SATELLITE COMMUNICATION

Dr. Pramod R. Bokde

Assistant Professor
Priyadarshini Bhagwati College of Engineering, Nagpur

March 23, 2021

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DISCLAIMER

This document does not claim any originality

and cannot be used as a substitute for prescribed
textbooks. The information presented here is merely a
collection by the teacher for his respective teaching
assignments. Various sources as mentioned at the end
of the document as well as freely available material
from internet were consulted for preparing this
document. The ownership of the information lies with
the respective authors or institutions.
Contents

1 Satellite Orbits 1

1.1 Introduction to satellite communication . . . . . . . . . . . . . . . 1

1.2 Kepler’s laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Kepler’s law Introduction . . . . . . . . . . . . . . . . . . . 2

1.2.2 Kepler’s First Law . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.3 Kepler’s Second Law . . . . . . . . . . . . . . . . . . . . . . 3

1.2.4 Kepler’sThird Law . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Newton’s law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 Newton’s first law . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.2 Newton’s second law . . . . . . . . . . . . . . . . . . . . . 5

1.3.3 Newton’s third law . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 orbital parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.5 Orbital Perturbations . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.5.1 Effects of non-Spherical Earth . . . . . . . . . . . . . . . . . 7

1.5.2 Atmospheric Drag . . . . . . . . . . . . . . . . . . . . . . . 8

1.6 Station Keeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.7 Geo stationary and Non Geo-stationary orbits . . . . . . . . . . . 9

1.7.1 Geo stationary . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3
4 CONTENTS

1.7.2 Non Geo-Stationary Orbit . . . . . . . . . . . . . . . . . . . 12

1.8 Look Angle Determination . . . . . . . . . . . . . . . . . . . . . . . 13

1.9 Limits of visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.10 Eclipse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.11 Sub satellite Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.12 Sun Transit Outage . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.13 Launching Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.13.1 Intoduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.13.2 Orbit Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.14 Launch vehicles and propulsion . . . . . . . . . . . . . . . . . . . . 21

1.14.1 Transfer Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2 Space Segment and Satellite Link Design 27

2.1 Spacecraft Technology- Structure . . . . . . . . . . . . . . . . . . . 27

2.2 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3 Attitude Control & Orbit Control . . . . . . . . . . . . . . . . . . . 29

2.3.1 Spinning satellite stabilization . . . . . . . . . . . . . . . . 31

2.3.2 Momentum wheel stabilization . . . . . . . . . . . . . . . . 33

2.4 Thermal Control and Propulsion . . . . . . . . . . . . . . . . . . . 35

2.5 Communication Payload & Supporting Subsystems . . . . . . . . 36

2.6 TT&C Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.6.1 Transponders . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.6.2 The wideband receiver . . . . . . . . . . . . . . . . . . . . . 41

2.6.3 The input demultiplexer . . . . . . . . . . . . . . . . . . . . 42

2.6.4 The power amplifier . . . . . . . . . . . . . . . . . . . . . . 45

CONTENTS 5

2.7 Satellite uplink and downlink Analysis and Design . . . . . . . . 47

2.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.7.2 Equivalent Isotropic Radiated Power . . . . . . . . . . . . 48

2.7.3 Transmission Losses . . . . . . . . . . . . . . . . . . . . . . 48

2.8 The Link-Power Budget Equation . . . . . . . . . . . . . . . . . . . 50

2.9 Amplifier noise temperature . . . . . . . . . . . . . . . . . . . . . . 51

2.10 The Uplink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

2.10.1 Input backoff . . . . . . . . . . . . . . . . . . . . . . . . . . 53

2.10.2 The earth station HPA . . . . . . . . . . . . . . . . . . . . . 53

2.11 Downlink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

2.11.1 Output back-off . . . . . . . . . . . . . . . . . . . . . . . . . 54

2.11.2 Effects of Rain . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.12 Inter modulation and interference . . . . . . . . . . . . . . . . . . 56

2.13 Propagation Characteristics and Frequency considerations . . . . 57

2.13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.13.2 Radio Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.14 System reliability and design lifetime . . . . . . . . . . . . . . . . . 58

2.14.1 System reliability . . . . . . . . . . . . . . . . . . . . . . . . 58

2.14.2 Design lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 59

3 Satellite Access 61

3.1 Modulation and Multiplexing: Voice, Data, Video . . . . . . . . . 61

3.1.1 Voice, Data, Video . . . . . . . . . . . . . . . . . . . . . . . 62

3.1.2 Modulation And Multiplexing . . . . . . . . . . . . . . . . 63

3.2 Analog – digital transmission system . . . . . . . . . . . . . . . . 63

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3.2.1 Analog vs. Digital Transmission . . . . . . . . . . . . . . . 63

3.2.2 Digital Data/Analog Signals . . . . . . . . . . . . . . . . . 64

3.3 Digital Video Broadcasting (DVB) . . . . . . . . . . . . . . . . . . . 66

3.4 Multiple Access Techniques . . . . . . . . . . . . . . . . . . . . . . 67

3.4.1 Frequency division duplexing (FDD) . . . . . . . . . . . . 68

3.4.2 Time division duplexing (TDD) . . . . . . . . . . . . . . . . 69

3.4.3 FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

3.4.4 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

3.4.5 Code Division Multiple Access (CDMA) . . . . . . . . . . 73

3.5 Channel allocation schemes . . . . . . . . . . . . . . . . . . . . . . 77

3.5.1 FCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.5.2 DCA and DFS . . . . . . . . . . . . . . . . . . . . . . . . . . 78

