Source and rights to: http://repairfaq.cis.upenn.edu/sam/icets/satellte.htm
Introduction
In a relatively short time, satellites have become an essential part
of global communication. In 1960, the first TV satellite, named Echo, was
launched. It was basically not much more than a reflector, which reflected
the TV signals it received from earth. Two years later Telstar followed,
which was the first so-called active TV satellite. Instead of only reflecting
the incoming signals, it also converted the signals in order to avoid interference
between the incoming and outgoing signals.
Telstar had a rotational speed which was different from the rotational
velocity of the earth, so it had to be followed very accurately by both
transmission and reception stations. In 1964, this problem was solved,
when the first earth-synchronous satellite, Syncom, was launched. Many
others have followed since. The most well known is probably Intelsat I,
which was launched in 1965. By 1969 the satellite net had expanded to a
worldwide communication and TV network.
In December 1982, the Astra I satellite was launched, which generated
new interest in satellites from the general public in Europe. With its
coming it has become possible for people in Europe to receive TV and radio
transmissions with a small dish antenna.
Communication Satellites
All current communication satellites are earth-synchronous or geo-stationary.
This means they circle the earth in a specified orbit, at the same speed
as the earth itself. As a result, they appear to stand still. All geo-stationary
satellites revolve around the earth at a height of 36,000 km, precisely
over the equator. Here, the centrifugal and gravitational forces of the
earth are in equilibrium, ensuring that the satellites stay in their position
and do not fall back to earth. Their speed is approximately 11,000 km per
hour and the distance to Central Europe is approximately 41,000 km. As
neither the distance nor the position over the equator changes, transmission
and receiving stations can remain fixed, maintaining their aim at the satellite.
The geo-stationary orbit where the satellites are in is also called the
Clarke Belt, named after Arthur C. Clarke. He was a British writer and
scientist who first proposed the idea of the geo-stationary orbit used
by today's satellites.
The Clarke Belt used by geo-stationary satellites.
Non-geo-stationary satellites are used for applications such as weather
observations, military surveillance and experiments. Most of them orbit
the earth at a lower altitude than the geo-stationary satellites. Their
orbital speed must therefore be faster, or else the earth's gravity would
pull them down.
Fixed Service Satellites
Fixed Service Satellites (FSS) are satellites designed to transport
telephone calls, data transmission and TV signals for broadcasting and
cable organizations. Because these satellites have a relatively low power
output of 10-20 watts per transmitted channel, it means that a large dish
antenna is required for good reception. (Less power means a weaker signal
which is harder to pick up, therefore requiring a larger antenna.) However,
the advantage of low power satellites is that more programs can be broadcast.
Consumer Satellites - DBS and MPS
A DBS, or Direct Broadcasting Satellite, is a satellite with high transmission
powers, especially designed to transmit radio and TV programs. Because
of its high power (up to ten times the power of a FSS satellite), its signals
can be received with smaller dish antennae of 25-40 cm in central receiving
areas.
Another kind of satellite is the Medium Powered Satellite (MPS), which
is a satellite with a transmission power of 50 watts. The advantage of
this type of satellite is that it has more power than a FSS and its signals
can therefore be received much easier. Although it has less power than
a DBS, its advantage over a DBS is that it allows the satellite to broadcast
more programs. The ASTRA satellite is an example of a MPS. MPS and DBS
satellites are also referred to as consumer satellites.
Satellite Positions
All the consumer satellites are located in the same geo-stationary orbit
36,000 km above the equator. Their positions vary from east to west in
accordance with international agreements. These agreements about orbital
positions allow several satellites to be placed in the same location, so
that TV viewers can receive a greater choice of programs with a fixed dish
antenna. Also, when a satellite needs to be replaced (the average lifetime
of a satellite is about 15 years) the replacement satellite can be put
in the same position, so that when the first one 'dies' and falls back
to earth, the next one is already in place and continues to broadcast the
same stations.
Consumer satellites are located above the equator, in different positions
from east to west.
