SATELLITE

COMMUNICATION Assignment

ABSTRACT Satellite has become popular as a media of

communication. In this paper some

terminology, methodology how it works

and uses of satellites are described in a very

easy and effective way.

Name: Aklima Zaman ID: 122400011 Wireless Network

Date of submission:

SUBMIT TO

ASIF IMRAN

Eastern University Dhaka

Satellite Communications

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Contents What is a Satellite? ....................................................................................................................................... 3

How do Satellites Work? .............................................................................................................................. 4

Some Terminology ....................................................................................................................................... 4

Uplink .................................................................................................................................................... 4

Transponder ......................................................................................................................................... 5

Downlink ............................................................................................................................................... 5

Advantages of Satellites ............................................................................................................................... 5

Disadvantages of Satellites .......................................................................................................................... 6

Basics: Factors in satellite communication ................................................................................................. 7

Azimuth angle ...................................................................................................................................... 7

Elevation Angle ................................................................................................................................... 7

Coverage Angle ..................................................................................................................................... 8

Free space loss ...................................................................................................................................... 9

Satellite Footprint ................................................................................................................................ 9

Atmospheric Absorption Loss .............................................................................................................. 9

What Are Satellites Used For ....................................................................................................................... 9

Television ................................................................................................................................................ 10

Telephones ............................................................................................................................................. 10

Navigation .............................................................................................................................................. 11

Business & Finance ................................................................................................................................. 11

Weather .................................................................................................................................................. 12

Climate and Environmental Monitoring ............................................................................................ 12

Safety ...................................................................................................................................................... 12

Land Stewardship ................................................................................................................................. 13

Development .......................................................................................................................................... 13

Space Science ......................................................................................................................................... 13

Satellites service types ............................................................................................................................... 14

1. Fixed Service Satellites (FSS) .......................................................................................................... 14

How Fixed Satellite Service Works .................................................................................................... 14

Applications of Fixed Satellite Service ............................................................................................... 14

Advantages of Fixed Satellite Service ................................................................................................ 14

2. Broadcast Service Satellites (BSS) or Direct Broadcast Service (DBS) .......................................... 15

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How BSS or DBS works ....................................................................................................................... 15

Advantages of Broadcast Service Satellites ....................................................................................... 16

3. Mobile Service Satellites (MSS) ..................................................................................................... 17

Applications of MSS............................................................................................................................ 17

Satellites Orbits .......................................................................................................................................... 17

Geostationary Earth Orbit (GEO) ........................................................................................................... 18

Advantages ......................................................................................................................................... 18

Limitations .......................................................................................................................................... 18

Low Earth Orbit (LEO) ............................................................................................................................. 19

How a Satellite Reaches LEO – Requirements ................................................................................... 20

Advantages of Low Earth Orbit .......................................................................................................... 20

Disadvantages of Low Earth Orbit ..................................................................................................... 20

Medium Earth Orbit (MEO) .................................................................................................................... 21

Advantage of Medium Earth Orbit .................................................................................................... 22

Disadvantage of Medium Earth Orbit ................................................................................................ 22

Molniya Orbit Satellites ......................................................................................................................... 22

Advantages ......................................................................................................................................... 23

Disadvantages .................................................................................................................................... 23

High Altitude Platform (HAP) ................................................................................................................. 23

HAP APPLICATIONS ............................................................................................................................ 23

Types of HAP ....................................................................................................................................... 23

Frequency Bands ........................................................................................................................................ 25

Capacity Allocation ..................................................................................................................................... 25

Frequency Division Multiple Access (FDMA) ......................................................................................... 25

Time Division Multiple Access (TDMA) ............................................................................................ 28

REFERENCES ................................................................................................................................................ 30

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What is a Satellite? A satellite is a moon, planet or machine that orbits a planet or star. For example, Earth is

a satellite because it orbits the sun. Likewise, the moon is a satellite because it orbits

Earth. Usually, the word "satellite" refers to a machine that is launched into space and

moves around Earth or another body in space.

Earth and the moon are examples of natural satellites. Thousands of artificial, or man-

made, satellites orbit Earth. Some take pictures of the planet that help meteorologists

predict weather and track hurricanes. Some take pictures of other planets, the sun,

black holes, dark matter or faraway galaxies. These pictures help scientists better

understand the solar system and universe.

Still other satellites are used mainly for communications, such as beaming TV signals and

phone calls around the world. A group of more than 20 satellites make up the Global

Positioning System, or GPS. If you have a GPS receiver, these satellites can help figure

out your exact location. [1]

Figure 1: A satellite

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How do Satellites Work?

Figure 2: How Satellite Works

Two earth stations wants to communicate with each other by wireless. But the range is

too long as the cant communicate. In that situation those station use satellite to

transferring data from each other. Satellite is an intermediate device which receives

data from earth stations and just relay the data to another earth station.

