(4) Repeater
The satellites, used for communication purposes, receive signals (from a transmitting station on the earth), amplify it and retransmit it (generally at a different carrier frequency) back towards the receiving stations on the earth. The role is, therefore, similar to that of a repeater. The repeater also picks up the signal from the transmitter, and retransmits it to the receiver, after amplifying it and (sometimes) changing its carrier frequency.