The positions of the satellites are controlled by international agreements
drawn up by the IRFB (International Radio Frequencies Board). The IRFB
also coordinates the frequencies used for satellite broadcasting, to prevent
interference which would be caused by two or more satellites using the
same frequency. The transmission frequencies used by consumer satellites
are in the KU-band, which roughly stretches from 10 to 17 GHz. The range
within the KU-band that is actually used by consumer satellites is between
10.7 GHz and 12.75 GHz.
10.7 - 11.7 GHzFSS+MPS
11.7 - 12.5 DBS
12.5 - 12.75 FSS (telecommunications)
Transmission Process
Signals are sent up to the satellite from the earth's surface. The transmission
station is called an uplink station. The transmission takes place via frequency
modulation (FM). The advantage of FM is that there are no problems regarding
the frequency and dynamic range that needs to be transmitted, plus, FM
is less sensitive to interference than AM. For practical reasons, conventional
TV stations broadcast in AM (called earth or terrestrial TV).
The outgoing transmission takes place at a very high frequency of 14,000
MHz (= 14 Gigahertz). To avoid any interference, the incoming signal (downlink)
is transmitted at a frequency between 10 and 12 GHz. This is the so-called
KU band, which covers the area from 10.7-12.75 GHz. The downlink signal
is sent to earth in a focused beam, via a parabolic antenna, that looks
quite similar to a receiving dish antenna. From there, it can be picked
up by private antenna, shared antenna installations and cable companies.
Footprint
Consumer satellites use a concentrated beam to give a stronger signal
over a smaller land area. The area over which the signals can be received
is called the footprint of a satellite. Footprint diagrams show the area
of coverage, including the antenna size which is needed for good reception
in the central and outlying areas. Under normal conditions, good reception
within the footprint area is possible for as much as 99.9% of the time.
However, exceptional weather conditions can have an adverse effect on reception
quality for short periods.
The footprint diagram shows the area of coverage and the required
antenna sizes in the central and outlying areas.
LNC
The signals received by the dish antenna are transferred to a frequency
converter called the LNC (Low Noise Converter), which is placed in the
focal point of the dish antenna. The LNC is also called the LNB (Low Noise
Block converter). The LNC converts the incoming signal to a lower frequency
in the area between 950 and 2150 MHz, and then amplifies the signal before
it is sent to the satellite tuner. Due to the very weak signal levels,
it is of vital importance that the amplification takes place free of noise.
During the amplification of the frequencies, all frequencies will be amplified,
including noise. An important performance parameter of the LNC is therefore
its noise factor. The lower the noise factor, the better the picture quality.
For good reception and image results, the quality of the LNC and the satellite
tuner are of vital importance.
A Low Noise Converter (Low Noise Block Converter) placed in the focal
point of the dish antenna.
Polarization
Polarization is a way to give transmission signals a specific direction.
It makes the beam more concentrated. Signals transmitted by satellite can
be polarized in one of four different ways: linear (horizontal or vertical)
or circular (left-hand or right-hand). FSS satellites use horizontal and
vertical polarization, whereas DBS satellites use left- and right-hand
circular polarization. To use the channels that are available for satellite
broadcast as efficiently as possible, both horizontal and vertical polarization
(and left- and right-hand circular polarization) can be applied simultaneously
per channel or frequency. In such cases the frequency of one of the two
is slightly altered, to prevent possible interference. Horizontal and vertical
transmissions will therefore not interfere with each another because they
are differently polarized. This means twice as many programs can be transmitted
per satellite. Consequently, via one and (almost) the same frequency the
satellite can broadcast both a horizontal and a vertical polarized signal
(H and V), or a left- and right-hand circular polarized signal (LH and
RH).
TV signals transmitted by satellites can be polarized in four different
ways: (1) vertical, (2) horizontal, (3) left-hand circular and (4) right-hand
circular.
Types of Polarizers
In order to select either a horizontal, vertical, right- or left-hand
circular signal, the LNC must be provided with a polarizer. There are three
types of polarizers: mechanical, ferrite/magnetic and electrically controlled
polarizers.
The mechanical polarizer is a small pulse-controlled motor which rotates
a metal probe between the horizontal and vertical polarization directions.
This system offers high switching precision, with low signal loss. It gives
wide-band reception, covering all the different frequency bands. By adding
a small circular depolarizer, the polarizer can be modified to also receive
circular polarized signals.
The ferrite-magnetic polarizer has no moving parts and gives effectively
instantaneous switching, combined with low signal losses. Channels need
to be pre-programmed. By adding the small circular depolarizer, this type
of polarizer can also be modified to receive circular polarized signals.
The 14/18V electrically controlled polarizer is integrated within the
LNC, and requires no additional connection other than to the LNC over a
coax cable.