Some Terminology

Uplink

Figure 3: Uplink

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Uplink is a transmission path by which radio or other signals are sent to a

communications satellite. This portion of a communications link used for the

transmission of signals from an Earth terminal to a satellite. An uplink is the converse of

a downlink. An uplink or downlink is distinguished from reverse link or forward link.

https://www.wordnik.com/words/Uplink

Transponder Transponder is an electrical device designed to receive a specific signal and

automatically transmit a specific reply. The satellite Transponder converts the signal

which it get from the earth station and sends it to the second earth station. [2]

Downlink

Figure 4: Downlink

Downlink is a transmission path for the communication of signals and data from a

communications satellite to the earth station. [3]

Advantages of Satellites If two stations can’t communicate in wireless medium than we have to use satellite. It

helps those stations to transfer data. It saves the cost of intercontinental cable laying and

the maintenance that would otherwise be necessary.

A satellite communications system is relatively inexpensive because there are no cable­

laying costs and one satellite covers a large area. Transmission cost of a satellite is

independent of the distance from the center of the coverage area.

Users can enjoy mobile communication anywhere within the satellite coverage area.

Satellite networks have simpler topology, which results in more manageable network

performance.

A satellite system will not suffer from disasters such as floods, fire, and earthquakes and

will, therefore, be available as an emergency service should terrestrial services be

knocked out.

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Because of the line-of-sight communication problem, the terrestrial or ground­ based

wireless requires many hops between a series of communication towers in order for the

signal to travel even I 00 km. Satellite communication requires only one hop from the

base station to the satellite and back down to the signal destination. Therefore, satellite

communication is a form of wireless communication.

With wireless technology, information is transferred between two antennas. Information

is encoded into radio signals at one end and decoded into data at the other.

Satellites provide seamless service. Broadcasting of a signal over a wide area makes the.

Simultaneous distribution of band width intensive information to thousands of locations.

Terrestrial microwave communications operate in low-GHz range, which limits all

communications to line- f-sight [13).The terrestrial microwave equipment such as

telephone relay towers are placed every few miles to relay telephone signals. The

coverage area of a satellite greatly exceeds that of a terrestrial system. Satellite

communications are available almost everywhere. A small constellation of three satellites

can cover the entire earth's surface as shown in Fig.

Figure 5: A Sample Satellite Network

Satellite communication can support all recently invented data like audio, video and all

type of data. Also broadband Internet satellites services are available to domestic and

business use.

Satellites can cover a much larger geographical area than traditional ground-based

systems. Satellites have the unique ability to cover the globe. For example TV, Fax,

Internet etc.

Satellite to Satellite communication is very precise.

Disadvantages of Satellites The parts of satellites are very costly ad to set it in the orbit a rock is needed. So, it costs

huge to build a rocket. That’s why lunching satellite is too expensive.

When a satellite starts its duty, people try to use that frequently. So day by day a lot more

users used that satellite so its bandwidth is fill up soon.

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As satellites are far from the earth base station and there are many obstacle (Clouds, High

storied building, etc.) between them the propagation delay increases for many reason. So

real time system faces many problems to use satellite.

Figure 6: Terrestrial Com. in Earth

Basics: Factors in satellite communication The coordinates to which the earth station antennas must be pointed to communicate with

satellite is called look angles. There are two types of Look angles.

Azimuth angle

Measured eastward from geographic north to the projection of satellite path on

the local horizontal plane at the earth station.

Elevation Angle

Measured upward from local horizontal plane at the earth station to the satellite

path. Coverage area of a satellite depends on Elevation Angle. The standard

condition is to keep Elevation Angle in 0 degree. So that so the transmission ray

spreads the skyline visible to the satellite in all directions. Due to earth

environmental factors like obstacle the transmission, atmospheric attenuation, and

the earth electrical background noise, the earth station should maintain a

minimum elevation angle.

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Figure 7: Elevation Angle

Coverage Angle Coverage Angle of satellite is the measure of the portion of the earth surface visible to a

satellite taking the minimum elevation angle into account.

R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)= cos(β + θ)/cos(θ)

o R = 6370 km (earth’s radius)

o h = satellite orbit height

o β = coverage angle

o θ = minimum elevation angle

Figure 8: Coverage Angle

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Free space loss The space between the earth station and satellite is known as free space loss. In satellite

communications, free-space loss is the major loss suffered by signals in travelling over

the Earth-satellite path. The loss is inversely proportional to the square of the distance

travelled and inversely proportional to the square of the frequency used. That is, as the

distance is doubled the received power is reduced by a factor of four. Similarly, as the

frequency is doubled the received power is reduced by a factor of four. Free-space loss

for geostationary satellite communications satellites varies between 190 to 210 dB

depending on the frequency used. [4]

Satellite Footprint The area of the Earth covered by the radiation from a satellite is called its footprint. The

size of the footprint depends on the location of the satellite in its orbit and the shape and

size of beam produced by its antennas. But the highest strength is in the center of

footprint and signal becomes weaker as long as far from center. [5]

Figure 9: Satellite Footprint

Atmospheric Absorption Loss In satellite communications, atmospheric Absorption loss results from the absorption of

the Earth-satellite signals as they pass through the Earth's atmosphere. The value of the

atmospheric loss is strongly dependent on frequency. And also it is caused by air and fog.

The transmission will so bad during rain, fog or air.

What Are Satellites Used For Satellites are manmade objects put into orbit. They often affect our lives without our

realizing it: they make us safer, provide modern conveniences, and broadcast

entertainment. Here are some of the jobs satellites do:

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Television

Figure 10: Dish TV

Satellites send television signals directly to homes, but they also are the backbone of

cable and network TV. These satellites send signals from a central station that generates

programming to smaller stations that send the signals locally via cables or the airwaves.