3.6 Spread spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

3.6.1 Spread spectrum Techniques . . . . . . . . . . . . . . . . . 79

3.7 Compression – Encryption . . . . . . . . . . . . . . . . . . . . . . . 81

3.7.1 Encryption and Transmission . . . . . . . . . . . . . . . . . 82

3.7.2 Video and Audio Compression . . . . . . . . . . . . . . . . 82

3.7.3 MPEG Standards . . . . . . . . . . . . . . . . . . . . . . . . 84

3.8 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

3.8.1 Symmetric key encryption . . . . . . . . . . . . . . . . . . . 86

4 Earth Segment 89

4.1 Earth Station Technology . . . . . . . . . . . . . . . . . . . . . . . . 89

4.1.1 Terrestrial Interface . . . . . . . . . . . . . . . . . . . . . . . 89

4.1.2 Transmitter and Receiver . . . . . . . . . . . . . . . . . . . 90

CONTENTS 7

4.1.3 Earth Station Tracking System . . . . . . . . . . . . . . . . 93

4.2 Antenna Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2.1 Feed System . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2.2 Antenna Reflector . . . . . . . . . . . . . . . . . . . . . . . 95

4.2.3 Antenna Mount . . . . . . . . . . . . . . . . . . . . . . . . . 98

4.2.4 Antenna Tracking System . . . . . . . . . . . . . . . . . . . 98

4.3 Receive-Only Home TV Systems . . . . . . . . . . . . . . . . . . . 99

4.3.1 The Indoor unit . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.3.2 The Outdoor Unit . . . . . . . . . . . . . . . . . . . . . . . . 102

4.4 Master Antenna TV System . . . . . . . . . . . . . . . . . . . . . . 104

4.5 Community Antenna TV System . . . . . . . . . . . . . . . . . . . 105

4.6 Test Equipment Measurements on G/T, C/No, EIRP . . . . . . . . 107

4.7 Antenna Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5 Satellite Applications 113

5.1 INTELSAT Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

5.2 INSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

5.2.1 INSAT System . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.2.2 Satellites In Service . . . . . . . . . . . . . . . . . . . . . . . 116

5.3 VSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

5.3.1 VSAT network . . . . . . . . . . . . . . . . . . . . . . . . . 121

5.3.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

5.4 Mobile satellite services . . . . . . . . . . . . . . . . . . . . . . . . 122

5.4.1 GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

5.4.2 Global Positioning System (GPS) . . . . . . . . . . . . . . . 128

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5.5 INMARSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

5.6 LEO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

5.7 MEO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

5.8 GEO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

5.9 Satellite Navigational System . . . . . . . . . . . . . . . . . . . . . 135

5.10 Direct Broadcast satellites (DBS) . . . . . . . . . . . . . . . . . . . . 136

5.10.1 Power Rating and Number of Transponders . . . . . . . . 136

5.10.2 Bit Rates for Digital Television . . . . . . . . . . . . . . . . 137

5.10.3 MPEG Compression Standards . . . . . . . . . . . . . . . . 138

5.11 Direct to home Broadcast (DTH) . . . . . . . . . . . . . . . . . . . 138

5.12 Digital audio broadcast (DAB) . . . . . . . . . . . . . . . . . . . . . 140

5.13 Worldspace services . . . . . . . . . . . . . . . . . . . . . . . . . . 141

5.14 Business Television (BTV) - Adaptations for Education . . . . . . 141

5.15 GRAMSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

5.16 Specialized services . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

5.16.1 Satellite-email services . . . . . . . . . . . . . . . . . . . . . 144

5.16.2 Video Conferencing (medium resolution) . . . . . . . . . . 145

5.16.3 Satellite Internet access . . . . . . . . . . . . . . . . . . . . . 146

Chapter 1

Satellite Orbits

1.1 Introduction to satellite communication

Satellites are specifically made for telecommunication purpose.

They are used for mobile applications such as communication to
ships, vehicles, planes, handheld terminals and for TV and radio
broadcasting.

They are responsible for providing these services to an

assigned region (area) on the earth. The power and bandwidth
of these satellites depend upon the preferred size of the footprint,
complexity of the traffic control protocol schemes and the cost of
ground stations.

A satellite works most efficiently when the transmissions

are focused with a desired area.

When the area is focused, then the emissions don’t go

outside that designated area and thus minimizing the interference
to the other systems. This leads more efficient spectrum usage.

Satellite’s antenna patterns play an important role and

must be designed to best cover the designated geographical area
(which is generally irregular in shape).

1
2 CHAPTER 1. SATELLITE ORBITS

Satellites should be designed by keeping in mind its

usability for short and long term effects throughout its life time.
The earth station should be in a position to control the satellite if
it drifts from its orbit it is subjected to any kind of drag from the
external forces.

Applications Of Satellites

1 Weather Forecasting

2 Radio and TV Broadcast

3 Military Satellites

4 Navigation Satellites

5 Global Telephone

6 Connecting Remote Area

7 Global Mobile Communication

1.2 Kepler’s laws

1.2.1 Kepler’s law Introduction

Satellites (spacecraft) orbiting the earth follow the same laws that
govern the motion of the planets around the sun. Kepler’s laws
apply quite generally to any two bodies in space which interact
through gravitation. The more massive of the two bodies is
referred to as the primary, the other, the secondary or satellite.
1.2. KEPLER’S LAWS 3

1.2.2 Kepler’s First Law

Figure 1.1: The foci F1 and F2 , the semi major axis a, and the semi minor axis b
of an ellipse.

Kepler’s first law states that the path followed by a satellite

around the primary will be an ellipse. An ellipse hast Two focal
points shown as F1 and F2 in Fig. 1.1. The center of mass of the
two-body system, termed the bary center, is always center of the
foci.

