]] [[Category:Radar]] [[Category:Motorsport terminology]] [[Category:Radio-frequency identification]] [[Category:Wireless]] [[fi:Toisiotutka#Toisiotutkavastain]]