Satellites are able fulfill a number of roles. One
of the major roles is for satellite communications. Here the satellite
enables communications to be established over large distances - well
beyond the line of sight. Communications satellites may be used for
many applications including relaying telephone calls, providing
communications to remote areas of the Earth, providing satellite
communications to ships, aircraft and other mobile vehicles, and there
are many more ways in which communications satellites can be used.
Satellite communications basics
When used for communications, a satellite acts as a
repeater. Its height above the Earth means that signals can be
transmitted over distances that are very much greater than the line of
sight. An earth station transmits the signal up to the satellite. This
is called the up-link and is transmitted on one frequency. The
satellite receives the signal and retransmits it on what is termed the
down link which is on another frequency.
The circuitry in the satellite that acts as the
receiver, frequency changer, and transmitter is called a transponder.
This basically consists of a low noise amplifier, a frequency changer
consisting a mixer and local oscillator, and then a high power
amplifier. The filter on the input is used to make sure that any out of
band signals such as the transponder output are reduced to acceptable
levels so that the amplifier is not overloaded. Similarly the output
from the amplifiers is filtered to make sure that spurious signals are
reduced to acceptable levels. Figures used in here are the same as
those mentioned earlier, and are only given as an example. The signal
is received and amplified to a suitable level. It is then applied to
the mixer to change the frequency in the same way that occurs in a
superheterodyne radio receiver. As a result the communications
satellite receives in one band of frequencies and transmits in another.
In view of the fact that the receiver and
transmitter are operating at the same time and in close proximity, care
has to be taken in the design of the satellite that the transmitter
does not interfere with the receiver. This might result from spurious
signals arising from the transmitter, or the receiver may become
de-sensitised by the strong signal being received from the transmitter.
The filters already mentioned are used to reduce these effects.
Signals transmitted to satellites usually consist of
a large number of signals multiplexed onto a main transmission. In this
way one transmission from the ground can carry a large number of
telephone circuits or even a number of television signals. This
approach is operationally far more effective than having a large number
of individual transmitters.
Obviously one satellite will be unable to carry all
the traffic across the Atlantic. Further capacity can be achieved using
several satellites on different bands, or by physically separating them
apart from one another. In this way the beamwidth of the antenna can be
used to distinguish between different satellites. Normally antennas
with very high gains are used, and these have very narrow beamwidths,
allowing satellites to be separated by just a few degrees.
Telecommunications satellite links
Communications satellites are ideally placed to
provide telecommunications links between different places across the
globe. Traditional telecommunications links used direct "cables"
linking different areas. As a result of the cost of installation and
maintenance of these cables, satellites were seen as an ideal
alternative. While still expensive to put in place, they provided a
high bandwidth and were able to operate for many years.
In recent years the bandwidth that can be offered by
cables has increased considerably, and this has negated some of the
gains of satellites. Additionally the geostationary satellites used for
telecommunications links introduce a significant time delay in view of
the very large distances involved. This can be a problem for normal
telephone calls.
Mobile satellite communications systems
There are many instances where communications need
to be maintained over wide areas of the globe. Ships, aircraft and the
like, need to be able to communicate from points all around the world.
Traditionally HF radio communications ahs been used, but this is
unreliable. Satellite communications provide an ideal solution to this
problem as satellite communications are much more reliable and they are
able to provide interference free stable communications links. As a
result, Satellite communications is now fitted as standard to all
maritime vessels, and it is becoming increasingly used by aircraft,
although it is not yet adopted for Air Traffic management (ATM).
In addition to these users, these services can be
sued by many land mobile or land portable radio users. Satellite
terminals provide are able to access the satellite and the users is
able to achieve communications from almost anywhere on the globe. As
these communications satellites are in geostationary orbits,
communications is not possible towards the poles as in these regions it
is not possible to see the satellites.
Direct broadcast communications satellites
Another variant of communications satellites is
those used for direct broadcasting. This form of broadcasting has
become very popular as it provides very high levels of bandwidth
because of the high frequencies used. This means that large numbers of
channels can be carried. It also enables large areas of the globe to be
covered by one delivery system. For terrestrial broadcasting a large
number of high power transmitters are required that are located around
the country. Even then coverage may not be good in outlying areas.
These DBS satellites are very similar to ordinary
communications satellites in concept. Naturally they require high
levels of transmitted power because domestic users do not want very
large antennas on their houses to be able to receive the signals. This
means that very large arrays of solar cells are required along with
large batteries to support the broadcasting in periods of darkness.
They also have a number of antenna systems accurately directing the
transmitted power to the required areas. Different antennas on the same
satellite may have totally different footprints.
Satellite phone systems
Satellites have also been used for cellular style
communications. They have not been nearly as successful as initially
anticipated because of the enormously rapid growth of terrestrial
cellular telecommunications, and its spread into far more countries and
areas than predicted when the ideas for satellite personal
communications was originally envisaged. Nevertheless these satellite
phone systems are now well established and have established a specific
market. Accordingly these satellite phone systems are now widely
available for mobile communications over wide areas of the globe.
The satellite phone systems that are available have
varying degrees of coverage. Some provide true global coverage,
although others are restricted to the more densely populated areas of
the globe.
The systems that were set up used low earth orbiting
satellites, typically with a constellation of around 66 satellites.
Handheld phones then communicated directly with the satellites which
would then process and relay the signals as required.
Other satellite phone systems use a number of
geostationary satellites, although these satellite phone systems
generally require the use of a directional antenna in view of the
larger distances that need to be covered to and from the satellite.
Additionally the levels of latency are higher (i.e. time delay for the
signal to travel to and from the satellite) in view of the much higher
orbit required. However as the satellites are geostationary, satellite
or beam handover is less of a problem.
The main
advantage of the satellite system is that it is truly global and
communications can be made from ships, in remote locations where there
would be no possibility of there being a communications network.
However against this the network is expensive to run because of the
cost of building and maintaining the satellite network, as well as the
more sophisticated and higher power handsets required to operate with
the satellite. As a result calls are more expensive than those made
over terrestrial mobile phone networks.
Satellite communications summary
Although the basics of satellite communications are
fairly straightforward, there is a huge investment required in building
the satellite and launching it into orbit. Nevertheless many
communications satellites exist in orbit around the globe and they are
widely used for a variety of applications from providing satellite
telecommunications links to direct broadcasting and the use of
satellite phone and individual satellite communication links.
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