INTRODUCTION
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 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.
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. The Boeing 601 is used for many purposes,
including direct broadcast TV. Direct broadcast TV is a system for
receiving television using a very small satellite dish. The Boeing 601 also
relays telephone, fax, and computer communications.
The most
powerful commercial satellite in the world is the Boeing 702.
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. Satellites in low earth orbit travel about 17,500 miles per hour.
These satellites can circle the whole 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. Instead of making circles around the earth,
these satellites make ovals. They orbit 6,000 to 12,000 miles above the
earth.
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. 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.
A satellite is
launched on a launch vehicle. 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
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.
FEATURES
.
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.
·
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
·
A satellite can relay your computer
message, your 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.
·
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
·
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.
What's Inside a Satellite?
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 motors that help keep the satellite in
its assigned place in 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.
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.
7.
The telemetry and command subsystem
provides a way for people at the ground stations to communicate with the
satellite.
Geosynchronous Orbit (GEO) Satellites
GEO
Satellite Definition:
A satellite in geosynchronous (or
geostationary) are positioned a fixed point at approximately 35,786
kilometers above the earth's surface. At this fixed height, the satellite
matches the Earth’s rotation speed and allows the satellites a full-disc
view at a stationary positionTo stay over the same spot on earth, a
geostationary satellite also has to be directly above the equator.
Otherwise, from the earth the satellite would appear to move in a
north-south line every day.
GEO satellites primary purpose is weather
imagery to optimize forecasting. In addition to weather imagery, these
satellites include instrumentation used in environmental monitoring
communications via a relay system. The world network of GEO satellites used
with weather imagery and environemental monitoring communications are as
follows:
Purpose
of GEO Satellites
GEO satellites provide the kind of continuous
monitoring necessary for intensive data analysis. By orbiting the
equatorial plane of the Earth at a speed matching the Earth's rotation,
these satellites can continuously over one position on the surface. Because
they stay above a fixed spot on the surface, they provide a constant vigil
for the atmospheric "triggers" for severe weather conditions such
as tornadoes, flash floods, hail storms, and hurricanes. When these
conditions develop these GEO satellites are able to monitor storm
development and track their movements.
The GEO satellite functions as a repeater of
the data back to an earth ground stations. Stevens also designs and
manufactures a GEO receive system called a Direct Readout Ground Station
(DRGS) and also provides an alternative Internet Access to the GOES data.
Advantages
of GEO Satellite Telemetry
·
Low
communications cost (Free for GOES)
·
Low
maintenance
·
Ideal
for remote locations
·
Data
easily shared among government users
·
Very
reliable data transmissions as system is supported governmental agencies
·
Available
for environmental or home-land security monitoring applications
Disadvantages of GEO
Satellite Telemetry
·
Scheduled
transmission times assigned by governmental agency and based on
Channel/Time availability
·
Interference
detection difficult
·
Troubleshooting
capabilities minimal
·
Data
is available to Government and public
·
Hardware
cost more expensive than other telemetry costs
·
One-way
transmissions
·
No
acknowledgement of successful data transmission.
·
If
a transmission fails, it cannot be repeated at a later time.
·
Primarily
available only to Federal, state or local governmental agencies or
government sponsored environmental monitoring applications.
LARGE GEO SATELLITES
Size Trends
During the past five years,
there has been a renewed emphasis on providing satellite-based services to
consumers. The acceptance of these services is determined to a great degree
by cost to the consumer, including the cost of the equipment as well as
monthly service charges. Consumer electronics benefits from competition as
well as cost decreases associated with volume manufacturing and
distribution and this is vividly demonstrated by the rapid decrease in the
cost of DBS home equipment. The power of the signal from the satellite is a
critically important factor in the determination of the cost of the ground
equipment or terminals. The more the power from the satellite, the less the
cost of the terminal. The size of the antennas and the cost of the
amplifiers decrease as the power from the satellite increases. Business
customers benefit from this increased power for the same reasons.
The need for more power and
bandwidth from commercial satellites is obvious to all the satellite
manufacturers. Typically, you would expect that increasing the power and
bandwidth from the satellite would require a larger, and thus heavier,
satellite. However, increasing the weight of the satellite adds to the cost
of the launch.
Thus today the increased demand for power
is the dominant factor in driving the development and utilization of new
GEO satellite technology, especially to meet these weight and
cost constraints. Bandwidth per satellite has been increasing as combined C
and Ku-band satellites become more common. The need for more bandwidth is
especially evident for the new data applications, which are expected to be
met with Ka-band and possibly V-band satellites. Here again, more total
power is needed to meet power per channel (or Hertz) requirements.