The semi major axis of the ellipse is denoted by a, and

the semi minor axis, by b. The eccentricity e is given by -
√
a2 − b2
e= (1.1)
a

1.2.3 Kepler’s Second Law

Figure 1.2: Kepler’s second law. The areas A1 and A2 swept out in unit time are
equal.
4 CHAPTER 1. SATELLITE ORBITS

Kepler’s second law states that, for equal time intervals, a

satellite will sweep out equal areas in its orbital plane, focused at
the barycenter. Referring to Fig. 1.2, assuming the satellite travels
distances S1 and S2 meters in 1 s, then the areas A1 and A2 will be
equal. The average velocity in each case is S1 and S2 m/s, and
because of the equal area law, it follows that the velocity at S2 is
less than that at S1 .

1.2.4 Kepler’sThird Law

Kepler’s third law states that the square of the periodic time of
orbit is proportional to the cube of the mean distance between the
two bodies. The mean distance is equal to the semi major axis a.

For the artificial satellites orbiting the earth, Kepler’s

third law can be written in the form
µ
a3 = (1.2)
n2

Where n is the mean motion of the satellite in radians per second

and is the earth’s geocentric gravitational constant µ = 3.986005 ×
1014 m3 /s2

1.3 Newton’s law

1.3.1 Newton’s first law

An object at rest will remain at rest unless acted on by an

unbalanced force. An object in motion continues in motion with
the same speed and in the same direction unless acted upon by
an unbalanced force. This law is often called ’the law of inertia’.
1.4. ORBITAL PARAMETERS 5

1.3.2 Newton’s second law

Acceleration is produced when a force acts on a mass. The greater

the mass (of the object being accelerated) the greater the amount
of force needed (to accelerate the object).

1.3.3 Newton’s third law

For every action there is an equal and opposite re-action. This

means that for every force there is a reaction force that is equal
in size, but opposite in direction. That is to say that whenever an
object pushes another object it gets pushed back in the opposite
direction equally hard.

1.4 orbital parameters

1 Apogee: A point for a satellite farthest from the Earth. It is

denoted as h a .

2 Perigee: A point for a satellite closest from the Earth. It is

denoted as h p .

3 Line of Apsides: Line joining perigee and apogee through

centre of the Earth. It is the major axis of the orbit. One-
half of this line’s length is the semi-major axis equivalents to
satellite’s mean distance from the Earth.

4 Ascending Node: The point where the orbit crosses the

equatorial plane going from north to south.

5 Descending Node: The point where the orbit crosses the

equatorial plane going from south to north.

6 Inclination: the angle between the orbital plane and the

Earth’s equatorial plane. Its measured at the ascending node
6 CHAPTER 1. SATELLITE ORBITS

from the equator to the orbit, going from East to North. Also,
this angle is commonly denoted as i.

7 Line of Nodes: the line joining the ascending and descending

nodes through the centre of Earth.

8 Prograde Orbit: an orbit in which satellite moves in the

same direction as the Earth’s rotation. Its inclination is
always between 0o to 90o . Many satellites follow this path as
Earth’s velocity makes it easier to lunch these satellites.

9 Retrograde Orbit: an orbit in which satellite moves in the

same direction counter to the Earth’s rotation.

10 Argument of Perigee: An angle from the point of perigee

measure in the orbital plane at the Earth’s centre, in the
direction of the satellite motion.

11 Right ascension of ascending node: The definition of an

orbit in space, the position of ascending node is specified.
But as the Earth spins, the longitude of ascending node
changes and cannot be used for reference. Thus for practical
determination of an orbit, the longitude and time of crossing
the ascending node is used.For absolute measurement, a
fixed reference point in space is required.
It could also be defined as ”right ascension of the ascending
node; right ascension is the angular position measured
eastward along the celestial equator from the vernal equinox
vector to the hour circle of the object”.

12 Mean anamoly: It gives the average value to the angular

position of the satellite with reference to the perigee.

13 True anamoly: It is the angle from point of perigee to the

satellite’s position, measure at the Earth’s centre.
1.5. ORBITAL PERTURBATIONS 7

Figure 1.3:

1.5 Orbital Perturbations

Theoretically, an orbit described by Kepler is ideal as Earth is

considered to be a perfect sphere and the force acting around the
Earth is the centrifugal force. This force is supposed to balance
the gravitational pull of the earth.

In reality, other forces also play an important role and

affect the motion of the satellite. These forces are the gravitational
forces of Sun and Moon along with the atmospheric drag.

Effect of Sun and Moon is more pronounced on

geostationary earth satellites where as the atmospheric drag
effect is more pronounced for low earth orbit satellites.

1.5.1 Effects of non-Spherical Earth

As the shape of Earth is not a perfect sphere, it causes some

variations in the path followed by the satellites around the
primary. As the Earth is bulging from the equatorial belt, and
keeping in mind that an orbit is not a physical entity, and it is the
forces resulting from an oblate Earth which act on the satellite
produce a change in the orbital parameters.
8 CHAPTER 1. SATELLITE ORBITS

This causes the satellite to drift as a result of regression of

the nodes and the latitude of the point of perigee (point closest to
the Earth). This leads to rotation of the line of apsides. As the orbit
itself is moving with respect to the Earth, the resultant changes are
seen in the values of argument of perigee and right ascension of
ascending node.

Due to the non-spherical shape of Earth, one more effect

called as the “Satellite Graveyard” is seen. The non-spherical
shape leads to the small value of eccentricity (10-5) at the
equatorial plane. This causes a gravity gradient on GEO satellite
and makes them drift to one of the two stable points which
coincide with minor axis of the equatorial ellipse.

1.5.2 Atmospheric Drag

For Low Earth orbiting satellites, the effect of atmospheric drag

is more pronounces. The impact of this drag is maximum at the
point of perigee. Drag (pull towards the Earth) has an effect on
velocity of Satellite (velocity reduces).

