satellite communication


satellite communication

Introduction
 in telecommunication, the use of artificial satellites to provide communications links between various points on Earth (Earth satellite). Communications satellites relay voice, video, and data signals between widely separated fixed locations (e.g., between the switching offices of two different national telephone networks), between a fixed location and numerous small fixed or mobile receivers in a designated area (e.g., direct satellite broadcasting of television programming), and between individual mobile users (e.g., aircraft, ships, motor vehicles, and personal handheld units). The technique involves transmitting signals from an Earth station to a satellite. Equipment onboard the satellite receives the signals, amplifies them, and retransmits them to a region of Earth. Receiving stations within this region pick up the signals, thus completing the link.

Satellites as radio repeaters
      Satellites provide communications links via microwave radio, most commonly in the superhigh-frequency band of 3 to 30 gigahertz (3 billion to 30 billion hertz, or cycles per second). These frequencies correspond to wavelengths ranging from 10 cm to 1 cm (4 inches to 0.4 inch). Radio waves this short diverge along straight lines in narrow beams, rather than propagating in an expanding spherical wave front in the manner of longer wavelengths. For communication by microwaves, therefore, transmitters and receivers must be within line of sight of one another. On land this can be achieved by using towers or hilltop locations, but microwave communication across oceans is impossible without the use of satellites.

      The specific frequency bands open to civilian satellite communication are assigned by the International Telecommunication Union, based in Geneva. Each band consists of an uplink (Earth-to-satellite) frequency and a downlink (satellite-to-Earth) frequency. The two bands that have been in use longest, and still carry the most traffic, are the C band, with uplink frequencies centred on 6 gigahertz and downlink frequencies centred on 4 gigahertz, and the Ku band, with uplink/downlink frequencies centred on 14/11 gigahertz. In order to relay signals in these frequencies, a typical communications satellite is equipped with several transponders, or repeaters. Each transponder consists of a receiver tuned to the uplink band, a frequency shifter that lowers the received signals to the downlink band, and a power amplifier that produces an adequate transmitting power. Multiple transponders allow a single satellite to provide a combination of wide-area beams for broadcasting and narrow-area “spot” beams for point-to-point communications.

      The most common source of microwave power for transmitting signals from communications satellites is the traveling-wave tube amplifier, the only remaining representative of vacuum tube technology in satellites. Solid-state power amplifiers are an economical alternative mainly for lower power transmissions. solar cells are the universal source of electric power in operational satellites. The cells can be placed on flat panels that extend from the body of the satellite, or they can cover the satellite's surface. Power for use when the satellite is in Earth's shadow is stored in rechargeable nickel-cadmium or nickel-hydrogen batteries.

      The strength of a signal reaching the intended area on Earth's surface depends on several factors. One is the satellite's transmitter power, which is subject to such limitations as the maximum practical size and weight of the solar panels that can be put into the desired orbit and the fairly low efficiency of the transmitter in converting input power into radiated power. Because the strength of a transmitter's signal decreases in proportion to the square of the distance from the transmitter, the satellite's altitude has a great effect on the received signal strength. For example, the signal from a satellite orbiting at an altitude of 30,000 km (18,600 miles) is only 1/10,000 as strong as a signal from an identical satellite orbiting 100 times closer (at 300 km altitude). To waste as little as possible of a transmitter's radiated power, it is advantageous to employ a narrow beam, pointed toward only those regions with which communication is desired. In order to achieve this concentration of power, the satellite's antenna must be quite large—as much as 2.5 metres (8 feet) in diameter. A typical satellite antenna is parabolic in shape, its concave surface reflecting microwave energy that is directed toward it by a complex array of feed horn antennas.

Orbit and attitude
      Communications satellites generally are carried into space by rocket-powered expendable launch vehicles, although in the 1980s a significant number were deployed during U.S. space shuttle missions. Most are placed ultimately into geostationary orbit, in which the satellite travels in a circular path around Earth in the plane of the Equator, at an altitude of about 35,800 km (22,250 miles). At this height the satellite orbits Earth with a period identical to Earth's rotation period, so that the satellite remains above the same spot on the globe. Three such satellites spaced equidistantly in orbit ensure complete coverage of Earth, with the exception of the polar latitudes.

      Throughout a satellite's service life, occasional use of small thruster motors maintain it in the proper geostationary orbit and in the correct attitude (i.e., pointing in the right direction). Attitude is controlled by one of three orientation methods: spin-stabilizing the entire satellite, including the antennas, by rotating it around its long axis like a top; spin-stabilizing the body of the satellite while the antenna platform is counterrotated, or despun, in order for it to continue to point at its coverage area on Earth; and maintaining three-axis stabilization of the entire satellite by means of onboard, electrically powered spinning wheels (called reaction, or momentum, wheels) and thrusters.

Development of satellite communication
      The idea of radio transmission through space is at least as old as the space novel Ralph 124C41+ (1911), by the American science fiction pioneer Hugo Gernsback (Gernsback, Hugo). Yet the idea of a radio repeater located in space was slow to develop. In 1945 the British author and scientist Arthur C. Clarke (Clarke, Sir Arthur C.) proposed the use of geostationary satellites for station-to-station and broadcast radio communication. Clarke assumed that these spacecraft would need to take the form of manned space stations with living quarters for crew that would be built in space of materials flown up by rockets and provided with receiving and transmitting equipment and directional antennas. Clarke suggested the use of solar power, either a steam engine operated by solar heat or photoelectric devices.

