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SATELLITE COMMUNICATIONS Compiled
By: Aditya Dave (99 EC 07) Himanshu
Chauhan (99 EC 06)
Manan Desai (99 EC 09) Udit Dave
(99 EC 08) Umang Doshi
(99 EC 12) Contents:- 1. History of Development Government satellite services Recent Development 2. Basic components in satellite communications Uplink Downlink Transmitters Antennas Power Generation 3. Advances in satellite technology and system concepts Non conventional technologies Space Station Concepts Millimeter Wave Satellites Direct Broadcast Satellites-Television 4. Applications in satellite technology Traditional Telecommunications Cellular Television Signals Marine Communications Spacebourne Landmobile Satellite Messaging for Commercial Jets Global Positioning Services 5. Technological Overview Orbits Constellations Global Voice Communications Global Broadband Networks 1.
History of development:
The first commercial satellite is believed to be Intelsat-1 or Early Bird. It weighed about 38 kg and had 240 channels. It had two functions: 1. to establish telecommunication links, specially between those countries which did not have international cable systems 2. to link national television broadcasters of different countries. These satellite services had three major benefits, which remain relevant even today 1. point to point and point to multipoint capabilities 2. relative insenstivity to long distances 3. ability to scan terrain of any type Governmental
satellite services: Intelsat, builder of the first commercial international satellite communications network, was established as an international co-operative organization with signatories to its treaty being national telecommunications operators or other entities nominated by national governments. The Soviet Union and its then client states formed a parallel organization called Intersputnik. Intelsat, in addition to selling capacity to countries for national purposes, supports corporate business users with offerings such as very small aperture terminal (VSAT) and satellite news gathering services. Recent developments: Basically, communication satellites are orbiting radio repeaters. The majority of Western examples to date have been lodged in geosynchronous earth orbits (GEOs) - sometimes called geostationary earth orbits - with spacecraft orbiting at around 36,000 km above the earth's surface in an equatorial plane. At that height and in that plane, satellites circle in time with the rotation of the planet and remain, to all intents and purposes, in the same spatial relationship to particular points on the earth's surface beneath. This arrangement has the advantage of providing 24-hour coverage of locations in the area"illuminated" by the satellite's transponders. A series of satellites, appropriately distributed, can provide near complete coverage of the globe. Over the years GEO satellites have become physically bigger and heavier - expanding, for example, from launch weights of tens of kilograms in the 1960s to nearly 1780 kilograms in the case of Intelsat-7 in 1989. The pay-off is higher circuit capacity - 120,000 on Intelsat-7 compared to 240 on Intelsat-1 - increased operational life through the ability to carry more on-board fuel, and more power. The latter, coupled with the use of higher frequencies, has led to a long term reduction in the size of earth terminals: for future land mobile satellite applications, these terminals are likely to be not much bigger than conventional terrestrial cellular telephones. To some extent, however, the introduction of new technologies and smaller components has acted as a brake on the requirement to build progressively larger orbiting vehicles in order to obtain higher performance. New generation satellites also have on-board switching and processing capabilities, along with in-space links to other spacecraft, and are sometimes dubbed "switchboards in the sky". A number of alternatives to GEO satellites have been developed. Highly eccentric orbit (HEO) spacecraft travel very rapidly around the earth in an elliptical path extending from altitudes of around 1000 km at the lowest point to 70,000 km at the highest. HEO systems have been popular with Russian operators and used by the Nordic countries. In recent years there has also been much interest in low earth orbit (LEO) and medium earth orbit (MEO) spacecraft. These orbit, respectively, at altitudes below 10,000 km and from 10,000 to 20,000 km.
A number of LEO and MEO systems are currently being planned to facilitate one of the satellite medium's newest applications - land mobile communications using small handheld terminals. One of the applications of the point to multi-point capability of satellite communication is VSAT (Very Small Aperture Terminals). Among the benefits claimed for VSAT technology are that it can cost less than conventional wireline and terrestrial systems for a given grade of service, that it can be relatively quickly implemented and is easy to use, and that it can offer improvements in reliability and availability over terrestrial networks in many parts of the world. A second, and sometimes related, use of satellites in thin
route, multi-destination VSAT applications is in the provision of dial tone to
rural and isolated communities, many of which are located in the less developed
countries (LDCs) of the world. Transponders on satellites providing thin route communication links can be shared by private VSAT network operators and public telecommunications operators furnishing rural services. In turn the latter services can mix voice, data and text with direct broadcast satellite (DBS) television traffic.
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