"At the scene" news broadcasts, whether live reporting on a vote at the Capitol or from

the scene of a traffic accident, are sent from the field to the studio via satellite, too.

Telephones

Figure 11: Flight phone communications on airplanes

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Satellites provide in-flight phone communications on airplanes, and are often the main

conduit of voice communication for rural areas and areas where phone lines are

damaged after a disaster. Satellites also provide the primary timing source for cell

phones and pagers. In 1998, a satellite failure demonstrated this dependence; it

temporarily silenced 80 percent of the pagers in the United States, National Public Radio

was not able to distribute its broadcasts to affiliates and broadcasted only via its

website, and on the CBS Evening News, the image of Dan Rather froze while the audio

continued.

Navigation

Figure 12: GPS Satellite

Satellite-based navigation systems like the Navistar Global Positioning Systems (known

colloquially as GPS) enable anyone with a handheld receiver to determine her location to

within a few meters. GPS locators are increasingly included in in-car direction services

and allow car-share services like Zip car to locate their cars. GPS-based systems are used

by civilians and the military for navigation on land, sea, and air, and are crucial in

situations like a ship making a difficult course in a harbor in bad weather or troops lost in

unfamiliar territory, where other navigation tools may not exist.

Business & Finance Communications satellites have the ability to rapidly communicate between a numbers of

widely dispersed locations. This is an important tool, allowing big manufacturing

companies and department stores to perform inventory management, provide instant

credit card authorization and automated teller banking services to even small towns, pay-

at-the-pump gas at freeway gas stations, and video conferencing for international

corporations.

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Weather

Figure 13: Satellite Winds marker

Satellites provide meteorologists with the ability to see weather on a global scale,

allowing them to follow the effects of phenomena like volcanic eruptions and burning gas

and oil fields, to the development of large systems like hurricanes and El Niño.

Climate and Environmental Monitoring

Satellites are some of the best sources of data for climate change research. Satellites

monitor ocean temperatures and prevailing currents; data acquired by satellite-borne

radars were able to show sea levels have been rising by three mm a year over the last

decade. Imaging satellites can measure the changing sizes of glaciers, which is difficult to

do from the ground due to the remoteness and darkness of the Polar Regions. Satellites

can determine long-term patterns of rainfall, vegetation cover, and emissions of

greenhouse gases.

Safety

Earth observation satellites can monitor ocean and wind currents as well as the extent of

forest fires, oil spills, and airborne pollution; together this information helps organize

emergency responders and environmental cleanup. Satellites can take the "search" out of

"search and rescue" for people in distress in remote regions. Distress radio beacons

directly linked to a search and rescue satellite can lead rescuers quickly and accurately to

a land, sea, or air emergency location.

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Land Stewardship

Satellites can detect underground water and mineral sources; monitor the transfer of

nutrients and contaminants from land into waterways; and measure land and water

temperatures, the growth of algae in seas, and the erosion of topsoil from land. They can

efficiently monitor large-scale infrastructure, for example fuel pipelines that need to be

checked for leaks, which would require enormous hours of land- or air-based inspection.

Imaging satellites produce high-resolution data of almost the entire landmass on earth;

such data used to be a closely guarded military capability, but now, nearly anyone with

an internet connection can find his house using Google Earth.

Development

Satellites are increasingly important to the developing world. For a country like India,

with populations separated by rough terrain and different languages, communications

satellites provides remote populations access to education and to medical expertise that

would otherwise not reach them. Earth observation satellites also allow developing

countries to practice informed resource management and relief agencies to follow refugee

population migrations.

Space Science

Before the Space Age, astrophysicists were limited to studying the universe via ground-

based telescopes, and so could only use information from the parts of the electromagnetic

spectrum that penetrated the Earth's atmosphere. Many of the most interesting

phenomena are best studied at frequencies that are best or only accessible from space—

satellite telescopes have been critical to understanding phenomena like pulsars and black

holes as well as measuring the age of the universe. The Hubble Space Telescope is

arguably the most valuable astronomical tool ever built! [6]

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Satellites service types

1. Fixed Service Satellites (FSS)

Figure 14: 1. Fixed Service Satellites

Fixed Satellite Service (FSS) is a type of mobile telephone service that allows

users in a specific area to make and receive cell phone calls. FSS systems or cell

phone towers are placed in strategic, fixed locations and provide service to

thousands of individual users simultaneously. Generally, FSS systems provide

reception for several square miles around them, which borders another FSS

system’s reception area. This allows users to communicate with multiple FSS

systems as they travel without losing reception.

How Fixed Satellite Service Works

FSS systems are essentially large transceivers that are able to both broadcast and

receive cell phone signals across large distances. FSS systems differ from MSS

(Mobile Satellite Service) systems as they are permanently placed in a specific

area, while MSS systems can be moved from one location to another or mounted

on a mobile vehicle. FSS systems are the most commonly used devices for

handling mobile cell phone communications and make up the majority of the

global telephony infrastructure.

