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Most satellites being launched
today are of the higher power variety operating
in the Ku-band frequency range. |
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These spin stabilized satellites
were among the first "medium" powered
birds. |
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The real promise of satellite
communications in now, finally, being realized.
Nearly every business in the world will soon
benefit from faster, cheaper and more efficient
communications. |
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"I believe that there'll be a satellite
terminal on top of every building in the world.
-- I once said there's going to be a terminal in
every building that has a Xerox machine or a PBX.
And it's happening, day after day, now." Sid
Topol, Scientific Atlanta. |
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Author and technical writer, Mark
Long takes us through an examination of
this high powered technology. "As a first
step in our investigation of Ku-band technology,
we need to examine some of the basic
communications building blocks."
|
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The frequency bands which lie between 3 and
30 MHz are known as short waves. At predictable
periods during each day, short wave radio signals
bounce off the upper layers of the earth's
atmosphere, which have been electrically charged
by radiation from the sun. Once reflected, these
signals can travel hundreds or even thousands of
miles. |
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The International Telecommunications Union of
the United Nations, serves as a clearing house
for the assignment of frequencies and orbital
positions for domestic, regional and
international telecommunications satellites. |
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The ITU has divided the
world into three separate regions, each with its
own Ku-band frequency assignments. |
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Like their c-band counterparts, Ku-band
satellites have their own, unique footprints,
with the signal strongest towards the center and
getting progressively weaker out towards the
edges. Here in Nashville,
Tennessee, this two foot dish is providing
excellent reception, where in other parts of the
country, larger antennas may be necessary. |
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On each footprint map, a
series of contour lines mark the points where
signal levels decrease from peak values. |
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Due to the short wavelengths involved,
raindrops are large enough to actually scatter or
absorb Ku-band satellite signals as they pass
through the earth's atmosphere. During downpours,
the resulting signal attenuation, can adversely
effect reception. |
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Lloyd Burns, VP, Harris Corp: "After
several months, we did determine that we could,
in fact, install a system within the United
States and overcome the rain attenuation problem.
|
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"There have been a few stations in
certain areas, maybe the Mobiles or Miamis, that
we were worried about. We overcame these problems
by installing larger antennas." |
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Ku-band uplink-downlink
equipment |
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Sporting events and occasional
transmissions, planned well in advance, have been
broadcast live, using large, C-band
mobile teleports, for years. But C-band
uplinks have not been able to cover fast breaking
news events, because the equipment can cause
interference with local terrestrial microwave
links. |
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Ku-band satellites, however, use a different
frequency range than do the terrestrial microwave
users, allowing news organizations to move
quickly from site to site, to uplink news
stories as they happen, without undue
limitations from FCC operation restrictions. |
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"Ku-band is advantageous, because the
amount of power that you can put into a
transponder is unconstrained because it does not
have to share the band of frequencies with
terrestrial users." C.J. Waylan, Pres. GTE
Spacenet
|
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"Since satellites don't share those
frequencies with terrestrial users, one can put
an earth station anywhere. You are unconstrained
by considerations such as interference with
microwave towers, and that sort of thing. |
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Fly-away Uplinks -- TV
networks also are using the latest Ku-band
technology to cover international events in ways
that were previously unthinkable.Any reporter can
now uplink live from literally anywhere on earth.
|
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Each video flight pack consists of a two way
satellite terminal and a modular antenna.
Contained in eight suitcases, it can be
transported as standard baggage on any commercial
airplane. |
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Videoconferencing is another
area where Ku-band satellites are playing a major
role. |
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Video schooling. Each day,
NTU transmits a series of 50 graduate level
courses in the fields of computer engineering,
computer science, electrical engineering, and
engineering management. More than 600 students
from at least 40 American corporations receive
the Ku-band programming.
|
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Another important role for today's Ku-band
satellites is the establishment of entertainment
services specifically for the lodging
industry. |
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The most successful lodging service to date
is Hi-Net, with their Satellite Cinema
pay movie channels. |
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The system manager computer,
behind the front desk, controls the system and
distributes the particular signals to each
individual guestroom. It also provides the
appropriate information so the hotel can bill the
guest. We then access that computer monthly from
our data center in Memphis. |
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Encryption The B-MAC
system by Scientific Atlanta is explained and
demonstrated by Pat Bohana.
|
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The development of VSATs, or Very Small
Aperture Terminals, represents another fast
growing area, integrating both satellite and
computer technologies. |
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"One of the most exciting applications
for VSATs is for retail stores, to be able to
have inventory control from every one of
thousands of branches centralized, through
satellite, to a central hub headquarters
location." C.J. Waylan.. |
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There are several different transmission
formats being used to uplink voice and
data signals. TDM, Time Division Multiplex, is
the most commonly used format for VSAT
operations. |
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VSATs usually operate in what is called a "star
network" configuration, where a
single hub station controls and monitors the
entire system. A 4.5 meter hub antenna can
support over 3000 earth stations. |
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The Hub's outdoor electronics is a very
compact but sophisticated system, explained by
Scientific Atlanta's Bob Reid. |
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Ku- band system installations
begins with understanding the differences between
different types of dishes. Deep or shallow, round
or spoon shaped, centered or offset feed, mesh or
solid, all important dish design factors are
explained. |
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Mark explains how Ku-band
installations differ from standard C-band
installs. All basic adjustments are covered. |
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Solid metal dishes are usually
the best reflectors for Ku-band signals, with the
one piece spun or hydroformed dish achieving the
highest accuracy. We see dishes being made at the
factory. |
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Mark wraps up a complete review of the
technical side of Ku-band earth stations by
looking at receivers and descramblers or IRDs. |
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