SATELLITE COMMUNICATION OVERVIEW

ABSTRACT.:
Satellite communication, in telecommunications, the use of artificial
satellites to provide communication links between various points on Earth.
Satellite communications play a vital role in the global telecommunications
system. Approximately 2,000 artificial satellites orbiting Earth relay analog
and digital signals carrying voice, video, and data to and from one or many
locations worldwide.
Satellite communication has two main components: the ground segment,
which consists of fixed or mobile transmission, reception, and ancillary
equipment, and the space segment, which primarily is the satellite itself. A
typical satellite link involves the transmission or up linking of a signal from
an Earth station to a satellite. The satellite then receives and amplifies the
signal and retransmits it back to Earth, where it is received and reamplified
by Earth stations and terminals. Satellite receivers on the ground include
direct-to-home (DTH) satellite equipment, mobile reception equipment in
aircraft, satellite telephones, and handheld devices.
SATELLITE COMMUNICATIONS - AN OVERVIEW
INTRODUCTION
The outer space has always fascinated people on the earth and
communication through space evolved as an offshoot of ideas for space
travel. The earliest idea of using artificial satellites for communications is
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found in a science fiction Brick Moon by Edward Evert Hale, published in
1869-70. While the early fictional accounts of satellite and space
communications bear little resemblance to the technology as it exists to day,
they are of significance since they represent the origins of the idea from
which the technology eventually evolved. In the area of satellite
communications, the technology has been responsive to the imaginative
dreams. Hence it is also expected that technological innovations will lead
the evolution of satellite communications towards the visions of today.
CONCEPT OF SATELLITE COMMUNICATIONS
Scientists from different countries conceived various ideas for
communications through space along with the technological breakthroughs
in different fields of science. The Russian scientist Konstantin Tsiolkovsky
(1857-1935) was the first person to study space travel as a science and in
1879 formulated his Rocket Equation, which is still used in the design of
modern rockets. He also wrote the first theoretical description of a man-
made satellite and noted the existence of a geosynchronous orbit. But he did
not identify any practical applications of geosynchronous orbit. The noted
German Scientist and rocket expert, Hermann Oberth, in 1923 proposed
that the crews of orbiting rockets could communicate with remote regions
on earth by signaling with mirrors. In 1928, Austrian Scientist Hermann
Noordung suggested that the geostationary orbit might be a good location
for manned space vehicle. Russian Scientists in 1937 suggested that
television images could be relayed by bouncing them off the space vehicles.
During 1942-1943, a series of articles by George O Smith were published in
Astounding Science Fictions concerning an artificial planet, Venus
Equilateral, which functioned as relay station between Venus and Earth
Station when direct communication was blocked by Sun. However, Arthur
C. Clarke, an electronic engineer and the well-known science fiction writer
is generally credited with originating the modern concept of Satellite
Communications. In 1945, Clarke, in his article `Extra Terrestrial
Relays: Can Rocket Stations give Worldwide Radio Coverage?’
published in Wireless World outlined the basic technical considerations
involved in the concept of satellite communications. Clarke proposed
orbiting space stations, which could be provided with receiving and
transmitting equipment and could act as a repeater to relay transmission
between any two points of the hemisphere beneath. He calculated that at
an orbital radius of 42,000 km. the space station’s orbit would coincide
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with the earth’s rotation on its axis and the space station would remain fixed
as seen from any point on the earth. He also pointed out that three such
synchronous stations located 120 degrees apart above the equator could
provide worldwide communications coverage. The concept was later
considered to be generating a billion dollar business in the area of
communications. However, Clarke did not patent the most commercially
viable idea of twentieth century as he thought satellites would not be
technically and economically viable until the next century.
REALISATION OF CONCEPT TO REALITY
In October 1957, the first artificial satellite Sputnik -I was launched by
former Soviet Russia in the earth’s orbit and in 1963 Clark’s idea became a
reality when the first geosynchronous satellite SYNCOM was successfully
launched by NASA.
The realization of the concept of satellite communications from an idea to
reality has been possible due to a large number of technological
breakthroughs and practical realization of devices and systems, which took
place during and after the World War II. The pressures of international
military rivalry during cold war period were also able to a great extent to
push scientific and technological research and development far faster than it
would have been possible if applied for peaceful purposes.
The successful launching of communications satellite in earth’s orbit was
possible because of keen interests shown by specific groups of people along
with the developments in diverse areas of science and technology. Some of
these factors, which are considered important in the realization of satellite
communications, are:
• Development of high power rocket technology and propulsion
systems capable of delivering satellites in high altitude orbits
• Scientific and military interests in Space Research
• Development of Transistors and miniaturization of electronic
circuitry.
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• Development of Solar Cells for providing sustained energy source
for the satellite.
• Development of high-speed computers for calculating and tracking
orbits.
• Government support in large-scale financial commitment to Space
Technology Development for Military, Scientific Experiments and
Civilian Applications.
• International military rivalry among super powers.
• The psychological impact of Sputnik Challenge leading to long
range program of scientific research and development undertaken
by US.
Before the transformation of the concept of communications by satellite to
blue print and subsequent development of the hardware took place it was
necessary to make the scientific communities convinced about the technical
feasibility of such a system. In US J.R. Pierce, of Bell Laboratories initiated
this by promoting the idea of transoceanic satellite communications within
the scientific and technical communities. In 1955 Pierce in a paper entitled
Orbital Radio Relays proposed detailed technical plan for passive
communications satellites, disregarding the feasibility of constructing and
placing satellites in orbit. He proposed three types of repeaters.
• Spheres at low altitudes
• A plane reflector
• An active repeater in 24 Hr. orbit
Pierce concluded his paper with a request to the scientific community to
develop rockets capable of launching communications satellite. Fortunately,
scientific and military interest in rocketry after World War II contributed in
the development of a number of rockets like Atlas, Jupiter and Thor rockets
in US and different multistage rockets in former USSR that ultimately made
the launching of satellites in orbit possible.
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On Oct. 4, 1957, Sputnik-1 was launched as part of Russia’s program for
International Geophysical Year. The launching of Sputnik marks the dawn
of the space age and the world’s introduction to artificial satellite. Mass of
Sputnik was only 184 lbs. in an orbit of 560 miles above the earth. It
carried two radio transmitters at 20.005 MHz and 40.002 MHz. However
this space craft was far more than a scientific and technical achievement as
it had a tremendous psychological and political impact particularly on
United States resulting in a technological competition between United
States and Russia, long term planning in Space Research and establishment
of NASA.
Four months after the launch of Sputnik, US Explorer-1 was launched in
January 1958 by a Jupiter rocket and the space race between Russia and US
began.
EVOLUTION OF COMMUNICATION SATELLITES
During early 1950s, both passive and active satellites were considered for
the purpose of communications over a large distance. Passive satellites
though successfully used in the early years of satellite communications, with
the advancement in technology active satellites have completely replaced
the passive satellites.
Passive Satellites
The principle of communication by passive satellite is based on the
properties of scattering of electromagnetic waves from different surface
areas. Thus an electromagnetic wave incident on a passive satellite is
scattered back towards the earth and a receiving station can receive the
scattered wave. The passive satellites used in the early years of satellite
communications were both artificial as well as natural.
In 1954, the US Naval Research Laboratory successfully transmitted the
first voice message through space by using the Moon to scatter radio signal.
These experiments resulted in the development of Moon-Relay System,
which became operational in 1959 for communications between
Washington, DC and Hawaii and remained operational till 1963.
The first artificial passive satellite Echo-I of NASA was launched in August
1960. Echo-I was 100-ft. diameter inflatable plastic balloon with aluminum
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coating that reflected radio signals transmitted from huge earth station
antennas. Echo-I had an orbital height of 1000 miles. Earth Stations
across US and Europe picked up the signal and contributed a lot in
motivating research in communication satellite.
Echo-I was followed by Echo-II in 1964. With Echo-II, Scientists of US
and Soviet Russia collaborated for the first time on international space
experiments. Signals were transmitted between University of Manchester
for NASA and Gorki State University in Russia. The orbit of Echo-II was
600 to 800 miles.
In 1963, US Air Force under Project West Ford launched an orbital belt of
small needles at 2000 miles height to act as a passive radio reflector.
