|
About satellites
The term satellite refers geographically to a
body that accompanies another heavenly body.
The moon is a satellite of the earth for example.
Besides natural satellites we have today also artificial satellites
that are used for defence, scientific purposes and communication.
The idea that radiodifused satellites should
be used to transmit TV and radio programs on long distances was
thought of by the writer Arthur C. Clark. That’s why the belt
with such satellites is called the Clark’s belt.
Artificial satellites can be divided by their
type:
1) telecommunication satellites which are used
to transmit information
2) radiodifused satellites which are used to emit TV and radio
programs
3) meteorological satellites which collect data about the earths
atmosphere and to predict the weather
4) military and espionage satellites which serve defence purposes
5) scientific satellites which are used for various scientific
experiments
the first satellite was launched in 1957. This
was the Russian satellite Sputnik that started the era of conquering
the communicational satellite area of the earth. A year later
the first American satellite Score was launched, afterwards followed
many satellites. The most important date is the year 1962. in
that year the American satellite Telstar 1 was launched. It was
used to change TV programs between America and Europe.
After that many satellite with many purposes
were launched.
The first satellites that were launched had a
emitting power of up to 10W. today satellites with emitting powers
of up to 150W are launched. The more power the satellite has the
smaller can be the antenna that receives the signal.
If a satellite has the purpose to circle around
the globe and be used for steady radio and TV program transmission
it has to be positioned in the geostationary orbit of the earth.
If the satellite is in a low orbit it will rotate around the earth
only a short time. If the satellite is in higher orbit he will
need more time to circle the globe. The first satellites travelled
around the world in only a few hours since they where launched
into very low orbits. It is characteristic that satellites that
are launched into low orbits travel in the shape of an ellipses
in a very short time. The higher the orbit a satellite is in the
more the ellipses becomes a circuit and the longer the satellite
travels to go around the planet.
All satellites that are exactly above the equator
at a height of 36.000km have a so-called geostationary orbit.
This means that the satellite is turning as fast as the earth
is turning and he’s always at the same point.
The sign and the number written next to the name
of satellite tells its position in regards to the 0th meridian.
The position of Astra 1 at 19.2°E means that this satellite is
exactly 19.2° to the east from the 0th meridian. (see picture).
For Amos 4°W this means that he’s 4° to west
from the 0 meridian.
If there more than one satellite at a position
this is called copositioning of satellites. This is the case with
the satellites Astra 19.2°E and HotBird 13°E. usually satellites
are positioned within a imaginary cube, which has 100km long sides.
Communication and radiodifused satellites contain
two parts: a service module and a control module. The first contains
electronic parts that serve the purpose of following and controlling
the satellite, devices for measuring and powering the satellite
with electric power. The communication module contains transponders.
Transponders are devices, which receive and send signals from
and to earth. In analogue technology you need one transponder
for one program, but digital technology can emit up to ten programs
(depending on content) via one transponder.
Most people noticed that radiodifused satellite
have also a few parabolic satellite antennas. A part of them is
used for receiving the programs from earth, another part is used
to reemit them back to earth and another part of them receives
order from the control station on earth that controls the satellite.
TV signals are transmitted to the ground satellite
station that distributes them to the satellite. Received signals
are transformed to frequencies of 14GHz and such signals are passed
on to the satellite. The signals that the satellite receives are
going into the control unit of the transponder where they get
strengthened by a local oscillator to frequencies from 10.700-12.750
GHz.
Afterwards these signals are directed to antennas,
which emit them back to earth where they can be received.
Satellites are not only used for the transmission
of radio and TV programs but also for the transmission of news
and sport and other important events. They are used to change
data between countries.
A satellite can’t send data to the earth as a
whole, but its send signals only cover a part of the earth. The
covered part of the earth has the shape of a circle or an ellipse
and the size depends on the satellites power. The most powerful
signals are in the centre. At the edges of the covered area the
signals are so weak that you need antennas with a scope of several
meters to receive the signals useful. Belt with the same power
are determined by concentric circles and they are referred to
as EIRP(Equivalent Isotropic Radiated Power).
Zones that are covered by satellite are not of
the same size. Some satellites have only a small territory that
they cover, for example Thor 1 at 1°W.
Data about the zones that are covered by satellites
and data about the satellites power are important for the size
of the receiving antenna. The more powerful the signal the smaller
the antenna can be.
Sometimes they don’t declare the power of the
satellites signal but they tell right away what scope has to be
used for reception.
|
