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CONTENTS

Serial no. Topics Page

No.

Chapter 1:

Introduction

History of satellites

Satellite Communication

Advantages of Satellite Communication

Disadvantages of Satellite Communication

1

2

5

5

Chapter 2:

Satellite

Subsystems

The spacecraft bus

Tracking, Telemetry and Command

Power Source

Propulsion Subsystem

Attitude determination and control

Radio, receivers and transponders

Thermal System

Payload

Computer control System

Ground Earth Station

6

11

11

11

Chapter 3:

Orbital Elements

&

Perturbations

Description of coordinate system

Orbital Elements

Eccentricity

Semi Major Axis

Inclination

Right Ascension of ascending mode

Argument of perigee

Mean anomaly

Look angles

Orbital perturbations

Effect of earth’s oblateness

Effect of sun and moon

13

14

18

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Chapter 4:

Satellite Orbits Introduction

Mechanism of launching a Satellite

Low Earth Orbit

Sun-synchronous Satellites

Medium Earth Orbit

Geostationary Orbit

High Earth Orbit

Polar Orbit

Graveyard Orbit

19

20

21

23

24

25

28

28

29

Chapter 5:

Frequency

allocation &

applications

Frequency allocation for satellites

Application of Communication Satellite

Weather forecasting

Telephone

Radio

Television

Internet access

Military service

Navigation service

Global mobile communication

30

31

Chapter 6:

Future of Satellite

Communication

High Altitude Platform

Reducing the size of satellites

Use of new technologies

35

36

37

Chapter 7:

Glossary

&

Bibliography

Glossary

Bibliography

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Chapter 1: INTRODUCTION

1.1 A PROBE INTO THE HISTORY OF SATELLITES

The Merriam-Webster dictionary defines a satellite as a celestial body orbiting another of larger

size or a manufactured object or vehicle intended to orbit the earth, the moon, or another celestial

body.

The vision of Socrates: "Man must rise above the Earth -- to the top of the atmosphere and beyond -- for only

thus will he fully understand the world in which he lives."

Socrates made this observation centuries before humans successfully placed an object in Earth's

orbit. And yet the Greek philosopher seemed to grasp how valuable a view from space might be,

even if he didn't know how to achieve it.

The Intelligence of Newton: Those notions -- about how to get an object "to the top of the atmosphere and beyond" --

would have to wait until Isaac Newton, who published his now-famous cannonball thought

experiment in 1729.

His thinking went like this: Imagine you place cannon atop a mountain and fire it horizontally.

The cannonball will travel parallel to Earth's surface for a little while but will eventually

succumb to gravity and fall to the ground. Now imagine you keep adding gunpowder to the

cannon. With the extra explosives, the cannonball will travel farther and farther before it falls.

Add just the right amount of powder and impart just the right velocity to the ball, and it will

travel completely around the planet, always falling in the gravitational field but never reaching

the ground.

Thoughts of a sci-fi writer: Newton may have worked through the mental exercise of launching a satellite, but it

would take a while before we actually accomplished the feat. One of the early visionaries was

sci-fi writer Arthur C. Clarke. In 1945, Clarke suggested that satellites could be placed into

orbit so that they moved in the same direction and at the same rate as the spinning Earth. These

so-called geostationary satellites, he proposed, could be used for communications.

Many scientists didn't fully embrace Clarke's idea -- until Oct. 4, 1957. That's when the Soviet

Union launched Sputnik 1; the first man-made satellite to orbit Earth. Sputnik was a 23-inch (58-

centimeter), 184-pound (83-kilogram) metal ball. Although it was a remarkable achievement,

Sputnik's contents seem meager by today's standards.

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Fig 1.1: Sputnik 1; the first artificial satellite

1.2 SATELLITE COMMUNICATION

1.2.1 What is a Communication Satellite:

A communications satellite or Comsat is an artificial satellite sent to space for the purpose of

telecommunications.

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.

Fig 1.2: Satellite communication system

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1.2.2 Early days of Satellite Communication:

Today's satellite communications can trace their origins all the way back to the Moon. A project

named Communication Moon Relay was a telecommunication project carried out by the United

States Navy. Its objective was to develop a secure and reliable method of wireless

communication by using the Moon as a natural communications satellite.

Sputnik 1 was launched as a step in the exploration of space and rocket development. While

incredibly important it was not placed in orbit for the purpose of sending data from one point on

earth to another. Hence, it was not the first "communications" satellite, but it was the first

artificial satellite in the steps leading to today's satellite communications.

ECHO 1: The first artificial satellite used solely to further advances in global communications

was a balloon named Echo 1. Echo 1 was the world's first artificial communications satellite

capable of relaying signals to other points on Earth. It soared 1,000 miles (1,609 km) above the

planet after its August 12, 1960 launch, yet relied on humanity's oldest flight technology —

ballooning. Launched by NASA, Echo 1 was a giant metallic balloon 100 feet (30 meters)

across.

Fig 1.3: ECHO 1; world’s first communication satellite

The world's first inflatable satellite — or "satelloon", as they were informally known — helped

lay the foundation of today's satellite communications. The idea behind a communications

satellite is simple: Send data up into space and beam it back down to another spot on the globe.

Echo 1 accomplished this by essentially serving as an enormous mirror 10 stories tall that could

be used to bounce communications signals off of.

Telstar : was the first active, direct relay communications satellite. Belonging to AT&T as part

of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British

General Post Office, and the French National PTT (Post Office) to develop satellite

communications, it was launched by NASA from Cape Canaveral on July 10, 1962, the first

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privately sponsored space launch. Relay 1 was launched on December 13, 1962, and became the

first satellite to broadcast across the Pacific on November 22, 1963.

Fig 1.4: Telstar; the first direct relay communication satellite

An immediate antecedent of the geostationary satellites was Hughes' Syncom 2, launched on

July 26, 1963. Syncom 2 revolved around the earth once per day at constant speed, but because it

still had north-south motion, special equipment was needed to track it.

Since Sputnik, several nations, led predominantly by the United States, Russia and China, have

sent some 2,500 satellites into space [source: National Geographic]. Some of these man-made

objects, such as the International Space Station, are massive. Others might fit comfortably in

your kitchen breadbox. We see and recognize their use in weather reports, television

transmission by DIRECTV and DISH Network, and everyday telephone calls. Even those that

escape our notice have become indispensable tools for the military.

Of course, launching and operating satellites leads to problems. Today, with more than 1,000

operational satellites in orbit around Earth, our immediate cosmic neighborhood has become

busier than a big city rush hour. And then there's the discarded equipment, abandoned satellites,

pieces of hardware and fragments from explosions or collisions that share the skies with the

useful equipment. This orbital debris has accumulated over the years and poses a serious threat to

satellites currently circling Earth and to future manned and unmanned launches.

