History of radio § Broadcasting

                                     

                                        Space communication

This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft. In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic “dish antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwav band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L,C, S, K, and k band are allocated for space communication. A radio link which transmits data from the Earth's surface to a spacecraft is called an uplink, while a link which transmits data from the spacecraft to the ground is called a downlink.

Communication Satellite:

An artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth. These are used because the microwave used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth. There are currently over 2000 communication satellites in orbit around the Earth. Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it. In a satellite ground station a microwave transmitter and large statellite dish antenna transmits a microwave uplink beam to the satellite. The uplink signal carries many channels of telecommunications traffic, such as long distance telephone calls, television programs, and internet signals, using a technique called frequency- dividion  multiplexing (FDM). On the satellite a transponder  receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first. There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines. Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.