Microsoft Encarta Encyclopedia 2000. (PICTURE ABOVE)
TYPE IN THE WORDS: "Communications satelite"
(Another Name for E.L.F & THE Link Above)
Article ON Microsoft Contributed By: S.Joseph Campanella
(Below)
==================================================================
Communications Satellite, any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.
HISTORY
Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could pick up by receiving stations around the world. ECHO 1 launched by the United States in 1960, consisted of an aluminized plastic balloon 30m (100ft) in diameter. Launched in 1964, Echo 2 was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas.
Satellite communications currently make exclusive use of active systems, in which each satellite carries its own equipment for reception and transmission. Score, launched by the United States in 1958, was the first active communications satellite. It was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station. Telestar 1, Launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred voice channels. Launched into an elliptical orbit inclined 45 degrees to the equatorial plan, Telestar could only relay signals between two ground stations for a short period during each revolation, when both stations were in it's line of sight.
Hundreds of active communications are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at different frequency to another. Satellites uses ranges of different frequencies, measured in hertz (Hz) or cycles per second, for receiving and transmitting signals. Many Satellites use a band of frequencies of about 6 billion hertz, or 6 gigahertz (GHZ) for upward, or uplink, transmission and 4 ghz for downward, or downlink, transmission. Another band at 14 GHz (uplink)and 11 or 12 GHz (downlink) is also much in use, mostly with fixed (nonmobile) ground stations. A band at about 1.5 GHz (for both uplink and Downlink) is used with small, mobile grounds stations (Ships, Land Vehicles, and aircrafts, Plus Humans, Etc.) Solar Energy cells (SOLAR PANELS) mounted on large panels attached to the satellite provide power for reception and transmission.
GEOSYNCHRONOUS ORBIT
A Satellite in a geosynchronous orbit follows a circular orbit over the equator in an altitude of 35,000 km (22,300 mi), Completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby, providing uninterrupted contact between ground stations in it's line of sight. The first communications satellite to be placed in this type of orbit was Syncom 2, launched by the National aeronautics and Space Administration (NASA) in 1963. Most Communications satellites that followed were also placed in geosynchronous orbit.
Commercial Communications Satellites
Deployment and operation of communications satellites on a commercial basis began with the founding of the Communications Satellites Corporation (COMSAT) in 1963. When the International Telecommunications Satellite Organization (INTELSAT) was formed in 1964, COMSAT became the U.S. member. Based in Washington DC, INTELSAT is owned by more than 120 nations. Intelsat 1, known as early bird, launched in 1965, provided either 240 voices circuits or one two-way television channel between the united states and Europe.
_________________________________________________________________________________
Copyright 1993-1999 Microsoft Corporation. All rights reserved, "Communications Satellite,"
pg.1
Durning the 1960's and the 1970's, message capacity and transmission capacity and transmission power of Intelsat 2, 3, and 4 generations were progressively increased. The First of the Intelsat 4s, launched in 1971, provided 4,000 voice circuits. With the intelsat 5 series (1980), innovations in signal focusing resulted in additional increases in capacity. A satellite's power could now be concentrated on small regions of the earth, making it possible smaller-aperture, lower grounds stations. An Intelsat 5 satellite can typically carry 12,000 voice circuits. The Intelsat 6 satellites, which entered service in 1989, can carry 24,000 circuits and feature dynamic on-board switching of telephone capacity among six beams, using a technique called SS-TDMA (SATELLITE-Switched time division multiple access). In the late 1990's, Intelsat had 19 satellites in orbit, providing the world's most extensive telecommunications system. Other systems also provide international service in competition with Intelsat. The growth of International systems has been paralleled by domestic and regional systems, such as the U.S. Telstar, Galaxy, and Spacenet programs and Europe's Eutelsat and Telecom.
SERVICES
Commercial Satellites provide a wide range of communication services. Television programs are relayed internationally, giving rise to the phenomenon known as the "Global Village." Satellites also relay programs to cable television systems as well as to homes equipped with dish antennas. In addition, very small aperture terminals (VSAT'S) relay digital data for a multitude of business services. Intelsat satellites now carry over 100,000 telephones circuits, with growing use of digital transmission. Digital source coding methods (SEE TELECOMUNNICATIONS) have resulted in a ten-fold reduction in the transmission rate needed to carry a voice channel, thus enhancing the capacity of exsisting facilities and reducing the size of ground stations that provide telephone service.
The International Mobile Satellite Organization (INMARSAT), Founded in 1979 as the International Maritime Satellite Organization, is a mobile telecommunications network, providing digital data links, telephone, and facsimile transmission, or fax, service between ships, offshore facilities, and shore-based stations throughout the world. It is also now extending satellite links (internet) for voice and fax transmission to aircraft on international routes.
RECENT TECHNICAL ADVANCES
Communications Satellite systems have entered a period of transition from point-to-point high capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low cost stations. The development of multiple access methods has both hastened and facilitated this transition. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed by them. By amplifying a single carrier frequency in each satellite repeater, TDMA, ensures the most efficient use of the satellite's onboard power supply.
A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams (Of Light) pointed toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams (of Light) in different directions, using the same reflector.
A Method for interconnecting many ground stations spread over great distances was demonstrated in 1993 with the launch of NASA'S ACTS (Advanced Communications Technology Satellite). The Satellite uses what is known as the hopping spot beam technique to combine the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have similar antennas and reduced power (Solar Panels and Generator).
The Concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Intelsat, developed by the Italian Research Council. With six spot beams operating at 30 GHz (uplink) and 20 GHz (Downlink), the Satellite interconnects TDMA transmissions between ground stations in all major economic centers of Italy.
_________________________________________________________________________________
Copyright 1993-1999 Microsoft Corporation. All rights reserved, "Communications Satellite,"
pg2
It does this by demodulating uplink signals, routing them between up-and -downlink beams, and combining and remodulating them for downlink transmission.
LASER BEAMS can be used to transmit signals between a satellite and the earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the Blue-Green Wavelength, which penetrates water, have been used for Communication between satellites and submarines.
NOTE: I SEE THIS BLUE-GREEN COLOR ALL THE TIME. PLUS PURPLE-PINK, WHITE-GOLD.
The latest development in satellites is the use of networks of small satellites in low earth orbit (2,000 km or (1,200 mi or less) to provide global telephone communication. The Iridium system uses 66 satellites in low earth orbit, while other groups have or are developing similar systems. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Anticipated customers of these systems include International business travelers and people living or working in remote
areas.
SEE ALSO SATELLITE, ARTIFICAL, SPACE, EXPLORATION, TELEPHONE.
CONTRIBUTED BY: S. Joseph Campanella
Mocrosoft (COPYRIGHT) Encarta (COPYRIGHT) Encyclopedia 2000 (COPYRIGHT) 1993-1999 Microsoft Corporation. All Rights Reserved.
_________________________________________________________________________________
Copyright 1993-1999 Microsoft Corporation. All rights reserved, "Communications Satellite,"
pg3
.jpg)
.jpg)