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1.3 ADVANTAGES OF SATELLITE COMMUNICATION

Until the advent of communication satellite, the long distance communication through

space was done by using cascading radio relays, VLF radio and HF or SW radio. The

latter two are internally low capacity media. Thus satellite has filled a huge void in the

sense that it has been capable of transmitting high capacities over long distance either

overland or water.

The satellite relays are internally wide-area broadcast, i.e. point-to-multipoint whereas

all the terrestrial relays are point-to-point. Besides the cost of transmitting information is

independent of the distance involved.

The satellite circuits can be installed rapidly. Once the satellite is in position the earth

stations can be installed and communication can be established in days or even hours.

Thus a station can be easily relocated from one place and installed in another.

Mobile communication can be easily achieved by satellite communication as it has a

unique degree of flexibility and reliability in inter-connecting mobile vehicles. Thus the

satellite has become an alternative to shortwave radio in the specialized area and has

significant reliability advantages.

1.4

DISADVANTAGES OF SATELLITE COMUNICATIONS

With the satellite in position the communication path between the terrestrial transmitter

and receiver is approximately 75,000km long. Since the velocity of electromagnetic wave

is 3x105 km/s, there is a delay of ¼ sec between the transmission and reception of a

signal. Thus between talks there is an elapse of ½ sec.

The delay produces echo which is actually caused owing to imperfect impedance

matching. Thus there is an audible reflection with ½ sec delay.

The time delay of ½ sec reduces the efficiency of the satellite in data transmission and

long files transfers.

Besides the cost of installing a satellite is too high and the system requires high output

power for earth station transmitters and sensitive receivers.

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Chapter 2: SATELLITE SUBSYSTEMS

The modern satellite is an extremely complicated piece of equipment composed of more than a

half-dozen major subsystems and thousands of parts. Satellites live and die in space and are

subjected to an extremely hostile environment. Below is the list of what goes into building a

satellite.

Fig 2.1: Satellite Subsystem

Within the satellite are two major sections:

The spacecraft bus (likewise, the bus).

The communications payload (or simply, the payload).

2.1 The STRUCTURAL SUBSYSTEM OR BUS:

The bus is a metal or composite frame on which the other elements are mounted. Because it

bears the stresses of launch, the bus is generally resilient. It may be painted with reflective paint

to limit the solar heat it absorbs, which could also provide some protection from laser attacks.

Within the bus we find:

2.1.1 Tracking Telemetry and Command:

The TT&C system is essential to the successful operation of a communication satellite.

Telemetry

The telemetry system collects data from many sensors within the spacecraft and sends these data

to the controlling earth station. Typically as many as 100 sensors may be located on the

spacecraft to monitor pressure in the fuel tanks, voltage and current in the power conditioning

unit, current drawn by each subsystem, and critical voltages and current sin the communications

electronics. The temperature of the subsystems must be kept within pre-determined limits so

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many temperature sensors are fitted and the status and positions of the switches in the system are

also reported back by the telemetry system. The sighting devices used to maintain spacecraft

attitude are also monitored via the telemetry link: this is essential in case one should fail and

cause the satellite to point in the wrong direction.

The telemetry data are usually digitized and transmitted as frequency or phase shift keying (FSK

or PSK) of a low power telemetry carrier using TDM techniques. A low data rate is normally

used to allow the receiver at the earth station to have a narrow bandwidth and thus maintain a

high carrier-to-noise ratio. The entire TDM frame may contain thousands of bits of data and take

several seconds to transmit. At the earth station a computer is used to monitor, store and decode

the telemetry data so that the status of any system or sensor on the satellite can be determined

immediately by the controller on earth.

Tracking

A number of techniques can be used to determine the current orbit of a spacecraft. Velocity and

acceleration sensors on the spacecraft can be used to establish the change in orbit from the last

known position, by integration of the data. Together with accurate angular measurements from

the earth station antenna, range is used to determine the orbital elements. Active determination of

range can be achieved by transmitting a pulse or sequence of pulses to the satellite and observing

the time delay before the pulse is received again. If a sufficient number of earth stations with an

adequate separation are observing the satellite, its position can be established by triangulation

from the earth station look angles or by simultaneous range measurements.

Fig 2.2: Typical tracking, telemetry and command system

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Command

A secure and effective command structure is vital to the spacecraft launch and operation of any

communications satellite. The command system is used to make changes in attitude and

corrections to the orbit and to control the communication system. During launch, it is used to

control the firing of the apogee boost motor and to spin up a spinner or extend the solar sails of a

three axis stabilized spacecraft.

The control word is converted into a command word, which is sent in a TDM frame to the

satellite. After checking for validity in the spacecraft, the word is sent back to the control station

via the telemetry link where it is checked again in the computer. If it is found to have been

received correctly, an execute instruction will be sent to the satellite so that the command is

executed.

2.1.2 A Power Source:

Power is often supplied by arrays of solar cells (―solar panels‖) that generate electricity, which is

stored in rechargeable batteries to ensure a power supply while the satellite is in shadow.

Technological improvements in battery technology have led to new battery types with high

specific energy (energy stored per unit mass) and high reliability.

Fig 2.3: Power source

Solar cells are mounted on the body of a satellite or on flat panels. Mounting the solar cells on

the satellite’s body results in a more compact configuration but since not all cells will be

illuminated by the Sun at any one time; the power generated is less than it would be from large

panels made of solar cells that are continually positioned to face the Sun. The solar panels often

have a large surface area compared with the rest of the satellite, so they sustain a relatively large

number of collisions with debris particles. Solar panels are fragile and can be damaged easily,

but partial damage to a solar panel may not disable the satellite. Satellites often can continue to

function with partially working solar panels, albeit with diminished capacity. However, if the

solar panels fail to deploy or are torn off, a satellite without another power source would cease

functioning fairly quickly. A malfunction of the power distribution system could also totally

impair the satellite.

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2.1.3 A Propulsion Subsystem:

The satellite’s propulsion system may include the engine that guides the spacecraft to its proper

place in orbit once it has been launched, small thrusters used for station keeping and attitude

control, and possibly larger thrusters for other types of maneuvering. If the propulsion system

does not function, because of damage or lack of propellant, the satellite may still be functional.

However, in orbits dense with other satellites, such as geostationary orbit, satellites must be able

to maintain their position very accurately or they will be a danger to their neighbors and to

themselves.

2.1.4 Attitude determination and control:

Spacecraft attitude determination is the process of estimating the orientation of a spacecraft by

making remote observations of other celestial bodies or reference points. Combinations of these

sensor observations are used to generate a more accurate estimate of spacecraft rotational

attitude.

