|What Is a Satellite?|
1 What Is a Satellite?
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.
Some satellites send and receive telephone, fax, and computer communications.
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 show 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.
This is one kind of satellite—a Boeing 376, built by Boeing Satellite
Systems. The Boeing 376 is used mostly for broadcast television
and cable television.
This is another, larger kind of satellite—the Boeing 601—
which is also built by Boeing Satellite Systems. The Boeing
601 is used for many purposes, including direct broadcast
TV, such as DIRECTV. Direct broadcast TV is a system for
receiving television using a very small satellite dish. The television
signal is relayed by a Boeing 601 satellite. The Boeing 601 also relays
telephone, fax, and computer communications.
The most powerful commercial satellite in the world is the Boeing 702.
Designed and built by Boeing Satellite Systems, this giant has a
wingspan of nearly 133 feet—more than a Boeing 737 jet plane.
© Boeing Satellite Systems, Inc. • 04/01 • 1000 • BSS 010196
GEO, or Geostationary Earth Orbit
A satellite in geosynchronous orbit circles the
earth in 24 hours—the same time it takes the
earth to rotate one time. If these satellites are
positioned over the equator and travel in the
same direction as the earth rotates, they appear
"fixed" with respect to a given spot on earth—
that is, they hang like
lanterns over the same spot
on the earth all the time.
Satellites in GEO orbit 22,282
miles above the earth. In this
high orbit, GEO satellites are
always able to "see" the
receiving stations below, and
their signals can cover a
large part of the planet.
Three GEO satellites can
cover the globe, except for
the parts at the North and
2 What Is an Orbit?
LEO, or Low Earth Orbit
A satellite in low earth orbit circles the earth
100 to 300 miles above the earth’s surface.
Because it is close to the earth, it must
travel very fast to avoid being pulled out of
orbit by gravity and crashing into the earth.
Satellites in low earth orbit travel about
17,500 miles per hour. These satellites can
circle the whole earth in about an hour and
MEO, or Medium Earth Orbit
Satellites circling 6,000 to
12,000 miles above the
earth are in mediumaltitude
orbit. In these
larger orbits they
stay in sight of a
station for 2 hours
or more, compared
to about 10 minutes
for LEOs. It takes
from 4 to
8 hours to
When a satellite is launched, it
is placed in orbit around the
earth. The earth’s gravity
holds the satellite in a certain
path as it goes around the
earth, and that path is called
an "orbit." There are several
kinds of orbits. Here are three
How Does a Satellite Get Into Space?
A satellite is launched on a launch vehicle, which is like a taxicab for satellites. The satellite
is packed carefully into the vehicle and carried into space, powered by a rocket engine.
Satellites are launched from only a few places in the world, primarily Cape Canaveral,
Florida; Kourou, French Guiana; Xichang, China, and Baikonur, Kazakstan. The best places
to launch satellites are near the ocean, so that when the launch vehicle falls away, it lands
in the water and not on people.
Another launch site actually travels
to the perfect launch spot. The
Sea Launch company rebuilt a big
platform once used for oil drilling
at sea. The platform carries satellites
from Long Beach, California,
to the equator, far out in the
Pacific Ocean, where its rocket
Putting everything together for a launch is very complicated. Many people in many companies
and sometimes in many countries have to work together and coordinate their work so
that everything will be ready for a launch. One of the ways things are coordinated is the
countdown. In a countdown, we count down instead of up, because we are counting hours
or minutes until liftoff—the moment when the rockets fire and the launch vehicle rises into
the air. The last ten seconds of the countdown sound like this: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1,
liftoff! But the countdown starts long before the day of the launch. Everyone who participates
in the launch has a schedule, and knows what he or she should be doing at 144
hours until liftoff, or 64 hours, or 7 hours, or 10 minutes. That’s how the launch planners
make sure that everything will be ready at the right time.
At launch, the launch vehicle’s rockets lift the satellite off the launch pad and carry it into
space, where it circles the earth in a temporary orbit. Then the spent rockets and the
launch vehicle drop away, and one or more motors attached to the satellite move it into its
permanent geosynchronous orbit. A motor is started up for a certain amount of time,
sometimes just one or two minutes, to push the satellite into place. When one of these
motors is started, it’s called a "burn." It may take many burns, over a period of several
days, to move the satellite into its assigned orbital position.
When the satellite reaches its orbit, a motor points it in the right direction and its antennas
and solar panels deploy—that is, they unfold from their traveling position and spread out so
the satellite can start sending and receiving signals.
The earth, the moon, and all the planets are held in place in the solar system
by two opposing forces. One is called gravity. Gravity is a force that pulls
objects toward a planet or other large object. When you hold a book in
the air and let go, gravity pulls it toward the earth and it falls.
