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ABSTRACT:

 Not so long ago, satellites were exotic, top-secret devices. They were used

 primarily in a military capacity, for activities such as navigation and espionage.

 Now they are an essential part of our daily lives. We see and recognize their use in

weather reports, television transmission by DIRECTV and the DISH Network, in

everyday telephone calls,in GPS(global positioning system)and by the media in

transmitting their text and images to multiple printing sites for speed localdistribution.

In the context of a worldwide communications network, satellite communications

systems are very important. Satellite communications links add capacity to existing

communications capabilities and provide additional alternate routings for 

communications traffic. Satellite links, as one of several kinds of long-distance links,

interconnect switching centers located strategically around the world. They are part of 

the defense communication systems (DCS) network. One important aspect of the

satellite communications network is that it continues in operation under conditions

that sometimes render other methods of communications inoperable. Because of this,

satellites make a significant contribution to improved reliability of Navy

communications.

When satellite television first hit the market in the early 1990s, home dishes were

expensive metal units that took up a huge chunk of yard space. In these early years,

only the most die-hard TV fans would go through all the hassle and expense of 

 putting in their own dish. Satellite TV was a lot harder to get than broadcast and

CABLE AND TV.

Today, you see compact satellite dishes perched on rooftops all over the United

States. Drive through rural areas beyond the reach of the cable companies, and you'llfind dishes on just about every house. The major satellite TV companies are luring in

more consumers every day with movies, sporting events and news from around the

world and the promise of movie-quality picture and sound.

Satellite TV offers many solutions to broadcast and cable TV problems. Though

satellite TV technology is still evolving, it has already become a popular choice for 

many TV viewers.

SATELLITE

COMMNUICATION

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WHAT IS A SATELLITE AND IT’S WORKING:

A satellite is basically any object that revolves around a planet in a circular or 

elliptical path. The moon is Earth's original, natural satellite, and there are many

manmade (artificial) satellites, usually closer to Earth.• The path a satellite follows is an orbit. In the orbit, the farthest

 point from Earth is the apogee, and the nearest point is the perigee.

• Artificial satellites generally are not mass-produced. Most satellites

are custom built to perform their intended functions. Exceptions include

the GPS satellites (with over 20 copies in orbit) and the Iridium satellites

(with over 60 copies in orbit).

• Approximately 23,000 items of space junk-objects large enough to

track with radar that were inadvertently placed in orbit or have outlived

their usefulness -- are floating above Earth. The actual number varies

depending on which agency is counting. Payloads that go into the wrong

orbit, satellites with run-down batteries, and leftover rocket boosters allcontribute to the count. This online catalogs of satellite has almost 26,000

entries!

Although anything that is in orbit around Earth is technically a satellite, the term

"satellite" is typically used to describe a useful object placed in orbit purposely to

 perform some specific mission or task. We commonly hear about weather satellites,

communication satellites and scientific satellite

• THE SOVIET SPUTNIK SATELLITE WAS THE FIRST TO ORBIT

EARTH, LAUNCHED ON OCTOBER 4, 1957.

TYPES OF SATELLITES:

Satellites come in all shapes and sizes and play a variety of roles. For example:

• Weather satellites help meteorologists predict the weather or see

what's happening at the moment. Typical weather satellites include the

TIROS, COSMOS and GOES satellites. The satellites generally contain

cameras that can return photos of Earth's weather, either from fixedgeostationary positions or from polar orbits.

• Communications satellites allow telephone and data conversations

to be relayed through the satellite. Typical communications satellites

include Telstar and Intelsat. The most important feature of a

communications satellite is the transponder -- a radio that receives a

conversation at one frequency and then amplifies it and retransmits it

  back to Earth on another frequency. A satellite normally contains

hundreds or thousands of transponders. Communications satellites are

usually geosynchronous.

• Broadcast satellites broadcast television signals from one point to

another (similar to communications satellites).

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• Scientific satellites perform a variety of scientific missions. The

Hubble Space telescope is the most famous scientific satellite, but there

are many others looking at everything from sun spots to gamma rays.

• Navigational satellites help ships and planes navigate. The most

famous are the GPS NAVSTAR satellites.

• Rescue satellites respond to radio distress signals .• Earth observation satellites observe the planet for changes in

everything from temperature to forestation to ice-sheet coverage. The

most famous are the LANDSAT series.