Applications of Fixed Satellite Service

FSS systems handle communications for thousands of individual cell phones in a

specific area. FSS systems are capable of handling cell phone calls, text

messages, images, videos, and any other type of file that can be sent or received

via standard GSM signals.

Advantages of Fixed Satellite Service

FSS systems are advantageous because they provide guaranteed reception in the

area that they are placed. Since FSS systems are permanently installed, they

collectively provide users with reliable cell phone reception across the majority

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of a country. As the user gets closer to an FSS unit, his/her signal becomes

stronger. Likewise, as a user gets further away from an FSS unit, his/her signal

becomes weaker. However, when a user gets far enough away from one FSS

unit, his/her cell phone will automatically begin communicating with the next

FSS unit closest to it. [7]

2. Broadcast Service Satellites (BSS) or Direct Broadcast Service (DBS) Direct broadcast satellite (DBS) refers to satellite television (TV) systems in which

the subscribers, or end users, receive signals directly from geostationary

satellites. Signals are broadcast in digital format at microwave frequencies. DBS

is the descendant of direct-to-home (DTH) satellite services.

A DBS subscriber installation consists of a dish antenna two to three feet (60 to

90 centimeters) in diameter, a conventional TV set, a signal converter placed

next to the TV set, and a length of coaxial cable between the dish and the

converter. The dish intercepts microwave signals directly from the satellite. The

converter produces output that can be viewed on the TV receiver.

How BSS or DBS works

Satellite TV works as a simple triangular scheme. The uplink station collects

signals from various content providers (HBO, ESPN, Discovery, local providers,

etc.), transforms it into high-frequency signal and emits to TV satellites orbiting

the Earth. The satellites then send downlink signal back to Earth through

electronic devices at the satellite called transponders. Single satellite can have

up to four dozen transponders, each capable of transmitting up to a dozen TV

channels. It adds up to a few hundreds of channels transmission capacity per

single satellite possible.

Due to the very high altitude - 22.236 miles - TV satellite can cover much larger

ground area - so called footprint -than a ground based antenna. Signal from the

satellite is picked up by a small satellite dish antenna mounted at the

subscriber's residence. Filtered and amplified, the signal is then sent to the

satellite TV receiver, where it gets decoded, transformed radio-wave to a

"usable" electrical form, and fed to the TV set, becoming a video/audio stream.

Originally, DBS frequency of choice was the high-powered Ku-band. However,

the need for expanded programming resulted in use of other frequencies: the

lower-power Ku-band's FSS frequencies, as well as lower-power C-band FSS

frequency, and higher-frequency Ka-band. Use of lower-powered emissions

resulted in the increase of the minimum dish size needed for efficient reception.

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Good signal reception requires no obstruction between the satellite and dish

receptor: the dish must have clear "view" of the satellite. Quality of reception

also depends on the receiver position within the "footprint" (the signal is

strongest in the center and weakens toward the periphery), position of the

satellite (the higher it is in the sky, the better) and weather conditions (heavy

rains or snow can affect signal quality). [8]

Advantages of Broadcast Service Satellites More choice: DBS offers one very important element in the world of paid

television service: Competition! Until recently, your only choice was your

local cable company, and their monopoly on television service provided no

incentive for additions or improvements outside of federal regulation. Today,

you have at least two options for satellite television service in lieu of cable,

and both are reasonably customizable to your tastes.

More channels per dollar: The receiver components which interface the

satellite signal to your television also serve as decoders. Therefore, the DBS

providers are able to use digital compression techniques to carry up to ten

times as many channels on the satellite transponders as would otherwise be

possible. As a general rule, you will get many more channels per dollar spent

with a satellite service than with cable TV.

Rural availability: For some people in remote rural areas, the cable vs. DBS

argument is moot because cable is simply not available. Satellite service, on

the other hand, is available anywhere in the contiguous 48 states (with more

limited availability in Alaska and Hawaii) as long as there is a clear line of

sight to the position of the satellite in the sky.

Reliable service: The cable infrastructure is always at the mercy of accidents

resulting in downed lines or severed cables. The only anamolies that typically

affect satellite broadcasts (aside from accidents involving your own dish

antenna) are extremely severe weather, or solar interferences during the

equinoxes, and these are rare.

Digital picture/sound: The analog signals sent over standard cable lines are

subject to degradation, interference, and other factors that can result in a

less than stellar picture. Satellite signals are digital, and like a compact disc,

are not subject to depreciation in picture or sound quality. Satellite TV

generally looks and sounds far superior to cable transmissions.

Interactive channel guides: Some cable systems include a channel which

shows program listings. These usually scroll along at an unbearably slow pace

and cannot be advanced to show more than a couple of hours of upcoming

shows. DirecTV and EchoStar include highly interactive program guides that

can be manually advanced, or can display additional program information,

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and sometimes can provide one-touch timed VCR recording. Certain

programming can also be locked out based upon content ratings. [9]

3. Mobile Service Satellites (MSS) Mobile satellite services (MSS) refers to networks of communications satellites

intended for use with mobile and portable wireless telephones. There are three

major types: AMSS (aeronautical MSS), LMSS (land MSS), and MMSS (maritime

MSS).