This causes the satellite to not reach the apogee height

successive revolutions. This leads to a change in value of semi-
major axis and eccentricity. Satellites in service are maneuvered
by the earth station back to their original orbital position.

1.6 Station Keeping

In addition to having its attitude controlled, it is important that a

geostationary satellite be kept in its correct orbital slot. The
equatorial ellipticity of the earth causes geostationary satellites to
drift slowly along the orbit, to one of two stable points, at 75o E
and 105o W.
1.7. GEO STATIONARY AND NON GEO-STATIONARY ORBITS 9

Figure 1.4: Typical satellite motion

To counter this drift, an oppositely directed velocity

component is imparted to the satellite by means of jets, which are
pulsed once every 2 or 3 weeks.

These maneuvers are termed east-west station-keeping

maneuvers. Satellites in the 6/4-GHz band must be kept within
0.1o of the designated longitude, and in the 14/12-GHz band,
within 0.05o .

1.7 Geo stationary and Non Geo-stationary orbits

1.7.1 Geo stationary

A geostationary orbit is one in which a satellite orbits the earth at

exactly the same speed as the earth turns and at the same latitude,
specifically zero, the latitude of the equator. A satellite orbiting in
a geostationary orbit appears to be hovering in the same spot in
the sky, and is directly over the same patch of ground at all times.

A geosynchronous orbit is one in which the satellite is

synchronized with the earth’s rotation, but the orbit is tilted with
10 CHAPTER 1. SATELLITE ORBITS

respect to the plane of the equator. A satellite in a

geosynchronous orbit will wander up and down in latitude,
although it will stay over the same line of longitude. Although
the terms ’geostationary’ and ’geosynchronous’ are sometimes
used interchangeably, they are not the same technically;
geostationary orbit is a subset of all possible geosynchronous
orbits.

The person most widely credited with developing the

concept of geostationary orbits is noted science fiction author
Arthur C. Clarke (Islands in the Sky, Childhood’s End,
Rendezvous with Rama, and the movie 2001: a Space Odyssey).
Others had earlier pointed out that bodies traveling a certain
distance above the earth on the equatorial plane would remain
motionless with respect to the earth’s surface. But Clarke
published an article in 1945’s Wireless World that made the leap
from the Germans’ rocket research to suggest permanent
manmade satellites that could serve as communication relays.

Geostationary objects in orbit must be at a certain

distance above the earth; any closer and the orbit would decay,
and farther out they would escape the earth’s gravity altogether.
This distance is 35,786 kilometers (22,236 miles) from the surface.

The first geosynchrous satellite was orbited in 1963, and

the first geostationary one the following year. Since the only
geostationary orbit is in a plane with the equator at 35,786
kilometers, there is only one circle around the world where these
conditions obtain.

This means that geostationary ’real estate’ is finite. While

satellites are in no danger of bumping in to one another yet, they
must be spaced around the circle so that their frequencies do not
interfere with the functioning of their nearest neighbors.
1.7. GEO STATIONARY AND NON GEO-STATIONARY ORBITS 11

Geostationary Satellites

There are 2 kinds of manmade satellites in the heavens above:

One kind of satellite ORBITS the earth once or twice a day, and
the other kind is called a communications satellite and it is
PARKED in a STATIONARY position 22,300 miles (35,900 km)
above the equator of the STATIONARY earth. A type of the
orbiting satellite includes the space shuttle and the international
space station which keep a low earth orbit (LEO) to avoid the
deadly Van Allen radiation belts.

The most prominent satellites in medium earth orbit

(MEO) are the satellites which comprise the Global Positioning
System or GPS as it is called.

The Global Positioning System

The global positioning system was developed by the U.S.

military and then opened to civilian use. It is used today to track
planes, ships, trains, cars or literally anything that moves.
Anyone can buy a receiver and track their exact location by using
a GPS receiver.

These satellites are traveling around the earth at speeds

of about 7,000 mph (11,200 kph). GPS satellites are powered by
solar energy. They have backup batteries onboard to keep them
running in the event of a solar eclipse, when there’s no solar
power.

Small rocket boosters on each satellite keep them flying

in the correct path. The satellites have a lifetime of about 10 years
until all their fuel runs out.

At exactly 22,300 miles above the equator, the force of

gravity is canceled by the centrifugal force of the rotating universe.
This is the ideal spot to park a stationary satellite.
12 CHAPTER 1. SATELLITE ORBITS

Figure 1.6: At exactly 22,000 miles (35,900 km) above the equator, the earth’s
force of gravity is canceled by the centrifugal force of the rotating universe. .

1.7.2 Non Geo-Stationary Orbit

For the geo- stationary case, the most important of these are the
gravitational fields of the moon and the sun, and the non-spherical
shape of the earth.

Other significant forces are solar radiation pressure and

reaction of the satellite itself to motor movement within the
satellite. As a result, station keeping maneuvers must be carried
out to maintain the satellite within set limits of its nominal
geostationary position.

An exact geostationary orbit therefore is not attainable in

practice, and the orbital parameters vary with time. The two-line
orbital elements are published at regular intervals.

The period for a geostationary satellite is 23 h, 56 min, 4

s, or 86,164 s. The reciprocal of this is 1.00273896 rev/day, which
is about the value tabulated for most of the satellites in Fig.

Thus these satellites are geo-synchronous, in that they

rotate in synchronism with the rotation of the earth. However,
they are not geostationary. The term geosynchronous satellite is
used in many cases instead of geostationary to describe these
near-geostationary satellites. It should be noted, however, that in
1.8. LOOK ANGLE DETERMINATION 13

general a geosynchronous satellite does not have to be

near-geostationary, and there are a number of geosynchronous
satellites that are in highly elliptical orbits with comparatively
large inclinations (e.g., the Tundra satellites).