LAUNCH OF SATTILITE:-This list includes countries with an independent capability to place satellites in orbit,

including production of the necessary launch vehicle. Note: many more countries

have the capability to design and build satellites but are unable to launch them,

instead relying on foreign launch services. This list does not consider those numerous

countries, but only lists those capable of launching satellites indigenously, and thedate this capability was first demonstrated. Does not include consortium satellites or 

multi-national satellites. 

First launch by country

Order Country Year of first launch Rocket Satellite

1 Soviet Union 1957 Sputnik-PS Sputnik 1

2 United States 1958 Juno I  Explorer 1

3 France 1965 Diamant  Astérix

4 Japan 1970 Lambda-4S Ōsumi

5 China 1970 Long March 1 Dong Fang Hong I 

6 United Kingdom 1971 Black Arrow  Prospero X-3" 

7 India 1980 SLV  Rohini

8  Israel 1988 Shavit Ofeq 1

 _  Russia[1]  1992 Soyuz-U  Kosmos 2175

 _  Ukraine[1]  1992 Tsyklon-3 Strela

9 Iran 2009 Safir-2 Omid 

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Satellite Frequency Bands Chart:-

Letter designation for satellite

frequency band

Frequency Range

(GHz)

L 1-2

S 2-4C 4-8

X 8 - 12

(8 - 12.5 in North America)

KU 12 - 18

(12.5 - 18 in North America)

K 18 - 27

(18 - 25.5 in North America)

KA 27 - 40

(26.5 - 40 in North America)

O 40-50

V 50-75

Working Of Satellite:-Radio waves are electromagnetic waves of very low frequency. Their frequency is

generally less than 30 MHz. there are reflected back to the earth by the ionosphere,

the atmosphere around the earth. Therefore, there is no need of a satellite to reflect

 back to the earth. Microwaves are electromagnetic waves with frequency from

30MHz to 1 GB. The ionosphere cannot reflect microwaves back to the earth. They

 pass through the ionosphere. A satellite used to receive microwaves and then transmit

them back to the earth.

Satellites provide links in two ways. Firstly a satellite provide point to point

communication link between one ground station and the other. One ground station

transmit signal to the other satellite and next ground station receives them from the

satellite. Secondly, satellite receives signals from one ground station and transmits to

them to the number of ground receivers.

Most satellite use frequency bandwidth through from 5.92 to 6.4GHz from

transmission of data from earth to the satellite and a frequency bandwidth from 3.7 to

4.1GHz for transmission from satellite to the earth. A satellite can provide service to a

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certain part of the earth if it is in sight. This can be done only if the satellite remains

stationary with respect to the earth

SATELLITE TELEVISION-A MERGE:

Why do we have satellite television? What's wrong with the terrestrial broadcasts that

we have enjoyed for five decades or more? Well the real benefit lies in the fact that

you get more channels. You get more channels because satellite broadcasts are able to

utilise more bandwidth than conventional terrestrial systems and can therefore fit

more separate channels into the space allowed.

Sattellite television really becomes important in areas where it is not possible to

install cable and the broadcast television reception is poor. Both systems use radio

wave signals to transmit and the waves travel in straight lines. That means for 

 broadcast television, which is transmitted and received via land-based antenna, thatthe natural curvature of the earth will eventually break the signals' line of sight. It also

means that other land based signals and obstructions are likely to interfere with the

TV signal and cause some distortion.

Satellites that transmit TV are placed in orbit over 22,000 miles above the Earth and

they rotate around the planet once every 24 hours and in the same direction that the

Earth is rotating in. Because the Earth rotates once every 24 hours and the satellite

moves around the Earth at the same velocity in the same direction, the satellite always

stays over the same point on the surface. The satellites are said to be in geostationary

orbit. Because they are high in the sky a satellite beams signals over a wide area of the

 planets surface. The satellites are also in communication with each other so that they

can relay the same broadcast to all satellites in orbit over different parts of the world.

This is how we are able to get TV live from anywhere on the Earth and view it at the

same time as everyone else on the planet receiving a signal from a satellite. When you

think about it it's a pretty amazing feat of technology.

When satellite television was first introduced the dishes required to receive the signal

were very expensive and people were able to set them up to receive programmes that

were not really intended for everybody to see. Nowadays people tend to subscribe to a

'Direct Broadcast Satellite' (DBS) provider such as DirecTV or Dish Network in the

US and Sky in the UK. These providers are able to select programs and broadcastthem to people as set packages, for example the 'Family', 'Sports' or 'Movie' packages.