A telephone connection using MSS is similar to a cellular telephone link, except

the repeaters are in orbit around the earth, rather than on the surface. MSS

repeaters can be placed on geostationary, medium earth orbit (MEO), or low

earth orbit (LEO) satellites. Provided there are enough satellites in the system,

and provided they are properly spaced around the globe, an MSS can link any

two wireless telephone sets at any time, no matter where in the world they are

located. MSS systems are interconnected with land-based cellular networks.

Applications of MSS

As an example of how MSS can work, consider telephones in commercial

airliners. These sets usually link into the standard cellular system. This allows

communication as long as the aircraft is on a line of sight with at least one land-

based cellular repeater. Coverage is essentially continuous over most developed

countries. But coverage is spotty over less well-developed regions, and is

nonexistent at most points over the oceans. Using an MSS network, the aircraft

can establish a connection from any location, no matter how remote. [10]

Satellites Orbits

Figure 15: Satellite Coverage

There are some orbits where satellites are set and different orbits has different issues.

The orbits with advantages and disadvantages are given bellow.

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Geostationary Earth Orbit (GEO)

Figure 16: GEO Ring

A geostationary satellite is an earth-orbiting satellite, placed at an altitude of

approximately 35,800 kilometers (22,300 miles) directly over the equator, that

revolves in the same direction the earth rotates (west to east). At this altitude, one

orbit takes 24 hours, the same length of time as the earth requires to rotate once on

its axis. The term geostationary comes from the fact that such a satellite appears

nearly stationary in the sky as seen by a ground-based observer. BGAN, the new

global mobile communications network, uses geostationary satellites.

Advantages

A single geostationary satellite is on a line of sight with about 40 percent of the

earth's surface. Three such satellites, each separated by 120 degrees of longitude,

can provide coverage of the entire planet, with the exception of small circular

regions centered at the north and south geographic poles. A geostationary satellite

can be accessed using a directional antenna, usually a small dish, aimed at the spot

in the sky where the satellite appears to hover. The principal advantage of this type

of satellite is the fact that an earthbound directional antenna can be aimed and then

left in position without further adjustment. Another advantage is the fact that

because highly directional antennas can be used, interference from surface-based

sources, and from other satellites, is minimized.

Limitations

Geostationary satellites have two major limitations. First, because the orbital zone is

an extremely narrow ring in the plane of the equator, the number of satellites that

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can be maintained in geostationary orbits without mutual conflict (or even collision)

is limited. Second, the distance that an electromagnetic (EM) signal must travel to

and from a geostationary satellite is a minimum of 71,600 kilometers or 44,600

miles. Thus, a latency of at least 240 milliseconds is introduced when an EM signal,

traveling at 300,000 kilometers per second (186,000 miles per second), makes a

round trip from the surface to the satellite and back.

There are two other, less serious, problems with geostationary satellites. First, the

exact position of a geostationary satellite, relative to the surface, varies slightly over

the course of each 24-hour period because of gravitational interaction among the

satellite, the earth, the sun, the moon, and the non-terrestrial planets. As observed

from the surface, the satellite wanders within a rectangular region in the sky called

the box. The box is small, but it limits the sharpness of the directional pattern, and

therefore the power gain, that earth-based antennas can be designed to have.

Second, there is a dramatic increase in background EM noise when the satellite

comes near the sun as observed from a receiving station on the surface, because the

sun is a powerful source of EM energy. This effect, known as solar fade, is a problem

only within a few days of the equinoxes in late March and late September. Even

then, episodes last for only a few minutes and take place only once a day. [11]

Low Earth Orbit (LEO)

Figure 17: LEO And GEO Orbits

Most commonly, Low Earth Orbit or LEO is defined as the orbit that extends 160

- 2,000 km (100 - 1250 miles) above the Earth's surface. There are also other

definitions that make low Earth orbits extend to up to 3,000 km. Some human

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objects that operate from LEO are the Space Shuttle, the Hubble Telescope, the

International Space Station or ISS and a large number of satellites. Most of the

manned missions besides the Apollo program have also taken place in this

region.

How a Satellite Reaches LEO – Requirements

In order for an object to escape the Earth's gravity, it has to acquire a net velocity

of at least 7,814 m/s tangent to the curve of the Earth. This is a requirement to

reach an orbital altitude of about 150 km (92 miles). In order to keep a stable orbit

around the Earth, the object or satellite has to have enough speed to balance the

gravitational force pulling it back down. The lower a satellite orbits the Earth, the

more speed it needs to balance this force.

For example, at an altitude of 100 miles the satellite will need a velocity of about

17,500 miles per hour and will orbit the Earth in about 90 minutes.

Correspondingly, at an altitude of 22,000 miles, the satellite will need a speed of

about 7000 miles per hour and will make an orbit in about 24 hours. The other

types of orbit require much more rocket fuel and energy.

Advantages of Low Earth Orbit

Using LEO as an operational zone has certain advantages. It is closer to the

Earth's surface and much cheaper and convenient to place a satellite, perform

experiments, fix and install new equipment. After the operations are finished, the

return trip lasts for a very short time.

