Because of their price and scale of equipment, artificial satellites are not mass produced but rather custom built to perform their desired function. The communication satellite has connected the world and has become a technology that people use to obtain TV signals and telephone signals. The most important feature of the communication satellite is the transponder. The transponder is a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. There are passive and active transponders.
Passive transponders allow a computer or robot to identify an object.
Active transponders receive incoming signals over a range, or band, of frequencies, and then retransmit it on a different band. A satellite contains hundreds or thousands of sophisticated active transponders. The term for the incoming signal is the uplink and the signal that is retransmitted from the satellite is the downlink. Satellite Comparisons Telstar and Intelsat are two communication satellites that are very similar. Intelsat has 19 satellites and Skynet has eight high-powered satellites operating in its Telstar Fleet.