Komunikasi Satelite

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Komunikasi Satelite

• Risanuri Hidayat

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Pendahuluan

• World demand for communication facilities
  carrying many different types of real-time and
  non-real-time signals such as voice, data,
  facsimile, and video has been growing by leaps
  and bounds during the past few decades. The
  continuing increasing demand and the resulting
  large amount of world-wide communication traffic
  naturally calls for links with very large
  transmision bandwidth.
• Before the era of "communication satellites",
  long-distance transmission of information has
  relied principally on microwave and suboceanic
  cables links.

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Pendahuluan

• Microwave links can provide large usable
  bandwidth and their performances are generally
  good. The major constraint is that the system is
  one of line-of- sight (LOS) where the
  transmitting and receiving antennas MUST SEE ONE
  ANOTHER, as the microwaves travel in straight
  line. To transmit signals beyond the horizon,
  repeater stations are required. The normal
  distance between repeaters is between 30 to 50
  miles depending on the terrain. Having many
  repeaters mean high operating and maintenance
  cost, higher security risk. The biggest problem
  however is that global communication will require
  transmission over large distance across sea and
  ocean.

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• Suboceanic Coaxial Cables have been installed
  and used. The factors limiting their usage are
  the high costs, high signal attenuation and the
  rather limited bandwidth of the cables which is
  insufficient to cope with growing high traffic
  demand.

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• An answer that can meet the needs of global
  communication is"Satellite Communications" in
  which a satellite in space is used as a repeater
  station in the sky, a concept invented in the
  1940's by scientist and science fiction writer,
  Arthur C. Clarke. Though it is a very simple
  concept, it nevertheless has profoundly changed
  the world today.

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• Essentially a satellite acts as a radio relay in
  the sky. Signals such as voice, data, facsimile
  and video are sent to it from antennas on earth,
  it then amplifies these signals and send them
  back to other earth antennas.

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• The important advantages of satellite
  communications lies in the fact that they can
• handle a large amount of traffic (b/w of 500 MHz)
  
• receive/send signals over most of the populated
  earth regions.
• Insencitive to distance (same cost)

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• To summarizes, a communications satellite
  provides
• A means to reach isolated places on earth
• An alternative to suboceanic cables.
• Long distance telephone (voice) and television
  links.
• A data transmission link capable of
  interconnecting computers and data terminals
  everywhere.

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Sejarah Komunikasi Satelite

• In 1964,the Intelsat Consortium was formed to
  operate and maintain the International
  Telecommunication Satellite System.In 1965,the
  first commercial satellite Intelsat I (Early
  Bird) was launched.In 1967-1968,it was
  followed by Intelsat II and Intelsat III
  respectively.In 1971,it was followed by
  Intelsat IV.As of 1982,there were some 400
  earth stations with over 55,000 channels using
  the Intelsat System.

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1989
1986
1980
1992
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• These Intelsat satellites were placed in orbits
  at a height of 35,860 km ( 22,282 miles ) called
  "geostationary" or "geosynchronous" orbits. They
  appear to be stationary with respect to a point
  on earth, since they travel around the earth in
  exactly the earth's rotation time.In principle
  then, only three satellites in geostationary
  orbits above the equator are sufficient to cover
  the entire earth, except the uninhabited polar
  regions.Signals from several ground terminals
  known as "earth station" sent to the synchronous
  satellite are relayed to the appropriate
  destination earth-stations. Some signals must be
  relayed through a second satellite to reach their
  final destinations.

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• Satellite Orbits A satellite in orbit comes
  under the influence of two forces, the
  centrifugal and the gravitational forces.For
  the satellite to stay in a circular orbit of
  distance about 42,200 km, the two forces must be
  equal. In other words, a geostationary satellite
  is placed at an altitude of 35,860 km above the
  equator. They placed above the equator to cover
  the populated earth surface, leaving the blind
  reqions around north and south poles.

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Satellite Subsystem

• Geostationary communication satellites will need
  the following on-board subsystems to function as
  a signal relaying station
• Stationkeeping consisting of a thrust and a
  stabilization subsystem to keep the satellites in
  their proper orbital altitude, position and
  direction.Due to the small solar and lunar
  gravitational forces acting on the satellite, it
  tends to deviate from its geostationary orbit.
  Since tight control over the satellite's position
  is absolutely necessary to keep it geostationary,
  because most earth stations's antennas are of
  nontracking type therefore stationkeeping is
  necessary where occasional corrections to its
  orbit are accomplished by on-board thrusters. A
  certain amount of fuel or propellant is used each
  time an orbit correction is made. Therefore a
  communication satellite will only have a limited
  useful life-span of service.