The small inclination makes it difficult to locate the

position of the ascending node, and the small eccentricity makes
it difficult to locate the position of the perigee.

However, because of the small inclination, the angles ω

and Ω can be assumed to be in the same plane.The longitude of the
sub-satellite point (the satellite longitude) is the east early rotation
from the Greenwich meridian.

φss = ω + Ω + v − GST (1.3)

The Greenwich sidereal time (GST) gives the eastward

position of the Greenwich meridian relative to the line of Aries,
and hence the sub-satellite point is at longitude and the mean
longitude of the satellite is given by

φssmean = ω + Ω + M − GST (1.4)

Equation 1.4 can be used to calculate the true anomaly,

and because of the small eccentricity, this can be approximated as
v = M + 2e sin M.

1.8 Look Angle Determination

The look angles for the ground station antenna are Azimuth and
Elevation angles. They are required at the antenna so that it
points directly at the satellite. Look angles are calculated by
considering the elliptical orbit. These angles change in order to
track the satellite.
14 CHAPTER 1. SATELLITE ORBITS

For geostationary orbit, these angels values does not

change as the satellites are stationary with respect to earth. Thus
large earth stations are used for commercial communications.

For home antennas, antenna beamwidth is quite broad

and hence no tracking is essential. This leads to a fixed position
for these antennas.

Figure 1.7: The geometry used in determining the look angles for Geostationary
Satellites.

Figure 1.8: The spherical geometry related to figure 1.7

With respect to the figure 1.7 and 1.8, the following

information is needed to determine the look angles of
geostationary orbit.

1 Earth Station Latitude: λ E

1.8. LOOK ANGLE DETERMINATION 15

2 Earth Station Longitude: Φ E

3 Sub-Satellite Point’s Longitude: ΦSS

4 ES: Position of Earth Station

5 SS: Sub-Satellite Point

6 S: Satellite

7 d: Range from ES to S

8 ζ: angle to be determined

Figure 1.9: plane triangle obtained from figure 1.7

Considering figure 1.9, it’s a spherical triangle. All sides

are the arcs of a great circle. Three sides of this triangle are defined
by the angles subtended by the centre of the earth.

1 Side a: angle between North Pole and radius of the

sub-satellite point.

2 Side b: angle between radius of Earth and radius of the sub-

satellite point.

3 Side c: angle between radius of Earth and the North Pole.

16 CHAPTER 1. SATELLITE ORBITS

a = 90o and such a spherical triangle is called quadrantal

triangle. c = 90o –λ.

Angle B is the angle between the plane containing c and

the plane containing a. Thus, B = Φ E − ΦSS

Angle A is the angle between the plane containing b and

the plane containing c.
Angle C is the angle between the plane containing a and the plane
containing b.

Thus, a = 90o , c = 90o − λ E and B = Φ E − ΦSS

Thus, b = ar cos(cos B cos λ E )

And A = arcsin(sin | B|/ sin b)

Applying the cosine rule for plane triangle to the triangle

of figure 1.9
q
d = R2 + a2GSO − 2RaGSO cos b (1.5)

Applying the sine rule for plane triangles to the triangle

of figure 1.9 allows the angle of elevation to be found:
a 
GSO
El = arccos sin b (1.6)
d

1.9 Limits of visibility

The east and west limits of geostationary are visible from any
given Earth station. These limits are set by the geographic
coordinates of the Earth station and antenna elevation.

The lowest elevation is zero (in theory) but in practice,

to avoid reception of excess noise from Earth. Some finite
minimum value of elevation is issued. The earth station can see a
satellite over a geostationary arc bounded by ± (81.30) about the
earth station’s longitude.
1.10. ECLIPSE 17

1.10 Eclipse

It occurs when Earth’s equatorial plane coincides with the plane f

he Earth’s orbit around the sun.

Near the time of spring and autumnal equinoxes, when

the sun is crossing the equator, the satellite passes into sun’s
shadow. This happens for some duration of time every day.

These eclipses begin 23 days before the equinox and end

23 days after the equinox. They last for almost 10 minutes at the
beginning and end of equinox and increase for a maximum
period of 72 minutes at a full eclipse. The solar cells of the
satellite become non-functional during the eclipse period and the
satellite is made to operate with the help of power supplied from
the batteries.

A satellite will have the eclipse duration symmetric

around the time t = Satellite Longitude/15 + 12 hours. A
satellite at Greenwich longitude 0 will have the eclipse duration
symmetric around 0/15

UTC + 12 hours = 00:00 UTC (1.7)

The eclipse will happen at night but for satellites in the

east it will happen late evening local time.

For satellites in the west eclipse will happen in the early

morning hour’s local time.

An earth caused eclipse will normally not happen during

peak viewing hours if the satellite is located near the longitude
of the coverage area. Modern satellites are well equipped with
batteries for operation during eclipse.
18 CHAPTER 1. SATELLITE ORBITS

Figure 1.10: A satellite east of the earth station enters eclipse during daylight
busy) hours at the earth station. A Satellite west of earth station enters eclipse
during night and early morning hours (non busy time).

1.11 Sub satellite Point

1 Point at which a line between the satellite and the center of

the Earth intersects the Earth’s surface.

2 Location of the point expressed in terms of latitude and

longitude.

3 If one is in the US it is common to use.

(a) Latitude - degrees north from equator

(b) Longitude - degrees west of the Greenwich meridian

4 Location of the sub satellite point may be calculated from

coordinates of the rotating system as:
!
π Zr
Ls = − cos−1 p (1.8)
2 xr2 + y2r + z2r
1.12. SUN TRANSIT OUTAGE 19

Figure 1.11: Sub satellite Point

1.12 Sun Transit Outage

Sun transit outage is an interruption in or distortion of

geostationary satellite signals caused by interference from solar
radiation.

