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.