Speech in digitized form was transmitted intelligently via this belt of
needles. However, further work in this area was discontinued due to strong
protests from the astronomers.
Although passive satellites were simple, the communications between two
distant places were successfully demonstrated only after overcoming many
technical problems. The large attenuation of the signal while traveling the
large distance between the transmitter and the receiver via the satellite was
one of the most serious problems. The disadvantages of passive satellites
for communications are:
• Earth Stations required high power (10 kW) to transmit signals
strong enough to produce an adequate return echo.
• Large Earth Stations with tracking facilities were expensive.
• Communications via the Moon is limited by simultaneous
visibility of the Moon by both the transmit and the receive
stations along with the larger distance required to be covered
compared to that of closer to earth satellite.
• A global system would have required a large number of
passive satellites accessed randomly by different users.
• Control of satellites not possible from ground.
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Active Satellites
In active satellites, which amplify and retransmit the signal from the earth
have several advantages over the passive satellites. The advantages of
active satellites are:
• Require lower power earth station
• Less costly
• Not open to random use
• Directly controlled by operators from ground.
Disadvantages of active satellites are:
• Disruption of service due to failure of electronics components
on-board the satellites
• Requirement of on-board power supply
• Requirement of larger and powerful rockets to launch heavier
satellites in orbit
World’s first active satellite SCORE (Satellite Communication by Orbiting
Relay Equipment) was launched by US Airforce in 1958 at orbital height of
110 to 900 miles. It transmitted a pre-recorded message of Christmas
Greetings from US President Eisenhower. However, the satellite did not
function as a true repeater.
The first fully active satellite was Courier launched into an orbit of 600 -
700 mile, by Department of Defense in 1960. It accepted and stored upto
360,000 Teletype words as it passed overhead and rebroadcast them to
ground station farther along its orbit. It operated with 3 watts of on-board
output power and it was also the first satellite to use solar cells for
generating electrical power.
In July 1962 AT&T’s active satellite Telstar was developed and launched.
Telstar was placed in an elliptical orbit with orbital height of 682 to 4030
miles circling the earth in 2 hours and 40 min. Through Telstar, the first live
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transatlantic television was transmitted. Voice, television, fax and data
were transmitted between various sites in UK, France, Brazil Italy and US
at 6/4 GHz frequency range.
Relay-I satellite of RCA & NASA, was the first satellite to carry redundant
system for increasing the reliability. Telephone & Television signals were
transmitted to Europe, South America and Japan. Frequency bands of
4.2/1.7 GHz and orbit heights of 942 to 5303 miles were used.
Syncom, the first geosynchronous satellite of NASA was built by Hughes
Aircraft Co. and was launched in July 1963 and was used for conducting
many experiments. Most famous of the series Syncom-III was launched in
1963 and was used to transmit Tokyo Olympic games to United States,
demonstrating the commercial market for space technology. Syncom-I and-
II were used by Department of Defense for military purpose. The Syncom
Satellites marked a turning point in the development of Satellite
Communications as most of the commercial satellites that followed were
designed to operate from geosynchronous orbit.
Table-1 gives the major milestones of Space Radio Communications events,
prior to the start of commercial satellite communications service by
INTELSAT.
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TABLE - 1
MAJOR MILESTONES OF SPACE RADIO COMMUNICATIONS
Category Year Activity Person/Agency
/ Country.
Geostationar
y concept
1945 Suggestion of Geostationary
satellite communication
feasibility.
A. Clark
( U.K )
Moon
Reflection
1946 Detection of Lunar Echo by
Radar
J. Mofenson
(U.S.A.)
1954 Passive relaying of voice by
moon reflection.
J.H. Trexler
( U.S.A. )
1960 Hawaii-Washington, D.C.
Communication by Moon
Reflection.
U.S.A. Navy.
Low altitude
orbit.
1957 Observation of signals from
Sputnik -1 Satellite.
U.S.S.R., Japan
and others.
1958 Tape-recorded voice
transmission by Satellite
SCORE.
U.S.A. Air
Force.
1960 Meteorological facsimile Trans
mission by Satellite Tiros-1.
U.S.A. NASA
1960 Passive relaying of telephone
and television by Satellite
Echo-1.
U.S.A. Army.
1960 Delayed relaying of recorded
voice by Satellite Courier-1B.
U.S.A. Army.
1962 Active transatlantic relaying of
communication by Satellite
Telstar-1.
U.S.A., U.K.,
France.
1962 Communication between
manned Satellites Vostok-3
and 4; Space television
transmission.
U.S.S.R.
1963 Scatter communication by tiny
needles in Orbit.
( West Ford Project 6 )
U.S.A. MIT.
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1963 Active transpacific relaying of
Relay 1.
U.S.A. NASA,
Japan.
Synchronous
Satellite.
1963 USA-Europe-Africa
Syncom 2.
U.S.A. NASA
1964 Olympic Games television
relaying by Satellite Syncom 3
U.S.A., NASA
Japan.
1965 Commercial Communication
(Semi-experimental) by
Satellite Early Bird.
INTELSAT.
SATELLITE COMMUNICATIONS SYSTEMS
Historically, commercial operational satellite communications systems were
developed after having working experience with a large number of
experimental satellite systems launched to demonstrate the various aspects
of satellite communications. Initially the commercial satellite
communications systems were meant for meeting the needs of international
transoceanic communications. The trend was for establishing only a few
large earth stations in any country for overseas communications. In the early
years of satellite communications, the earth stations were large due to low
transmit power available from the satellites. Over the years the trend has
been reversed as with the advancement of technology, higher transmitted
power is available from the satellites. This has reduced the size and the cost
of the earth stations. Thus the trend is now on the use of thousands of small
earth stations and portable hand held terminals, for meeting the various
specialized communications needs. Moreover, apart from international
system a number of Regional, Domestic, and military systems are now in
operations worldwide. From the traffic point of view, emphasis was initially
more on point-to-point telephone, telex etc, and to some extent on
Television broadcasting. The present trends however are on Direct To
Home television broadcasting and VSAT based data communications using
small antenna systems deployed on rooftops or on one’s backyards. Finally,
it is expected that the satellite communications will meet the ultimate goal
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of hand held personal communications of voice and data for anyone from
anywhere and anytime.
Different types of Satellite Communications Systems are:
• Experimental
• International
• Regional
• Domestic
• Military
• Navigational and Radio Determination
• Personal Communications System
• Broadband Satellite System
Experimental Satellite Communications Systems
For the purpose of test and evaluation of new technologies a number of
satellites have been designed and operated for technical experiments.
Various experiments have also been conducted using these satellites for
demonstrating different applications of communications satellites.
Prominent among these experimental satellites are:
• Applications Technology Satellite Series (ATS-1, ATS-3,
ATS-5 & ATS-6) of NASA.
• Joint Canadian - US Communications Technology Satellite
(CTS or Hermes)
• Advanced Communications Technology Satellite (ACTS) of
NASA.
• APPLE (Ariane Passenger Payload Experiment) Satellite of
India.
• Symphonie Satellite (France & Germany).
• SIRIO (Italy)
• LES (US military)
• OTS (ESA)
• JBS, CS (Japan)
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International Satellite Systems
Currently only a part of the world’s long distance telecom traffic is handled
by different international satellite communications systems. However, for
international broadcasting of television there is no alternative to satellite
communications. Examples of various international satellite systems are:
• INTELSAT
• New Skies Satellites
• PanAmSaT
• INTERSPUTNIK
• INMARSAT
• COSPAS-SARSAT
INTELSAT: Recognizing that Satellite Communications would be an
important means for international cooperation, in July 1961, President
Kennedy of US invited all nations to participate in a communication satellite
system in the interest of world peace and brotherhood among peoples
throughout the world.
In Dec. 1961, UN endorsed the US proposal regarding the desirability of a
global system of communication satellites because it could
• Forge new bonds of mutual knowledge and understanding
between people everywhere
• Offer a powerful tool to improve literacy and education in
developing areas
• Support world weather services by speedy transmittal of data
• Enable leaders of nations to talk face to face on a convenient
and reliable basis
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The UN unanimously adopted General Assembly Resolution, which stated
that:
`Communications by means of satellite should be available to the nations
of the world as soon as practicable on a global and nondiscriminatory
basis’.