Sun appears to be an extremely noisy source which

completely blanks out the signal from satellite. This effect lasts
for 6 days around the equinoxes. They occur for a maximum
period of 10 minutes.

Generally, sun outages occur in February, March,

September and October, that is, around the time of the equinoxes.

At these times, the apparent path of the sun across the

sky takes it directly behind the line of sight between an earth
station and a satellite. As the sun radiates strongly at the
microwave frequencies used to communicate with satellites
(C-band, Ka band and Ku band) the sun swamps the signal from
the satellite.

The effects of a sun outage can include partial

degradation, that is, an increase in the error rate, or total
destruction of the signal.
20 CHAPTER 1. SATELLITE ORBITS

Figure 1.12: Earth Eclipse of a Satellite and Sun transit Outage

1.13 Launching Procedures

1.13.1 Intoduction

Low Earth Orbiting satellites are directly injected into their orbits.
This cannot be done incase of GEOs as they have to be positioned
36,000 kms above the Earth’s surface.

Launch vehicles are hence used to set these satellites in

their orbits. These vehicles are reusable. They are also known as
’Space Transportation System’ (STS).

When the orbital altitude is greater than 1,200 km it

becomes expensive to directly inject the satellite in its orbit. For
this purpose, a satellite must be placed in to a transfer orbit
between the initial lower orbit and destination orbit. The transfer
orbit is commonly known as Hohmann-Transfer Orbit.
1.14. LAUNCH VEHICLES AND PROPULSION 21

1.13.2 Orbit Transfer

Figure 1.13: Orbit Transfer positions

About Hohmann Transfer Orbit: This manoeuvre is named for

the German civil engineer who first proposed it, Walter
Hohmann, who was born in 1880. He didn’t work in rocketry
professionally (and wasn’t associated with military rocketry), but
was a key member of Germany’s pioneering Society for Space.

Travel that included people such as Willy Ley,

Hermann, and Werner von Braun. He published his concept of
how to transfer between orbits in his 1925 book, The Attainability
of Celestial Bodies.)

The transfer orbit is selected to minimize the energy

required for the transfer. This orbit forms a tangent to the low
attitude orbit at the point of its perigee and tangent to high
altitude orbit at the point of its apogee.

1.14 Launch vehicles and propulsion

The rocket injects the satellite with the required thrust** into the
transfer orbit. With the STS, the satellite carries a perigee kick
motor*** which imparts the required thrust to inject the satellite in
its transfer orbit. Similarly, an apogee kick motor (AKM) is used
22 CHAPTER 1. SATELLITE ORBITS

to inject the satellite in its destination orbit.

Generally it takes 1-2 months for the satellite to become

fully functional. The Earth Station performs the Telemetry
Tracking and Command function to control the satellite transits
and functionalities.

(Thrust: It is a reaction force described quantitatively by

Newton’s second and third laws. When a system expels or
accelerates mass in one direction the accelerated mass will cause
a force of equal magnitude but opposite direction on that
system.)

Kick Motor refers to a rocket motor that is regularly

employed on artificial satellites destined for a geostationary orbit.
As the vast majority of geostationary satellite launches are
carried out from spaceports at a significant distance away from
Earth’s equator.

The carrier rocket would only be able to launch the

satellite into an elliptical orbit of maximum apogee
35,784-kilometres and with a non-zero inclination approximately
equal to the latitude of the launch site.

TT&C: it’s a sub-system where the functions performed

by the satellite control network to maintain health and status,
measure specific mission parameters and processing over time a
sequence of these measurement to refine parameter knowledge,
and transmit mission commands to the satellite.

1.14.1 Transfer Orbit

It is better to launch rockets closer to the equator because the

Earth rotates at a greater speed here than that at either pole. This
extra speed at the equator means a rocket needs less thrust (and
therefore less fuel) to launch into orbit.
1.14. LAUNCH VEHICLES AND PROPULSION 23

In addition, launching at the equator provides an

additional 1,036 mph (1,667 km/h) of speed once the vehicle
reaches orbit. This speed bonus means the vehicle needs less fuel,
and that freed space can be used to carry more pay load.

Figure 1.14: Hohmann Transfer Orbit

Figure 1.15: Launching stages of a GEO (example INTELSAT)

24 CHAPTER 1. SATELLITE ORBITS

Rocket Launch

A rocket launch is the takeoff phase of the flight of a rocket.

Launches for orbital spaceflights, or launches into interplanetary
space, are usually from a fixed location on the ground, but may
also be from a floating platform (such as the Sea Launch vessel)
or, potentially, from a superheavy An-225-class airplane.

Launches of suborbital flights (including missile

launches), can also be from:

1 a missile silo

2 a mobile launcher vehicle

3 a submarine

4 air launch:

(a) from a plane (e.g. Scaled Composites Space Ship One,

Pegasus Rocket, X-15)
(b) from a balloon (Rockoon, da Vinci Project (under
development))

5 a surface ship (Aegis Ballistic Missile Defense System)

6 an inclined rail (e.g. rocket sled launch)

“Rocket launch technologies” generally refers to the

entire set of systems needed to successfully launch a vehicle, not
just the vehicle itself, but also the firing control systems, ground
control station, launch pad, and tracking stations needed for a
successful launch and/or recovery.

Orbital launch vehicles commonly take off vertically, and

then begin to progressively lean over, usually following a gravity
turn trajectory.
1.14. LAUNCH VEHICLES AND PROPULSION 25

Once above the majority of the atmosphere, the vehicle

then angles the rocket jet, pointing it largely horizontally but
somewhat downwards, which permits the vehicle to gain and
then maintain altitude while increasing horizontal speed. As the
speed grows, the vehicle will become more and more horizontal
until at orbital speed, the engine will cut off.

