Satellite Communication - A General Discussion PART 1

[sherry schneider]

By the terms of the dictionary, a satellite is an object which
revolves around another object. For example, the Moon is a satellite
of the Earth, and the Earth is one of the Sun. The phenomenon
interested mankind to investigate more on this idea and experiment on
man-made satellite en-circling the Earth. It was thought that if such
a satellite was put in the space, communication could be set up from
one part of the world to the other. A communication satellite orbits
around the earth and in effect is an artificial satellite, stationed
in space for the purpose of telecommunication. These satellites use
the geosynchronous orbits, Molniya orbits or low polar Earth orbits.
A geosynchronous orbit is essentially a geocentric orbit, which takes
the same time as the Earth to complete one orbit. If this satellite
could be seen from ground, it would seem that the satellite is
stationary and not moving at all. Satellites in such orbits are useful
for telecommunication applications. Satellites in Molniya orbit have a
highly elliptical, set at an angle, taking 12 hours to make a complete
the orbit. A satellite in a low polar Earth orbit passes right above
both the poles, inclined at a certain angle relative to the equator.
How are the objects kept in orbit?
Since man have been able to lift his head towards the sky, he has
looked up at the sky and wondered how that Sun is held up high above
there, and why does not the Moon fall on us. It has only been about
300 years now, that we have developed the scientific reason as to the
things we were wondering about in the past. It was in the late 17th
century that Sir Isaac Newton put forth this fundamental law. The
first law says that, every object of matter in this universe attracts
another, with a force, which is proportional to the product of their
masses and inversely proportional to the square of the distance
between the two. Therefore, with the larger mass having greater power
to attract, this attraction gets weaker as the distance between the
two objects are increased.
Newton's law of gravity means that the Sun pulls the Earth and the
other entire planet in this universe, and the Earth also pulling on
the Sun. Since both are quite large masses, the force of attraction
must be large. The question comes, if the Sun is pulling all other
planets, why don't the planets fall on the Sun?
The explanation to this is, that the planets are moving sideways too
at a great velocity. Therefore, taking the example of the Earth, by
the time the Earth has fallen the 93,000,000 miles to the Sun, it has
moved 93,000,000 miles sideways, far enough to miss the Sun. Likewise,
by the time the Moon has fallen 240,000 miles to the Earth, it has
moved 240,000 miles sideways, missing the Earth. This process is
repeated continually and the Earth orbits the Sun and the Moon the
earth, in a never-ending way. If any planet stops moving sideways and
finds itself closer to the Sun, it will fall quickly without missing
the Sun. This sideways movement, called the "angular velocity"
prevents the Earth in falling into the Sun. The same is true for all
planets en-circling the Sun. The Moon would fall on to Earth if it had
not had that angular velocity.
If the gravity of the Earth were to be turned off, the Earth would
leave its circular trajectory and travel straight with a high
velocity, reaching 50 billion miles out from the Sun in a century's
time. Hence, it is the gravitational forces of both the Sun and the
Earth, that holds the Earth in a orbit around the Sun. The orbiting
Earth may be compared to a piece of stone tied to a string, which is
swung in a circle holding the string at one end. If you were to let go
of the string, the stone, tied at the other end of the string, will
fly off at a straight trajectory, just as Earth would do, if the
gravity were to be turned off. This force is termed as the
'Centrifugal force.'
There can be a question about the time required to complete one orbit.
Does that depend upon the distance at which the object is orbiting?
After years of experiments, it was found out, that the greater the
distance, the more time it takes to complete one orbit. This was
deduced from the formula, that the time taken, is directly
proportional to the distance of the object around which it is
orbiting. Thus, the planet which is at a larger distance from the Sun,
takes longer to complete one orbit. This time taken to complete one
orbit is termed as 'orbital period.'
Artificial Satellites
Looking at the first law of Newton, it was understood that in
principle it should be possible to put up a man-made satellite in the
sky, which would orbit the Earth. As has been seen, this satellite
would need a sideways velocity, like the earth, so that it does not
fall back on Earth. If such a satellite is put up in the sky at a
distance of 4,000 miles, it will have an orbital period of
approximately 90 minutes. To miss falling on to the Earth, this
satellite would have to have a sideways velocity of 17,000 miles per
hour.
Let us consider firing of a cannon ball. If the firing is quite weak,
the cannon ball describes a parabolic path and lands up few hundred
yards away. If we bring a heavier cannon ball, and try and shoot it a
bit further with greater force, the ball describes a parabolic path to
the earth's surface and lands up a few hundred miles away. This time
the cannon ball goes over the Earth's curvature to describe its path.
If now, the a super-heavy cannon ball is shot very forcefully to land
a few thousand miles away, the ball would travel much further than
what it would if the Earth was flat. Clearly it can be deduced that
the Earth's curvature had some effect on the distance that this cannon
ball traveled. Imagine that the velocity of this cannon ball is 5
miles per second, i.e., 17,000 miles per hour. As it falls to the
Earth, it misses the Earth's surface and the Earth's gravitational
force makes the ball continuously change its course in its the fall to
the Earth. Thus this cannon ball starts orbiting the Earth. Till the
concept of rocket came in, such velocity was unthinkable and putting
up a satellite in the sky remained a dream. Eventually the technology
evolved and the first artificial satellite 'Sputnik,' was launched by
the Russians in 1957. This satellite was not much more than a basket
ball and had a radio transmitter on board. It made 'Beep. Beep. Beep'
sounds and would appear and disappear, again to re-appear in 90
minutes time.
A few years before the Americans put John Glen into the orbit, on-
board an artificial satellite, the Russians had already launched Yuri
Gagarin, the first man in space. All these satellites were launched at
the same altitude, give or take a few hundred miles, and both had the
orbital time of 90 minutes. If these satellites were launched higher,
for example at a height of 22,300 miles, the orbital time would have
been orbiting the Earth every 24 hours. The skill required to launch
the artificial satellites, so as to achieve a geosynchronous orbit,
did not happen till 1963. This required the satellite to be launched
at a much higher altitude.
When you, look back to the dawn of space travel, the landing on the
Moon by Apollo in 1969, seemed to be a giant step forward in space
travel. Since that time, weather forecasting has technologically
improved with geostationary meteorological satellites, sending the
pictures that we see on television everyday. The television broadcast
has taken a leap with these satellites, where live telecasts are being
beamed from one side of the world to the other. The satellite aided
voice communication has had a remarkable effect in getting help in a
remote area, where other communication is not available, delivering
the highest speech quality with reliability.
With globalisation, factories and offices have come up in the remote
part of the world. These locations are often in those parts where
modern communication network does not exist. Despite this, business
needs to go on and communication is vital. Satellite communication has
virtually brought these places quite close, bringing the communication
network virtually to any location around the world. This has been done
without the need of an infrastructure of wireless network, fiber
optics, or even copper cabling.
End of Part I
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