However, there are disadvantages as well. The amount of atmospheric gases is

low but still significant in these altitudes. These gases cause atmospheric drag that

result in orbital decay over time. The satellite will eventually slow down and be

pulled back by the Earth's gravity. Another serious problem is the dwell time, or

the time a satellite remains above a region. LEO satellites orbit the Earth too

quickly (~18,000 miles per hour) and this may cause problems during the

operation of a weather or communications satellite over a certain part of the

globe. A low Earth orbit is the simplest and most cost effective of satellite

placement and provides high bandwidth and low latency. [12]

Disadvantages of Low Earth Orbit

On the other hand LEO systems face a serious problem. Over 35 million bits of

debris primarily pieces of old satellites, launch vehicles and solid rocket fuel

currently orbit the earth, at speeds of up to seven kilometers per second. In

comparison, a bullet fired from a high-speed rifle travels at only 0.8 kilometers

per second. This forces LEO companies such as Iridium and Globalstar to worry

not only about making sure their satellites work but that flying garbage doesn't

knock them out of commission. Studies conducted by the National Aeronautics

and Space Administration show that a collision with a fragment the size of a

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marble would do serious damage to a satellite. A 10-centimeter piece would be

enough to permanently put the satellite out of order.

Collisions with such debris are not rare events; space shuttles, for example,

regularly return to earth with cracked windows and pockmarks presumed to have

been caused by encounters with space junk. In July 1996, a French military

satellite was destroyed by a fragment from the Pegasus rocket that had carried it

into space. The problem is particularly acute for LEO satellites. Lower orbits must

pass through the debris from the rockets that carry higher-orbiting satellites

farther into space. The lower altitudes are also strewn with pieces of other

satellites 130 of which have broken up in space. To make matters worse, as the

numbers of satellites taken to the skies increases, the probability of collisions

increases, which in turn creates more junk to threaten increasingly crowded orbit

paths. Already high insurance premiums for these enterprises will get worse.

Figuring out ways to protect satellites and space vehicles from space junk is not

an easy task. Shields and insulation material are standard parts of satellites in low

earth orbit. Furthermore companies such as Iridium and Globalstar have factored

in the risk of space junk by launching extra satellites that can replace disabled

ones. This raises the multibillion-dollar price tag of network deployment, but it

also protects against downtime, and thus lost revenue. [13]

Medium Earth Orbit (MEO)

Figure 18: Medium Earth Orbit (MEO)

A medium earth orbit satellite (MEO) is a satellite that orbits the earth in between

Low Earth Orbit Satellites (LEO), which orbit the earth at a distance from the

earth of about 200-930 miles (321.87-1496.69 km) and those satellites which orbit

the earth at geostationary orbit, about 22,300 miles (35,888.71 km) above earth.

Each type of satellite can provide a different type of coverage for communications

and wireless devices. Like LEOs, these satellites don’t maintain a stationary

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distance from the earth. This is in contrast to the geostationary orbit, where

satellites are always approximately 22,300 miles from the earth.

Any satellite that orbits the earth between about 1000-22,000 miles (1609.34-

35,405.57 km) above earth is an MEO. Typically the orbit of a medium earth orbit

satellite is about 10,000 miles (16,093.44 km) above earth. In various patterns,

these satellites make the trip around earth in anywhere from 2-12 hours, which

provides better coverage to wider areas than that provided by LEOs.

Advantage of Medium Earth Orbit A MEO satellite’s longer duration of visibility and wider footprint means fewer

satellites are needed in a MEO network than a LEO network.

Disadvantage of Medium Earth Orbit A MEO satellite’s distance gives it a longer time delay and weaker signal than a

LEO satellite, though not as bad as a GEO satellite.

Molniya Orbit Satellites

Figure 19: Molniya Orbit

Molniya orbit is a highly elliptical orbit with an inclination of 63.4 degrees, an argument

of perigee of -90 degrees and an orbital period of one half of a sidereal day. Molniya

orbits are named after a series of Soviet/Russian Molniya communications satellites

which have been using this type of orbit since the mid-1960s.

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Advantages

Figure 20: Molniya - 1

Molniya orbits required less rocket power to achieve than a geosynchronous orbit, and

were better suited to communications with northern latitudes. Since satellites in such

an orbit move very slowly at apogee, they appear to 'hover' for hours at a time over

northern latitudes. A series of three Molniya satellites can act like a GEO satellite.

Disadvantages The disadvantage was that the sending/receiving dish must track the satellite, whereas

for a geosynchronous satellite a fixed dish could be used.

High Altitude Platform (HAP) In current era demand for wireless communication is notoriously increases. A terrestrial

and satellite system provides wireless communication services. Terrestrial systems are

used in mobile applications while satellite systems are used where terrestrial system not

reached. HAPs are airship or airplanes which altitudes at 17-22km above earth surface.

HAPs have been proposed mobile services in stratosphere. It have advantages of both

terrestrial as well as satellite. It also provides services like 3G, emergency services and

Wi-MAX. HAP networks are provides different services like military application, earth

monitoring, traffic monitoring and control. In terms of services, HAP offering low cost

and high facility services.