Attitude estimates must be calculated quickly and continuously during the entire operational life

of the mission. During normal operations, the problem is recursive—the attitude filter basing

new predictions on present and prior sensor information. The attitude filter must also estimate

from activation when the spacecraft is first initiated and no prior data is available.

Fig 2.4: Attitude determination

The systems designed to carry out 3-axis attitude determination are inevitably complex, but must

still be designed with the utmost care to perform the task as reliably as possible. Any, even

temporary malfunction is potentially serious, damaging fragile instruments, breaking

communications links, upsetting measurements and disrupting power generation.

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2.1.5 Radio receivers, transmitter and transponders:

In addition to the communication equipment needed to operate the satellite, a satellite may carry

similar equipment for other tasks. It may carry a radio antenna to collect radio signals, such as

telephone or television signals, and to relay or rebroadcast them. The antenna serves to receive

and transmit signals. It may be a parabolic dish (similar to satellite TV dishes), a feed horn (a

conical or cowbell shaped structure), or a minimal metal construction (similar to a rooftop TV

antennae).

Fig 2.5: Radio receiver, transmitter and transponder

When a system is designed to automatically receive a transmission, amplify it, and send it back

to Earth, possibly at a different frequency, it is called a transponder .A satellite-based radar

system is also composed in part of transmitters and receivers used to send and then receive the

radio waves. Receivers are also used by the military for signals intelligence i.e., eavesdropping

on military communications, detecting the operating frequencies of enemy radar, or collecting

telemetry from ballistic missile tests. Similarly, a satellite may carry transmitters to send out

radio signals, such as the navigation signals from the Global Positioning System. A satellite may

be designed to transmit a signal to a specific receiver on the Earth, or to broadcast it over a large

area.

2.1.6 Thermal regulatory system:

The whole point of the thermal system is to regulate the temperature of the satellite's

components. Too hot or too cold, or too great a swing in temperature will prematurely end the

useful life of a satellite. This system dissipates the heat away from earth, out into space, so as not

to interfere with the satellite's operation.

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2.2 PAYLOAD:

The payload is the business-end of the satellite, consisting of:

Repeater (microwave receivers, RF multiplexers, power amplifiers, channel processing and

switching). Contained within the repeater are the transponders,

Antennas (reflectors, feeds, feed networks, support structure and pointing mechanisms). The

antennas create "footprint" coverage but require the repeater to receive and transmit the actual

signals from and to the ground.

2.3 A COMPUTER CONTROL SYSTEM:

The on-board computer monitors the state of the satellite subsystems, controls its actions, and

processes data. High-value satellites may incorporate sophisticated anti-jamming hardware that

is operated by the computer. If someone gained control of the satellite’s computer, the satellite

could be made useless to its owners. Computer systems are also sensitive to their

electromagnetic environment and may shut down or reboot during solar storms or if barraged by

high levels of electromagnetic radiation.

2.4 GROUND EARTH STATION:

Satellites are monitored and controlled from their ground stations. One type of ground station is

the control station, which monitors the health and status of the satellite, sends it commands of

various kinds, and receives data sent by the satellite. The antenna that the control station uses to

communicate with the satellite may be located with the station, but it need not be: to maintain

constant contact with a satellite not in geostationary orbit, and which therefore moves relative to

the Earth, the station needs to have antennae or autonomous stations in more than one location.

Satellites may also have other types of ground stations.

Military communications satellites have ground stations that range from large, permanent

command headquarters to small, mobile field terminals. Ground stations are generally not highly

protected from physical attack. Disabling a control station may have an immediate disruptive

effect, but the disruption can be reduced by having redundant capabilities, such as alternate

control centers. Computers at control centers may be vulnerable to attack and interference,

especially if they are connected to the Internet. However, high value command computers will

have high security, and many of the military command center computers are isolated from the

Internet.

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Fig 2.6: Ground Station Telemetry

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Chapter 3:ORBITAL ELEMENTS and PERTURBATIONS

3.1 Description of co-ordinate system:

The coordinate system used is called geocentric coordinate system. The origin of

the system is at the centre of the earth. The zi axis coincides with the earth’s axis

of rotation and extends through the geographic north-south pole. The xi axis

points towards a fixed location in space called “the first point of Aries”. This is the

direction of a line from the center of the earth through the center of sun at

vertical equinox, the instant when the sub solar point crosses the equator south

to north. The coordinate system moves through space along with the revolution

of earth.

The plane (xi, yi) containing the earth’s equator is called equatorial plane.

Fig 3.1: The geocentric coordinate system

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3.2 Orbital Elements

All of the satellites follow the same physical laws that were first described by Johannes Kepler.

A satellite's position at a specific time can be determined using seven distinct orbit

characteristics named the "Keplerian orbit elements". These orbit elements define the satellite

orbit's orientation with respect to the Earth, the satellite's last known position within its orbit, the

shape of the satellite's orbit and the satellite's orbital speed.

Orbital elements are used to specify the absolute or inertial coordinates of the satellite at time t.

The set commonly used in satellite communications is –

1. Eccentricity(e)

2. Semi major axis (a)

3. Inclination (i)

4. Right ascension of ascending mode (Ω)

5. Argument of perigee (ω)

6. Mean anomaly (at epoch) (M)

7. Look angles

The main two elements that define the shape and size of the ellipse:

3.2.1 Eccentricity:

The Eccentricity (e) defines how oval the satellite's orbit is. It is mathematically defined as the

ratio of the orbit's focus distance (c) to the orbit's semi-major axis (a).

e = c / a

Where , e = the orbit eccentricity;

c = the focus distance; and

a = the satellite orbit's semi-major axis.

The eccentricity of a satellite orbit is a unit less value that lies between 0 (circular orbit) and 1

(parabolic orbit).

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Fig 3.2: Eccentricity

3.2.2 Semi Major Axis:

Semi major axis (a) is the sum of the perigee and apogee distances divided by two. For circular

orbits, the semi major axis is the distance between the centers of the bodies, not the distance of

the bodies from the center of mass.

Two elements define the orientation of the orbital plane in which the ellipse is embedded:

3.2.3 Inclination:

Inclination (i): The angle that the orbital plane makes with the equatorial plane is the inclination

angle i. It can also be thought as the vertical tilt of the ellipse with respect to equatorial

plane(reference plane).

3.2.4 Right Ascension of ascending node :

Right ascension of ascending node (Ω) is the Angular distance measured eastward in the

equatorial plane from the axis pointing towards first point of Aries (i.e., the direction of line from

center of earth through the center of sun at the vertical equinox) is called Right Ascension (RA).

The two points at which the orbit penetrates the equatorial plane are called nodes.

Vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node

(where the orbit passes upward through the reference plane).

3.2.5 Argument of Perigee:

Argument of perigee (ω): It defines the orientation of the ellipse in the orbital plane, as an angle

measured from the ascending node to the perigee (the closest point the second body comes to the

first during an orbit).

