To meet this requirement smaller dish antennas having

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Unformatted text preview: systems permit data transmission rates of about 16 Giga (1 Giga = 10 9) bits per second. Furthermore, it can carry literally thousands of voice channels at the same time. The link can support 250,000 voice channels. The initial installation cost of microwave links being very high, they are mostly used to link big cities with heavy telephone traffic between them. Communications Satellite The main problem with microwave communication is that the curvature of the earth, mountains, and other structures often block the line-of-sight. Due to this reason several repeater stations are normally required for long distance transmission, which increases the cost of data transmission between two points. This problem is overcome by using satellites, which are relatively newer, and more promising data transmission media. A communication satellite is basically a microwave relay station placed in outer space. These satellites are launched either by rockets or by space shuttles and are precisely positioned 36,000 kms above the equator with an orbit speed that exactly matches the earth's rotation speed. Since a satellite is positioned in a geosynchronous orbit, it is stationary relative to earth and always stays over the same point on the ground. This allows a ground station to aim its antenna at a fixed point in the sky. Each satellite can receive and retransmit signals to slightly less than half of the earth's surface. Therefore, at least three satellites are needed in geosynchronous orbit to provide worldwide data transmission service. Hundreds of satellites are now in orbit to handle international and domestic data, voice, and video communications needs. The INSAT series of Indian satellites are positioned in such a way that they are accessible from any place in India. As shown in Figure 17.6, in satellite communication, microwave signal at 6 GHz or 14 GHz (read as gigahertz = 10 Hz) is transmitted from a transmitter on earth to the satellite positioned in space. By the time this signal reaches the satellite, it becomes weak due to 36,000 kms travel. A transponder mo...
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This document was uploaded on 04/07/2014.

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