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Komunikasi Satelite Risanuri Hidayat
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.
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.
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.
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)
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.
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.
1989 1980 1986 1992
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
