Megan G. / Physics 337 / April 17,1997
Communications Satellites have been around since 1958. A communications
satellite is a spacecraft that orbits the Earth and relays messages, radio,
telephone and television signals. Stations on the ground, called earth
stations, transmit signals to the satellite, which then relays the signal
to other earth stations. As a newer form of communications, communications
satellites are very useful in bringing the people in the world together.
Communications between people that used to take days or even months, now
take only minutes or seconds using satellites.
The first satellite to relay messages from one Earth Station to another
was SCORE (Signal Communicating by Orbiting Relay Equipment) launched December
18, 1958. These early satellites, because they were visible from both sides
of the Atlantic Ocean simultaneous for only a short time could provide
only a period of five hours a day for communications.
Early United States Satellites
In 1960 the United States launched the Echo satellite, a metallic balloon
that reflected signals. Later satellites, such as Telestar and Relay, included
electronic relay equipment called transponders. Syncom II, the first satellite
to be placed in a synchronous orbit, was launched in 1963. The first commercial
communications satellites were launched in 1965.
Passive and Active Communication
Communications through satellites are either passive or active. The first
communications satellites were passive. Signals from Earth were merely
reflected from the orbiting metallic sphere. Later types of satellites
are active. Active communication satellites receive signals from Earth,
electronically strengthen the signals, and transmit the signals to Earth.
Relaying of Signals
This relaying of signals from one Earth Station to another is done through
the satellite's transponder. Most communications satellites have more than
one transponder and antenna so that they can relay several users of radio
waves or signals at the same time.
Launch of Communication Satellites
Communications satellites are launched by rockets or carried into space
by the Space Shuttle. Once in space, small engines on the satellites guide
the satellite into orbit and help keep them there. Most communications
satellites are placed in orbit at an altitude of 22,300 miles above the
Earth. This is known as a geostationary or synchronous orbit. This allows
the satellite to orbit the Earth at the same speed as the rotation of the
Earth. As a result, the satellite appears to be stationary above the same
location on Earth.
Satellite Broadcast Range
Broadcasts from a satellite in synchronous orbit can cover only about one
third of the Earth's surface. To send signals anywhere in the world, three
communications satellites in geostationary orbit are needed.
Communications satellites will be used to link all the regions and people
of the world. This is a giant step from the early uses of communication
satellites. "What at the beginning of the decade, was no more than a concept
in the minds of a few engineers had, by the end, become a fully commercial
system providing global communication system" (Fishlock 23). This global
system will consist of many satellites, positioned in geostationary orbit,
providing high bandwidth capacity, interconnect many highly specialized
Earth Stations operating in more than thirty countries. This network, already
in progress by consortiums headed by Motorola (Iridium) will provide the
framework and capability for anyone in the world to communicate with anyone
else, regardless of location.
The experimental communication satellites were Score, Echo, Telestar, Relay,
Syncom. They were all launched by the U.S. between 1958 and 1963. Since
then, with the advances in satellite communications, many countries have
developed or purchased their own communications satellites, with many others
planning to do the same. "More and more large companies with extensive
telephone and computer needs are turning to an economical alternative to
conventional transmissional methods" (Beatty 275).
Intelsat Satellite Telecommunication
Intelsat Telecommunication is the biggest worldwide satellite communication
network. "It has over 110 nations, and operates powerful communications
satellites, such as Intelsat 6, over the Atlantic, Pacific, and Indian
Oceans" (Cayne 50). The Intelsat spans the globe, and domestic satellites
such as the USSR's Molniya satellites. Western Union's Westar, and Canada's
Anik - serve individual countries. The Intelsat V is the latest in its
space-craft series, it can handle 12,000 telephone circuits and two color
television transmission simultaneously.
Teledesic Satellite Network
A company called Teledesic intends to create a $9 billion wireless data
network requiring the launch of more than 810 satellites or "birds". Other
proposed systems would employ fewer, but still significant number of birds.
Telestar, the experimental satellite designed by Bell Telephone Laboratories
and placed in an elliptical inclined medium altitude orbit by NASA in 1962.
LEO Satellites take less than two hours to orbit the planet. This spacecraft
maybe able to serve a user on the ground only a few minutes before it travels
out of range.
Satcomes, a different name for communication satellites are being used
increasingly to handle long distance telephone, television, and other transmission
around the world.
Weather forecasting has been revolutionized by the use of satellites. They
are able to scan the whole Earth and the atmosphere continuously. The entire
Earth has been photographed at least once daily on a continuous basis for
use in weather prediction. Satellite's data provide information about the
ocean, desert, and polar areas of the Earth where conventional weather
reports are unavailable or limited. The satellites can show how weather
systems are developing anywhere in the world, even where there are no ground
stations. Some weather satellites such as the U.S. Goes and European Meteosat,
circle in geostationary orbit. Other satellites, such as the U.S. NOAA
series, are launched into a polar orbit, over the North and South Poles.
These satellites can scan the whole Earth every 12 hours.
Radio transmissions are also conducted through satellite facilities. Amateur
radio operators, or "hams" have built several Orbital Satellites Caring
Amateur Radios (OSCARS) that take advantage of leftover payload space on
other missions to get a free ride into orbit. Other communications satellites
listen for distress signals from ships and aircraft, then relay the victims'
location to rescue teams.
TV Broadcasting System
One of the best known uses of communications satellites is for the transmission
of video, television. High bandwidth can easily be provided using satellites
which allow easy sending of television broadcasts. In addition, developments
in broadcast technology allow different types of transmissions to be sent,
taking advantage of sharing the same satellite signal. "The cost to the
viewers must be small, and this requirement could best be met if satellites
transmissions employed vestigial sideband amplitude modulation of frequencies
in or near the existing UHF broadcasting bands" (Fishlock 45). Using receivers,
amplifiers and transmitters and the electronic techniques of multiplying,
these communication satellites can simultaneously relay many telephone
and television signals.
For telecommunications, orbiting switchboards (communications satellites)
have helped to reduce costs of long distance telephone calls dramatically.
It is predicted that users in the future will spend 65 cents a minute to
be hooked via satellites into the wired phone network. Long- distance phone
charges over land line networks will be added to that figure. "Iridium,
which will bypass long distance land lines with its inter-satellite communication,
expects to charge $3 per minute for delivering a call "from anywhere to
anywhere" (Mannes 69).
Satellite vs. Terrestrial
Communications satellites are more economical to build and use than more
traditional terrestrial methods. For example, in 1983, seven undersea cables
linked North America and Europe. The last cable cost $175 million. In comparison,
a modern communications satellite costs roughly $80 million (including
payment for launching it into orbit). It handles more than 3 times as many
The use of satellite technology, particularly in the use of communications
satellites has grown rapidly in the past thirty years. Each day more and
more uses for the satellites are being discovered. Feeding this is the
rapid advancement of technology that allows the quick implementation of
Communications satellites will not only help out a person in distress but
allow a person walking the street in Manhasset N.Y. USA to use their cellular
phone to speak with someone in China. More and more satellites are being
launched each year to support new and growing uses for business, military
and communication needs. Satellite communications will continue in the
right direction, UP.
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CT: Electronic Publishing, 1996 Ed.
Beatty, Kelly J. The New Book of Popular
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Cayne, Berrards. The New Book of Popular
Science. USA: Grolier Incorporated, 1987.
Cavendish, Marshall. Out of This World. New
York: Arco Publishing, 1985.
Fishlock, David. A Guide to Earth Satellites.
New York: Macdonald London and American Elsevies Inc, 1971.
Mannes, George. "A Wireless World". Popular Mechanics. September