The Satellite Tuner
Signals come in to the satellite tuner via the LNC. A satellite tuner
next to the TV tuner is required for satellite reception. Normal TV tuners
can only handle signals between 47 to 870 MHz, whereas satellite transmission
takes place between 950 and 2150 MHz. TV sets cannot generate specific
LNC control signals, nor handle polarization switching. Furthermore, TV
tuners cannot process the audio signals from the satellite. Some TV sets
and VCRs have satellite tuners built in. In addition to a satellite tuner,
one may also need an additional antenna positioner (in case of a polar
mount dish), a descrambler box and a smart card reader in order to receive
encoded transmissions, all of which can be built into the satellite tuner.
All satellite tuners are equipped with a special connection for the
existing antenna or cable, which makes replugging unnecessary if you want
to switch from conventional to satellite TV and vice versa.
Scrambling and Conditional Access
Not all signals picked up by a dish antenna are suitable for viewing.
For several reasons TV signals can be scrambled or given conditional access
and can only be watched with the help of a decoder or descrambler. These
reasons might be that:
- Programs are financed by viewer subscription rather than advertising
revenues.
- Programs are meant for a selected audience.
- Programs to be broadcast have been acquired with copyright clearance
for specific geographical areas only.
There is a distinction between scrambling and conditional access, although
for the viewer without a decoder the result is the same: unclear video
and/or audio signals. Scrambling is the jumbling up of a picture and/or
a sound channel to make it impossible to watch or listen to a program without
a decoder. Conditional access is a form of encoding to protect information
with a scrambled signal that tells the decoder how to decode it. Scrambling
is therefore applied to the picture, whereas conditional access is applied
to the control signal. Scrambled signals require additional decoder boxes
or a smart card reader for access.
The Dish Antenna
Types of Dish Antennae
There are a number of dish antenna types. The first and simplest is
the Prime Feed Focus dish, which is a parabolic dish with the LNC mounted
centrally at the focus. Because the LNC is mounted centrally, it means
that a lot of the incoming signals are blocked by the LNC. Its efficiency
of 50% is low compared with the other types. The Prime Feed Focus dishes
are mainly used for antennae with diameters over 1.4 meters. Because of
its relatively larger surface, the parabolic antenna is less sensitive
to small directional deviations and there is a better chance of receiving
signals outside the normal footprint. On the other hand, rain and snow
can easily collect in the dish and could interfere with the signal.
Prime Feed Focus Dish.
The Offset Dish Antenna, has its LNC not mounted centrally, but to the
side of the dish. Because the LNC no longer obstructs the signal path,
the dish has a better performance than the Prime Feed Focus dish. This
allows the dish diameter to be smaller. Another advantage of this type
of dish is that it can be positioned almost vertically, whereas the Prime
Feed Focus dish needs to be positioned more obliquely. The problem that
it could collect rain and snow and give disturbance to the signals is therefore
less likely to happen.
Offset Dish Antenna.
The Dual Offset Dish Antenna is an improvement on the Offset Dish antenna
and has an even better performance. Its efficiency is about 80%. The main
feature of this antenna is that it has two dishes: a larger receiving dish
and a smaller dish facing the opposite direction which collects the signals
from the larger dish and directs it to the LNC.
Dual Offset Dish Antenna.
The Flat Antenna is the most compact type and visually the least obtrusive.
This type is best suited for receiving signals from DBS satellites in central
footprint areas. The LNC is built-in.
Flat Antenna with built-in LNC.
Antenna Sizes
Dish antennae come in various types and sizes, each with their specific
characteristics and purposes. The size of the dish required depends upon
whether you live in a central footprint area or in an outlying area. There
are three sizes: small - 60 to 70 cm diameter, medium - 90 cm and large
- 1.20 to 1.50 meters. There are also smaller sized dishes, with a 45 cm
diameter, but these are specifically designed for DBS satellites, which
because of their high transmission power permit smaller dish antennae.
Small dish - 60 to 70 cm
- Wide opening angle (comparable with wide angle lens) and therefore
quite easy to install and tune.
- Not very selective, with possibility of interference if the number
of satellites is increased.
- Not very sensitive, but sufficiently sensitive to receive MPS satellites
in central receiving areas.
- Thanks to its small size, it can be mounted almost anywhere, such as
on a balcony.
- A relatively cheap alternative for satellite reception. For a reasonable
price a complete installation including a dish antenna, LNC and satellite
tuner can be purchased.
Medium-sized dish - 90 cm
- Acceptable, practical intermediate size between large and small dishes.
- Capable of receiving from many satellites.