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Fig 3.3: Orbital parameters

In this diagram, the orbital plane (yellow) intersects a reference plane (gray). For earth-orbiting

satellites, the reference plane is usually the Earth's equatorial plane, and for satellites in solar

orbits it is the ecliptic plane. The intersection is called the line of nodes, as it connects the center

of mass with the ascending and descending nodes. This plane, together with the Vernal Point (ɣ),

establishes a reference frame.

3.2.6 Mean Anomaly:

Mean Anomaly (M): The Mean Anomaly indicates where the satellite was located within its

orbit at a particular Epoch. The Mean Anomaly at any time t, M(t), can be determined by adding

the last known Mean Anomaly, Mo, to the orbit's Mean Motion multiplied by the time that has

elapsed (t - to):

M(t) = M0 + n (t - t0)

Where M(t) = the Mean Anomaly at time t;

M0 = the Mean Anomaly at t=0; n = the satellite orbit's Mean Motion;

t = the time chosen; and t0 = the time of the last known Mean Anomaly.

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For a perfectly circular orbit (Eccentricity of 0), the Mean Anomaly is exactly equal to the True

Anomaly throughout the orbit.

The Mean Anomaly can range anywhere from 0 to 360 degrees.

3.2.7 Look Angles:

The coordinate to which the earth station antennas must be pointed to communicate with a

satellite are called look angles. These are –

a) Azimuth angle (Az): The angle measured eastward from geographic north to the

projection of the satellite path on a local horizontal plane on earth station.

b) Elevation (ε): is defined between the centre of the satellite beam and the plane

tangential to the earth’s surface. A so called footprint can be defined as the area on earth

where the signals of the satellite can be received.

c) Inclination Angle (δ): is defined between the equatorial plane and the plane

described by the satellite orbit. An inclination angle of 0 degrees means that the satellite

is exactly above the equator.

Fig 3.4: Azimuthal Angle Fig 3.5 Angle of Inclination

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3.3 ORBITAL PERTURBATIONS:

If the earth and the satellite are considered as point masses influenced only by mutual

gravitational attraction, then Keplerian Orbit results.

In reality, the earth and satellites respond to many other influences like asymmetry of Earth’s

gravitational field, the gravitational field of sun and moon, solar radiation pressure, atmospheric

drag and so on. These interfering forces cause the true orbit to be different from a Keplerian

ellipse.

3.3.1 Effects of the earth’s oblateness:

Since the earth is not a perfect sphere with symmetric distribution of mass, its gravitational

potential does not have 1/r dependence assumed be Keplerian orbit. The earth’s gravitational

potential is represented more accurately by expansion in Legendre polynomial Jn in ascending

powers of re/r (earth’s radius/orbital radius). In this expansion, the dominant term is J2(re/r)2. The

effect of this term is to cause an unconstrained geosynchronous satellite to drift toward and

circulate around the nearer of two stable points. These correspond to sub satellite longitudes of

105o

W and 75oE, known as ―graveyards‖ as they collect old satellites whose station keeping fuel

is exhausted.

3.3.2Effect of Sun and Moon:

Gravitational attraction by the sun and moon causes the orbital inclination of a geosynchronous

satellite to change with time. If not countered by north-south station keeping, these forces would

increase the orbital inclination from an initial 0o at launch to 14.67

oin 26.6 years.

The rate of change varies with the inclination of the moon’s orbit, but values of about 0.86o per

year (calculated in 1970-80).

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CHAPTER 4: Satellite Orbits

4.1 Introduction:

This unit discusses the basics of satellite and elaborating the parameters which are needed to

calculate the distance of an orbit to which a satellite is to be launched and the other factors which

are necessary to define an orbit. Further this unit discusses the applications of satellites and

elaborates on the global communication which has now become possible due to the presence of

satellites. Going further, this unit also elaborates on the types of orbits a satellite can follow to

provide communication.

Satellites orbit around the earth. Depending on the application, these orbits can be circular or

elliptical. Satellites in circular orbits always keep the same distance to the earth’s surface

following a simple law:

Kepler’s Law:

1. The orbit of a satellite is an ellipse with the centre of the earth at one of the foci.

2. The line joining the centre of the earth and the satellite sweeps over equal areas in equal

intervals of time.

3. The square of the orbital period of 2 satellites have the same ratio as the cubes of the

mean distance from the centre of the earth, i.e. .

The attractive force Fg of the earth due to gravity equals: (

)

.

The centrifugal force Fc trying to pull the satellite away equals .

[The variables have the following meaning: m is the mass of the satellite; R is the radius of earth

= 6,370 km; r s the distance of the satellite to the centre of the earth; g is the acceleration of

gravity = 9.81 m/ ; ω is the angular velocity with ω = 2·π·f, where f is the frequency of the

rotation.]

To keep the satellite in a stable circular orbit, the following equation must hold: Fg = Fc, i.e.,

both forces must be equal.

Looking at this equation the first thing to notice is that the mass m of a satellite is irrelevant (it

appears on both sides of the equation). Solving the equation for the distance r of the satellite to

the centre of the earth results in the following equation:

The distance r = (

( ) )

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From the above equation it can be concluded that the distance of a satellite to the earth’s surface

depends on its rotation frequency.

Fig 4.1: Kepler’s 2nd

Law

4.2 Mechanism of Launching a Satellite:

All satellites today get into orbit by riding on a rocket or in the cargo bay of the space shuttle.

Satellites have no independent means of breaking through the atmosphere and reaching space, so

they require an external vehicle, such as a rocket or space shuttle, to carry them to their

orbit. Several countries and businesses have rocket launch capabilities, and satellites as large as

several tons make it into orbit regularly and safely.

For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the

rocket through the thickest part of the atmosphere most quickly and best minimizes fuel

consumption.

After a rocket launches straight up, the rocket control mechanism uses the inertial guidance

system to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course

described in the flight plan.

The IGS determines a rocket's exact location and orientation by precisely measuring all of the

accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals,

the gyroscopes' axes stay pointing in the same direction. This gyroscopically stable platform

contains accelerometers that measure changes in acceleration on three different axes. If it knows

exactly where the rocket was at launch and the accelerations the rocket experiences during flight,

the IGS can calculate the rocket's position and orientation in space.

In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east,

giving the launch vehicle a free boost. The strength of this boost depends on the rotational

velocity of Earth at the launch location. The boost is greatest at the equator, where the distance

around Earth is greatest and so rotation is fastest.

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Once the rocket reaches extremely thin air, at about 120 miles (193 kilometres) up, the rocket's

navigational system fires small rockets, just enough to turn the launch vehicle into

a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure

some separation between the launch vehicle and the satellite itself.

