Introduction. For
many years, the notion of satellite communications was a fantasy produced by
fertile mind of Arthur C Clarke; a brilliant idea but rather impractical. As
with so many creative ideas, technology eventually caught up and now satellites
are commonplace. For the communication engineer, however, they represent
challenging and stimulating field of work. Within satellite communication
system we find the whole gamut of technologies operating in a strange and
demanding as little energy to run as possible, the system must be capable of
withstanding the arduous journey from earth to orbit. Consequently a careful
balance must be struck between the mechanical, structural, electronic,
electrical and electromagnetic engineering requirements of the system.
Satellite communications provide opportunities, and pose problems, in communication
methods. Their large area of access (footprint) allows a single transmission to
cover an enormous number of receivers, thus allowing broadcast signals to be
transmitted simultaneously to large number of people. However, this feature
itself creates difficulties; partly political and partly economic. National boundaries
are no barrier whatsoever, and the charging mechanism required to allow the
satellite operator to recover the cost of development and provide continuous
support requires a novel solution. Satellites can travel in variety orbits,
basically elliptical in shape. A geostationary satellite system which orbit is
fixed and essentially circular. It maintains a fixed position relative to
points on earth, and this allows a cheap receiving antenna to be set up once
and then fixed position.
The disc will be as
small as possible and the amplifier as cheap as possible, consistent with
obtaining an adequate signal for most of the year. Outage will be high, compare
with that for a better quality receiver, but must still be at reasonable level.
The preoccupation in satellite link design is the power budget; how much power
can be obtained from transmitter, how much power can be directed towards the
receiver, how much power is lost over the link, and at the transmitter and
receiver, and how much is left at the detector? What margin is necessary above
the minimum detectable signal for the detector being used? Essentially the
problem is to ensure that the signal level at the detector is large enough to
produce a satisfactory output for a large part of the year. Since the satellite
distance is fixed, the attenuation (except for affect of rain) is also fixed,
and it is high.
In satellite
communications the direction of transmission is indicated by the term used: the
downlink is transmission from satellite to earth station and the uplink is
transmission in the opposite direction.
As we have
mentioned before, communications by satellite impose enormous problems on
systems designer. In the uplink we have availability of high intensity
transmission beam; because the transmitter is ground based and therefore power
is not a problem, coupled with a sensitive receiver on the satellite. While in
the downlink we have a low power source in the satellite and a highly sensitive
receiver in the earth station. However, this presupposes that the earth station
cost is not a constraint. For public utility applications such as telephony,
cost will not be a limiting factor; the high cost of an expensive receiving
station can be spread over a very large number users. In direct broadcast
television however, the story is different. A single user has to pay for receiver
and therefore its cost must be low. This means that the earth station will be
‘basic’.
****Source
: Telecommunication Engineering ; J Dunlop and D G Smith; Chapman &Hall;
London