HAP APPLICATIONS

Broadband Fixed Wireless Access Applications 2G/3G and 4G applications

Emergency and disaster scenarios

Military Communications

Earth monitoring and positioning

Types of HAP

A terrestrial-HAP-satellite system

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Figure 21: A terrestrial-HAP-satellite system

It is a mixed infrastructure, includes a HAPs network using a satellite as a

link to the terrestrial networks to the final users. It provides best features

of both HAPs and satellite communications. It can support high QOS

(Quality Of services). First, the capability of the satellites of broadcasting

and multicasting are used to transmit information from fiber networks to

the HAP network deployed below the satellite. Second, HAPs are used to

improve the satellite performance over the earth.

A integrated terrestrial – HAP system

This system works without the satellite-HAP link. Haps are considered to

project one or more macro cells Here HAP network can be connected to

terrestrial network through gateway.

Figure 22: An integrated terrestrial – HAP system

A standalone HAP system

This system is used in many applications. For example broadband for all.

In rural or remote area, it is expensive to deploy terrestrial systems.

Satellite system is costly to be launched if traffic demand is small. This

system may be deployed economically and efficiently. [14]

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Frequency Bands

Figure 23: Different kinds of satellites use different frequency bands

Capacity Allocation Capacity of satellite data transfer is allocated via two techniques. They are

FDMA

o FAMA-FDMA

o DAMA-FDMA

TDMA

o Advantages over FDMA

Frequency Division Multiple Access (FDMA) The FDMA technique is traditional in radio communications, since it relies on frequency

separation between carriers. All that is required is that the Earth stations transmit their

traffic on different microwave frequencies and that the modulation not cause the carrier

bandwidths to overlap. Three such independent transmissions are indicated at the left of

Figure 5.17 by long rectangles extending along the same "time" dimension but on

different frequencies (indicated by different shading). A constraint in PDMA is that the

sum of the bandwidths of the individual carriers cannot exceed the satellite's available

bandwidth. Consequently, all three carriers in the uplink pass cleanly through the satellite

repeater and are radiated toward the area of coverage on the ground.

The principle behind FDMA is that each Earth station or user terminal is assigned a

separate frequency on which to transmit. That assignment can be fixed for time

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(permanently assigned), possibly changeable only by a manual command at either the

uplink or the downlink or from a central control point. Alternatively, the assignment of

the frequency can be dynamic (demand assigned, or demand assignment multiple access,

DAMA ), responding to user requests for service .Permanently assigned FDMA channels

are useful for dedicated bandwidth services, which also are known as leased lines.

Demand assigned channels are suitable for circuit­ switched services, notably for

telephony. Special signaling channels, also provided on an FDMA basis, are added to

allow stations to request connections and to alert stations to incoming calls. In either case,

the bandwidth of the channel must match the information bandwidth of the signal and

allow for some guard band to prevent adjacent channel interference. The guard band may

be as small as 5% or 10% of the channel signal (occupied) bandwidth, to allow for proper

channel filtering and frequency errors.

An example of FDMA for a transponder with several carriers is presented in Figure 5.18.

Each carrier is transmitted by a single Earth station, and thus there are a total of four such

stations involved in this FDMA network. Indicated are the station location, frequency,

and satellite EIRP. There are also smaller FDMA carriers used for narrowband data. This

is a real spectrum representation for a C-band transponder used for distance learning,

where each carrier and associated Earth station originates two classes at the same time.

The lumps at the bottom are intermodulation distortion (IMD) produced by the active

carriers in the transponder, which is being operated within 3 dB of multicarrier saturation.

The overall efficiency of FDMA is affected by intermodulation distortion (IMD), which

results from multiple carriers in a common nonlinear amplifier like a TWTA or an SSPA.

Figure 24 offers an example of how C/N varies, depending on the total input power of a

common spacecraft TWT power amplifier. The X-axis displays the TWT input power

with respect to saturation. The Y-axis represents carrier to noise ratio, broken down into

uplink, downlink, combined uplink and downlink, intermodulation ion, and total link

C/N. We see that there is an optimum operating point in terms of TWT input, for which

the total C/N is maximized. As discussed at the end of this chapter, that type of analysis

requires knowledge of the particular signal characteristics and a detailed link budget.

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Figure 24: FDMA service optimization of the TWTA operating point for maximum C/N total (all curves are representative but not accurate).

Figure 25: FDMA carriers within a C-band transponder with a total bandwidth of 36 MHz. Each large carrier provides 3 Mbps of data for video conferencing and the smaller carriers are for voice and data return channels

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Over the years, FDMA became viewed as antiquated compared to TDMA and CDMA.

However, the fact that FDMA carriers can be transmitted without coordination or

synchronization gives it longevity in telecommunications in general and satellite

communications in particular. Many mesh network applications still rely on the

simplicity and the efficiency of FDMA. Due to a variety of practical limitations on

equipment power and cost, FDMA often is combined with the other multiple access

methods to provide smaller networks that can operate more or less independently of one

another. The basic principle of the channelized repeater and the transponder itself reflect

the principle of FDMA.