Most ELV (Expendable Launch Vehicles) launchers put the satellite in an inclined elliptical

orbit called a transfer orbit with an apogee at geosynchronous altitude and a 185-370 perigee. At

the transfer orbit apogee, a rocket engine called the Apogee Kick Motor (AKM) puts the

satellite into a circular geosynchronous orbit with (ideally) zero inclination. The AKM must be

capable of increasing the satellite velocity from 1.58 km/s to 3 km/s in geosynchronous orbit

while simultaneously reducing the orbital inclination to zero.

Geosynchronous satellites launched by STS (Space Transportation System) are moved from this

orbit to a transfer orbit by an additional stage frequently called a PAM (Payload Assist Module)

or a perigee motor.

Fig 4.2: Placing of a satellite in orbit

4.3 Low Earth Orbit:

A Low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometres

(99 mi), with an orbital period of about 88 minutes, and 2,000 kilometres (1,200 mi), with an

orbital period of about 127 minutes. Objects below approximately 160 kilometres (99 mi) will

experience very rapid orbital decay and altitude loss. The altitude is usually not less than 300 km

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for satellites, as that would be impractical due to atmospheric drag. With the exception of the

manned lunar flights of the Apollo program, all human spaceflights have taken place in LEO.

These satellites in this orbit are placed 500-1500 kilometres above the surface of the earth. As

LEOs circulate on a lower orbit, hence they exhibit a much shorter period that is 95 to 120

minutes. LEO systems try to ensure a high elevation for every spot on earth to provide a high

quality communication link. Each LEO satellite will only be visible from the earth for around ten

minutes.

The orbital velocity needed to maintain a stable low earth orbit is about 7.8 km/s, but reduces

with increased orbital altitude. The delta-v needed to achieve low earth orbit starts around

9.4 km/s. (In astrodynamics a Δv or delta-v (literally "change in velocity") is a measure of the

amount of "effort" that is needed to change from one trajectory to another by making an orbital

maneuver. It is a scalar that has the units of speed.

Delta-v is produced by the use of propellant by reaction engines to produce a thrust that

accelerates the vehicle). Atmospheric and gravity drag associated with launch typically adds 1.5-

2.0 km/s to the delta-v launch vehicle required to reach normal LEO orbital velocity of around

7.8 km/s (28,080 km/h). Present day mobile communication systems use LEO satellites.

The advantages of LEO satellites are:

1. The delay for packets delivered via a LEO is relatively low (approximately 10

milliseconds). The delay is comparable to long-distance wired connections (about 5–10

milliseconds).

2. Link diversity is better and satellites are relatively small.

3. Using advanced compression schemes, transmission rates of about 2,400 bit/s can be

enough for voice communication. LEOs provide this bandwidth for mobile terminals with

Omni-directional antennas using low transmit power in the range of 1W.

4. Smaller footprints of LEOs allow for better frequency reuse, similar to the concepts used

for cellular networks. LEOs can provide a much higher elevation in Polar Regions and so

better global coverage.

5. Much less power is required for satellite communication because of reduced height.

6. These satellites are mainly used in remote sensing an providing mobile communication

services

7. The LEO satellites less area coverage saves bandwidth.

The disadvantages of LEO satellites are:

1. The biggest problem of the LEO concept is the need for many satellites if global

coverage is to be reached. Several concepts involve 50–200 or even more satellites in

orbit.

2. The short time of visibility with a high elevation requires additional mechanisms for

connection handover between different satellites. The high number of satellites combined

with the fast movements resulting in a high complexity of the whole satellite system.

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3. Objects in LEO encounter atmospheric drag in the form of gases in

the thermosphere (approximately 80-500 km up) or exosphere (approximately 500 km

and up), depending on orbit height. Objects in LEO orbit Earth between the atmosphere

and below the inner Van Allen radiation belt.

4. Doppler offset is higher (because of relative movement) than that in GEO and MEO.

5. There is a need for routing of data packets from satellite to another satellite if a user

wants to communicate around the world, due to small footprint. Due to the large

footprint, a GEO typically does not need this type of routing, as senders and receivers are

most likely in the same footprint.

6. Total System development cost is higher than GEO and MEO.

Examples of LEO are: TELEDESIC, IRIDIUM, GLOBSTAR, ARIES etc.

4.4 Sun-Synchronous Satellites:

A Sun-synchronous orbit (sometimes called a heliosynchronous orbit) is a geocentric

orbit which combines altitude and inclination in such a way that an object on that orbit ascends

or descends over any given Earth latitude at the same local mean solar time.

These satellites rise and set with the sun. Their orbit is defined in such a way that they are always

facing the sun and hence they never go through an eclipse.

For these satellites, the surface illumination angle will be nearly the same every time. (Surface

illumination angle is the angle between the inward surface normal and the direction of light. This

means that the illumination angle of a certain point of the Earth's surface is zero if the Sun is

precisely overhead and that it is 90 degrees at sunset and at sunrise). This consistent lighting is a

useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths

(e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean

and atmospheric remote sensing instruments that require sunlight).

Typical sun-synchronous orbits are about 600 to 800 km in altitude, with periods in the 96 to

100 minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the

direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).

Examples of Sun-Synchronous Satellites: Yohkoh, TRACE, Hinode and Proba-2,

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4.5 Medium Earth Orbit:

Medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), is the

region of space around the Earth above Low Earth orbit (altitude of 2,000 kilometres (1,243 mi))

and below geostationary orbit (altitude of 35,786 kilometres (22,236 mi)).

The satellites in these orbits are can be positioned somewhere between LEOs and GEOs, both in

terms of their orbit and due to their advantages and disadvantages.

These satellites move more slowly relative to the earth’s rotation. Period of revolution of a

satellite is 12 hours. Using orbits around 10,000 km, the system only requires a dozen satellites

which is more than a GEO system, but much less than a LEO system. 24 satellites constellation

placed in 6 orbits at 55 inclination with the equator. Depending on the inclination, a MEO can

cover larger populations, so requiring few handovers. Minimum 4 satellites are always visible

from the Earth. Two frequencies are used for transmission: L1=1.575 GHz and L2=1.226 GHz.

The latter is reserved for secure operations. Communications satellites that cover the North and

South Pole are also put in MEO.

Advantages of MEO satellites:

1. A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites

are needed in a MEO network than a LEO network.

2. It gives uniform global coverage and link diversity can be employed.

3. Propagation delay (~100 millisecond) is less than GEO.

4. Main purpose is position fixing, but velocity and acceleration can also be determined

which is not possible with GEO.

Disadvantages of MEO satellites:

1. Propagation delay is more than LEO.

2. Doppler offset is larger.

3. Ground segment and network control is more complex than GEO.

4. The signal is weaker than LEO.

Example of MEO satellites: GPS, Oddyssey, etc.