In August 1962, US Government passed Communications Satellite Act. Its
purpose was to establish a commercial communications system utilizing
satellites, which would serve the needs of the US and other countries and
contribute to world peace and understanding. The significance of the choice
of a single system for international communications is economic, technical
and political.
In August 1964, the final negotiations for the international satellite system
were completed and nineteen nations became the founding members of
International Telecommunications Satellite Organization (INTELSAT) with
Headquarters in Washington D.C, USA. These nineteen nations are
Australia, Austria, Belgium, Canada, Denmark, France, Germany, Ireland,
Italy, Japan, the Netherlands, Norway, Portugal, Spain, Sweden,
Switzerland, the United Kingdom, United States and Vatican City. Over the
years the number of member governments grew to 144.
In April 1965, Early Bird (INTELSAT-I) was launched starting the
commercial international satellite services. Within four years the
INTELSAT system grew from the single transatlantic link to the global
network with high capacity INTELSAT satellites positioned over the
Atlantic, Pacific and Indian Ocean Regions. The 240 voice circuit capacity
of the Early Bird is miniscule in comparison to the channel capacity of the
latest INTELSAT satellites which caters to tens of thousands of telephone
channels in addition to providing TV, data, fax, telex and Internet services
to more than 200 countries and territories.
With the improvement in life of the satellites, introduction of latest
communication techniques, and the availability of more channel capacity,
the tariff of Intelsat has also been reduced considerably over the years.
In the 1960’s at the time of establishment of Intelsat, the satellite
Communication Industry was not well developed. The international
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telecommunications was also not considered suitable for handling by private
companies. However, the skepticism changed after successful privatization
of telecommunications departments in many countries during the last few
decades of the twentieth century. Since in highly competitive
telecommunications market, private enterprises are in a position to provide
better and cheaper services compared to the international organizational set
up of Intelsat, ideas for privatization of Intelsat were mooted. Privatization
places Intelsat on a level playing field to better address opportunities of the
telecommunications marketplace. Streamlined decision-making is expected
to make it easier to expand the business and introduce new services.
Considering these, in November 2000, the representatives of all member
governments of Intelsat unanimously approved a plan to privatize Intelsat.
The approved plan endorses the transfer of all assets, liabilities and
operations to a private Bermuda based company known as Intelsat Ltd.,
and its 100 % subsidiaries. In accordance with its heritage as a global
satellite communications services provider to all countries, Intelsat Ltd. will
continue to honour a clear set of public service commitments on a
commercial basis. These include
• Global coverage and global connectivity
• Service to “lifeline” customers around the world
according to specific Lifeline Connectivity Obligation
Contracts
• Non-discriminatory access to the Intelsat Ltd. Satellite
fleet
A small separate and independent inter governmental office will monitor the
private company’s implementation of these public service commitments.
Privatization of INTELSAT is expected to be completed in 2001.
New Skies Satellites N.V (New Skies) is formed through the partial
privatization of Intelsat. It is a wholly independent satellite service provider
starting its services through five in orbit satellites transferred from Intelsat
fleet. New Skies, with headquarters at The Hague, Netherlands, began
operations as a commercial spin off from Intelsat in November 1998, with
three satellites in Atlantic Ocean Region, one each in the Indian & Pacific
Ocean Region and appropriate ground facilities around the world. These
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satellite and the ground facilities provide complete global coverage at C
band and high powered Ku band spot beams over most of the world’s
principal population centers. It offers video, voice, data and Internet
communications links for broadcast networks, telephone carriers, enterprise
customers and ISPs.
PanAmSat Corporation, based in Greenwich, Conn. / USA, is a leading
provider of global video and data broadcasting services via satellites. The
company builds, owns and operated networks that deliver entertainment and
information to cable television systems, TV broadcast affiliates,
telecommunications companies and corporations with a large fleet of 21
satellites in orbit as on 2001.
INTERSPUTNIK: The Intersputnik International Organization of Space
Communications with headquarters at Moscow was established in 1971,
according to an intergovernmental agreement submitted to UN, for
operating a global satellite communication system. However, Intersputnik
became operational only in 1974 with Molniya and Statsionar Satellites for
providing telephony, telegraph, radio, data, telex and direct broadcasting of
television. Initially Intersputnik had nine member countries, which has
grown to twenty-three over the years. These are Afghanistan, Belarus,
Bulgaria, Cuba, Czech Republic, Georgia, Germany, Hungary, Kazakhstan,
Kyrghizstan, DPR Korea, Laos, Mongolia, Nicaragua, Poland, Tajikistan,
Turkmenistan, Romania, Russia, Syria, Ukraine, Yemen, Vietnam.
Intersputnik’s user base exceeds 100 state run and private companies.
Due to the changes in the geopolitical conditions in the 1990’s and rapid
development of telecommunications market with the introduction of new
services created severe competitions among the telecom service providers.
Under the changed circumstances, Intersputnik reviewed its strategy to get
adapted to the dynamically developing environment. In order to keep the
competitive edge, Intersputnik established strategic alliances with different
satellite communications operators, manufactures, launch vehicle service
providers, ground equipment manufacturers and international entities. One
of these alliances is the joint venture Lockheed Martin Intersputnik (LMI)
established in 1997. First LMI satellite with 44 high power C and Ku band
transponders and lifetime of 15 years was launched at 75 deg E. in
September 1999. With strategic alliance with Lockheed Martin corporation,
Intersputnik is able to provide high quality services, which include digital
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video, high bit rate access to Internet, use of VSATs, Telemedicine, Tele-
education, banking on a global scale etc.
INMARSAT: INMARSAT was established as an international
cooperative organization similar to INTELSAT, for providing satellite
communications for ships and offshore industries. INMARSAT, a
specialized agency of UN, was established in 1979 and became operational
in 1982 as a maritime focused intergovernmental organization with
headquarters located at London. INMARSAT has forty-four members and
also provides services to nonmember countries.
INMARSAT has become a limited company since 1999. INMARSAT Ltd
is a subsidiary of the INMARSAT Ventures plc holding company, which
operates a constellation of geosynchronous satellites for worldwide mobile
communications. The satellites are controlled from INMARSAT
headquarters at London, which is also home to INMARSAT ventures and a
small Intergovernmental office created to supervise the company’s public
service duties for the maritime community i.e. Global Maritime Distress and
Safety System and Air traffic Control communications for the aviation
industry.
Starting with a user base of about 900 ships in early 1980’s, the user base of
INMARSAT grew to 210,000 ships, vehicles, aircrafts and portable
terminals in 2001. INMARSAT Type A mobile terminals meant for
installation in large ships are quite expensive, whereas, portable
INMARSAT mini-M terminals are small, cost effective and easy to operate.
The services provided by INMARSAT include telephone, fax and data
communications up to 64 kbps. Other services include videotext,
navigation, weather information and Search & Rescue. INMARSAT
Satellites can also be used for emergency Land Mobile Communications for
relief work and to re-establish communications or to provide basic service
where there is no alternative. INMARSAT can also be used to alert people
on shore for coordination of rescue activities. Apart from maritime and
Land Mobile Satellite Service, INMARSAT also provide aeronautical
satellite service for passenger communications.
INMARSAT system operates at C-band and L-band frequencies. The
INMARSAT system uses allocations in the 6 GHz band for the ground
station to satellite link, 1.5 GHz for satellite terminal downlink, 1.6 GHz for
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terminal to satellite uplink and 4 GHz for the satellite to ground station
down link.
COSPAS-SARSAT: COSPAS-SARSAT is a joint venture of Canada,
France, Russia and US. It is a satellite based international Search and
Rescue, alert and location system using different low earth orbit satellites
operating in the frequency of 120 MHz and 406 MHz. Local Users
Terminals operating in different parts of the world pick-up the alert signals
from Emergency Location Beacons carried by ships and airplanes and pass
on the information regarding the location of accident to the nearest rescue
centres for carrying out the rescue operations. Since the operationalisation
of the satellite based Search and Rescue System, hundreds of lives have
been saved due to timely deployment of prompt rescue operations.
Domestic Satellite Communications Systems
In the initial years of implementation of commercial Satellite
Communications the emphasis was mainly on the transoceanic and
international communications. However use of satellite communications for
improvement of domestic communications also emerged as a distinct
possibility. Countries like, USSR, Canada, Indonesia took initiatives in
implementing domestic satellite communications systems for the respective
countries. Satellite Communications System for domestic communications
is cost effective compared to the conventional terrestrial systems under the
following conditions.
