- ▪ 1997by Robert StoffelsBy the end of 1996, several thousand artificial satellites were circling the Earth. About 1,000 of them were in a geosynchronous orbit—that is, they were located over the Equator—at an altitude of 35,900 km (22,300 mi). At that distance they circled the Earth once every 24 hours, and since the Earth rotates on its axis once each 24 hours, the satellites appear to be stationary.A primary function of many of these satellites is to provide television service. Programs are transmitted up to a satellite (the uplink), where they are received, modified, and retransmitted to the Earth (the downlink). Because the majority of such systems operate at low power, the dish-shaped receiving antenna must be large, 3 or more metres (10 or more feet) in diameter. By 1992, however, improved technology allowed smaller antennas to be used. These systems were called VSATs (very small aperture terminals), and the dishes were only 1.2 m (4 ft) in diameter.Although most of the downlinks are terminated at a commercial cable TV company's facility, it is possible for individuals to subscribe directly to TV service by capturing a signal from one of the downlinks. This, however, requires the purchase and installation of an antenna. Most people consequently choose the commercial cable TV companies. These organizations receive signals from the downlink and distribute them to subscribers via a coaxial cable network.Some 64% of the households in the United States subscribed to such television service in 1996, and most found it satisfactory. However, many potential customers lived far away from a coaxial cable and therefore could not take advantage of the service. For these people there is now an alternative, direct-broadcast satellite television (DBS).The first U.S. high-powered DBS satellite was made by Hughes Aircraft Co. and owned by Hughes Electronic Corp.'s DirecTV and Hubbard Broadcasting's United States Satellite Broadcasting (USSB). Providing 150 television channels, it was launched on Dec. 17, 1993, from Kourou, French Guiana. Some months earlier, on July 22, 1993, Spain launched DBS Hispasat 1B into a geosynchronous orbit. The power of DBS permits receivers to use an antenna that is only 46 cm (18 in) in diameter. Such an antenna can be clamped to a windowsill or mounted on the roof or on a pedestal outside the house. It is easier to mount than previous antennas and is also more secure—and much less expensive. A June 1996 promotional campaign by one company offered antennas for $199. By 1996, according to the National Cable Television Association, DBS subscribership was doubling each year. Satellites continue to be launched, and improvements in technology allow increasingly larger numbers of TV channels to be downlinked. The orbital slots available over the Equator are carefully controlled by the International Telecommunications Union (ITU), however. The ITU requires each nation to coordinate its satellite slots with its neighbours'. The last available slot for DBS satellites that could reach all of the U.S. was sold in January 1996 to MCI Communications Corp. for $682.5 million.DBS systems were expanding rapidly not only in the U.S. but also in many other countries. Among the systems in operation, or slated for operation, were: British Sky Broadcasting (BSkyB), Galaxy Latin America, Japan Sky Broadcasting, the News Corp./TCI Latin America, the News Corp./TCI Australia, Star TV (Asia and Japan), and China Aerospace Corp.Significant advantages of DBS are the high quality of its signals, its capacity to handle 120 or more channels, and the access it provides to programming regardless of a customer's location. Supporters of DBS continue to insist that it is indeed the wave of the future and will compete successfully with cable TV. Detractors, on the other hand, suggest that its sudden upsurge is nothing more than the embracing of a new technology by "early adopters." They also note that DBS provides little local programming; legal restraints prevent DBS from delivering local broadcast signals.One other disadvantage to DBS—one that seems likely to become increasingly significant—is its lack of interactivity. DBS has, as its middle name, "broadcast." That means that communication is essentially one way—the same set of signals is sent to all potential receivers, and the channel selection is made at the receiver. But how does one send information from the receiver to the broadcaster? In a cable TV system such a channel can be installed (though at a high price), but with DBS it becomes necessary to use the telephone to send information to the originator.Robert E. Stoffels, formerly editor of Telephone Engineer & Management magazine, is a telecommunications consultant.
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satellite — [ satelit ] n. m. • satelite v. 1265; lat. satelles, satellitis « garde du corps » I ♦ 1 ♦ (1665) Corps céleste gravitant sur une orbite elliptique autour d une planète. La Lune est le satellite de la Terre. Les satellites de Jupiter. Satellite… … Encyclopédie Universelle
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