Radio and Satellite Technologies
The post-World War II era ushered in the
development of satellites and their launch systems, and with it, the
communications revolution. In 1958, the "space race" began when the
Soviets launched Sputnik. The first American communications satellite, Telstar
1, was launched in 1962. It could handle 600 telephone conversations
simultaneously. Today, over 200 international communications satellites,
similar to the Intelstat VI family, can carry 33,000 telephone calls at once.
This capability will again be exceeded with the development of future
satellites.
Isaac Newton first discovered how gravity keeps satellites in orbit when he observed how the moon orbits the
earth. Any object, be it the moon or a communications satellite, is pulled
toward the earth by the force of gravity. But, if it is moving fast enough and
is moving perpendicular to the pull of gravity, it falls in a curved path and
circles the earth. Although it's always falling, it stays in orbit. (It's
similar to a ball on a string: if you swing it fast enough, it will travel in a
circle around your hand.) If a satellite does not move fast enough, it
eventually spirals closer and closer to the earth and burns up in the earth's
atmosphere.
Satellites are sent into space and placed in
orbit in one of two ways. The first way is with an expendable rocket that
launches the satellite out of the earth's atmosphere. A smaller rocket attached
to the satellite then places the satellite into the proper orbit. The second
way of getting a satellite into space is on board a space shuttle. After the
shuttle reaches a low earth orbit, astronauts release the satellite. The
shuttle backs away and a transfer engine on the satellite places it in its
proper orbit.
Satellites orbit the earth primarily in four
ways: polar orbits, geosynchronous orbits, low earth orbits, and molniya
orbits. In a polar orbit, a satellite orbits the earth from pole to pole. It
covers different parts of the earth as the earth turns beneath it. This orbit
is used for observing the weather and mapping earth resources. Most
communications satellites are put into a geosynchronous orbit. This kind of
orbit positions a satellite over the equator; the satellite orbits the earth at
the same speed as the earth is turning, giving the satellite the appearance of
being stationary. Low earth orbits vary with the type of satellites and their
primary purposes. The uses of satellites in low earth orbit may range from
scientific research to surveillance. Molniya orbits are highly elliptical and are used primarily by Soviet
communications satellites.
Satellites will continue to grow in
popularity, complexity, and uses, limited only by the imaginations of those
involved in the technology
http://www2.thomas.edu/faculty/hansenj/Satellite Transmission.htm
http://www.tropian.com/tech/tech_docs/RF_transmission.pdf
http://www.tropian.com/tech/tech_docs/tropiantechnologywp.pdf
http://roland.lerc.nasa.gov/~dglover/sat/satcom2.html
http://www.ask.com/main/askjeeves.asp?ask=Number+of+satellites+in+orbit&ori=0