Other factors
driving the increased size and weight of the satellite are the needs for
larger antennas, onboard processing electronics, and intersatellite links.
The
demand for increased microwave power from the satellite is probably the
most important factor in driving the insertion of new technology into
modern GEO satellites. Higher power at the customers' antenna translates
into lower cost equipment and the availability of new services and thus the
need for the manufacture of more satellites and their associated launches.
The demand for more power from satellites is driving the development of
considerable new technology, with the requirement that this new technology
does not add to the cost of the satellite or its weight, which translates
into increased launch costs.
The power subsystem is
composed of the solar array (solar cells on the supporting structure
including pointing devices), batteries and the power conditioning
electronics. Considerable progress has been made in the last five years.
MARKET FORCES
AND FUTURE DRIVERS
MARKET FORCES
Satellites are uniquely
suited to certain applications. These include (1) broadcasting, (2) service
to mobile users (including ships, aircraft, land mobile and emergency
services), and (3) providing nearly "instant infrastructure" in
underserved areas. This last feature is the basis for a large number of
recent filings in the
This Section discusses
three telecommunication trends that are fueling interest in satellite
systems. These are direct-to-the-home television (DTH) broadcasting, or
direct broadcast satellite (DBS); the enormous growth in wireless hand-held
phone usage (cellular, personal communication services (PCS) and paging);
and the growth in the number of personal computers (PC's) in the world,
increasing numbers of which are multimedia ready and are being used to
interconnect with the Internet and/or collect information from the World
Wide Web. These three topics are treated in turn in the sections that
follow.
Direct Broadcast Satellite
The distribution of TV
signals via satellite began in the
It is widely believed that
a small size receiving antenna-something that can readily be mounted on the
side of a house, for example-is necessary to reach a large subscriber base.
Hughes has been the first to approach this market. In 1994 it launched a
high-power (~120 watts/transponder) 16-transponder satellite (DBS-1)
capable of beaming over 100 digitally-compressed TV channels to viewers,
who receive the signals with a 45 cm (18") diameter antenna and set
top box converter costing initially about $700. (Prices have since dropped
because the service providers have begun to subsidize the purchase).
Further capacity increases were achieved with the launches of DBS-2 and
DBS-3, and this service (known as DirecTV) was expecting to have over 3
million subscribers by the end of 1997. Hughes DirecTV and Stanley
Hubbard's United States Satellite Broadcasting (USSB) both use these
satellites.
Primestar, which is owned
by the five biggest
MCI and News Corp. won the
rights (at a cost of $682.5 million) to occupy the last Ku-band slot from
which to broadcast over 200 channels across the nation via a partnership
known as American Sky Broadcasting (ASkyB), but MCI has since indicated its
desire to scale back its involvement. This forced Time News to seek a
merger with Echostar, which is due to receive its powerful Echostar III
satellite (being built by Lockheed Martin) in 1998. In addition, TCI plans
to inaugurate DBS service at the end of 1997 with a high-power satellite
launched into an orbital slot it already controlled, and to use digital
compression to deliver Primestar programming to smaller dishes, as well as
to cable head ends for distribution on existing cable networks that have
limited capacity. Current expectations are that U.S. DBS subscribers will
number about 6 million by the end of 1997 and could be double this number
by the end of the year 2000.
DBS has enjoyed an even
more solid growth in
Satellite PCS
In 1996, the
For a small
number of channels this can be raised to 11 dB.
The Iridium system is being built by Motorola, together with
subcontractors (e.g., Lockheed Martin, Raytheon, COM DEV). It consists of a
fleet of 66, low earth orbiting satellites at 780 kilometer altitude.
Eleven satellites will be equally spaced in each of six, circular, nearly
polar orbits. Subscribers access the satellites via L-band spot beams (each
satellite can activate up to 48) using a TDMA scheme for transmitting
voice, coded at 2.4 kbps, or data. Each satellite can handle up to 1,100
simultaneous calls. TDMA packets arriving at a satellite are demodulated
and, depending on their destination, routed (at 20 GHz) to a gateway earth
station (if one is in view), or (at 23 GHz) to the satellite ahead or
behind in the same orbital plane, or the satellite to the east or west in
the adjacent orbital plane.