Figure 1.16: STS-7/Anik C2 mission scenario.

26 CHAPTER 1. SATELLITE ORBITS
Chapter 2

Space Segment and Satellite Link

Design

2.1 Spacecraft Technology- Structure

A satellite communications system can be broadly divided into

two segments—a ground segment and a space segment.

Figure 2.1: Satellite Structure

The space segment will obviously include the satellites,

27
28 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

but it also includes the ground facilities needed to keep the

satellites operational, these being referred to as the tracking,
telemetry, and command (TT&C) facilities. In many networks it is
common practice to employ a ground station solely for the
purpose of TT&C.

The equipment carried aboard the satellite also can be

classified according to function. The payload refers to the
equipment used to provide the service for which the satellite has
been launched. In a communications satellite, the equipment
which provides the connecting link between the satellite’s
transmit and receive antennas is referred to as the transponder.
The transponder forms one of the main sections of the payload,
the other being the antenna subsystems.In this chapter the main
characteristics of certain bus systems and payloads are described.

2.2 Power Supply

The primary electrical power for operating the electronic

equipment is obtained from solar cells. Individual cells can
generate only small amounts of power, and therefore, arrays of
cells in series-parallel connection are required. Figure shows the
solar cell panels for the HS 376 satellite manufactured by Hughes
Space and Communications Company.

In geostationary orbit the telescoped panel is fully

extended so that both are exposed to sun- light. At the beginning
of life, the panels produce 940 W dc power, which may drop to
760 W at the end of 10 years.

During eclipse, power is provided by two

nickel-cadmium (Ni-Cd) long life batteries, which will deliver
830 W. At the end of life, battery recharge time is less than 16 h.
2.3. ATTITUDE CONTROL & ORBIT CONTROL 29

Figure 2.2: Satellite eclipse time as a function of the current day of the year.

Capacity of cylindrical and solar-sail satellites, the cross-

over point is estimated to be about 2 kW, where the solar-sail type
is more economical than the cylindrical type (Hyndman, 1991).

2.3 Attitude Control & Orbit Control

The attitude of a satellite refers to its orientation in space. Much of

the equipment carried aboard a satellite is there for the purpose
of control- ling its attitude. Attitude control is necessary, for
example, to ensure that directional antennas point in the proper
directions.

In the case of earth environmental satellites, the

earth-sensing instruments must cover the required regions of the
earth, which also requires attitude control. A number of forces,
referred to as disturbance torques, can alter the attitude, some
examples being the gravitational fields of the earth and the moon,
solar radiation, and meteorite impacts.

Attitude control must not be con- fused with station

keeping, which is the term used for maintaining a satellite in its
correct orbital position, although the two are closely related.

To exercise attitude control, there must be available some

30 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

measure of a satellite’s orientation in space and of any tendency

for this to shift. In one method, infrared sensors, referred to as
horizon detectors, are used to detect the rim of the earth against the
background of space.

With the use of four such sensors, one for each quadrant,
the center of the earth can be readily established as a reference
point. Usually, the attitude-control process takes place aboard the
satellite, but it is also possible for control signals to be transmitted
from earth, based on attitude data obtained from the satellite.

Also, where a shift in attitude is desired, an attitude

maneuver is executed. The control signals needed to achieve this
maneuver may be transmitted from an earth station.

Controlling torques may be generated in a number of

ways. Passive attitude control refers to the use of mechanisms
which stabilize the satellite without putting a drain on the
satellite’s energy supplies; at most, infrequent use is made of
these supplies, for example, when thruster jets are impulsed to
provide corrective torque. Examples of passive attitude control
are spin stabilization and gravity gradient stabilization.

The other form of attitude control is active control. With

active attitude control, there is no overall stabilizing torque
present to resist the disturbance torques. Instead, corrective
torques are applied as required in response to disturbance
torques. Methods used to generate active control torques include
momentum wheels, electromagnetic coils, and mass expulsion
devices, such as gas jets and ion thrusters.
2.3. ATTITUDE CONTROL & ORBIT CONTROL 31

Figure 2.3: (a) Roll, pitch, and yaw axes. The yaw axis is directed toward the
earth’s center, the pitch axis is normal to the orbital plane, and the roll axis is
perpendicular to the other two. (b) RPY axes for the geostationary orbit. Here,
the roll axis is tangential to the orbit and lies along the satellite velocity vector.

The three axes which define a satellite’s attitude are its

roll, pitch, and yaw (RPY) axes. These are shown relative to the
earth in Fig. 2.3. All three axes pass through the center of gravity
of the satellite. For an equatorial orbit, movement of the satellite
about the roll axis moves the antenna footprint north and south;
movement about the pitch axis moves the footprint east and west;
and movement about the yaw axis rotates the antenna footprint.

2.3.1 Spinning satellite stabilization

Spin stabilization may be achieved with cylindrical satellites. The

satellite is constructed so that it is mechanically balanced about
one particular axis and is then set spinning around this axis. For
geostationary satellites, the spin axis is adjusted to be parallel to
the N-S axis of the earth, as illustrated in Fig. 2.4. Spin rate is
typically in the range of 50 to 100 rev/min. Spin is initiated during
the launch phase by means of small gas jets.

In the absence of disturbance torques, the spinning

satellite would maintain its correct attitude relative to the earth.
32 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Disturbance torques are generated in a number of ways, both

external and internal to the satellite.

Figure 2.4: Spin stabilization in the geostationary orbit. The spin axis lies along
the pitch axis, parallel to the earth’s N-S axis.

Solar radiation, gravitational gradients, and meteorite

impacts are all examples of external forces which can give rise to
disturbance torques. Motor bearing friction and the movement of
satellite elements such as the antennas also can give rise to
disturbance torques. The overall effect is that the spin rate will
decrease, and the direction of the angular spin axis will change.
Impulse-type thrusters, or jets, can be used to increase the spin
rate again and to shift the axis back to its cor- rect N-S orientation.