Cost will depend on which one you choose to buy.

SATELLITE TV SYSTEM:

Today, most satellite TV customers get their programming through a direct

broadcast satellite (DBS) provider, such as DirecTV or DISH Network. The provider 

selects programs and broadcasts them to subscribers as a set package. Basically, the

 provider's goal is to bring dozens or even hundreds of channels to your TV in a formthat approximates the competition, cable TV.

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Unlike earlier programming, the provider's broadcast is completely digital, which

means it has much better picture and sound quality. Early satellite television was

 broadcast in C-band radio -- radio in the 3.7-gigahertz (GHz) to 6.4-GHz frequency

range. Digital broadcast satellite transmits programming in the Ku frequency range

(11.7 GHz to 14.5 GHz ).

The Components:

There are five major components involved in a direct to home (DTH) or  directbroadcasting (DBS) satellite system: the programming source, the broadcast center,

the satellite, the satellite dish and the receiver.

• Programming sources are simply the channels that provide

  programming for broadcast. The provider doesn't create original

  programming itself; it pays other companies (HBO, for example, or 

ESPN) for the right to broadcast their content via satellite. In this way,

the provider is kind of like a broker between you and the actual

  programming sources. (Cable TV companies work on the same

 principle.)

• The broadcast center is the central hub of the system. At the

  broadcast center, the TV provider receives signals from various  programming sources and beams a broadcast signal to satellites in

geosynchronous orbit.

• The satellites receive the signals from the broadcast station and

rebroadcast them to Earth.

• The viewer's dish picks up the signal from the satellite (or multiple

satellites in the same part of the sky) and passes it on to the receiver in

the viewer's house.

• The receiver processes the signal and passes it on to a standard TV.

SATELLITE TV PROGRAMMING:

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Satellite TV providers get programming from two major sources: national

turnaround channels (such as HBO, ESPN and CNN) and various local channels

(the ABC, CBS, Fox, NBC and PBS affiliates in a particular area). Most of the

turnaround channels also provide programming for cable TV, and the local channels

typically broadcast their programming over the airwaves.

Turnaround channels usually have a distribution center that beams their programming

to a geosynchronous satellite. The broadcast center uses large satellite dishes to pick 

up these analog and digital signals from several sources.

Most local stations don't transmit their programming to satellites, so the provider has

to get it another way. If the provider includes local programming in a particular area,

it will have a small local facility consisting of a few racks of communications

equipment. The equipment receives local signals directly from the broadcaster 

through fiber-optic cable or an antenna and then transmits them to the central

 broadcast center.

The broadcast center converts all of this programming into a high-quality,

uncompressed digital stream. At this point, the stream contains a vast quantity of data

-- about 270 megabits per second (Mbps) for each channel. In order to transmit the

signal from there, the broadcast center has to compress it. Otherwise, it would be too

 big for the satellite to handle. In the next section, we'll find out how the signal is

compressed

Satellite TV Signal:

Satellite signals have a pretty long path to follow before they appear on your TVscreen in the form of your favorite TV show. Because satellite signals contain such

high-quality digital data, it would be impossible to transmit them without

compression. Compression simply means that unnecessary or repetitive information is

removed from the signal before it is transmitted. The signal is reconstructed after 

transmission.

Standards of Compression:

Satellite TV uses a special type of video file compression standardized by the MovingPicture Experts Group (MPEG). With MPEG compression, the provider is able to

transmit significantly more channels. There are currently five of these MPEG

standards, each serving a different purpose. DirecTV and DISH Network, the two

major satellite TV providers in the United States, once used MPEG-2, which is still

used to store movies on DVDs and for digital cable television (DTV). With MPEG-2,

the TV provider can reduce the 270-Mbps stream to about 5 or 10 Mbps (depending

on the type of programming).

 Now, DirecTV and DISH Network use MPEG-4 compression. Because MPEG-4 was

originally designed for streaming video in small-screen media like computers, it can

encode more efficiently and provide a greater bandwidth than MPEG-2. MPEG-2remains the official standard for digital TV compression, but it is better equipped to

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analyze static images, like those you see on a talk show or newscast, than moving,

dynamic images. MPEG-4 can produce a better picture of dynamic images through

use of spatial (space) and temporal (time) compression. This is why satellite TV using

MPEG-4 compression provides high definition of quickly-moving objects that

constantly change place and direction on the screen, like in a basketball game.