- Rotor required.
- Stations which are more difficult to receive do not come through so
well.
- The price is between the prices of the small and the large dishes.
By purchasing a good quality LNC and a good satellite tuner, the various
stations can be received at remarkably good quality.
Large dish - 1,20 to 1,50 m
- Small opening angle (comparable with a telephoto lens) and therefore
must be installed and tuned by an expert.
- Very selective, and therefore little chance of interference.
- Only effective with a rotor.
- Much more sensitive than the small dish (hence better quality). Is
also suitable to receive satellites which orbit further below the horizon
and therefore transmit weaker signals.
- Wind resisting construction required due to the size.
- A large dish with a corresponding high quality LNC and a good satellite
tuner will cost considerably more than a small dish.
Mounting Dishes
Before mounting a dish, there are some aspects to be taken into consideration.
The dish antenna must have a clear path to the southern skies. There should
not be any obstacles between the dish antenna and the satellite, such as
buildings and trees. The view should be absolutely clear. The dish must
be able to "see" the satellite. Provided these points are taken
into consideration, it does not matter whether the antenna is installed
on top of a building, on a balcony, or simply on the ground.
An antenna needs to be aligned in two planes, namely horizontally and
vertically. It should make an upright angle of 30 degrees. This upright
angle is called the elevation of the dish in the vertical plane. The azimuth
angle is the position in the horizontal plane and determines how much the
dish needs to be turned to the east or the west in order to receive the
signals from the desired satellite. For optimum reception quality, the
two angles must be adjusted to within a range of _1_. After the first alignment,
the dish needs to be fine-tuned by trial and error, until the best signal
is received.
When mounting a dish antenna, the elevation and azimuth angles must
be carefully adjusted within a 1 degree range.
Polar Mount Principle
With most dish antennae you have to decide at which satellite the dish
antenna is to be aimed at before installing it. An alternative to this
is to have a polar mount dish. This is a dish that rotates automatically
to the position of the satellite selected by the tuner, thus making it
easy to tune in to new satellites without having to reinstall the dish
antenna.
A polar mount antenna can rotate to the position of the satellite
selected by the tuner.
Considerations Regarding Choices
A small dish can only be used to receive satellites in orbit not too
far away from the south line. Outside that line the distance to the satellites
soon gets much bigger and consequently the reception quality worsens. For
this reason small dishes are always fixed and are only tuned once.
A large dish antenna has a larger focus which can be aimed much more
accurately at a cluster of satellites that orbit closely together, and
therefore is more selective than a small dish. Consequently, a large dish
will be less troubled than a small antenna by interference problems of
the various signals. On the other hand, a mini-dish is comparatively cheap
and can always be replaced by a larger dish (and another LNC). Such a small
dish can be installed easily, if need be on a windowsill, and is less sensitive
to various influences, such as weather conditions.
Cable or Satellite?
Which is better: cable or satellite? This is a question to be considered
if you have cable TV, or if you live in an area or place where a cable
system will be installed. Should one choose cable, with its subscription
costs, or a satellite system, with its purchase costs and the possibly
additional costs of a decoder? The following aspects may be relevant:
- A dish antenna might be of interest to people that do not have cable
TV. The conventional TV antenna will still be needed to receive earth stations.
- Satellite TV offers a wider program range than cable. The cable network
organization has already made a selection out of the available satellite
programs, but however extensive the cable offer, its capacity remains limited.
- The quality of satellite reception will often be much better than the
quality of the cable signal, provided one uses a good dish antenna plus
corresponding reception installation.
Digital Satellite
As more and more information is being handled in digital format, the
future for satellite is also digital. In the near future, transmissions
will take place in digital format and this offers some advantages. The
prime reason for digital broadcasting is that with analog broadcasting
only one channel per transponder can be transmitted, whereas with digital
broadcasting this can be 10 channels per transponder. This means a substantial
cost reduction per channel. Due to compression techniques, more information
can be put on the same channel bandwidth currently being used, which allows
more flexibility. For instance, the sender can opt for higher resolution,
or for a lower resolution but more channels. In general, digital broadcasting
will bring an increase of choices to consumers. Besides a likely increase
of the number of programs, the same programs will also be broadcast several
times per hour or day, to give the consumer more flexibility in when to
watch a program. Also, channels will become increasingly focused on specific
subjects, such as documentaries, movies, sports, and perhaps even more
specific than that (for example only football or nature documentaries).