There are some possible constraints to the effective use of FDMA, so satellite

communication engineers have to do their homework before making a selection (that is

true when doing the tradeoffs, because there are so many factors to consider in that type

of decision). Some applications use a very low data rate per user, such as 4-kbps speech

or text transfer. Using the SCPC FDMA technique, the RF bandwidth per carrier is

comparable to those data rates. At only 4 kHz of band­ width, the carrier is subject to two

sources of carrier frequency error: LO drift, and Doppler frequency shift. Both could

cause carriers to run over each other if not controlled. LO drift can be improved through

the use of accurate and stable frequency sources, something that gradually is being

introduced both in space and on the ground. Doppler shift depends on the relative motion

of the satellite and the user. Therefore, systems in which both are in motion (e.g., non -

GEO MSS) tend not to use narrowband FDMA.

Time Division Multiple Access (TDMA) Earth station transmissions in a common TDMA network are all on the same frequency,

and each employs the full bandwidth of the RF channel, which may consist of an entire

transponder (full transponder TOMA) or a segment of band­ width within a transponder

(narrowband TDMA). In the center of Figure 26, the wide rectangles represent full-

bandwidth transmissions from Earth stations within the same satellite coverage beam.

Interference between transmissions, which are on the same frequency, is prevented by

synchronizing the transmission so they do not overlap in time. That is a much more

complex process than FDMA because a common system of timing and control must be

employed by the Earth stations sharing the same satellite channel. Individual Earth

stations, therefore, transmit their traffic in the form of bursts of information, necessitating

compression of the traffic in time at the transmitting end and the complementary

expansion at the receiving end. A similar technique is used in PC-based LANs, allowing

several PCs to "talk" to one another on a common cable loop. In TDMA, the most

appropriate modulation is digital i n nature, typically QPSK, since that is compatible with

the compression and timing requirements of burst transmission.

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Figure 26: An example of a typical TOMA timeframe.

An example of a TDMA frame assignment is provided in Figure 5.20. The only

inefficiencies are due to the need for guard times between bursts, analogous to the guard

bands used in FDMA, "preamble" frame overhead bits used for synchronization and

network control data. Full transponder saturated operation allows the bit rate to be run up

to the theoretical maximum, with no significant loss due to IMD. (The latter decreases

FDMA channel capacity by 50% or more.) There are a few degradations that apply to full

transponder TDMA. The first is that the bursts themselves produce pulses of de current

demand by the transponder output amplifier; some form of compensation typically is

needed in the spacecraft power subsystem to prevent the pulses from affecting operation

of other equipment on the same power line. Another concern is sidebands that can be

generated in the transponder, potentially causing adjacent transponder interference.

However, both factors can be dealt with effectively in the design of the modulation

structure and the repeater itself, or by backing off the output amplifier.

Full-transponder TDMA networks were first introduced in the INTELSAT system and

provided a maximum throughput of about 60 Mbps (based on a transponder bandwidth of

36 MHz, QPSK modulation), and no FEC. In a current system, link performance can be

improved with FEC at the expense of some throughput. Moving to BEM will offset such

a loss.

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Full-transponder TDMA networks were first introduced in the INTELSAT system and

provided a maximum throughput of about 60 Mbps (based on a transponder bandwidth of

36 MHz, QPSK modulation), and no FEC. In a current system, link performance can be

improved with FEC at the expense of some through put. Moving to BEM will offset such

a loss.

The alternative of narrowband TDMA has found wide application in satellite networks as

well as the popular GSM mobile radio system. It is a blend of TDMA and FDMA,

designed to reduce the power and bandwidth requirements for the Earth stations and user

terminals. That is because the power required is directly proportional to the data rate;

narrowband TDMA is much less demanding on the user terminal, which can be designed

for very low power operation. That being the case, why not simply use FDMA and forget

about having to provide accurate timing and synchronization? Even without considering

the throughput efficiency of TDMA, there are a number of benefits to using the hybrid

technique. First, the bandwidth of each carrier is proportional to the number of station s

that share the same channel multiplied by the average data rate that each station is

afforded. The wider bandwidth, which can still be well under 100 kHz for mobile voice

applications, reduces the demand for maintaining a precise frequency; that saves guard

band. Also, user terminals can frequency-hop to grab unused time slots, thereby

increasing the total throughput. Frequency hopping has the additional advantage of

decreasing access delay and setup time. Thus, new applications (e.g., mobile data) and

addition al user scan be inserted into what would have been a wasted resource.

REFERENCES [1] http://www.nasa.gov/audience/forstudents/5-8/features/what-is-a-satellite-

58.html

[2] https://www.wordnik.com/words/transponder

[3] https://www.wordnik.com/words/downlink

[4] http://www.argospress.com/Resources/satellite/frespaclos.htm

[5] http://www.argospress.com/Resources/satellite/footpr.htm

[6] http://www.ucsusa.org/nuclear_weapons_and_global_security/solutions/space-

weapons/what-are-satellites-used-for.html

[7] http://www.tech-faq.com/fixed-satellite-service.html

[8] http://dish-cable.com/DBS.htm

[9] http://www.maplenet.net/~trowbridge/DBSintro.htm

[10] http://searchmobilecomputing.techtarget.com/definition/mobile-satellite-

services

[11] http://searchmobilecomputing.techtarget.com/definition/geostationary-satellite

[12] http://www.brighthub.com/science/space/articles/114687.aspx#imgn_0

[13] http://www.123helpme.com/view.asp?id=132309

[14] http://www.ijetae.com/files/Volume3Issue4/IJETAE_0413_38.pdf