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Fig 4.3 - LEO, Sun-Synchronous & MEO orbits and their respective distances from Earth

4.6 Geostationary Earth Orbit:

A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with

an orbital period of one sidereal day (approximately 23 hours 56 minutes and 4 seconds),

matching the Earth's sidereal rotation period. The synchronization of rotation and orbital period

means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns

to exactly the same position in the sky after a period of one sidereal day. Over the course of a

day, the object's position in the sky traces out a path, typically in the form of an analemma,

whose precise characteristics depend on the orbit's inclination and eccentricity.

A special case of geosynchronous orbit is the geostationary orbit, which is a circular

geosynchronous orbit at zero inclination (that is, directly above the equator). A satellite in a

geostationary orbit appears stationary, always at the same point in the sky, to ground observers.

Popularly or loosely, the term "geosynchronous" may be used to mean

geostationary. Specifically, Geosynchronous Earth orbit (GEO) may be a synonym for

geosynchronous equatorial orbit, or geostationary earth orbit. A geostationary orbit is a circular

geosynchronous orbit in the plane of the Earth's equator with a radius of approximately

42,164 km (26,199 mi) (measured from the centre of the Earth).

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There is a difference between the geostationary and geosynchronous orbits. We should note that

while other orbits may be many, there is ONLY ONE Equatorial orbit, i.e. the orbit which is

directly above the earth's equator. Sometimes we send a satellite in the space which though has a

period of revolution is equal to period of rotation of earth, but its orbit is neither equatorial nor

Circular. So, this satellite will finish one revolution around the earth in exactly one day i.e. 23

hours, 56 Minutes and 4.1 seconds, yet it does NOT appear stationary from the earth. It looks

oscillating but NOT stationary and that is why it is called Geosynchronous.

Features of a geosynchronous satellite:

1. The orbit is NOT circular

2. The orbit is NOT in equatorial plane i.e. directly above the equator, it's in inclined

orbit

3. The angular velocity of the satellite is equal to angular velocity of earth

4. Period of revolution is equal to period of rotation of earth.

5. Finish one revolution around the earth in exactly one day i.e. 23 hours, 56 Minutes

and 4.1 seconds

6. There are many geosynchronous orbits.

Features of geostationary satellite:

1. The orbit is circular

2. The orbit is in equatorial plane i.e. directly above the equator and thus inclination is

zero.

3. The angular velocity of the satellite is equal to angular velocity of earth

4. Period of revolution is equal to period of rotation of earth.

5. Finish one revolution around the earth in exactly one day i.e. 23 hours, 56 Minutes

and 4.1 seconds

6. There is ONLY one geostationary orbit.

A satellite in such an orbit is at an altitude of approximately 35,786 km (22,236 mi) above mean

sea level. It maintains the same position relative to the Earth's surface. If one could see a satellite

in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not

exhibit diurnal motion, while the Sun, Moon, and stars would traverse the heavens behind it. The

theoretical basis for this novel phenomenon of the sky goes back to Newton's theory of motion

and gravity. In that theory, the existence of a geostationary satellite is made possible because the

earth rotates (with respect to an inertial frame in which Newton's laws of motion and gravity

hold). However, as a practical device, the geostationary satellite owes much for its realisation

to Arthur C. Clarke who proposed it during the 20th century and in whose honour the orbit is

called a Clarke orbit. Such orbits are useful for telecommunications satellites as the satellite

antennas that communicate with them do not have to move, but can be pointed permanently at

the fixed location in the sky where the satellite appears.

A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the

satellite drifts out of this orbit because of perturbations such as the solar wind, radiation pressure,

variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun, and

thrusters are used to maintain the orbit in a process known as station-keeping.

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Fig 4.4 – Geostationary Orbit

Advantages of GEO satellites:

1. Small number of satellites (4 to 6) is required for global coverage. However with only 3

satellites global and continuous coverage can easily be achieved.

2. Minimal Doppler Shift takes place here.

3. Simpler ground station antenna is required which does not need fast tracking.

4. GEO systems have significantly greater available bandwidth than the LEO and MEO

systems. This permits them to provide two-way data, voice and broadband services that

may be unpractical for other types of systems.

5. Because of their capacity and configuration, GEOs are often more cost-effective for

carrying high-volume traffic, especially over long-term contract arrangements. For

example, excess capacity on GEO systems often is reserved in the form of leased circuits

for use as a backup to other communications methods.

6. Because they stay above a fixed spot on the surface, they provide a constant vigil for the

atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail

storms, and hurricanes. When these conditions develop these GEO satellites are able to

monitor storm development and track their movements.

7. GEO satellites are proven, reliable and secure - with a lifespan of 10-15 years.

8. Available for environmental or home-land security monitoring applications

Disadvantages of GEO satellites:

1. The biggest problem for voice and also data communication is the high latency as without

having any handovers, the signal has to at least travel 72,000 kilometres.

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2. Northern or southern regions of the Earth (poles) have more problems receiving these

satellites due to the low elevation above a latitude of 60°, i.e., larger antennas are needed

in this case.

3. Gravitational pull of sun and moon makes these satellites deviate from their orbit. Over

the period of time, they go through a drag. (Earth’s gravitational force has no effect on

these satellites due to their distance from the surface of the Earth.)

4. These satellites experience the centrifugal force due to the rotation of Earth, making them

deviate from their orbit.

5. The non-circular shape of the earth leads to continuous adjustment of speed of satellite

from the earth station.

6. The transmit power needed is relatively high which causes problems for battery powered

devices.

7. Huge propagation delay (~300 milliseconds) occurs.

8. Degradation of SNR takes place as huge power is to be provided for communication via

GEO satellite from earth.

9. Due to the large footprint, either frequencies cannot be reused or the GEO satellite needs

special antennas focusing on a smaller footprint.

10. There is no link diversity.

11. Expensive launching and maintenance mechanism.

Examples of GEO satellites: INMARSAT (used for VSAT).

4.7 Higher Earth Orbit:

A high Earth orbit is a geocentric orbit with an altitude entirely above that of a geosynchronous

orbit (35,786 kilometres (22,236 mi)). The orbital periods of such orbits are greater than twenty-

four hours, therefore satellites in such orbits have an apparent retrograde motion, that is, even if

they are in a prograde orbit (90° > inclination >= 0°), their orbital velocity is lower than

Earth's rotational speed, causing their ground track to move westward on Earth's surface.

Characteristic features:

1. It provides service at high altitudes and it gives selected area coverage.

2. The satellite has to pass through Van Allen belt.

3. It has the highest propagation delay.

4. The satellite is complex.

5. The launching and maintenance cost is highest.

Example of HEO satellites: Vela 1A.

