BEHAVIOR
The advantage of having satellite radio reception is being able to drive
completely across the country and never having to change your radio
dial. Or being able to listen to every NFL football game regardless
of where you are. Satellite radio delivers dozens of music and entertainment
channels, plus sports, news, and traffic information nearly
everywhere in the United States, including southern Alaska. Television
service for backseat viewers will soon be available by satellite radio.
HABITAT
These antennas can take one of several shapes. Most common is a vertical
wire sheathed in plastic about a foot long that has a plastic base
attached to the car. Another model added after market is a small plastic
box with wires that can be fed into the trunk. All are mounted on the
roof or other parts high enough to receive signals from overhead.
HOW IT WORKS
The two satellite companies operating in the United States, Sirius
and XM Satellite Radio, merged in February 2007. Because the two
companies use incompatible technology, they will have redundant
equipment and services until they introduce radio receivers that can
receive signals from both systems. The combined company has seven
satellites in space plus one spare for each of the two technologies.
XM satellites are geostationary, while Sirius satellites are geosynchronous.
A geostationary satellite revolves around the Earth at the
same rate that the Earth is spinning, so it stays over the same point
relative to Earth. These are located above the equator. Geosynchronous
satellites return to the same location above Earth at the same
time every day. Having multiple geosynchronous satellites allows the
radio company to have one above the center of the United States at all
times. This reduces the number of repeaters they need on the ground.
The spares are kept on hand to replace a satellite should it fail.
In addition to the satellites, there is a network of ground repeaters
that fill in the signal in locations that don’t have good reception from
the satellite. A typical U.S. city might have 20 repeaters. XM operates
about 800 repeaters in the United States.
The satellites broadcast (and the repeaters repeat) a signal within
the frequency band centered at 12.5 MHz. They broadcast on two carrier
waves within the 12.5 MHz band and use four other bands
to repeat the signal. A complex system allows one signal to fill in
for another.
The visible receivers catch the radio signals from either satellite
or ground repeater, filter out unwanted radio signals, and amplify the
signal. The second component of the system decodes the radio signals
and lowers the frequency of the signals so the car radio can play
the songs.
© 2009 by Ed Sobey
All rights reserved
Published by Chicago Review Press, Incorporated
814 North Franklin Street
Chicago, Illinois 60610
ISBN: 978-1-55652-812-5
Printed in the United States of America
5 4 3 2 1
Library of Congress Cataloging-in-Publication Data
Sobey, Edwin J. C., 1948–
A field guide to automotive technology / Ed Sobey.
p. cm.
Includes index.
ISBN 978-1-55652-812-5
1. Automobiles—Popular works. 2. Mechanics—Popular works. I. Title.
TL146.5.S63 2008
629.2—dc22
2008046620
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