In telecommunication, a transponder is one of two types of devices. In air navigation or radio frequency identification, a 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 satellite transponder receives signals over a range of uplink frequencies usually from a satellite ground station, 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.
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. The original analogue 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 (analogue 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.
In optical fibre communications, a transponder is an element that sends and receives the optical signal from a fibre. A transponder is typically characterised 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:
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 ITU standardised for DWDM communication). As such, transponders can be considered as two transceivers placed back-to-back. In this view, transponders provide easier-to-handle lower-rate parallel signals, but are bulkier and consume more power than transceivers.
As a result, the difference in transponder functionality also might influence the functional description of related optical modules like transceivers and mux-ponders.
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 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 to avoid 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 gliders, the power requirements during long flights.
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