Transcript Satelite Communication

Satelite Communication
• 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.
• 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
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over large distance across sea and ocean.
Introduction
• 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.
• 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.
• 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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Introduction
The important advantages are 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)
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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History of Communication 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.
1980
1986
1989
1992
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History of Communication Satelite
•
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 earthstations. Some signals must be relayed through a second satellite to
reach their final destinations.
•
Satellite Orbits : A satellite in orbit comes under the influence of two
forces, the centrifugal and the gravitational forces. Three kinds orbit are
geostationary, eliptic circle, circular-pole
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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:
1. 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.
2. 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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Satellite Subsystem
3. 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.
4. 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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Satellite Subsystem
5. 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
1. C-Band
The bandwidth allocated for commercial satelite
communications is limited to 500 MHz in the CBand 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.
2. KU-Band
Most commercial satellites today use the CBand. However future satellites are being
designed for the 12/14 GHz or KU-Band with uplink frequency fu of 14.0 to 14.5 GHz; and downlink 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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3. C-Band VS KU-Band
The 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
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areas.
Advantage of KU Band
1. Its earth station antennas can operate in any large
city centers.
2. The gain of antennas are greater on both the uplink 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.
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