4.8 Polar Orbit:

A polar orbit is one in which a satellite passes above or nearly above both poles of the body

being orbited (usually such a planet as the Earth, but possibly another such body as the Sun) on

each revolution. It therefore has an inclination of (or very close to) 90 degrees to the equator. A

satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.

Polar orbits are often used for earth-mapping, earth observation, capturing the earth as time

passes from one point, reconnaissance, as well as for some weather satellites. The Iridium

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satellite constellation also uses a polar orbit to provide telecommunications services. The

disadvantage to this orbit is that no one spot on the Earth's surface can be continuously sensed

from a satellite in a polar orbit. A satellite can hover over one polar area much the time, albeit

from far away, using a polar highly elliptical orbit with its apogee above that area.

Fig 4.5 – Polar Satellite

4.9 Graveyard Orbit:

A graveyard orbit, also called a junk orbit or disposal orbit, is a super synchronous orbit that lies

significantly above synchronous orbit, where spacecraft are intentionally placed at the end of

their operational life. It is a measure performed in order to lower the probability

of collisions with operational spacecraft and of the generation of additional space debris (known

as Kessler syndrome).

A graveyard orbit is used when the change in velocity required to perform a de-

orbit manoeuvre is too high. De-orbiting a geostationary satellite requires a delta-v of about

1,500 metres per second (4,900 ft/s), whereas re-orbiting it to a graveyard orbit only requires

about 11 metres per second (36 ft/s).

For satellites in geostationary orbit and geosynchronous orbits, the graveyard orbit is a few

hundred kilometres above the operational orbit. The transfer to a graveyard orbit above

geostationary orbit requires the same amount of fuel that a satellite needs for approximately three

months of station keeping. It also requires a reliable attitude control during the transfer

manoeuvre. While most satellite operators try to perform such a manoeuvre at the end of the

operational life, only one-third succeed in doing so.

In order to obtain a license to provide telecommunications services in the United States,

the Federal Communications Commission (FCC) requires all geostationary satellites launched

after March 18, 2002, to commit to moving to a graveyard orbit at the end of their operational

life.

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Chapter 5: FREQUECY ALLOCATION AND

APPLICATIONS

5.1 FREQUENCY ALLOCTION FOR SATELLITES

Allocation of frequencies to satellite services is a complicated process which requires

international coordination and planning. This is done as per the International Telecommunication

Union (ITU).

International Telecommunication Unit:

The International Telecommunication Union (ITU), a specialized agency of the United Nations,

regulates satellite communications. The ITU, which is based in Geneva, Switzerland, receives

and approves applications for use of orbital slots for satellites. Every two to four years the ITU

convenes the World Radio Communication Conference, which is responsible for assigning

frequencies to various applications in various regions of the world. Each country’s

telecommunications regulatory agency enforces these regulations and awards licenses to users of

various frequencies.

To implement this frequency planning, the world is divided into three regions:

Region1: Europe, Africa and Mongolia.

Region 2: North and South America and Greenland.

Region 3: Asia (excluding region 1 areas), Australia and south-west Pacific.

Within these regions, frequency bands are allocated to various satellite services, although a given

service may be allocated different frequency bands in different regions. Some of the services

provided by the satellite are:

Fixed satellite service: Provides Links for existing Telephone Networks Used for

transmitting television signals to cable companies.

Broadcasting satellite service: Provides Direct Broadcast to homes. E.g. Live Cricket

matches etc.

Mobile satellite services: This includes services for:

Land Mobile, Maritime Mobile and Aeronautical mobile.

Navigational satellite service: Include Global Positioning systems.

Meteorological satellite services: They are often used to perform Search and Rescue

service.

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There are 6 frequency bands that have been allocated for use of satellite communication. Based

on the satellite service, following are the frequencies allocated to the satellites:

Bands Field of Service Downlink bands

(MHz)

Uplink bands (MHz)

VHF [Very High Frequency] Military 250-270 (approx.) 272-312 (approx.)

C Band Commercial 3700-4200 5925-6425

X Band Military 7250-7750 7900-8400

Ku Band Commercial 11700-12200 14000-14500

Ka Band Commercial 17700-21200 27500-30000

Ka Band Military 20200-21200 43500-45500

5.2 APPLICATIONS OF COMMUNICATION SATELLITES

5.2.1 Weather Forecasting:

Certain satellites are specifically designed to monitor the climatic conditions of earth. They

continuously monitor the assigned areas of earth and predict the weather conditions of that

region. This is done by taking images of earth from the satellite. These images are transferred

using assigned radio frequency to the earth station. These satellites are exceptionally useful in

predicting disasters like hurricanes, and monitor the changes in the Earth's vegetation, sea state,

ocean color, and ice fields.

5.2.2 Telephone:

The first and historically most important application for communication satellites was in

intercontinental long distance telephony. The fixed Public Switched Telephone Network

relays telephone calls from land line telephones to an earth station, where they are then

transmitted to a geostationary satellite.

Instead of using cables it was sometimes faster to launch a new satellite. But, fiber optic cables

are still replacing satellite communication across long distance as in fiber optic cable, light is

used instead of radio frequency, hence making the communication much faster (and of course,

reducing the delay caused due to the amount of distance a signal needs to travel before reaching

the destination.).

Using satellites, to typically reach a distance approximately 10,000 km away, the signal needs to

travel almost 72,000 km, that is, sending data from ground to satellite and (mostly) from satellite

to another location on earth. This results in substantial amount of delay and this delay becomes

more prominent for users during voice calls.

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Satellite communications are still used in many applications today. Remote islands such as

Ascension Island, Saint Helena, Diego Garcia, and Easter Island, where no submarine cables are

in service need satellite telephones. There are also regions of some continents and countries

where landline telecommunications are rare to nonexistent, for example large regions of South

America, Africa, Canada, China, Russia, and Australia. Satellite communications also provide

connection to the edges of Antarctica and Greenland. Other land use for satellite phones are rigs

at sea, a backup for hospitals, military, and recreation. Ships at sea often use satellite phones, and

planes

5.2.3 Radio:

Satellite radio offers audio services in some countries, notably the United States. Mobile services

allow listeners to roam a continent, listening to the same audio programming anywhere. A

satellite radio or subscription radio (SR) is a digital radio signal that is broadcast by a

communications satellite, which covers a much wider geographical range than terrestrial radio

signals. Satellite radio offers a meaningful alternative to ground-based radio services in some

countries.

Radio services are usually provided by commercial ventures and are subscription-based. The

various services are proprietary signals, requiring specialized hardware for decoding and

playback. Providers usually carry a variety of news, weather, sports, and music channels, with

the music channels generally being commercial-free.

