Geostationary Satellite
If
a satellite is made to revolve from west to east with a period of revolution
equal to 24 hours in a circular orbit concentric and coplanar with the
equatorial plane of the earth, its relative velocity with respect to the earth
will be zero. Such a satellite is called geostationary satellite because it
appears stationary to an observer on the earth. It is called synchronous
satellite because its angular speed is same as that of the earth about its own
axis. When such a satellite is used for communication purposes, it is known as
communication satellite.
A
satellite which revolves around the earth in its equatorial plane with the same
angular speed and in the same direction as the earth rotates about its own axis
is called a geostationary or synchronous satellite.
Height of a Geostationary Satellite:
The height of a satellite
above the earth‘s surface is given by
h = 
−
R
but T
= 24 h = 86400 s
R = radius
of the earth = 6400 km
g = 9.8 ms−2
= 0.0098 kms−2
∴ h
= 
= 42330 – 6400
= 35930 km
Necessary Condition for a
Geostationary Satellite:
These are as follows:
1.
It should revolve in an orbit
concentric and coplanar with the equatorial plane of the earth.
2.
Its sense of rotation should be
same as that of the earth i.e., from west to east.
3.
Its period of revolution around
the earth should be exactly same as that of the earth about its own axis i.e.,
24 hours.
4.
It should revolve at a height of
nearly 36000 km above the earth’s surface.
Uses
of Geostationary Satellites:
1.
In communication radio, T.V. and
telephone signals across the world. Geostationary satellite acts as reflectors
of such signals.
2.
In studying upper region of the
atmosphere.
3.
In forecasting weather.
4.
In determining the exact shape
and dimensions of the earth.
5.
In studying meteorites.
6.
In studying solar radiation and
cosmic rays.
Use
of Geostationary Satellite in Global Transmission:
A
satellite cannot establish communication link over the entire earth. This is
because, the curvature of the earth keeps a large part of the earth out of sight.
However, three uniformly speed satellite (120° apart from each other) placed in
a geostationary orbit and equipped with radio transponders can be used to
provide line of sight communication between any two points on the earth, as
shown in figure below.
Such
satellite are called synchronous communication satellite (SYNCOMS). The
geostationary orbit is also called Clarke geosynchronous orbit or Clarke arc
after the name of famous science writer Arthur C. Clarke who first proposed the
idea of communication satellite in 1945.
Problems:
1. To what latitude
does the SYNCOMS coverage extend? What is the orbital speed of a SYNCOMS?
Solution:
Clearly, the latitude
of the coverage extends upto the tangent SP, as shown
in below figure.
From right 
cos λ = 
= 
= 
= 0.151
∴ λ = 81.3°
Thus a circular arc of
about 90° is left uncovered around the pole. That is why we need three
satellite to cover the entire earth.
Orbital speed of the
SYNCOMS
= 
= 
= 3067 ms−1
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