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• A Power subsystem to supply power to the
  electronics.The satellites is normally powered
  by solar cells capturing solar energy. These
  solar cells are mounted around INTELSAT's
  cylindrical body surface and are capable of
  giving about 400 Watts of power for Intelsat IV
  satellite. During any period when it is eclipsed,
  on-board batteries take over the function.

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• A Command and Telemetry subsystem for
  transmitting data about it to earth and receiving
  commands from earth. On board instrumentation
  continuously sends to a control earth station
  details of its subsystems and position. From this
  station, necessary commands are sent to it to
  maintain its orbital position and to keep it
  functioning correctly. Each transponder can be
  switched on or off as required.

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• An Antenna subsystem for receiving and
  transmitting signals.Most communication
  satellites contain several transponders utilizing
  the whole available 500 MHz of bandwidth, and
  serveral antennas. Some antennas have wide beams
  (17.3 degree) for earth coverage, while some have
  narrow beams (4.5 degree) for densely populated
  reqions. The narrow or spot beam antennas will
  have increase ERP (Effective Radiated Power) and
  hence a larger antenna gain. Either
  earth-coverage or spot-beam antennas can be used
  on the down-link by switching.

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• Transponders containing necessary electronics
  subsystem to receive signals, amplify and change
  their frequencies, then retransmitting them to
  earth.The Radio Frequency,RF, relay section of
  a communication satellite is called a
  "transponder" (acronym for transmitter and
  responder). The transponder and associated
  antennas form the primary subsystem. This
  transponder differs from conventional microwaves
  (LOS) repeaters in that many separate ground
  earth stations can access it simultaneously. The
  transponders operate on different frequencies for
  receiving and transmitting to avoid interference
  to weak incoming signal by powerful transmitted
  signal. Most satellites have more than one
  transponder to fill the whole 500 MHz bandwidth
  allocated. The individual transponder bandwidth
  may vary according to designs.

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Frequency Band

• C-BandThe bandwidth allocated for commercial
  satelite communications is limited to 500 MHz in
  the C-Band frequency region, known as 4/6 GHz
  band. In this band 3.7 to 4.2 GHz forms the
  down-link (transmit) frequency fd, and 5.925 to
  6.425 GHz the up-link (receive) frequency fu.
  KU-BandMost commercial satellites today use the
  C-Band. However future satellites are being
  designed for the 12/14 GHz or KU-Band with
  up-link frequency fu of 14.0 to 14.5 GHz and
  down-link frequency fd of either 11.7 to 12.2 GHz
  or 10.95 to 11.2 or 11.45 to 11.7 GHz.

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C-Band VS KU-BandThe selection for suitable
operating frequency depends on such factors as
size and gain of antenna, bandwidth allocation,
atmospheric attenuation or losses, various
sources of noise, different types of loss and
noise point of view, the C-Band can provide
high-quality transmission and is used exclusively
by commercial satellite communication. However
there is an increasing usage of this band in
large urban areas because they also constitute
the frequencies used for terrestrial microwave
links. Thus a severe drawback of the C-Band is
that of "The problem of interference between
satellite link and terrestrial microwave links".
By far the most serious interference is that from
an earth station interfering with a microwave
receiver nearby. This is because an earth station
must transmit high power signal to make up the
large transmission distance loss. Some of the
signal spilled may therefore be substantial to
interfere with a microwave receiver, hence an
earth station should not be located in large
urban areas.
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• On the other hand, the KU-Band are seen to offer
  the following advantages
• Its earth station antennas can operate in any
  large city centers.
• The gain of antennas are greater on both the
  up-link and the down-link than those of the
  C-Band having the same size.

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• The improvement in antenna gain could be used to
  allow the earth station and the stellite antennas
  to be made smaller and cheaper or to make up for
  the increased signal loss and noise in bad
  weather. Also it would mean that for a same size
  antenna, the beam-width is less than the C-Band,
  thus lessening the interference effects. The
  disadvantage of the higher frequency is the
  increase in signal loss and noise under poor
  weather condition with heavy rain, fog or clouds.