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satellite communication by timothy pratt pdf free download

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver(s) at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are over 2,000 communications satellites in Earth’s orbit, used by both private and government organizations.[1]

Wireless communication uses electromagnetic waves to carry signals. These waves require line-of-sight, and are thus obstructed by the curvature of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated points.[2] The electromagnetic signals that communication satellites work with, have a large spectrum of wavelengths and frequencies. To keep these waves from interfering with one another, international organizations have certain rules and regulations describing which wavelength a certain company or group can use. By separating out wavelengths, communication satellites will have minimal interference and be able to communicate effectively.[3]

There are two major classes of communications satellites, passive and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, and only a very small amount of the transmitted energy reflects toward the receiver. Since the satellite is so far above Earth the radio signal is attenuated on its long journey due to free-space path loss, so the signal received on Earth is very weak. Active satellites, on the other hand, amplify the received signal before re-transmitting it to the receiver on the ground[3] Passive satellites were the first communications satellites, but are little used now.