The other force is called centrifugal force.
It makes a rotating object fly outward
from the center. If you ride a merrygo-
round very fast, and you fall off
your horse, you will probably fall on
the outside of the circle because of
In our solar system, nine planets revolve around the sun. Gravity and centrifugal force must
be in balance for the planets to stay in their positions. If gravity stopped working, the planets
would fly off on their own into space. If centrifugal force stopped working, the planets
would be pulled toward the sun. The two forces are balanced, so the planets remain in
The sun is a star. Its natural satellites, including the earth,
are called planets. The earth and the other planets
revolve around the sun. Some planets have natural
satellites, called moons. Our moon revolves around
The same two forces keep the moon in its place as it
revolves around the earth: the earth’s gravity keeps
the moon from flying away into space, and centrifugal
force keeps the moon from being pulled down to the
The sun and the earth are not the only sources of gravity. Everything in space has its own
gravity. Large objects like planets have stronger gravity, and small objects like meteors
have weaker gravity. So just as earth’s gravity holds the moon in position, the moon’s gravity
holds in place a spaceship that lands on the moon.
The moon is smaller than the earth, so the pull of its gravity is weaker than that of the earth.
If you went to the moon, you would not weigh as much as you do on the earth—but you
would still weigh enough to keep from falling off the moon.
4 When a Spaceship Lands on the Moon,
Why Doesn’t It Fall Off?
What Does a Satellite Do?
Satellites do many things for people. Their most important job is helping people
communicate with other people, wherever they are in the world.
• A satellite can carry a camera as it travels in its orbit and take pictures of the whole
earth. Mapmakers can use these pictures to make more accurate maps. Satellite pictures
can also help experts predict the weather, because from the satellite, the camera
can actually see the weather coming. When you watch the weather forecast on TV, you
are seeing 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 message, 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, or sending, information
• 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 Inglewood. 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
called 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
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.
How Big Is a Satellite?
Different kinds of satellites are used in different situations, for different purposes. To
talk about the sizes of satellites, we’ll use two examples: the Boeing 601, which is
used mostly for direct broadcast TV and business communication networks, and the
Boeing 702, which is used mostly for video distribution, satellite telephone and
Internet services, and digital radio.
The Boeing 601 has a box-shaped center with several antenna
reflectors that look like big dinner plates. Long, wing-like structures
attach on two sides. These are the solar panels. The outside
of the solar panels is covered with solar cells, which convert
the sun’s energy to electricity. The bigger the panels, the more
solar cells are exposed to the sun, so the satellite can generate
When the Boeing 601 is first launched, its antenna reflectors and
solar panels are stowed—that is, put away—so it can fit inside a
launch vehicle. After launch, the satellite travels through space
until it reaches its assigned orbital position. Then its reflectors
and solar panels deploy—that is, open or unfold into the right
position for doing their work. A stowed Boeing 601 is 12.6 feet
(3.8) meters high. When the satellite is deployed, the solar panels
extend to a width of 86 feet (26 meters).
Satellites weigh more at the beginning of life (BOL) in orbit than at the
end. This is because they carry fuel for the thruster engines that keep
them in place in their orbits. As the fuel is used up, the satellite gets
lighter. The average Boeing 601 weighs 3800 pounds (1727 kg) at its
BOL in orbit. Satellites are built to weigh as little as possible, because
lifting a satellite into orbit costs about $15,000 per pound. Some satellites
use special electric thrusters called XIPS, short for xenon ion
propulsion system. XIPS fuel is a gas that weighs much less than
ordinary liquid fuel, yet XIPS is ten times more efficient.
The typical Boeing 702 is a larger, more powerful satellite. With a 702,
you can put two or three satellites’ worth of communication electronics
in orbit using one satellite and one launch. When the Boeing 702 is
stowed for launch, it is 23 feet (7 meters) high. When the satellite is
deployed, the solar panels extend to a width of 132.5 feet (40.4 meters).
The average Boeing 702 weighs 6505 pounds (2950 kg) at its BOL in orbit.
Who Owns the Satellites?
Satellites are usually owned by companies or countries. The companies that own satellites
usually want to make money by renting out part of the satellite to other companies.
The countries or government agencies that own satellites want to make people’s lives
better by improving the communication networks in their countries.
For example, Indonesia is a country made up of 13,677 islands whose people speak
more than 250 languages. Imagine how much time and money it would take to connect
them all with wires and telephone poles. Using satellites, Indonesia bridged all the
islands at once and helped people learn the national language.
