The problem with microwave communications is of line of sight. Because of the curvature of earth, mountains and other high structures often block the line of sight. So you require several repeater stations for long-distance transmission, which increases the cost of data transmission. This problem is overcome by using satellites.
A communication satellite is an electrical device positioned in an orbit around the earth. It can be thought of a big microwave repeater in the sky. I contains one or more 'transponders', each of which listens to some portion of the frequency spectrum, amplifies the incoming signal and then rebroadcasts it at another frequency. Different frequencies are used for 'uplinking' and 'downlinking' to avoid any interference of signals. Uplink refers to data flow from the earth to the satellite. Here the earth station works as a transmiter and the satellite transponder as a receiver. Downlink refers to data flow from the satellite to the earth. Here the saellite works as a transmitter and the earth station as a receiver.
Advantages
There is no line of sight restriction, so transmission and reception is possible between any two randomly chosen places.
Disadvantages
Launching a satellite into an orbit costs a lot.
A signal sent to a satellite is broadcasted to all receivers within the satellite's range. So, measures are required to prevent unauthorized tampering of information.
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