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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.
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.
- Spring '14