Satellite TV Encoding and Encryption:

At the broadcast center, the high-quality digital stream of video goes through an

MPEG encoder, which converts the programming to MPEG-4 video of the correct

size and format for the satellite receiver in your house.

Encoding works in conjunction with compression to analyze each video frame and

eliminate redundant or irrelevant data and extrapolate information from other frames.

This process reduces the overall size of the file. Each frame can be encoded in one of 

three ways:

• As an intraframe, which contains the complete image data for that

frame. This method provides the least compression.

• As a predicted frame, which contains just enough information to

tell the satellite receiver how to display the frame based on the most

recently displayed intraframe or predicted frame. A predicted frame

contains only data that explains how the picture has changed from the

 previous frame.

• As a bidirectional frame, which displays information from the

surrounding intraframe or predicted frames. Using data from the closest

surrounding frames, the receiver  interpolates the position and color of 

each pixel.

This process occasionally produces artifacts -- glitches in the video image. One

artifact is macroblocking, in which the fluid picture temporarily dissolves into

 blocks. Macroblocking is often mistakenly called pixilating, a technically incorrect

term which has been accepted as slang for this annoying artifact. Graphic artists and

video editors use "pixilating" more accurately to refer to the distortion of an image.

There really are pixels on your TV screen, but they're too small for your human eye to

 perceive them individually -- they're tiny squares of video data that make up the

image you see.

Satellite Dish:

When the signal reaches the viewer's house, it is captured by the satellite dish. A

satellite dish is just a special kind of antenna designed to focus on a specific broadcast

source. The standard dish consists of a parabolic (bowl-shaped) surface and a central

feed horn. To transmit a signal, a controller sends it through the horn, and the dish

focuses the signal into a relatively narrow beam.

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The curved dish reflects energy from the feed horn, generating a narrow beam.

The dish on the receiving end can't transmit information; it can only receive it. The

receiving dish works in the exact opposite way of the transmitter. When a beam hits

the curved dish, the parabola shape reflects the radio signal inward onto a particular 

 point, just like a concave mirror focuses light onto a particular point.

The curved dish focuses incoming radio waves onto the feed horn.

In this case, the point is the dish's feed horn, which passes the signal on to the

receiving equipment. In an ideal setup, there aren't any major obstacles between the

satellite and the dish, so the dish receives a clear signal.

In some systems, the dish needs to pick up signals from two or more satellites at the

same time. The satellites may be close enough together that a regular dish with a

single horn can pick up signals from both. This compromises quality somewhat,

 because the dish isn't aimed directly at one or more of the satellites. A new dishdesign uses two or more horns to pick up different satellite signals. As the beams from

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different satellites hit the curved dish, they reflect at different angles so that one beam

hits one of the horns and another beam hits a different horn.

The central element in the feed horn is the low noise blockdown converter, or LNB.

The LNB amplifies the radio signal bouncing off the dish and filters out the noise

(radio signals not carrying programming). The LNB passes the amplified, filtered

signal to the satellite receiver inside the viewer's house.

Satellite Receiver:

The end component in the entire satellite TV system

is the receiver. The receiver has four essential jobs:• It de-scrambles the encrypted

signal. In order to unlock the signal, the

receiver needs the proper decoder chip for 

that programming package. The provider 

can communicate with the chip, via the satellite signal, to make necessary

adjustments to its decoding programs. The provider may occasionally

send signals that disrupt illegal de-scramblers as an electronic counter

measure (ECM) against illegal users.

• It takes the digital MPEG-2 or MPEG-4 signal and converts it

into an analog format that a standard television can recognize. In the

United States, receivers convert the digital signal to the analog NationalTelevision Systems Committee (NTSC) format. Some dish and receiver 

setups can also output an HDTV signal.

• It extracts the individual channels from the larger satellite

signal. When you change the channel on the receiver, it sends just the

signal for that channel to your TV. Since the receiver spits out only one

channel at a time, you can't tape one program and watch another. You

also can't watch two different programs on two TVs hooked up to the

same receiver. In order to do these things, which are standard on

conventional cable, you need to buy an additional receiver.

• It keeps track of pay-per-view programs and periodically

phones a computer at the provider's headquarters to communicatebilling information. 

Receivers have a number of other features as well. They pick up a programming

schedule signal from the provider and present this information in an onscreen

 programming guide. Many receivers have parental lock-out options, and some have

 built-in digital video recorders(DVRs), which let you pause live television or record it

on a hard drive.