• A large country without basic terrestrial communications
• Population is spread over mountains, deserts and a large
number of islands
• Thinly populated remote areas
Former USSR was the first country to adopt satellite communications for its
domestic use. However, because of its geographic location, where large
landmass was in the high latitude region, the geosynchronous satellite
systems were not found to be suitable. Thus a system with a series of
Molniya Satellites operating in highly elliptical non-geosynchronous
inclined orbits was introduced in 1965 to meet the country’s domestic
requirement of telecommunications and television transmission.
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Canada became the first country to use a geosynchronous satellite for
domestic communications with the launching of Anik-1 in 1972. With the
advent of Anik Satellite it was possible to cover for the first time the whole
of Canada particularly thinly populated northern region under the live TV
coverage. Apart from TV, Anik Satellites are also used for radio
broadcasting to remote locations and interactive distance education.
Indonesia is the first developing country to have its own domestic satellite
system. Because of its limited infrastructure and widely scattered
population dispersed over more than 13,600 islands, a satellite
communications system is an ideal technology to deliver
telecommunications and broadcasting throughout the country.
Telecommunications services using PALAPA-A, the first Indonesian
domestic satellite started in 1976.
Some of the other countries with their own domestic satellite
communications systems are:
• United States of America (Wester, SBS, Etc.)
• India (INSAT)
• Brazil (Brazilsat)
• Mexico (Morelos)
• China (Chinasat)
• Japan (CS, BS)
Many countries where operating an exclusive domestic satellite
communications system is not economical, the domestic requirements of
communications can be met by leasing capacity from Intelsat or other
satellites.
Regional Satellite Communications Systems
Regional Satellite Communications Systems have been an ideal means to
deliver telecommunications and broadcasting services to a number of
countries in a region for meeting their domestic and regional
telecommunications and broadcasting requirements rather than having
separate domestic system for each of these countries. A number of
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regional satellite communications systems are presently in operations and
quite a few of them are in the planning stages.
Some of the regional Satellite Communications Systems are:
• EUTELSAT
• ARABSAT
• AUSSAT
• PALAPA
Eutelsat is a consortium of twenty-six European nations, established for
operations and maintenance of space segment of the Eutelsat Satellite
System, and providing its members with the space segment capacity
necessary for meeting their telecommunications services requirements.
Eutelsat provides services that are not available via Intelsat in Europe.
These include:
• Transmission of television networks to eighteen countries for
cable distribution and transmission of Eurovision programs
• Intra-European telephony and telegraphy
• Multi-service data communications for computer networking,
facsimile, remote printing of newspapers, teleconferencing etc.
Eutelsats’ space segment is coordinated by the European Space Agency
(ESA) which procures spacecraft from European manufactures.
Arabsat evolved from 1953 Arab league agreement to develop regional
telephone, telex and telegraph communications. Arab Space
Communication Organization was established in 1976 and had twenty-two
members. However, by the time two Arabsat Satellites were launched in
1985, Egypt, a leader in the System’s planning had been expelled from Arab
league. Each Arabsat Satellite has twenty-five C-band transponders and
one C/S band transponder for community television reception. However,
the Arabsat Satellites are extremely under utilized as only six countries have
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earth stations. Several countries are still working with Intelsat lease rather
than switch to Arabsat.
AUSSAT systems of Australia designed for meeting domestic
communications needs of Australia for Radio, TV broadcasting and long
distance Telephony is also used by Papua New Guinea for
telecommunications and broadcasting services.
Since 1979, PALAPA system of Indonesia became a regional satellite
system, after Philippines, Thailand and Malaysia signed agreement to use
PALAPA.
Military Satellite System
For military communications Army, Air force and Navy use both fixed and
mobile satellite systems. In addition to the normal communications, military
communications are also required for tactical communications from remote
and inhospitable locations.
The special requirements of military communication terminals are high
reliability, ruggedness, compact, operations under hostile environment,
immunity to jamming, ease of portability and transportation, etc. Examples
of military satellite communications systems are:
• DSCS (US AF)
• SKYNET (UK)
• NATO (NATO)
• FLTSATCOM (US NAVY)
• MILSTAR
Because of the special frequency band used in Military satellite system and
other special requirements, Military satellite Systems are always much
costlier and it takes longer time to design and develop compared to
commercial satellite communications systems. Realizing that not all
communications are strategic in nature, there is a trend now to use
commercial communications system as far as possible. US Department of
Defense is one of the major users of commercial Iridium satellite system
with their own gateway.
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Navigational System
Satellites have now replaced the stars and terrestrial systems for the purpose
of navigation and radiolocation. The Transit Satellite system of US Navy
was the first satellite navigational system with satellites orbiting in low
polar orbits. By means of triangulation, the crews could establish the
location of the ship and submarine by picking up the signals transmitted by
different Transit satellites. US agreed to allow civilian use of Transit
Navigational System for use by merchant marine shipping industry
throughout the world.
Transit system is now replaced by the Global Positioning System (GPS) of
US Navstar Satellite System consisting of eighteen low earth orbiting
satellites operating at L-band. GPS receiver calculates the position
(latitude, longitude, height) with extremely high accuracy by receiving
signals from at least three-satellite passes. Apart from its use in ships, the
miniaturized GPS receiver has also found many applications related to land
based fleet monitoring.
The Russian Glonass system is the other navigational satellite system.
However, the system is not being maintained properly by timely
replacement of the satellites.
Personal Communications System
Introduction of terrestrial personal communications in many countries
since1980’s saw a very rapid growth of wireless telecommunications.
Considering that the terrestrial cellular telephone systems are limited only to
urban and sub-urban areas, the business potential in providing global
satellite based personal communications system was realized by many. It
was thought that a satellite based personal communication system would
provide not only communications to remote locations from anywhere, it
would also provide a seamless roaming system integrating the scattered
pockets of terrestrial system. A large number of global and regional
personal satellite communications systems using both geosynchronous and
non-geosynchronous satellites were planned during the 1990’s. Most of
these proposed systems were for voice communications with non-
geosynchronous satellites in order to avoid the long delay associated with
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geosynchronous satellites. However, quite a few of these proposed systems,
never took off and a few ran into financial problems at the implementation
stage casting serious doubts about the commercial viability of satellite based
personal communications system using non geosynchronous satellites.
Examples of regional satellite based personal communications systems
providing voice and data services through geosynchronous satellites are
• Thuraya
• Asia Cellular Satellite (ACeS)
Both these systems use satellites with large antenna systems and cover a
large area of Asia and Europe.
Thuraya: Thuraya Satellite Company is a regional satellite system that
provides satellite telephone services to a region covering 99 countries
through a dynamic mobile phone that combines satellite, GSM cellular
system and GPS. Thuraya was established in 1997 in UAE as a private joint
venture with shareholders from 18 national telecommunications operators
and investment houses. Thuraya meets the demand for seamless coverage of
mobile communications to 2.3 billion people residing in India, Middle
East,Central Asia, North & central Africa and Europe. Thuraya handsets
offer voice, data, fax messaging and position location. It enables the user to
use GSM service in local networks and automatically switch on to satellite
mode whenever out of local terrestrial reach. The first Thuraya satellite
operating in L band has been launched in Oct 2000 and commercial service
started from 2001.
ACeS is another regional geo-mobile personal satellite communications
system providing digital voice, fax and data communications using small
dual mode (satellite and GSM) handsets. Users of ACeS are able to roam
between terrestrial GSM cellular and satellite networks and can interface
with public switched telephone networks. ACeS is jointly owned by PT
Pacifik Satelit Nusantara of Indonesia, Lockheed Martin Global
Telecommunications of USA, Philippines Long Distance Telephone
Company and Jasmine International of Thailand. ACeS coverage area
extends from Pakistan & India in the west to Philippines & Papua New
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Guinea in the east and from China & Japan in the north to Indonesia in the
south. ACeS started its operations from November 2000.
