 - A transponder is a wirelesscommunications, monitoring, or control device that picks up and automatically responds to an incoming signal. The term is a contraction of thewords transmitter and responder. Transponders can be eitherpassive or active.A passive transponder allows a computer or robot to identify an object. Magnetic labels, such as those on credit cards and store items, are commonexamples. A passive transponder must be used with an active sensor that decodes andtranscribes the data the transponder contains. The transponder unit can bephysically tiny, and its information can be sensed up to several feet away. Simple active transponders are employed in location, identification, andnavigation systems for commercial and private aircraft. An example is an RFID (radio-frequency identification) devicethat transmits a coded signal when it receives a request from a monitoring or controlpoint. The transponder output signal is tracked, so the position of the transpondercan be constantly monitored. The input (receiver) and output (transmitter) frequencies are preassigned. Transpondersof this type can operate over distances of thousands of miles. Sophisticated active transponders are used in communications satellites and on board space vehicles. They receive incoming signals over a range, or band, of frequencies, and retransmit thesignals on a different band at the same time. The device is similar to a repeater of the sort used in land-based cellulartelephone networks. The incoming signal, usually originating from a point on theearth's surface, is called the uplink. The outgoing signal, usually sent toa point or region on the surface, is the downlink. These transponderssometimes operate on an interplanetary scale.
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11 Apr 2003
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