How Do Satellites Work?
Kayleen M. / Physics 336 #18 / 17 April 1997
Satellites perform many functions. In order to carry out these functions,
satellites use different subsystems. Basic Subsystems are groups of devices
that help the instruments aboard a satellite work together and keep the
satellite operating properly.
Communications
The communications subsystem of a satellite is essential to the function
of all satellites. There are many different components used in spaceborne
communications subsystems including: special antennas, receivers, and transmitters.
All components must be highly reliable and low weight. Most satellites
are fitted with beacons or transponders, which help with easy ground tracking.
Global receive and transmit horns receive and transmit signals over wide
areas on earth. Many satellites and ground stations have radio dishes that
transmit and receive signals to communicate. The curved dishes reflect
outgoing signals from the central horn and also reflect signals in incoming
beams. Transmitting antennas transmit pictures at different frequencies
and varying coverage. They also receive command signals from earth. A satellites
ability to receive signals is also necessary in order to trigger the return
of data or to correct a malfunction if possible.
Command and Data
Command and data subsystems are also extremely important to the functions
of satellites. The command and data handling subsystems consist of computers
that gather and process data from aboard a satellite. These computers then
execute commands from the earth to the satellite.
The process of making physical measurements from a distance is called
telemetry. The inputs of a satellites telemetric are received by any one
of the satellites many sensors. The outputs are the "encoded information"
that the satellite transmits to ground receivers for storage and later
analysis (Urdway 289). Encoding is the transfer of output in a useful manner
to earth. The communication of this data is key to the jobs of many satellites.
Power Supply
The power supply of a satellite is essential to all satellite functions.
A satellite could not function at all without a power source. "The
task of a satellites power supply subsystem is to accept the original power
in whatever form it comes, condition it properly, and then relay it to
the necessary on-board equipment" (Urdway 288). The power supply subsystem
generates, stores, and distributes a satellites electrical power. The amount
of electrical power required ranges, depending on the instrumentation and
communications load on a satellite. It also depends on the lifetime of
a satellite. There are many different types of power that may be used for
a satellites power supply subsystem. Power can be supplied by short-lived
fuel cells or batteries, by longlived solar cells, or by radioisotope devices.
Solar cells are very common in satellite power subsystems. They take
solar energy available in space and convert it directly to electrical energy.
Solar cells are often used with chemical batteries, which provide energy
when the satellite is in the shadow of the earth. Solar cells are reliable
and are usually used on scientific and applications satellites. Although,
they are heavy, expensive, and take up a large volume. Fuel cells take
fuel and oxidize it to produce power. The comparison of weight to the energy
provided makes fuel cells less attractive, but they are often used in short
satellite missions. Turbogenerators produce power from the heat of the
sun or of nuclear reactions. Thermionic generators produce power from sunlight.
In both of these, radioisotopes or small reactors produce heat. This is
then converted into electricity. Radioisotope devices have been used, but
are still in the developmental stages.
Conclusion
The functions of a satellite are carried out by different subsystems.
The power supply subsystem of a satellite is the basis for all satellites
functions. From the power source, satellites can communicate and receive
and send data to and from earth. These subsystems together perform the
functions that make satellites work.
Reference Section
Beatty, Kelly. "Galileo: An Image Gallery." Sky and Telescope.
November 1996: cover.
Charyk, Joseph V., and Metzger, Sidney. "Satellite, Artificial."
Groliers Multimedia
Encyclopedia. CDROM. Grolier Publishing, 1995.
"Nasa Communicates With Lost Satellite." New York Times.
February 1996: A15.
Urdway, Frederick I. "Satellites, Artificial." Encyclopedia
Americana: International
Edition. New York: Grolier Inc, 1996.