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A satellite is something that goes around and around a larger something,
like the earth or another planet. Some satellites are natural, like the
moon which is a natural satellite of the earth. Other satellites are made
by scientists and technologists to go around the earth and do certain jobs.

Some satellites send and receive television signals. The signal is sent
from a station on the earth's surface. The satellite receives the signal
and rebroadcasts it to other places on the earth. With the right number of
satellites in space, one television program can be seen all over the world.

Satellites make it possible to communicate by telephone, fax, Internet,
or computer with anyone in the world. Other satellites observe the world's
weather, feeding weather information into giant computer programs that
help scientists know what the weather will be. The weather reporters on
your favorite TV news program get their information from those scientists.
Still other satellites take very accurate pictures of the earth's surface,
sending back images that tell scientists about changes that are going on
around the world and about crops, water, and other resources.

A satellite can carry a camera as it travels in its orbit and take pictures
of Earth. Mapmakers can use these pictures to make the most accurate
maps. Satellite pictures can also help predict the weather, because from
the satellite, the camera can actually see the weather coming. When you
watch the weather forecast on TV, you see pictures of the earth taken by
a camera riding on a satellite. Satellites in orbit can send messages to a
special receiver carried by someone on a ship in the ocean or in a truck in
the desert, telling that person exactly where he or she is.

A satellite can relay your telephone call across the country or to the
other side of the world. If you decide to telephone your friend in Mexico
City, your call can be sent up in space to a satellite, then relayed to a
ground station in Mexico and sent from there to your friend's telephone.
A satellite can relay your computer or fax message, or Internet data as
well. With the help of satellites, we can fax, e-mail, or download
information anyplace in the world. When the satellite sends a message
from your computer or fax to another computer or fax, it's called data
transmission. The satellite is transmitting information, or data.

A satellite can transmit your favorite TV program from the studio where
it is made to your TV set—even if the studio is in Japan and your TV
set is in Iowa. From the studio where it is made, a TV program is
broadcast to a satellite. This is called an uplink. Then it is rebroadcast
from the satellite to another place on the earth. This is a downlink. To
link means to connect. So uplink is connecting upward to the satellite
and downlink is connecting downward to earth. When words or pictures
or computer data are sent up to a satellite, they are first converted to an
invisible stream of energy, called a signal. The signal travels up through
space to the satellite and then travels down from the satellite to its
destination, where it is converted back to a voice message, a picture,
or data, so that the receiver can receive it. Some satellites have a
digital signal processor, which is like a very powerful computer. While
they orbit, these satellites can change the kind of work they do and the
places they send signals.

Satellites have a great deal of equipment packed inside them. A satellite
has seven subsystems, and each one has its own work to do.
The propulsion subsystem includes the electric or chemical motor that
brings the spacecraft to its permanent position, as well as small thrusters
(motors) that help keep the satellite in its assigned place in orbit. Satellites
drift out of position because of solar wind or gravitational or magnetic
forces. When that happens, the thrusters are fired to move the satellite
back into the right position in its orbit.

The power subsystem generates electricity from the solar panels on the
outside of the spacecraft. The solar panels also store electricity in storage
batteries, which provide power when the sun isn't shining on the panels.
The power is used to operate the communications subsystem.

The communications subsystem handles all the transmit and receive
functions. It receives signals from the earth, amplifies or strengthens them,
and transmits (sends) them to another satellite or to a ground station.

The structures subsystem distributes the stresses of launch and acts as
a strong, stable framework for attaching the other parts of the satellite.

The thermal control subsystem keeps the active parts of the satellite
cool enough to work properly. It does this by directing the heat that is
generated by satellite operations out into space, where it won't interfere with
the satellite.

The attitude control subsystem maintains the communications "footprint"
in the correct location. Satellites can't be allowed to jiggle or wander,
because if a satellite is not exactly where it belongs, pointed at exactly the
right place on the earth, the television program or the telephone call it
transmits to you will be interrupted. When the satellite gets out of position,
the attitude control system tells the propulsion system to fire a thruster
that will move the satellite back where it belongs.

Operators at the ground station need to be able to transmit commands
to the satellite and to monitor its health. The telemetry and command
subsystem provides a way for people at the ground stations to
communicate with the satellite.

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