 - In general, a satellite is anything that orbits something else, as, for example, the moon orbits the earth. In a communications context, a satellite is a specialized wireless
receiver/transmitter that is launched by a rocket and placed in
orbit around the earth. There are hundreds of satellites
currently in operation. They are used for such diverse purposes
as weather forecasting, television broadcast, amateur radio
communications, Internet communications, and the Global Positioning System, (GPS).
The first artificial satellite, launched by
Russia (then known as the Soviet Union) in the late 1950s, was
about the size of a basketball. It did nothing but transmit a
simple Morse code signal over and over. In contrast, modern
satellites can receive and re-transmit thousands of signals
simultaneously, from simple digital data to the most complex
television programming.
There are three types of communications satellite
systems. They are categorized according to the type of orbit they
follow.
A geostationary satellite orbits the
earth directly over the equator, approximately 22,000 miles up. At this
altitude, one complete trip around the earth (relative to the
sun) takes 24 hours. Thus, the satellite remains over the
same spot on the earth's surface at all times, and stays fixed
in the sky from any point on the surface from which it can be
"seen." So-called weather satellites are
usually of this type. You can view images from some of these
satellites on the Internet via the Purdue Weather
Processor. A single geostationary satellite can "see" approximately
40 percent of the earth's surface. Three such satellites, spaced
at equal intervals (120 angular degrees apart), can provide
coverage of the entire civilized world. A geostationary satellite
can be accessed using a dish antenna aimed at the spot in the sky
where the satellite hovers.
A low-earth-orbit (LEO) satellite system
employs a large fleet of "birds," each in a circular orbit
at a constant altitude of a few hundred miles. The
orbits take the satellites over, or nearly over, the geographic
poles. Each revolution takes approximately 90 minutes to a few
hours. The fleet is arranged in such a way that, from any point
on the surface at any time, at least one satellite is on a line
of sight. The entire system operates in a manner similar to the
way a cellular telephone functions. The main difference
is that the transponders,
or wireless receiver/transmitters, are moving rather than fixed,
and are in space rather than on the earth. A well-designed LEO
system makes it possible for anyone to access the Internet via
wireless from any point on the planet, using an antenna no more
sophisticated than old-fashioned television "rabbit
ears."
Some satellites revolve around the earth in elliptical
orbits. These satellites move rapidly when they are near perigee,
or their lowest altitude; they move slowly when they are near apogee,
or their highest altitude. Such "birds" are used by
amateur radio operators, and by some commercial and government
services. They require directional antennas whose orientation
must be constantly adjusted to follow the satellite's path across
the sky.
| LAST UPDATED: |
17 Oct 2008
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