
SUPERYACHT #490 February 2003
Article selected from our quarterly magazine dedicated to the largest
and most luxurious boats with information, interviews, technical
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Video Nautica

Article by Luca Gigli
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SATELLITE TELEVISION SYSTEMS
A simple description of the assemblies and the devices they comprise,
going a little into more detail for the only apparatus specially
designed to be used at sea: the parabolic aerial
Walking along harbour quays you will now notice large white plastic
structures shaped like rotating solids, fairly frequently on the
rollbars of the larger boats, commonly known as "balloons" or "radomes".
These structures are really just containers that in most cases have
parabolic aerials inside them. Such aerials are the primary components
of more complex systems for decoding and visualising information from
orbiting satellites.
HOW A PARABOLIC AERIAL WORKS
A parabolic aerial can be compared to a "bowl" that collects the very
high frequency signals from satellites located a great distance away
from the Earth (tens of thousands of kilometres) and concentrates them
in a single point called the "focus" of the aerial where a second
element in the system called "LNB"is positioned. This device amplifies
the signal received and reduces its frequency, enabling the signal to be
transferred along a coaxial cable to another part of our system - the
receiver". It can therefore be guessed that the size of the parabolic
disc plays an important role in the quantity and thus in the quality of
the final result: the greater the quantity of signal collected, the more
intense the signal reaching the LNB itself will be, good for
guaranteeing better audio and better video images.
AERIAL TRACKING SYSTEMS
For a parabolic aerial to gather the satellite signals and transfer them
to the LNB it must be constantly pointed in the orbital direction in
which the satellite constellations gravitate. On land it is a simple
matter to direct the aerial towards the fleet of Hot Bird satellites,
which, located at thirteen degrees east, transmit Italian channels, both
encrypted and not. On boats the question is much more complicated; to
compensate for movement caused by to rolling, pitching and yawing both
special tracking systems and relative management software are necessary,
which once the position of the satellites has been determined and
recognised, are able to maintain the correct orientation of the
parabolic aerial and in extreme cases of total signal loss manage to
restore the link in the shortest time possible. There are tracking
systems of various complexities: the most simple and economic just
maintain alignment while berthed at the quay, while the most
sophisticated, often of military origin, enable the satellites to be
traced while sailing. The problem with the system is therefore that of
tracing one satellite out of many basing on a special "ID" associated to
the latter and constantly maintaining alignment with the same. This
characteristic imprint, on first generation systems, derived from a
series of special frequencies transmitted by the satellite that, once
they are received by the system, univocally identify it. Now, thanks to
digital technology it is possible to recognise each satellite based on
the digital ID it transmits together with all the other signals and
information, meaning that tracking errors are impossible.
SATELLITE FOOTPRINTS
As already mentioned above, the larger the aerial the better the signal
reception quality. There is however a minimum threshold of signal
intensity below which the system does not work. The zone covered by the
satellites, called "footprint" is made up of concentric areas with an
irregular shape that give a gradually weaker signal as it passes from
the central to the peripheral zones. It goes without saying that to be
able to receive a satellite channel at the extreme edges of the
"footprint" a larger satellite disc will be necessary to compensate for
the lack of the signal itself. It is obviously impossible to install
huge aerials even onboard boats of a certain size, even if with modern
technologies a high standard of performance has been reached due to the
excellent geometries of the discs and the improved performance of the
converters that compensate for size. Therefore, once the channels of
interest have been established, it is necessary to examine the zones
covered by the satellite that transmits them and evaluate if an aerial
of a size compatible with that of the boat is adequate for the zones in
which the boat sails. Bearing in mind that Italian channels are mainly
transmitted by the Hot Bird fleet, it is helpful to remember that each
satellite has its own footprint, so in determined zones it will be
possible to receive some channels at the expense of others. The system
on board must therefore be able to recognise and track the different
satellites tracked on at least the Mediterranean basin, as some Italian
channels are duplicated and transmitted by some of these (for this
purpose it is possible to consult the internet site www.satcodx.com to
check on channels and cover).
ANALOGUE AND DIGITAL RECEIVERS
The interface between the aerial/LNB group and the video is a signal
decoding instrument called receiver. The receiver can be either analogue
or digital and may include a "decoder", an instrument useful for
decoding encrypted transmissions. Although some systems still need
analogue receivers for tracking the satellite, digital receivers are
currently the most widespread, "Gold Box" models in particular are at
present those most marketed on the wave of Pay TV. It is also expedient
to add that despite the fact that some channels are not encrypted, that
is they do not need any special decoders, other programmes may however
be encrypted for reasons linked to payment of transmission rights in the
different zones covered.
CIRCULAR AND LINEAR POLARISATION
If you should just happen to want to take a trip to the Caribbean from
the Mediterranean, it might not be possible to use the television system
there because of the different polarisation of the signals emitted by
the American satellites. In fact, while in the Mediterranean the
electromagnetic waves have linear polarisation, in America the waves
have circular polarisation. To be able to receive these signals you will
have to replace the existing LNB and the receiver connected to it.
SATELLITE INTERNET
Several "satellite" providers have been created recently, which have
elaborated a link up system to Internet that, when downloading using a
device called "satellite modem" that takes advantage of the considerable
bandwidth guaranteed by the television satellites, permits the reception
of a considerable volume of data, while when uploading it exploits a
standard terrestrial link, a telephonic twisted pair or any mobile phone
or even a satellite terminal. So, through Astra or Hot Bird large
volumes of data can be received in a short time. This approach, given
the low speed of data exchange and the high operative cost of normal
satellite systems, can be considered a small step forward in technology,
even if it has some limits. The initial link, in fact, is set up using
very slow devices, restricted by the network cover (for GSM) or that
work at a "baud rate" so low (see some satellite systems) that it is
necessary to consult your Internet provider to find out if it supports
this transmission speed. Furthermore the instructions given by your
station are sent to the land provider server, bounce back to the
satellite provider server and are finally sent to the satellite. These
passages slow down surfing on-line considerably, while they do not have
any effect on downloading files or large programmes. Some satellite
providers offer an interesting possibility - that of being able to book
the web pages in advance and then receive them updated at your
workstation at a pre-set time, without having to connect to the network.
Naturally, the rules regarding satellite coverage defined above are
still valid, being conditions that cannot be ignored for correct working
of the whole system.
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