Examples of a few Personal Satellite communications Systems providing
services or planning to provide services using non-geosynchronous satellite
constellations are:
• Iridium (66 Satellites)
• Globalstar (48 satellites)
• Orbcomm (35 satellites)
• New ICO (10 satellites)
Non-geosynchronous satellites in low and medium earth orbits need a large
number of satellites in a constellation to provide global coverage and the
number of satellites in the constellation increases with decreasing orbital
height. The non-geosynchronous low earth orbit satellites appear to be
attractive for providing two-way voice and data communications and
location positioning to small handheld terminals from the points of view of
higher available power from the satellite, low time delay etc. However,
launching and maintaining a large number of satellites on orbit and
operations of corresponding ground system pose technical as well as
operational problems. Financial problems faced by a few of these systems in
providing services at the early stages of operations, have made people
rethinking on the commercial viability of such systems. The frequency
allocations for personal communications systems are in the VHF, L band
and S band.
Iridium: The Iridium system was the first satellite based Personal
communications system to start commercial global wireless digital voice
communications operations in November 1998 with its 66 Low Earth Orbit
satellite constellation. But the infamous original Iridium service did not pick
up and it failed in its attempt to attract the target subscriber base. This
caused financial problems and bankruptcy of the company within a few
months of starting the operational service. Iridium’s failure despite a
sophisticated on board technology and compatibility of hand sets with
different terrestrial mobile telephone standards, is considered largely due to
poor marketing and a service that was too costly. Moreover, by the time
Iridium system was launched the cellular phone coverage also improved
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worldwide, thus reducing the target service area of Iridium that was
uncovered by terrestrial cellular service.
After acquiring the assets of the bankrupt Iridium LLC, a privately held
corporation Iridium Satellite LLC, launched its commercial global satellite
communications service in March 2001. Iridium Satellite provides voice,
paging, and messaging services to mobile subscribers using handheld user
terminals. Frequency of operations of Iridium system is L band.
Globalstar: Globalstar is a consortium of leading international
telecommunications companies originally established in 1991 to deliver
satellite telephony services through a network of exclusive service
providers. Globalstar system designed with a constellation of 48 low-earth
orbiting satellites, started its commercial phone service using multimode
handsets from October 1999. Other Globalstar services include voice mail,
short messaging service, fax and supporting terrestrial IS 41 and GSM
systems.
Calls from a Globalstar wireless handsets are transmitted in L band to the
satellite and the receive frequency is in S band. Calls via satellites are
routed through the appropriate gateway, from where they are passed on to
existing fixed and cellular telephone network. The service is available in
more than 100 countries in 6 continents.
Orbcomm: Orbcomm Global LP is the first commercial provider of global
low earth orbit satellite data and messaging communications system.
Globalstar started its commercial service in November 1998 with 28 out of
a constellation of 35 low earth orbit satellites and 14 gateway Earth stations
in 5 countries. Orbcomm provides two-way monitoring, tracking and
messaging services to both fixed and mobile terminals. The system is
capable of sending and receiving two-way alphanumeric packets, similar to
two-way paging and e-mail.
VHF frequency bands are used for providing two-way messaging services at
low data rates. The orbiting satellites pick up small data packets from
sensors in vehicles, containers, vessels or remote fixed sites and relay these
to the destination through a tracking Earth Station and Gateway Control
Centre.
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Orbcomm was originally formed as a partnership company owned by
Orbital Sciences Corp (USA), Teleglobe Inc (Canada) and Technology
Resources Industries Bhd (Malaysia). In April 2001, International
Licensees, a consortium of Orbcomm licensees and other investors
purchased all the assets of Orbcomm Global LP and its other entities that
were under protection of bankruptcy since September 2000.
New ICO: New ICO, formerly of ICO global communications is working
on a Medium Earth Orbit(MEO)/ Intermediate Circular Orbit (ICO) satellite
system designed for both fixed and mobile operations around the world.
ICO Global Communications was founded in 1995 and contracts for
satellites launch services and ICO network were awarded. In August 1999,
due to financial problems, the company was declared bankrupt. However,
with fresh investment from a group of international investors New ICO
emerged from the bankruptcy. New ICO system consists of a constellation
of 10 on-orbit ICO satellites, 2 on orbit spares at an orbit of 10,390 km.
Target launch of service of New ICO system is 2003.
New ICO is based in London with offices in different countries. The goal of
New ICO is to provide global Internet protocol services, including Internet
connectivity, data, voice and fax services. The system operates in both
circuit switched mode based on GSM standard and packet switched Internet
protocol mode. New ICO plans to target markets like maritime,
transportation Government, oil, gas, construction & other industries,
individuals and small & medium size businesses.
Broadband Satellite System
Broadband satellite service is an emerging service which has caught the
fancy of many for meeting the demand of worldwide fiber like access to
telecommunications services such as computer networking, broadband
Internet access, interactive multimedia and high quality voice. These
systems use advanced satellite technology at Ka band or Ku band
frequencies to achieve the high bandwidth requirements.
Examples of proposed Broadband Satellite systems are:
• Teledesic
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• SkyBridge
• Spaceway
Teledesic satellite network is designed with 288 plus spare satellites at low
earth orbits. The operating frequency is in the Ka band of the frequency
spectrum with 30 GHz uplink and 20 GHz downlink. The network will
enable millions of simultaneous users to access the two-way network using
standard user equipment providing up to 64 Mbps on the down link and up
to 2 Mbps on the up link. The fixed user equipment will be mounted out
door and connect inside to a computer network or PC.
Teledesic is a private company based in Bellevue, Washington (USA)
attracting investment from many reputed companies and individuals. The
ambitious Teledesic service targeted to begin in 2005 will enable broadband
connectivity for businesses, schools and individuals everywhere on the
planet and expected to facilitate improvements in education, healthcare and
other crucial global issues.
SkyBridge is a satellite-based broadband global telecommunications
system designed to provide business and residential users with interactive
multimedia applications as well as LAN interconnection or ISDN
applications, thus allowing services such as high speed Internet access and
video conferencing to take place anywhere in the world. The system is
based on a constellation of 80 low earth orbiting satellites, which link
professional and residential users equipped with low cost terminals and
terrestrial gateways. The satellite network operates at Ku band and will
deliver asymmetric broadband connection to fixed network at up to 60
Mbps (in steps of 16 kbps) to the user and up to 2 Mbps (in increments of
16 kbps) on the return link via a gateway.
SkyBridge LP was formed in Delaware, USA in 1997. The partners of
SkyBridge LP are Alcatel and leading industries from North America,
Europe and Asia.
Spaceway is another advanced broadband satellite system offered by
Hughes Network system of USA that will make high-speed broadband
applications available on demand to the businesses and to consumers
around the world. Operating in the Ka band spectrum, SPACEWAY will
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consist of interconnected regional satellite systems providing service to
nearly all of the world’s population. The first North American regional
service will start in 2002 with two geosynchronous satellites plus an on
orbit spare. Using a globally deployed system of satellites in conjunction
with a ground-based infrastructure, users will transmit and receive video,
audio, multimedia and other digital data at uplink rates between 16 kbps to
16 Mbps. The access to the system will be provided through a family of low
cost easily installed 66 cm terminals.
ORBITS FOR COMMUNICATION SATELLITE
The path a Satellite or a planet follows around a planet or a star is defined
as an orbit. In general the shape of an orbit of a satellite is an ellipse with
the planet located at one of the two foci of the ellipse. The circular orbit
may also be considered as an ellipse where the two foci of the ellipse
coincide at the center of the circle. Satellite Orbits are classified in two
broad categories i.e.
• Non-Geostationary Orbit (NGSO)
• Geo Stationary Orbit (GSO)
Non-Geostationary Orbit (NGSO)
Early ventures with satellite communications used satellites in Non-
geostationary low earth orbits due to the technical limitations of the launch
vehicles in placing satellites in higher orbits. With the advancement of
launch vehicles and satellite technologies, once the Geo Stationary Orbit
(GSO) was achieved, majority of the satellites for telecommunications
started using GSO due to its many advantages. During 1990s the interests in
NGSOs were rekindled due to several advantages of NGSO in providing
global personal communications in spite of its many disadvantages.
Advantages of NGSO are:
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• Less booster power required to put a satellite in lower orbit
• Less space loss for signal propagation at lower altitudes
(<10,000 km) leading to lower on board power
requirement
• Less delay in transmission path – reduced problem of echo in
voice communications
• Suitability for providing service at higher latitude
• Lower cost to build and launch satellites at NGSO
• Use of VHF and UHF frequency bands at NGSO permits low
cost antennas for hand-held terminals
Disadvantages of NGSO are:
• Requirement of a large number of orbiting satellites for global
coverage as each low earth orbit satellite covers a small
portion of the earth’s surface for a short time.