Satellites for Fixed Services
Several factors are driving
an explosion of interest in fixed satellite service (FSS) systems. These
include:
·
Strong growth in demand for telecommunications services
worldwide, and especially for data service (fueled by use of the Internet)
·
Liberalization of telecommunications markets through
deregulation and through the WTO agreements
·
The ability of satellites to provide "instant
infrastructure" requiring little in the way of civil works (which can
be expensive)
·
A number of large
·
The fact that large players, such as Motorola, have chosen to
enter the market
Proposed services include
voice, data, video, imaging, video teleconferencing, interactive video, TV
broadcast, multimedia, global Internet, messaging, and trunking. A wide
range of applications is planned through these services, including distance
learning, corporate training, collaborative workgroups, telecommuting,
telemedicine, wireless backbone interconnection (i.e., wireless LAN/WAN),
video distribution, direct-to-home video, and satellite news-gathering, as
well as the distribution of software, music, scientific data, and global
financial and weather information.
|
PORTABLE AND MOBILE TERMINALS FOR
MULTIMEDIA & BUSINESS USE
Satellite-based multimedia
service for the consumer is an important part of the business plans of many of
the Ka-band systems now under construction. This has developed as an important
activity since the 1992/1993 study. Research programs like ACTS, Japan's
program in highly intelligent communications, Italy's ITALSAT, and the European
DIGISAT, ISIS and MMIS projects, demonstrate that the feasibility of
satellite-based interactive multimedia services, are laying the necessary
groundwork. The development of portable and mobile terminals for these
applications should proceed rapidly along an evolutionary path since--except
for reducing terminal size and cost--few hardware innovations are involved.
LAUNCH
SYSTEMS
The increased use of commercial
satellites to meet the burgeoning worldwide market for telecommunications has
placed increased demands on the launch service industry. The capacity of this
industry will not be adequate to meet the needs of all the proposals for new
satellites. Even though not all the proposals will get to the marketplace,
there appears to be a shortage of launch capacity. In addition, this industry
has new challenges to meet. In contrast to the past when most of the commercial
satellites were placed into GEO, new satellites will also be placed into LEO
and MEO. These latter orbits will be used by constellations of satellites
requiring the launch of numerous satellites at a time and the launch of
satellites to replace failed satellites, with little lead time. In addition,
there is considerable pressure on the launch industry to make a considerable
decrease in the price of entrance into space as well as to increase the
reliability of the launches, a point that has been watched with considerable
interest by the investment banking community.
Background
Ten GEO launches per year were
adequate to satisfy the satellite communications business a few years ago. It
is now up to thirty and appears that it will increase to almost 100 during the
next decade. Launching to LEO will soon exceed launches to GEO. Constellations
composed of many satellites, in some cases over one hundred, will put pressures
on the industry for timely launches. The launch of commercial satellites is no
longer the sole province of the United
States . Europe
(Arianespace), Russia ,
China ,
Japan
and the Ukraine
have entered this business, with Arianespace replacing the United States
as the dominant launch provider. No longer do satellite manufacturers and
service providers purchase a launch at a time. They purchase blocks of launches
from numerous vendors to ensure the availability of launches when needed. The
launch industry has then responded positively to these assured future orders by
upgrading the capability of existing rockets and by proposing new launch
systems. An example of bulk ordering is the 1997 Hughes purchase of 5 launch
options aboard the Chinese Long March rocket. Hughes followed this up with the
purchase of 10 launches from Japan 's
Rocket System Corporation. Space Systems Loral followed a similar path and
purchased several launches on the Proton from International Launch Services.
The inaugural contracts by Hughes have been important factors in establishing
the viability of the Boeing (McDonnell Douglas) Delta III and Sea Launch as
well as the Japan H-IIA and the upgrading of the Proton launch facilities. The
July 1997 Motorola RFP to provide launch services for its new Celestri system
(since cancelled), Iridium replacements, Iridium follow-ons and other
satellites totaled 516 satellites, quite an impressive number.
CONCLUSION
Satellites
must cope with the high temperatures of sitting in direct sunlight, and the
near absolute-zero temperatures they drop down to in the shadow of the earth.
The very fact that they move rapidly between the two extremes in temperature
means their lifespans are very short. As if this were not enough, they must
cope with the solar wind, which creates a buildup of static electricity on the
satellite.
All these
factors contribute to requiring that the components of the satellite be very
durable and therefore very, very expensive. This is why very few companies and
governments operate satellites, and only a handful of companies build them.
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