Intelecommunications, atransponder is a device that, upon receiving a signal, emits a different signal in response.[1] The term is ablend oftransmitter andresponder.[2][3]
Inair navigation orradio frequency identification, aflight transponder is an automatedtransceiver in an aircraft that emits a coded identifying signal in response to an interrogating received signal.
In acommunications satellite, asatellite transponder receives signals over a range of uplink frequencies, usually from asatellite 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.
Acommunications satellite’schannels are called transponders because each is a separatetransceiver orrepeater. Withdigital videodata compression andmultiplexing, severalvideo andaudio channels may travel through a single transponder on a singlewidebandcarrier. Originalanalog video only has one channel per transponder, withsubcarriers for audio and automatic transmission identification service (ATIS). Non-multiplexedradio stations can also travel insingle 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 usinglandlines to send it to anearth station for multiplexing with other stations.
Infiber-optic communications, a transponder is the element that sends and receives the optical signal from afiber. 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:
As a result, differences in transponder functionality also might influence the functional description of related optical modules like transceivers andmuxponders.
Another type of transponder occurs inidentification friend or foe (IFF) systems inmilitary aviation and inair traffic controlsecondary surveillance radar (beacon radar) systems forgeneral aviation andcommercial aviation.[7]
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]
Depending on the type of interrogation, the transponder sends back atransponder 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 arebackward compatible with Modes A and C. Mode S is mandatory incontrolled airspace in many countries. Some countries have also required, or are moving toward requiring, that all aircraft be equipped with Mode S, even inuncontrolled 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 ofballoons andgliders, the power requirements during long flights.[citation needed]
Transponders are used on some military aircraft to ensure ground personnel can verify the functionality of a missile’sflight termination system prior to launch. Such radar-enhancing transponders are needed as the enclosed weapon bays onmodern aircraft interfere with prelaunch, flight termination system verification performed byrange safety personnel during training test launches. The transponders re-radiate the signals allowing for much longer communication distances.[8]
TheInternational Maritime Organization'sInternational Convention for the Safety of Life at Sea (SOLAS) requires theAutomatic Identification System (AIS) to be fitted aboard international voyaging ships with 300 or more gross tonnage (GT), and all passenger ships regardless of size.[9] AIS transmitters/receivers are generally calledtransponders, but they generally transmit autonomously, althoughcoast stations can interrogateclass B transponders on smaller vessels for additional information.[citation needed] In addition,navigational aids often have transponders calledRACON (radar beacons) designed to make them stand out on a ship's radar screen.[citation needed]
Sonar transponders operate under water and are used to measure distance and form the basis of underwater location marking, position tracking andnavigation.
Electronic toll collection systems such asE-ZPass in the eastern United States useRFID transponders to identify vehicles.[10]
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]
Passive and active RFID systems are used inmotor sports, and off-road events such asEnduro andHare 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]
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 dangerousrace back to the start-finish line.[citation needed]
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.[11][12]
Transponders may also be used by residents to enter theirgated communities.[citation needed] 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 thegate interrogation signal, which may get an acceptable response from the resident's car. Such units properly installed might involvebeamforming, unique transponders for each vehicle, or simply obliging vehicles to be stored away from the gate.[citation needed]
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188). This article incorporatespublic domain material fromDictionary of Military and Associated Terms.United States Department of Defense.