• Complex hand over problem of transferring signal from one
satellite to another
• Less expected life of satellites at NGSO requires more
frequent replacement of satellites compared to satellite in GSO
• Compensation of Doppler shift is necessary
• Satellites at NGSO undergoes eclipse several times a day
necessitates the requirement of robust on board battery system
for the satellite for operations without solar power during
eclipse
• Complex network management for a constellation of satellites
and corresponding ground system
• Problem of increasing space debris in the outer space
There are different types of Non Geostationary Orbits (NGSO), depending
on the orbital height and the inclination of the orbital plane. Inclination is
the angle that the orbital plane makes with the equatorial plane at the time
of crossing the equator moving from south to north of the earth and is
measured from 0 to 180 degrees. NGSOs are classified in the following
three types as per the inclinations of the orbital plane
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• Polar Orbit
• Equatorial Orbit
• Inclined Orbit
In polar orbit the satellite moves from pole to pole and the inclination is
equal to 90 degrees. In equatorial orbit the orbital plane lies in the
equatorial plane of the earth and the inclination is zero or very small. All
orbits other than polar orbit and equatorial orbit are called inclined orbit.
A satellite orbit with inclination of less than 90 degrees is called a prograde
orbit. The satellite in prograde orbit moves in the same direction as the
rotation of the earth on its axis. Satellite orbit with inclination of more than
90 degrees is called retrograde orbit when the satellite moves in a direction
opposite to the rotational motion of the earth. Orbits of almost all
communication satellites are prograde orbits, as it takes less propellant to
achieve the final velocity of the satellite in prograde orbit by taking
advantage of the earth’s rotational speed. Example of retrograde orbit is the
sun synchronous orbit where the orbital parameters are such that that the
satellite crosses the same latitude at the same local time. This type of orbit
is used for earth observation satellites where repeated observations are
required to be made under the same sun angle. It needs more propellant to
launch a satellite in retrograde orbit as it is launched in a direction opposite
to the direction of the earth’s rotation.
Satellite orbits are also classified in terms of the orbital height. These are:
• Low Earth Orbit (LEO)
• Medium Earth Orbit (MEO) / Intermediate Circular Orbit
(ICO)
• Highly Elliptical Orbit (HEO)
• Geosynchronous Earth Orbit (GEO)
Satellite orbits with orbital height of approximately 1000 km or less are
known as Low Earth Orbit (LEO). LEOs tend to be in general circular in
shape. Satellite orbits with orbital heights of typically in the range of 5000
km to about 25,000 km are known as Medium Earth Orbit (MEO) /
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Intermediate Circular orbit (ICO). MEO and ICO are often used
synonymously, but MEO classification is not restricted to circular orbits.
Satellites in Highly Elliptical Orbit (HEO) are suitable for communications
in the higher latitudes. Russian Molnya satellites have highly inclined
elliptical orbits with a perigee of about 1000 km, apogee of 40,000 km,
inclination of 63.435 deg and orbital period of 12 hours. In Geosynchronous
Earth Orbit (GEO) the satellite is in equatorial circular orbit with an altitude
of 35,786 km and orbital period of 24 hours. Three satellites in GEO placed
1200
apart over equator cover most of the world for communications
purposes.
Fig.1 shows different types of orbits.
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Geostationary Orbit (GSO)
There is only one geostationary orbit possible around the earth, lying on the
earth’s equatorial plane and the satellite orbiting at the same speed as the
rotational speed of the earth on its axis. For a Satellite to have an orbital
period equal to that of earth’s rotation i.e. a sidereal day (23 Hrs 56 min.
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4.09 sec.) an altitude of 35,786 km is required. Such a satellite orbiting at a
velocity of 3.075 km/sec remains fixed relative to any point on earth or
geostationary. With the idealized assumptions that the geostationary
satellite is at rest relative to the earth the conditions required to be satisfied
for geostationary orbit are:
• The orbit shall be circular
• The period of the orbit shall be equal to the period of rotation
of the earth about itself
• The plane of the orbit shall be the same as the equatorial plane
but the sub-satellite longitude, i.e. the longitude of the
projection of the satellite on the Earth’s surface can be selected
arbitrarily.
The principle of satellite communications based on this concept of
geostationary orbit was originated by Arthur C Clarke. Main advantage of
geostationary satellite being the permanent contact between the ground
segment and the satellite with fixed directional antennas at both the earth
station and the satellite.
The ITU (International Telecommunications Union), recognizing the
importance of the GSO along with the frequency spectrum as limited natural
resources available on the earth, set out the procedures for all radio
communications services, regarding the use of GSO/spectrum through ITU
Radio Regulations, a binding international treaty. With respect to the use of
the GSO and frequency spectrum, the ITU space regulations laid down in
the ITU Constitution is as follows:
In using frequency bands for radio services, Member states shall
bear in mind that radio frequencies and any associated orbits,
including the geostationary-satellite orbit, are limited natural
resources and they must be used rationally, efficiently and
economically, in conformity with the provisions of Radio
Regulations, so that countries or groups of countries may have
equitable access to those orbits and frequencies, taking into account
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the special needs of developing countries and the geographical
situation of particular countries.
Table-3 outlines the salient features, advantages and disadvantages of
Geostationary Satellite Orbit (GSO).
TABLE – 3
GEOSTATIONARY SATELLITE ORBIT
Attitude 35,786 km.
Period 23 Hr. 56 min. 4.091 sec. (One sidereal day)
Orbit inclination. 00
Velocity 3.075 km per sec.
Coverage 42.5% of earth’s surface.
Sub satellite point On equator.
Area of no
coverage
Beyond 810
North and South latitude.
(77º if angle of elevation below 5º are eliminated )
Advantages - Simple ground station tracking.
- No hand over problem
- Nearly constant range
- Very small doppler shift
Disadvantages - Transmission delay of the order of 250 msec.
- Large free space loss
- No polar coverage
A perfect geostationary orbit is a mathematical abstraction that could be
achieved only by a spacecraft orbiting around a perfectly symmetric earth
and no other forces are acting on the spacecraft other than the central
gravitational attraction from the earth. The abstraction is however, useful as
an approximate description of real case, since all other forces or
perturbations due to attractive forces of the Moon and the Sun and the non-
sphericity of the Earth’s gravity are small.
In real life due to gravitational pull of the Moon & the Sun, the equatorial
orbital plane of the satellite makes an angle of inclination with respect to the
equatorial orbital plane. For a satellite with orbital period equal to a sidereal
day and non-zero inclination, the footprint of the satellite will move in
North-South direction over its sub satellite point instead of remaining
stationary. The non-spherical shape of the earth also causes movement of
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the satellite in the east-west direction. Thus the trace of the satellite on earth
appears to roam in both North-South and East-West direction around the
sub-satellite point.
The inclination of the satellite can be corrected by firing appropriate
thrusters on-board the satellite and is known as North-South station
keeping. Similarly the correction of East-West drift of the satellite is called
East-West Station keeping. Without any station keeping the inclination
plane drifts to about 0.86 deg per year. Thus the satellite orbital position is
required to be corrected periodically to keep the drift from the desired
location within a certain limit. Considering the drift in the satellite position
in North-South and East-West direction around the sub-satellite point, it is
more appropriate to designate such an orbit as geosynchronous orbit.
GEOSYNCHRONOUS COMMUNICATION SATELLITE
Geosynchronous Satellites have now become almost synonymous for
communications satellites, because of its wide use in telecommunications
due to the advantages over non-geosynchronous satellites. Because of the
availability of a number of communication satellites over the
geosynchronous arc, the communications between different parts of the
world have become possible and affordable. The communication satellites
have played a significant role in converting the world into a global village.
Salient features of Geosynchronous Communications Satellite
Salient features of Geosynchronous Satellite are:
• Wide Coverage
• Stationary Position
• Multiple Access
• Suitability for transcontinental telecommunications,
broadcasting, mobile and thin route communications.
• Frequency reuse capability
• Very low Doppler Shift
• Reliability.
• Cost effectiveness.
