CH 1 Introduction Class notes.pdf - Chapter1 Introduction...

Chapter1
Introduction to Satellite Communication
1.1 INTRODUCTION
Satellite communication is an application of radio
communication. Radio communication has been
used in telecommunication and broadcast for
many decades. Since the invention of radio waves
we have realized that communication over long
distances is more economical using radio media
than the copper wire. Radio communication is
carried out in different frequency bands such as
HF, VHF, UHF, SHF, microwaves, millimeter
waves etc. In terrestrial radio communication the
media is the earth’s atmosphere. The propagation
characteristics, such as the attenuation, dispersion
etc. is different in different frequency bands. The
wavelength of radio waves is inversely
proportional to the frequency of the wave and is
shorter at higher frequency than at lower
frequency. At wavelengths comparable to the size
of the water molecules in atmosphere, for
example, the attenuation in the corresponding
frequency band is large. At certain frequency
bands the radio waves change the direction when they enter from higher density to lower density of the
medium. This phenomenon aids the radio broadcast in medium-wave bands. The waves are refracted within
the propagation medium resulting in longer coverage of the area by the radio signals. At higher frequencies,
VHF and beyond, the radio waves traverse in straight lines, and not refracted back to earth. At such
frequency bands communication is possible only when the transmitter and receiver have no obstacles
between them and the waves are able to travel straight from transmitter to receiver. Such communication is
termed line-of-sight communication.
Due to earth’s curvature the radio waves transmitted from a source with certain height can only
reach a certain distance that is not obstructed by earth itself. This distance, called the range of coverage,
cannot be increased even if we increase the power of the transmitted signal. The range of the coverage can
be increased by increasing the location of the antenna above the surface of the earth. The largest possible
area of coverage on earth from a transmitter placed sufficiently high above the earth is less than one third of
the earth surface. Arthur C. Clerk published an article titled “Extraterrestrial Relays” in 1945 in the
Wireless World magazine in which he suggested that if three satellites are placed in a circular orbit at 120
degrees separation at a radius of 42,242 km in the equatorial plane they could cover the entire world
[Clarke 1945]. This is equivalent to having an antenna with a height of approximately 35,871 km.
Obviously, we cannot have a physical structure of this height to hold an antenna. That is the reason why a
satellite is used. The satellite at this altitude acts as a repeater, or a transceiver, relaying the signals received
from one part of the earth to another part. A satellite at this altitude in space can see nearly a third of the
earth and, hence, three satellites at this altitude cover most of the earth surface. There is some part of the
earth that will remain uncovered by three satellites. However, when the satellites are in equatorial orbit the
uncovered are lie in the Polar Regions which are not inhibited.