These receiver features are just added bonuses to the technology of satellite TV. With

its movie-quality picture and sound, satellite TV is becoming a popular investment for 

Photo courtesy DirecTV

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consumers. Digital cable, which also has improved picture quality and extended

channel selection, has proven to be the fiercest competitor to satellite providers. The

TV war is raging strong between satellite and digital cable technologies as well as

 between the providers who offer these services. Once considered luxuries in most

households, satellite and digital cable are becoming quite common as providers

 bundle TV with Internet and phone services to offer competitive deals and win over customers.

Problems with Broadcast TV:

Conceptually, satellite TV is a lot like broadcast TV. It's a wireless system for 

delivering television programming directly to a viewer's house. Both broadcast

television and satellite stations transmit programming via a radio signal.

Broadcast stations use a powerful antenna to transmit radio waves to the surrounding

area. Viewers can pick up the signal with a much smaller antenna. The main

limitation of broadcast TV is range. The radio signals used to broadcast televisionshoot out from the broadcast antenna in a straight line. In order to receive these

signals, you have to be in the direct line of sight of the antenna. Small obstacles like

trees or small buildings aren't a problem; but a big obstacle, such as the Earth, will

reflect these radio waves.

If the Earth were perfectly flat, you could pick up broadcast TV thousands of miles

from the source. But because the planet is curved, it eventually breaks the signal's line

of sight. The other problem with broadcast TV is that the signal is often distorted,

even in the viewing area. To get a perfectly clear signal like you find on cable, you

have to be pretty close to the broadcast antenna without too many obstacles in the

way.

The Satellite TV Solution: 

Satellite TV solves the problems of range and distortion by transmitting broadcast

signals from satellites orbiting the Earth. Since satellites are high in the sky, there are

a lot more customers in the line of sight. Satellite TV systems transmit and receive

radio signals using specialized antennas called satellite dishes.

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Satellites are higher in the sky than TV antennas, so they have a much larger line

of sight range. 

The TV satellites are all in geosynchronous orbit, meaning that they stay in one

 place in the sky relative to the Earth. Each satellite is launched into space at about

7,000 mph (11,000 kph), reaching approximately 22,200 miles (35,700 km) above the

Earth. At this speed and altitude, the satellite will revolve around the planet once

every 24 hours -- the same period of time it takes the Earth to make one full rotation.

In other words, the satellite keeps pace with our moving planet exactly. This way, you

only have to direct the dish at the satellite once, and from then on it picks up the

signal without adjustment, at least when everything works right.

At the core, this is all there is to satellite TV. But as we'll see in the next section, there

are several important steps between the original programming source and your TV

set.

ADVANTAGES OF SATELLITE COMMUNICATIONS:

Satellite communications have unique advantages over conventional long distance

transmissions. Satellite links are unaffected by the propagation variations that

interfere with hf radio. They are also free from the high attenuation of wire or cablefacilities and are capable of spanning long distances. The numerous repeater stations

required for line-of-sight or troposcatter links are no longer needed. They furnish the

reliability and flexibility of service that is needed to support a military operation.

Capacity:

The present military communications satellite system is capable of communications

  between backpack, airborne, and shipboard terminals. The system is capable of 

handling thousands of communications channels.

Reliability:

Communications satellite frequencies are not dependent upon reflection or refractionand are affected only slightly by atmospheric phenomena. The reliability of satellite

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communications systems is limited only by the equipment reliability and the skill of 

operating and maintenance personnel.

Vulnerability:

Destruction of an orbiting vehicle by an enemy is possible. However, destruction of a

single communications satellite would be quite difficult and expensive. The costwould be excessive compared to the tactical advantage gained. It would be

 particularly difficult to destroy an entire multiple-satellite system such as the twenty-

six random-orbit satellite system currently in use. The earth terminals offer a more

attractive target for physical destruction. These can be protected by the same

measures that are taken to protect other vital installations.

A high degree of freedom from jamming damage is provided by the highly directional

antennas at the earth terminals. The wide bandwidth system that can accommodate

sophisticated anti-jam modulation techniques also lessens vulnerability

Flexibility

Most operational military satellite earth terminals are housed in transportable vans.

These can be loaded into cargo planes and flown to remote areas. With trained crews

these terminals can be put into operation in a matter of hours. Worldwide

communications can be established quickly to remote areas nearly anywhere in the

free world.