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Brief description of each of these features are given below:
Wide Coverage: From the geosynchronous orbit the satellite can cover an
area equal to about 42% of the area of the earth (38% if angles
of elevation below 5º are not used). Thus three satellites placed
120º apart can cover almost the whole world for the purpose of
communications. INTELSAT Satellites strategically placed
over Atlantic Ocean Region (AOR), Indian Ocean Region (IOR)
and Pacific Ocean Region (POR) covers the whole world for
International Telecommunications. With worldwide satellite TV
coverage, any incidence happening in any part of the world can
now be viewed live in the TV throughout the world.
Stationary Position: The orbital velocity of the geosynchronous satellite
being equal to the rotational velocity of the earth on its own
axis, the satellite in the geosynchronous orbit appears to be
stationary with respect to any location from the earth. Thus the
satellite is always visible from any earth station situated in its
coverage region and the tracking of the satellite is simple and
there is no hand over problem of transferring signal from one
satellite to another as in the case of satellites in NGSO. The
constant visibility of the satellite also enables both the satellite
and the earth station to use highly directive antennas. High gain
of the antennas on-board the satellite and the earth station,
enhances the transmit and receive capabilities.
Multiple Access: Multiple Access is the ability of a large number of users
to simultaneously interconnect their respective voice, data and
television links through a satellite. The wide geographic
coverage and broadcast nature of satellite channel are exploited
by means of multiple access. Multiple access also helps in
optimum use of satellite capacity, satellite power, spectrum
utilization and interconnectivity among different users at
reduced cost.
A satellite in geosynchronous orbit can link multiple earth stations within its
coverage area and separated by great circle distances up to 17,000 Km.
Multiple access is the unique feature of satellite communications not
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possible to get by any other means. For m earth stations visible from a
Satellite, the number of potential available communication circuits is given
by
n = m (m-1)/2
compared to non flexible 2-port network of conventional cable or land
based networks.
Suitability for Transcontinental Telecommunications, Broadcasting,
Mobile and Thin Route Communications: TV Broadcasting via Satellite
is perhaps the most common use of geosynchronous satellite. In developing
countries where the terrestrial TV distribution is very limited, the
communications satellites can be very effectively utilized for TV
distribution. Geosynchronous satellites handle a large portion of
transcontinental telecommunications traffic.
Geosynchronous Satellites along with other NGSO satellites are found to be
suitable for reliable mobile communications for ships and aircrafts, as the
ship and the aircraft can continuously maintain the communication link with
the satellites while moving. However, GEO based satellite systems are
much simpler to operate and maintain compared to other system.
Geosynchronous Satellites are also the most suitable means of providing
reliable and cost effective communications to thin route rural areas,
interconnecting small islands, and providing communications to hilly and
difficult terrain.
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Frequency Reuse: The frequency bands of a geosynchronous satellite can
be reused by different methods for increasing the channel
capacities of the communications satellite. By using specially
designed spatially separated shaped beams the same frequency
and polarizations can be reused. By using orthogonal
polarizations the same frequency bands can be reused for the
same coverage area of the satellite. By using orthogonal linear
and circular polarizations and shaped beams covering different
regions, the same frequency band can be reused many folds thus
increasing the communication capacity of geosynchronous
satellite. Different techniques of frequency reuse of the same
frequency band are found in INTELSAT series of satellite.
Very Low Doppler Shift: Compared to low earth orbit satellites, in
geosynchronous satellite there is almost no Doppler Shift i.e.
change in the apparent frequency of operations to and from
Satellite, caused by the relative motion of the Satellite and the
earth station. Satellites in elliptical orbits have different
Doppler shifts for different earth stations and this increases the
complexities of the receivers especially when a large number of
earth stations intercommunicate.
Reliability: The reliability of long distance telecommunication links
improves considerably when geosynchronous satellites are used.
The path loss in the satellite links although very high; these
remain almost constant, thus maintaining the performance
quality of the link.
Cost effectiveness: The geosynchronous satellite because of its long life of
twelve to fifteen years and wide-band operations shared by a
large number of users, makes the point to point service very cost
effective compared to the service provided by land based
terrestrial system. No viable alternatives to geosynchronous
satellites are presently available, so far as the broadcasting and
mobile services are concerned.
Problems of Geosynchronous Satellite Communications Systems
The problems of geosynchronous satellite communications systems are:
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• No coverage of polar region.
• Long time delay.
• Echo.
• Eclipse due to the earth and the sun.
• Sun Transit outage
No Coverage Region: The geosynchronous satellite from its location of
35,786-Km altitude above equator is not found suitable for
communications beyond the latitude of 81 deg. Thus the polar
region of the earth cannot be properly covered by
geosynchronous satellite.
Time Delay: In Satellite Communications System using geosynchronous
satellite, the signal has to travel a long distance while travelling
from the transmit earth station to the receive earth station via
satellite. From the geometry of the geosynchronous satellite
orbit it is found that the single hop time required for the signal to
travel from one point to another varies from 230 m sec. (90 deg.
elevation) to 278 m sec. (0 deg. elevation). This time delay
does not pose any problem in data and broadcasting services,
but this delay is quite perceptible in two-way telephone
conversations. ITU-T specifies a delay of less than 400 msec
to prevent echo effects and delay variation of upto 3 msec.
Although the propagation and intersatellite delays of LEOs are
lower, LEO systems exhibit high delay variation due to
connection handovers, satellite and orbital dynamics and
adoptive routing.
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Echo: Generally a long distance telephone circuit is accompanied by echo
due to mismatch at the terminal point where circuits are
converted from four wire to two wire system. As the delay of
the echo is increased, the effect of the echo becomes
increasingly disagreeable to the talker. The echo can be
attenuated by using echo suppressor or echo canceller. By using
echo suppressor of excellent quality, a two hop satellite link can
be utilized for practical communications, provided the delay is
acceptable.
Eclipse of Satellite: A Satellite is said to be in eclipse when the positions
of the earth, the Sun and the Satellite are such that the earth
prevents sun light from reaching the satellite i.e when the
satellite is in the shadow of the earth. For geosynchronous
satellites, eclipses occur for 46 days around equinox (March 21
and September 23). During full eclipse, a satellite does not
receive any power from solar array and it must operate entirely
from batteries. In case the power available from battery is not
enough, some of the transponders may be required to be shut
down during the eclipse period. The satellite passes through
severe thermal stress during its passage into and out of the
earth’s shadow. The solar power also fluctuates sharply at the
beginning and end of an eclipse. For these reasons the
probability of failure of satellite is more during eclipse than at
any other time.
Sun Transit Outage: Sun transit outage takes place when the sun passes
through the beam of an earth station. During vernal and
autumnal equinox, the sun approaches toward a geosynchronous
satellite as seen from an earth station and this increases the
receiver noise level of the earth station very significantly and
prevent normal operations. This effect is predictable and can
cause outage for as much as 10 min. a day for several days. The
sun transit outage is about 0.02 percent in an average year. A
receiving earth station cannot do anything about it except wait
for the sun to move out of the main lobe.
ELEMENTS OF SATELLITE COMMUNICATIONS SYSTEM
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Two major elements of Satellite Communications Systems are
• Space Segment
• Ground Segment
The Space Segment includes
• Satellite
• Means for launching satellite
• Satellite control centre for station keeping of the satellite
The functions of the ground segment are to transmit the signal to the
satellite and receive the signal from the satellite. The ground segment
consists of
• Earth Stations
• Rear Ward Communication links
• User terminals and interfaces
• Network control centre
Schematic block diagram showing the elements of Satellite Communications
System is shown in fig. 2.
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SPACE SEGMENT
Communication Satellite
Communication satellites are very complex and extremely expensive to
procure & launch.
The communication satellites are now designed for 12 to 15 years of life
during which the communication capability of the satellite earns revenue, to
recover the initial and operating costs. Since the satellite has to operate
over a long period out in the space the subsystems of the satellite are
required to be very reliable. Major subsystems of a satellite are:
• Satellite Bus Subsystems
• Satellite Payloads
Satellite Bus subsystems:
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• Mechanical structure
• Attitude and orbit control system
• Propulsion System
• Electrical Power System
• Tracking Telemetry and Command System
• Thermal Control System
Satellite Payloads
• Communication transponders
• Communication Antennas
Since the communications capacity earns revenue, the satellite must carry as
many communications channels as possible. However, the large
communications channel capacity requires large electrical power from large
solar arrays and battery, resulting in large mass and volume. Putting a
heavy satellite in geosynchronous orbit being very expensive, it is logical to
keep the size and mass of the satellite small. Lightweight material optimally
designed to carry the load and withstand vibration & large temperature
cycles are selected for the structure of the satellite.