In areas with a relatively high population density, it is easier and less expensive to reach the bulk

of the population with terrestrial broadcasts. Thus in the UK and some other countries, the

contemporary evolution of radio services is focused on Digital Audio Broadcasting (DAB)

services or HD Radio, rather than satellite radio.

Amateur radio operators have access to the amateur radio satellites that have been designed

specifically to carry amateur radio traffic. Most such satellites operate as space borne repeaters,

and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly

directional antennas such as Yagis or dish antennas. Due to launch costs, most current amateur

satellites are launched into fairly low Earth orbits, and are designed to deal with only a limited

number of brief contacts at any given time. Some satellites also provide data-forwarding services

using the X.25 or similar protocols.

5.2.4 Television:

As television became the main market, its demand for simultaneous delivery of relatively few

signals of large bandwidth to many receivers being a more precise match for the capabilities of

geosynchronous comsats. Two satellite types are used for North American television and radio:

Direct broadcast satellite (DBS), and Fixed Service Satellite (FSS).

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Fixed Service Satellites use the C band, and the lower portions of the Ku bands. They are

normally used for broadcast feeds to and from television networks and local affiliate stations

(such as program feeds for network and syndicated programming, live shots, and backhauls), as

well as being used for distance learning by schools and universities, business television (BTV),

Videoconferencing, and general commercial telecommunications. FSS satellites are also used to

distribute national cable channels to cable television head ends.

Free-to-air satellite TV channels are also usually distributed on FSS satellites in the Ku band.

The Intelsat Americas 5, Galaxy 10R and AMC 3 satellites over North America provide a quite

large amount of FTA channels on their Ku band transponders.

A direct broadcast satellite is a communications satellite that transmits to small DBS satellite

dishes (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally

operate in the upper portion of the microwave Ku band. DBS technology is used for DTH-

oriented (Direct-To-Home) satellite TV services

Some manufacturers have also introduced special antennas for mobile reception of DBS

television. Using Global Positioning System (GPS) technology as a reference, these antennas

automatically re-aim to the satellite no matter where or how the vehicle (on which the antenna is

mounted) is situated. These mobile satellite antennas are popular with some recreational vehicle

owners.

5.2.5 Internet Access:

After the 1990s, satellite communication technology has been used as a means to connect to the

Internet via broadband data connections. This can be very useful for users who are located in

remote areas, and cannot access a broadband connection, or require high availability of services.

5.2.6 Military Services:

Military satellites are often used for gathering intelligence, as a communications satellite used for

military purposes, or as a military weapon. A satellite by itself is neither military nor civil. It is

the kind of payload it carries that enables one to arrive at a decision regarding its military or

civilian character.

5.2.7 Navigation Services:

The system allows for precise localization world-wide, and with some additional techniques, the

precision is in the range of some meters. Ships and aircraft rely on GPS as an addition to

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traditional navigation systems. Many vehicles come with installed GPS receivers. This system is

also used, e.g., for fleet management of trucks or for vehicle localization in case of theft.

5.2.8 Global Mobile Communication:

The basic purpose of satellites for mobile communication is to extend the area of coverage.

Cellular phone systems, such as AMPS and GSM (and their successors) do not cover all parts of

a country. Areas that are not covered usually have low population where it is too expensive to

install a base station. With the integration of satellite communication, however, the mobile phone

can switch to satellites offering world-wide connectivity to a customer. Satellites cover a certain

area on the earth. This area is termed as a „footprint‟ of that satellite. Within the footprint,

communication with that satellite is possible for mobile users. These users communicate using a

Mobile-User-Link (MUL). The base-stations communicate with satellites using a Gateway-

Link (GWL). Sometimes it becomes necessary for satellite to create a communication link

between users belonging to two different footprints. Here the satellites send signals to each other

and this is done using Inter-Satellite-Link (ISL).

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Chapter 6: FUTURE OF SATELLITE

COMMUNICATION

Satellite communications faces certain problems and challenges. Since the beginning of the era

of satellite communication numerous satellites have been launched by different nations. The

average operational lifetime of a satellite is 15-20 years after which they are de-orbited and

turned into space debris. Thus the outer space surrounding earth is so full of satellites and space

debris that it is becoming difficult to find proper orbital space for the new launches to take place.

6.1 High Altitude Platform

Thus an alternative has been sought in the form of HAP [High Altitude Platform] technology.

A high-altitude platform (HAP) is a quasi-stationary aircraft that provides means of delivering

a service to a large area while staying thousands of feet above in the air for long periods of time.

HAP differs from other aircraft in the sense that it is specially designed to operate at a very high

altitude (17-22 km) and is able to stay there for hours, even days. The new generation of HAPs,

however, will expand this period to several years.

Since HAPs operate at much lower altitudes than satellites, it is possible to cover a small region

much more effectively. Lower altitude also means much lower link budget (hence lower power

consumption) and smaller round trip delay compared to satellites. Furthermore, deploying a

satellite drains significant time and monetary resources, in terms of development and launch.

HAPs, on the other hand, do not cost much and are rapidly deployable. Another major difference

is that a satellite, once launched, does not allow for full maintenance, while HAPs do

Fig 6.1 – High Altitude Platform

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6.2 Reducing the size of satellites

Many experts fear that building and deploying large satellites is simply not sustainable, at least

not by taxpayer-funded government agencies. One solution is to turn over satellite programs to

private interests, such as SpaceX, Virgin Galacti.

Another solution involves shrinking the size and complexity of satellites. Scientists at

California Polytechnic State University and Stanford University have been working since 1999

on a new type of satellite, called CubeSat that relies on building blocks as small as 4 inches (10

centimetres) on a side. Each cube receives off-the-shelf components and can be combined with

other cubes, usually from different teams, to make a more complex payload. By standardizing

the design and spreading the development costs to multiple parties, the costs of the satellite don't

escalate as greatly. A single CubeSat spacecraft might cost less than $100,000 to develop, launch

and operate.

Fig 6.2 – CubeSat1

This illustration demonstrates how CubeSat1 could use its radar and laser cross-track sensor to

measure the distance and relative motion of the other satellite (CubeSat2 on left).

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6.3 Use of new technology

Big or small, future satellites must be able to communicate efficiently with Earth-based stations.

Historically, NASA has relied on radio frequency (RF) communication, but RF is reaching its

limit as demand for more capacity increases. To overcome this obstacle, NASA scientists have

been developing a two-way communication system based on lasers instead of radio waves.

The equipment to run the test hitched a ride on NASA's Lunar Atmosphere and Dust

Environment Explorer, which launched in September 2013 and headed for the moon, where it

began to orbit and collect information on the lunar atmosphere. On Oct. 18, 2013, researchers

made history when they used a pulsed laser beam to transmit data over the 239,000 miles

(384,633 kilometres) between the moon and Earth at a record-breaking download rate of 622

megabits per second.