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 theplanet located at one of the two foci of the ellipse. The circular orbit may alsobe considered as an ellipse where the two foci of the ellipse coincide at thecenter 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 launchvehicles in placing satellites in higher orbits. With the advancement of launchvehicles and satellite technologies, once the Geo Stationary Orbit (GSO) wasachieved, majority of the satellites for telecommunications started using GSOdue to its many advantages. During 1990s the interests in NGSOs wererekindled due to several advantages of NGSO in providing global personalcommunications in spite of its many disadvantages.

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Advantages of NGSO are:

• 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 permitslow cost antennas for hand-held terminals

 Disadvantages of NGSO are:

•Requirement of a large number of orbiting satellites for globalcoverage as each low earth orbit satellite covers a small portionof the earth’s surface for a short time.

• Complex hand over problem of transferring signal from onesatellite to another 

• Less expected life of satellites at NGSO requires more frequentreplacement of satellites compared to satellite in GSO

• Compensation of Doppler shift is necessary

• Satellites at NGSO undergoes eclipse several times a daynecessitates the requirement of robust on board battery systemfor the satellite for operations without solar power during eclipse

• Complex network management for a constellation of satellitesand corresponding ground system

• Problem of increasing space debris in the outer space

There are different types of Non Geostationary Orbits (NGSO), depending onthe orbital height and the inclination of the orbital plane. Inclination is theangle 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 measuredfrom 0 to 180 degrees. NGSOs are classified in the following three types asper the inclinations of the orbital plane

 • Polar Orbit

• Equatorial Orbit

• Inclined Orbit

In polar orbit the satellite moves from pole to pole and the inclination is equalto 90 degrees. In equatorial orbit the orbital plane lies in the equatorial planeof 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 therotation of the earth on its axis. Satellite orbit with inclination of more than 90

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degrees is called retrograde orbit when the satellite moves in a directionopposite to the rotational motion of the earth. Orbits of almost allcommunication satellites are prograde orbits, as it takes less propellant toachieve the final velocity of the satellite in prograde orbit by taking advantageof the earth’s rotational speed. Example of retrograde orbit is the sun

synchronous orbit where the orbital parameters are such that that the satellitecrosses the same latitude at the same local time. This type of orbit is used for earth observation satellites where repeated observations are required to bemade under the same sun angle. It needs more propellant to launch a satellitein 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. Theseare:- 

• 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 knownas 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 about25,000 km are known as Medium Earth Orbit (MEO) / Intermediate Circular orbit (ICO). MEO and ICO are often used synonymously, but MEOclassification is not restricted to circular orbits. Satellites in Highly EllipticalOrbit (HEO) are suitable for communications in the higher latitudes. RussianMolnya satellites have highly inclined elliptical orbits with a perigee of about1000 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 equatorialcircular 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 theworld for communications purposes.

Geostationary Orbit (GSO)

There is only one geostationary orbit possible around the earth, lying on theearth’s equatorial plane and the satellite orbiting at the same speed as therotational speed of the earth on its axis. For a Satellite to have an orbitalperiod equal to that of earth’s rotation i.e. a sidereal day (23 Hrs 56 min. 4.09sec.) an altitude of 35,786 km is required. Such a satellite orbiting at avelocity of 3.075 km/sec remains fixed relative to any point on earth or geostationary. With the idealized assumptions that the geostationary satelliteis at rest relative to the earth the conditions required to be satisfied for geostationary orbit are:

• The orbit shall be circular 

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• 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 equatorialplane but the sub-satellite longitude, i.e. the longitude of the

projection of the satellite on the Earth’s surface can be selectedarbitrarily.

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 groundsegment and the satellite with fixed directional antennas at both the earthstation and the satellite.

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 nocoverage

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

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IT’S FUTURE-THE CONCLUSION:

In the future SATELLITE TV will open up so many possibilities that it is hard to

imagine what our lives will be like in 30, or even just 20 years from now. To give an

idea of how fast things are going. 50 years ago, there was nothing in space that wasmade by humans. Now there are even satellite graveyards (specific orbits where

obsolete satellites are "parked"). The possibilities of satellite TV technology are

growing faster every year. What took 10 years to develop 30 years ago is now done in

2 years.

Satellite TV is one of the driving forces for satellite technologies because the need to

 please million of subscribers is much stronger than the need to please the relative

limited needs of communications for commercial purposes. The future of satellite TV

is so bright, that a supernova would pale in comparison.