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| Satellite Communication |
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| It is mainly done through geostationary satellites. These satellites are fitted with special devices. These devices can receive signals from an Earth station and transmit them again in different directions. These special devices are called transponders. |
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| A TV transmitter on an Earth station transmits signals in the form of electromagnetic waves. These waves carry audio (sound) or video (picture) message. The satellite receives these waves. The transponders in the satellite retransmit the message either in the same frequency or a different frequency in different directions. These signals are then received by a large number of ground stations. These stations are located in various regions. The signals received from the satellites are very weak. The ground stations amplify the signals before transmitting them again. It is these signals, which are received by the TV sets located far away from the programme-transmitting stations. |
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| Other means of telecommunication like telephone (mainly for long distances), telex (i.e., printed messages) and FAX (i.e., facsimile of printed matter or pictures) use a similar mechanism. In the case of intercontinental telecommunication, two or more satellites are linked together. |
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| High frequency radio and submarine cable systems have been widely used for long- distance overseas telecommunications. The high frequency band from 3 MHz to 30 MHz has provided worldwide telephone and telegraph circuits. But it has following two drawbacks. |
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| (i) High noise levels |
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| (ii) Limited bandwidth |
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| To a limited extent, cables can provide high quality circuits and can carry up to 1000 or more telephone circuits and even television signals. |
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| Ionospheric reflections are not possible for frequencies in the very high frequency and microwave bands. So, line-of-sight links are required with an array of repeaters. With the advent of Earth satellites, we can provide a microwave repeater at almost any desired altitude. So, distances of thousands of kilometer can be covered by a single Earth-satellite-Earth link. |
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| The feasibility of wide-band communication over large distances has been demonstrated by the Telstar, Relay, Syncom and Early Bird satellites. A good quality service can be continuously ensured by a global operational satellite system. Thus, communication satellites would give a valuable service in addition to the submarine cable and high frequency radio systems. |
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| Geostationary communication satellites have enabled not only national but also international television programmes to be relayed among many ground stations around the world. Three artificial satellites placed in equatorial orbits at 120o from one another can cover practically the whole populated land area of the world. |
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| The transmitters transmit wide band microwave signals to the geostationary satellite above the transmitter. These land-based transmitters are high-power and highly directive. Each microwave channel has a large bandwidth, which can accommodate many TV signals and thousands of TV channels. |
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| The satellites (generally powered by solar batteries) receive the transmission, demodulate and amplify it and remodulate on a different carrier before transmitting again. The transmitting antenna on the satellite, by the use of a suitable reflector, directs the beam to a narrow region on the Earth. For national distribution, the transmission is downward from a wide angle antenna so that the whole national area is "illuminated" by the transmission. For international distribution, the transmission is also towards the other one or two satellites (in line of sight direction) from highly directive antennas. The demodulation amplification-remodulation-transmission process is repeated in the second satellite. The final "down channel" transmission is received in the same or in a different country by a large cross-section antenna. It is then processed in low-noise receivers and finally re-radiated from regular TV transmitters. |
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| There are a number of INTERSAT satellites over the Atlantic, Pacific and Indian oceans operating as relay stations to as many as forty ground stations around the world. The international system of satellite communication caters to the continental 625/50 as well as the American 525/60 systems. Since television standards differ from country to country, the transmitting station adopts the standard of the originating country. The ground station converts the received signal with the help of digital international conversion equipment to the local standard before relaying it. |
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| Frequency modulation is used for both 'up channel' and 'down channel' transmission. Though FM needs a larger bandwidth, it offers good immunity from interference and requires less power in the satellite transmitter. |
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For global communication, a communication satellite should move  |
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| where r is the radius of the orbit of the communication satellite and R is the radius of the Earth. |
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| A circular orbit of the communication satellite is specified in terms of the following: |
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| (i) The orbit radius |
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| (ii) The angle of inclination of the orbit plane to the Earth's equatorial plane |
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| (iii) The position of the ascending mode |
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| (iv) The phase angle of the satellite. |
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| The area of the Earth from which a satellite is visible increases with the altitude. So, satellites at higher altitudes give larger coverage. At altitudes below 10,000 km, the number of satellites required for global coverage would be excessive. At altitudes above 20,000 km, the time taken by signals may be large enough to cause confusion in telephone conversation. If time-delay difficulties are ignored, then a synchronous satellite at 36000 km height can be advantageously used. |
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| If several satellites are spaced around the same orbit in space, the tracks of the satellites will be different due to Earth's rotation about its own axis. But if, for example, four satellites are placed into different orbits with their ascending modes displaced successively by 30° intervals to the east direction, the difference, in effects of Earth's rotation, can be counteracted and the paths of all the satellites relative to the Earth will be the same. Such Earth-track integrals systems can be arranged to have the satellite period an integral factor of the sidereal day in order to have the same track repeated day after day. |
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| Satellite technology is very useful in collecting information about various factors of the atmosphere, which governs the weather and climatic conditions. An aerial photograph of the Country is shown daily on the television. It is also published daily in the newspapers. The data collected by INSAT satellite enables our scientists to make short-term and long-term predictions about weather. We are able to get prior information about an emerging cyclone or floods or drought conditions. This enables the government to take suitable measures to minimize the loss of lives and property. |
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| The satellite communication is costly because of the limited life of a satellite. A geosynchronous satellite using high gain antenna requires close control of both its position and altitude. The position and altitude control rockets require fuel that has to be put in once for all before launch. With a given payload, the longer the life, heavier the satellite and more the, cost. All communication satellites are designed for a maximum operating life limited by its positioning fuel capacity. |
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| The successive generations of communication satellites can incorporate the latest developments in electronics and communication engineering technology. This would help to pack much more capacity into satellites of comparable size. It is possible that with the advances in technology, the cost of satellite communication would be considerably reduced in the near future. |
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