Attitude and orbit control system maintains the orbital location of the
satellite and controls the attitude of the satellite by using different sensors
and firing small thrusters located in different sides of the satellite.
Liquid fuel and oxidizer are carried in the satellite as part of the propulsion
system for firing the thrusters in order to maintain the satellite attitude and
orbit. The amount of fuel and oxidizer carried by the satellite also
determines the effective life of te satellite.
The electrical power in the satellite is derived mainly from the solar cells.
The power is used by the communications payloads and also by all other
electrical subsystems in the satellite for house keeping. Rechargeable
battery is used for supplying electrical power during ellipse of the satellite.
Telemetry, Tracking and Command system of the satellite works along with
its counterparts located in the satellite control earth station. The telemetry
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system collects data from sensors on board the satellite and sends these data
via telemetry link to the satellite control centre which monitors the health of
the satellite. Tracking and ranging system located in the earth station
provides the information related to the range and location of the satellite in
its orbit. The command system is used for switching on/off of different
subsystems in the satellite based on the telemetry and tracking data.
The thermal control system maintains the temperature of different parts of
the satellite within the operating temperature limits and thus protects the
satellite subsystems from the extreme temperature conditions of the outer
space.
The communications subsystems are the major elements of a
communication satellite and the rest of the space craft is there solely to
support it. Quite often it is only a small part of the mass and volume of the
satellite. The communications subsystem consists of one or more antennas
and communications receiver - transmitter units known as transponders.
Transponders are of two types, Repeater or Bent pipe and processing or
regenerative. In Repeater type, communications transponder receives the
signals at microwave frequencies and amplifies the RF carrier after
frequency conversion, whereas in processing type of transponder in addition
to frequency translation and amplification, the RF carrier is demodulated to
baseband and the signals are regenerated and modulated in the transponder.
Analog communication systems are exclusively repeater type. Digital
communication system may use either variety. Fig. 3(a) and 3(b) show the
schematic diagrams of repeater type and regenerative type transponders
respectively.
1 - 43

The actual reception and retransmission of the signals are however,
accomplished by the antennas on board the satellite. The communications
antennas on board the satellite maintain the link with the ground segment
and the communications transponder. The size and shape of the
communications antenna depend on the coverage requirements and the
antenna system can be tailor made to meet the specific coverage
requirements of the system.
Launch Vehicle
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The function of the launch vehicle is to place the communication satellite in
the desired orbit. The size and mass of the satellite to be launched is
limited by the capability of the launch vehicle selected for launching the
satellite. The satellite launch vehicle interface is also required to be
provided as per the launch vehicle selected. Satellite launch vehicles are
classified in two types i.e.
• Expendable
• Reusable
In expendable type the launch vehicle can be used only once and most of
the launch vehicles are expendable type. Space Transportation System
(STS) or Space Shuttle of NASA, USA is the only available operational
reusable launch vehicle. Although most of the launches take place from
ground, Sea Launch has embarked on the launching of satellites from off
shore platforms and Peagasus launch vehicles can launch small satellites
from aircrafts. Launching of a satellite in orbit being a costly affair a number
of programs have been undertaken by NASA to make the future launching
of satellites in orbit as cost effective and routine as commercial air travel.
Satellite Control Centre
Satellite Control Centre performs the following function.
• Tracking of the satellite
• Receiving Telemetry data
• Determining Orbital parameters from Tracking and Ranging
data
• Commanding the Satellite for station keeping
• Switching ON/OFF of different subsystems as per the
operational requirements
• Thermal management of satellite.
• Eclipse management of satellite
• Communications subsystems configuration management.
• Satellite Bus subsystems configuration management etc.
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GROUND SEGMENT
The ground segment of satellite communications system establishes the
communications links with the satellite and the user. In large and medium
systems the terrestrial microwave link interfaces with the user and the earth
station. However, in the case of small systems, this interface is eliminated
and the user interface can be located at the earth station. The earth station
consists of
• Transmit equipment.
• Receive equipment.
• Antenna system.
Fig. 4 shows the schematic block diagram of an earth station.
In the earth station the base band signal received directly from users’
premises or from terrestrial network are appropriately modulated and then
transmitted at RF frequency to the satellite. The receiving earth station after
demodulating the carrier transmits the base band signal to the user directly
or through the terrestrial link.
The baseband signals received at the earth stations are mostly of the
following types.
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• Groups of voice band analog or digital signals
• Analog or digital video signals
• Single channel analog or digital signal
• Wide band digital signal.
In satellite communications, in early days FM modulation scheme was most
frequently used for analog voice and video signal transmission. However,
the trnd is now to use digital signals for both voice and video. Various
digital modulation schemes like Phase Shift Keying (PSK) and Frequency
Shift Keying (FSK) are adopted for transmission of digital signals.
The network operations and control centre for the communications network
monitors the network operations by different users, distribution of different
carriers within a transponder and allocation of bandwidth & EIRP of
different carriers. Proper functioning of Network operations and control
centre is essential where the number of users in the network is large.
Network operations & control centre is also responsible for giving clearance
to the ground system in respect of antenna radiation pattern, EIRP etc.
SATELLITE COMMUNICATIONS SERVICES
Different Satellite Communications services are classified as one way link
and two way link. One way link from transmitter Tx to receiver Rx on
earth’s surface is shown in fig.5.
1 - 47

Examples of satellite services where the transfer of information takes place
through one way link are:
• Broadcast Satellite Service (Radio, TV, Data broadcasting)
• Data Collection Service (Hydro meteorological data collection)
• Space operations service, (Tracking, Telemetry, Command)
• Safety services (Search & Rescue, Disaster Warning)
• Earth Exploration Satellite Service (Remote Sensing)
• Meteorological Satellite Service (Meteorological data
dissemination)
• Radio Determination Satellite Service (Position location)
• Reporting Service (fleet monitoring)
• Standard frequency and time signal satellite service
• Space Research Service.
In two-way Satellite Communications link the exchange of information
between two distant users takes place through a pair of transmit and receive
earth stations and a satellite. Fig.6 shows the elements of two-way link
Examples of two-way satellite services are
• Fixed Satellite Service (Telephone, telex, fax, high bit rate data
etc.)
• Mobile Satellite Service (Land mobile, Maritime, Aero-mobile,
personal communications)
1 - 48

• Inter Satellite Service.
• Satellite News Gathering (Transportable and Portable )
A new class of two-way fixed satellite network service known as Very
Small Aperture Terminal (VSAT) service has became very popular among
business and closed users group communities.
SAT networks are operated in two different configurations i.e. Mesh and
Star. While in Mesh configuration a VSAT terminal can communicate with
another VSAT terminal in a single hop connection, Star network involves
two hops via satellite and the hub station.
REFERENCES
1. Clarke, `Extra Terrestrial Relays’, Wireless World. Vol.51, pp 305-308,
October 1945.
2. Heather E. Hudson, Communication Satellites: Their Development and
Impact.
3. Delbert D. Smith, Communication via Satellite: A vision in Retrospect.
4. Lewis, Communications Services via Satellite.
5. Miya K. Satellite Communication Engineering.
6. Maral and M. Barsquet, Satellite Communications Systems.
7. Spilker, J.J. Digital Communication by Satellite.
8. Morrow Jr. (Ed) Satellite Communications, Proc. IEEE Vol.59, No.2
Feb.1971.
9. Podcaczky E.I.(Ed), Satellite Communications Proc. IEEE, Vol.65,
No.3, March 1977.
10.Harry L. Van Trees (Ed), Satellite Communications, IEEE Press selected
reprint series (1979).
11.Kadar I. (Ed), Satellite Communications Systems, AIAA Selected reprint
series Vol. 18, Jan. 1976.
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12.James Martin, Communications Satellite Systems.
13.Pratt and C.W.Bostian, Satellite Communications.
14.Bhargava et al, Digital Communications by Satellite.
15.Gagliardi, Satellite Communications
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SATELLITE COMMUNICATION OVERVIEW

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