Transponders

The transponder is essentially a receiver which receives the signal transmitted from the earth by the uplink, amplifies it and retransmits it with the downlink, with a different frequency. Thus the word “Transponder” is formed by combining the two words- TRANSmitter and resPONDER. Most satellites have anything between 10 to 30 transponders of different bandwidth on board.

Transponders can be either active or passive. A passive transponder allows a computer or robot to identify an object. Magnetic labels, such as those on credit cards and store items, are common examples. A passive transponder must be used with an active sensor that decodes and transcribes the data the transponder contains. The transponder unit can be physically tiny, and its information can be sensed up to several feet away. Simple active transponders are employed in location, identification, and navigation systems for commercial and private aircraft. An example is an RFID (radio-frequency identification) device that transmits a coded signal when it receives a request from a monitoring or control point. The transponder output signal is tracked, so the position of the transponder can be constantly monitored. The input (receiver) and output (transmitter) frequencies are pre-assigned. Transponders of this type can operate over distances of thousands of miles.