What is a satellite?
A satellite is something that orbits, or goes 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, or computer with
anyone in the world.
Other satellites observe the world's weather, feeding weather information into computer programs that help scientists know what the weather will be.
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--an HS 376, built by Hughes Space and Communications Company. The HS 376 is used mostly for broadcast television and cable
television.
This is another, larger kind of satellite--the HS 601--which is also built by Hughes Space and Communications Company. The HS 601 is used for many purposes,
including direct broadcast TV. Direct broadcast TV is a new system of receiving television using a very small satellite dish. The television signal is relayed by an HS
601 satellite. The HS 601 also relays telephone, fax, and computer communications.
What Does a Satellite Do?
A satellite orbits Earth. It is kept in place by gravity and centrifugal force. And while it is up there going round the Earth, it helps people communicate and learn
more about our planet.
A satellite can carry a camera and take pictures of the whole Earth as it travels in its orbit. These images can be used by cartographers to make more
accurate maps. Satellite pictures can also be used to 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 on a satellite.
Satellites in orbit can send messages to a special receiver carried by someone on a ship on the ocean or in a tank 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 someone in Mexico City, your call will
be sent up in space to a satellite, then relayed to a ground station in Mexico and sent from there to the telephone in Mexico City.
A satellite can relay your computer message or your fax message as well. With the help of satellites, we can fax or e-mail 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, data.
A satellite can transmit your favorite TV program from the studio where it is made to your TV set. 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.
When words or pictures or computer data are sent up to a satellite, they are first converted to an invisible stream of energy, 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.
Satellites do many things for people. Their most important job is to help people communicate--by telephone, telegraph, television, data transmission, and
photography--with other people, wherever they are in the world.
What is an orbit?
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 of them.
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 Earth in about
an hour and a half.
MEO, or Medium Earth Orbit
Communications satellites that cover the north pole and the south pole are placed in a medium altitude, oval orbit. Receivers on the ground must track these
satellites. Because their orbits are larger than LEOs, they stay in sight of the ground receiving stations for a longer time. They orbit 6,000 to 12,000 miles above the
Earth.
GEO, or Geostationary Earth Orbit
Satellites that provide continuous communications services or weather data are placed in geosynchronous orbit at a distance from the Earth of 22,282 miles. These
satellites circle 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 are always able to "see" the receiving stations below, and a satellite in this high orbit can cover a large part of the planet; three satellites can cover
the globe, except for the parts near the north and south poles.
click to view orbital diagram (32k)
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.
Certain places in the world are especially good for launching satellites, because of weather conditions, local geography, and where the satellite is intended to go.
Satellite destination is important, because the rotation of the Earth, prevailing winds, and other conditions can either help the satellite get where it's going or make it
more difficult.
Cape Canaveral, in Florida, is one of the good places to launch a satellite. Others include Kourou, French Guiana; Xichang, China; and Tanegashima, Japan.
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.
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 low Earth orbit. Then the
spent rockets and the launch vehicle drop away, and a small "kick" motor attached to the satellite moves it into an elliptical transfer orbit. From there, another small
motor is used to push the satellite into its permanent geosynchronous orbit. A satellite takes several days after launch to reach its permanent orbit.
When the satellite reaches its orbit, a motor is used to point it in the right direction and its antennas are deployed from their traveling position so the satellite can start
sending and receiving signals.
click to view illustration of launch vehicles (48k)
Who Owns the Satellites?
Satellites are usually owned by countries or companies. The companies that own satellites usually want to make money by renting out part of the satellite to other
companies. The countries or governments that own satellites want to improve the communication networks in their countries and build a national identity.
The government of Indonesia decided that the best way to communicate with its people and teach them about their country would be to put a satellite up in space
over Indonesia and put a television set in each village. Gradually the people would learn about themselves and their country, and eventually they would use their new
knowledge to increase trade and to communicate with people in other parts of the world by satellite. So the Indonesian government asked a satellite manufacturer to
build a satellite for them, and that satellite is now in orbit over Indonesia.
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, Thailand, and the United States--are now increasing
opportunities for their people by using satellites.
Many large companies also own and operate satellites. They 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, and businesses who 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 government, you might want your satellite 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.
The area of the Earth's surface that is covered by a satellite's signal is called the satellite's footprint or beam pattern.
What's Inside a Satellite?
Satellites have a great deal of equipment packed inside them. Most satellites have seven subsystems, and each one has special work to do.
1. The propulsion subsystem includes the rocket motor that brings the spacecraft to its permanent position, as well as small thrusters (motors) that help to 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
can provide power at times when the sun isn't shining on the panels. The power is used to operate the communications subsystem. The entire communications
subsystem can be operated with about the same amount of power as would be used by 10 light bulbs.
3. The communications subsystem handles all the transmit and receive functions. It receives signals from the Earth, amplifies them, and transmits (sends) them to
another satellite or to a ground station.
4. The structures subsystem helps provide a stable framework so that the satellite can be kept pointed at the right place on the Earth's surface. 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.
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 points the spacecraft precisely to maintain the communications "footprints" in the correct location. 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 system provides a
way for people at the ground stations to communicate with the satellite.
click to view cut away satellite illustration (48k)
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 HS 376, which is
used mostly for network and cable TV, and the HS 601, which is used mostly for direct broadcast TV and business communication networks.
The HS 376 is a small, barrel-shaped structure with an antenna reflector that looks like a lid on the barrel. When the HS 376 is first launched, its
antenna reflector 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 reflector and solar panels are deployed--that is, unpacked and put in the right position
for doing their work.
A typical HS 376 is 2.16 meters (7 feet 1 inch) in diameter, and 2.82 meters (9 feet 3 inches) high, in its stowed position. When it is deployed,
its diameter is the same, but it is much taller: 6.57 meters (21 feet 7 inches) tall. The height of the deployed satellite is more than twice its height
when stowed.
The satellite body is made like a telescope; when it is deployed, an outer cylinder is driven down by tiny electric motors to reveal the inner
cylinder and locks into place. All of the outside of the satellite body is made of solar cells, which take the sun's energy and convert it to
electricity. That means that when the outside is in its full telescope position, more solar cells are exposed to the sun, and the satellite can generate
more power. The deployed HS 376 can generate more than twice as much power as the stowed HS 376.
Satellites weigh more at the beginning of life in orbit than at the end. This is because they carry rocket fuel for the thruster engines that will keep
them in place in their orbits. As the fuel is used up, the satellite gets lighter. The HS 376 weighs 634 kg (or 1395 pounds) at the beginning of its
life in orbit.
The HS 601 is a larger and more powerful satellite. When it is stowed for launch, it looks like a big box, 3.8 meters (12.6 feet) high. The HS 601 carries its solar
panels on long wing-like structures. When the satellite is deployed, the solar panels are extended to a width of 26 meters (86 feet), and the antenna reflectors make
the middle of the satellite 7.1 meters (23.3 feet) wide.
The HS 601 weighs 1727 kg (3800 pounds) at the beginning of its life in orbit.