      In a paper published in April 1955, the American engineer and scientist John Robinson Pierce (Pierce, John Robinson) analyzed various concepts for unmanned communications satellites. These included passive devices, such as metallized balloons, plane reflectors, and corner reflectors, that would merely reflect back to Earth part of the energy directed to them. Active satellites, incorporating radio receivers and transmitters, were also considered. Pierce discussed satellites at synchronous altitudes, satellites at lower altitudes, and the use of Earth's gravity to control the attitude or orientation of a satellite.

  The first satellite to relay messages between Earth stations was the U.S. government's Project SCORE, launched December 18, 1958. Circling Earth in a low elliptical orbit, it functioned for 13 days until its batteries ran down. One of the best-known early satellites was Echo 1, a balloon made of Mylar plastic coated with a thin layer of aluminum, which was launched August 12, 1960. Successful communications tests carried out by reflecting radio signals from Echo 1's surface encouraged further experimentation. Telstar 1, launched July 10, 1962, was an active satellite and was the first to transmit live television signals and telephone conversations across the Atlantic Ocean. Syncom 2, launched July 26, 1963, was the first geostationary communications satellite, and Syncom 3, launched August 19, 1964, relayed the first sustained transpacific television picture.

      Experimental programs such as those described above represented the conjunction of a number of technological advances that were necessary for the era of satellite communication to begin. These included the development of reliable launch vehicles, solid-state electronic devices, spin stabilization for attitude control, efficient solar cells for power generation, and Earth-to-satellite telemetering and control techniques. In addition, the development of the low-noise maser and the traveling-wave tube amplifier was necessary for satellites to capture and amplify the weak uplink signals for retransmission to Earth. Intelsat 1 (also known as Early Bird), the first commercial communications satellite, was launched April 6, 1965; it provided high-bandwidth telecommunications service between the United States and Europe as a supplement to existing transatlantic cable and shortwave radio links. Intelsat 1 carried 240 voice circuits or one television channel. The Intelsat 2 series of satellites (launched 1967) together offered full coverage of the Atlantic and Pacific regions, and each satellite of the Intelsat 3 series (1968–70) provided more than 1,500 voice circuits or four television channels. The Intelsat 4 satellites (1971–75) each carried 6,000 voice circuits or 12 television channels. In contrast to these early series, the Intelsat 9 satellites launched in the first years of the 21st century each could handle 600,000 circuits or 600 television channels.

 The Intelsat satellites were developed in the United States under the aegis of the International Telecommunications Satellite Organization (now Intelsat, Ltd.), which was formed in 1964 to maintain the space segment of global satellite communications links. In 1972 the U.S. Federal Communications Commission issued its “open skies” order, which permitted any legal entity to develop a satellite system for specialized applications. As a result, private companies deployed a large number of satellites over the following decades. After the mid-1960s, with the introduction of the Soviet Union's Molniya satellites, many national and regional organizations established their own satellite services for domestic telecommunication. Within two decades these organizations included services in the Arab League, Australia, Brazil, Canada, China, the European Union, France, India, Indonesia, Japan, Mexico, and the United Kingdom. At the end of the 20th century, communications satellite operators existed in more than 50 countries.

      Since the beginning of the satellite era, advances in attitude-control thrusters, in solar cells, and in subsystem reliability have extended the service lifetimes of satellites to about 15 years. Payload weight has been reduced by very-large-scale integration of solid-state electronics and by the availability of lighter and more efficient microwave cavity filters, traveling-wave tubes, and solid-state power amplifiers. Improved antenna design has given fine control over both uplink and downlink beam patterns; the ability to transmit tighter beams has allowed satellites to deliver a higher percentage of their transmitted power to the desired coverage area, thus permitting significant reductions in the size of Earth receiving antennas. The combination of improved antenna design with high-gain, low-noise receivers also has led to significant reductions in the size of Earth transmitting antennas. Finally, the development of frequency reuse techniques has allowed satellites to transmit and receive multiple channels in the same frequency band.

      By the end of the 1970s, more than two-thirds of all international telephony (telephone) was routed through satellite channels. This situation began to reverse in the 1980s with the introduction of high-capacity optical fibre links (see cable), which introduce shorter delay times in signal transmission than do satellite links. The minimum propagation delay for geostationary satellite communication, with its nearly 72,000-km (44,500-mile) round-trip travel time, is about one-fourth of a second; in practice, delay times can be longer. Although such delays are acceptable for many kinds of data transfer, in voice communication they are quite noticeable and often annoying.

      A nongeostationary satellite in low Earth orbit (LEO; generally below 2,000 km [1,200 miles] in altitude) can provide much shorter propagation delays, and it has the additional advantage of allowing direct uplinking and downlinking with small, low-power portable stations. A LEO system, however, requires dozens of satellites for complete global coverage. Beginning in the late 1990s and spurred by the growth of personal mobile communications such as cellular telephone services, several companies attempted to establish their own constellations of LEO satellites for this purpose. All of them experienced difficulty competing with ground-based systems, and most were out of business by the early 21st century.

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Universalium. 2010.

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