Being able to communicate better with people all over the world helps countries develop
trading opportunities, increase business, and get information they need. Many countries—
including Australia, Brazil, Canada, China, Japan, Luxembourg, Malaysia, Mexico,
Norway, Sweden, Thailand, and the United States—are now increasing opportunities for
their people by buying satellites.
Many large companies also own and operate satellites. They may rent space on the
satellite to other companies and businesses. For example, a large communication company
might buy a satellite and then rent space on the satellite to television companies,
telephone companies, Internet service companies, and businesses that want to do business
in other parts of the world.
A satellite operator can let its satellite "see" as much as one-third of our planet at a
time, or it can shape the signal to reach a smaller area. For example, if you were the
Indonesian telecommunications company, you might want your satellite signal to cover
only Indonesia, and not spill over into other countries. The satellite signal can be
shaped to cover the exact area that the operator wants to reach. Some satellites can
divide their signal into many movable spot beams. Like a tight, bright flashlight beam, a
spot beam can be aimed at an exact small area on earth, then moved elsewhere to
reach new or different customers.
The area of the earth’s surface covered by a satellite’s signal is called the
What’s Inside a Satellite?
Satellites have a great deal of equipment packed inside them. A satellite has seven
subsystems, and each one has its own work to do.
1. 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.
2. 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. A Boeing 702 generates enough power at the end of its service
life to operate two hundred 75-watt light bulbs.
3. 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.
4. The structures subsystem distributes the stresses of launch and acts as a strong,
stable framework for attaching the other parts of the satellite.
5. 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.
6. 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.
7. 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.
ZENIT Sea Launch
Intelsat 707; Thor 1, 2A, 3
Intelsat 511 (I)
Marisat 2 (F3) (I)
PAS-3R, 6, 6B
Telstar 11 (Orion 1)
New Skies 806
GE Columbia 515
Marecs B2 (I)
Telecom 2A, 2D
Eutelsat II F-2
Nilesat 101, 102
SBS 6, Galaxy VI
Brasilsat A2 (I), B4
Inmarsat III F-3
Commercial Communications Satellites
in Geosynchronous Orbit
Insat 2E/Intelsat APR-1, 3B
GE Spacenet 4
Optus A2 (I), A3 (I)
Superbird B1, B2 (4)
Optus B3, Leasat F-5 (I)
New Skies 513
GE Satcom C5, GE-8/Aurora III
PAS Brasilsat A1 (I)
GE Satcom C1, GE-7
GE Satcom C4
GE Satcom C3
Morelos II (I)
Echostar V, Sirius 2
Asiasat 3S, Asiastar
BS-3N; BSAT-1A, B;
Inmarsat II F-4
Palapa C2, Koreasat 2
Anik E2, F1
GE Gstar 4
Inmarsat II F-2
Galaxy 3R, VIII-i
Inmarsat III F-4
Hispasat 1A, 1B, 1C
New Skies K, 803
Inmarsat III F-2
Sirius 2/GE-1E, 3
Eutelsat W1 (R)
Sirius W; Hot Bird 1, 2, 3, 4, 5
DFS Kopernikus 1 (F-3)
New Skies 703
Inmarsat III F-1, Intelsat 804
Inmarsat II F-3 (I)
Eutelsat I F-4 (I)
Inmarsat III F-5
HGS 5 (I) (aka SBS 4)
Nahuel -1, GE-6
JCSat 3, R(4)
LMI AP 1 (Gorizont 29)
JCSat 4A (6)
LMI AP 2 (Gorizont 30)
42.0°ETurksat-1C, GALS 1, EurasiaSat 144.0°E
Europe*Star B (aka Koreasat 1), 148.0°E
Eutelsat II F-1 (I)
Thaicom 2, 3
DirecTV-2, -3, 1R;
JCSat 1B (5)
Astra 1A, 1B, 1C, 1E, 1F, 1G, 1H
Eutelsat II F-3
Arabsat 2A, 3A
Sirius 1 (CD Radio)
119.0°WEchostar I, II, IV, VI; DirecTV-6
Insat 2A (I), 1D (I)
Inmarsat II F-1 (I)
TDRS-6 (GE Columbia)
Apstar 2R (Telstar 10)
Brasil 1(T) (aka Anik C1)
Sirius 3 (CD Radio)
Galaxy XI, Nimiq
GE Satcom Ku-2 (I),
GALS 2; Eutelsat W4, SESAT
Astra 1D, 2A, 2B, 2D
Eutelsat II F-4
DFS Kopernikus 2 (F-2)
108.0°EPalapa B2R, Cakrawarta 1,
Telkom 1, GE-1A
Telstar 12 (Orion 2)