Nutation, which is a form of wobbling, can occur as a

result of the disturbance torques and/or from misalignment or
unbalance of the control jets. This nutation must be damped out
by means of energy absorbers known as nutation dampers.

The antenna feeds can therefore be connected directly to

the transponders without the need for radio frequency (rf) rotary
joints, while the complete platform is despun. Of course, control
signals and power must be transferred to the despun section, and
a mechanical bearing must be provided.

The complete assembly for this is known as the bearing

and power transfer assembly (BAPTA). Figure 2.5 shows a
photograph of the internal structure of the HS 376.
2.3. ATTITUDE CONTROL & ORBIT CONTROL 33

Figure 2.5: HS 376 spacecraft.

Certain dual-spin spacecraft obtain spin stabilization

from a spinning flywheel rather than by spinning the satellite
itself. These flywheels are termed momentum wheels, and their
average momentum is referred to as momentum bias.

2.3.2 Momentum wheel stabilization

In the previous section the gyroscopic effect of a spinning satellite

was shown to provide stability for the satellite attitude.

Stability also can be achieved by utilizing the gyroscopic

effect of a spinning flywheel, and this approach is used in satellites
with cube-like bodies (such as shown in Fig. and the INTELSAT V
type satellites shown in Fig. These are known as body-stabilized
satellites.

The complete unit, termed a momentum wheel, consists

of a flywheel, the bearing assembly, the casing, and an electric
drive motor with associated electronic control circuitry.
34 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

The flywheel is attached to the rotor, which consists of a

permanent magnet providing the magnetic field for motor action.
The stator of the motor is attached to the body of the satellite.

Thus the motor provides the coupling between the

flywheel and the satellite structure. Speed and torque control of
the motor is exercised through the currents fed to the stator.

Figure 2.6: Alternative momentum wheel stabilization systems: (a) one-wheel,

(b) two- wheel, (c) three-wheel.

When a momentum wheel is operated with zero

momentum bias, it is generally referred to as a reaction wheel.
Reaction wheels are used in three axis stabilized systems. Here,
as the name suggests, each axis is stabilized by a reaction wheel,
as shown in Fig. 7.8c. Reaction wheels can also be combined with
a momentum wheel to provide the control needed (Chetty, 1991).

Random and cyclic disturbance torques tends to

produce zero momentum on average. However, there will
2.4. THERMAL CONTROL AND PROPULSION 35

always be some disturbance torques that causes a cumulative

increase in wheel momentum, and eventually at some point the
wheel saturates.

In effect, it reaches its maximum allowable angular

velocity and can no longer take in any more momentum. Mass
expulsion devices are then used to unload the wheel, that is,
remove momentum from it (in the same way a brake removes
energy from a moving vehicle). Of course, operation of the mass
expulsion devices consumes part of the satellite’s fuel supply.

2.4 Thermal Control and Propulsion

Satellites are subject to large thermal gradients, receiving the

sun’s radiation on one side while the other side faces into space.
In addition, thermal radiation from the earth and the earth’s
albedo, which is the fraction of the radiation falling on earth
which is reflected, can be significant for low altitude
earth-orbiting satellites, although it is negligible for geostationary
satellites.

Equipment in the satellite also generates heat which has

to be removed. The most important consideration is that the
satellite’s equipment should operate as nearly as possible in a
stable temperature environment. Various steps are taken to
achieve this. Thermal blankets and shields may be used to
provide insulation. Radiation mirrors are often used to remove
heat from the communications payload.

The mirrored thermal radiator for the Hughes HS 376

satellite can be seen in Fig. These mirrored drums surround the
communications equipment shelves in each case and provide
good radiation paths for the generated heat to escape into the
surrounding space.

One advantage of spinning satellites compared with

36 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

body stabilized is that the spinning body provides an averaging

of the temperature extremes experienced from solar flux and the
cold back- ground of deep space. In order to maintain constant
temperature conditions, heaters may be switched on (usually on
command from ground) to make up for the heat reduction which
occurs when transponders are switched off. The INTELSAT VI
satellite used heaters to maintain propulsion thrusters and line
temperatures (Pilcher, 1982).

2.5 Communication Payload & Supporting

Subsystems

The physical principle of establishing communication

connections between remote communication devices dates back
to the late 1800s when scientists were beginning to understand
electromagnetism and discovered that electromagnetic (EM)
radiation (also called EM waves ) generated by one device can be
detected by another located at some distance away.

By controlling certain aspect s of the radiation (through a

process called modulation ), useful information can be embedded
in the EM waves and transmitted from one device to another.

The second major module is the communication

payload, which is made up of transponders. A transponder is
capable of :

1 Receiving uplinked radio signals from earth satellite

transmission stations (antennas).
2 Amplifying received radio signals

3 Sorting the input signals and directing the output signals

through input/output signal multiplexers to the proper
downlink antennas for retransmission to earth satellite
receiving stations (antennas).
2.6. TT&C SUBSYSTEM 37

2.6 TT&C Subsystem

The TT&C subsystem performs several routine functions aboard

the spacecraft. The telemetry, or telemetering, function could be
interpreted as measurement at a distance. Specifically, it refers to
the overall operation of generating an electrical signal
proportional to the quantity being measured and encoding and
transmitting this to a distant station, which for the satellite is one
of the earth stations.

Data which are transmitted as telemetry signals include

attitude information such as that obtained from sun and earth
sensors; environmental information such as the magnetic field
intensity and direction, the frequency of meteorite impact, and so
on; and spacecraft information such as temperatures, power
supply voltages, and stored-fuel pressure.

Telemetry and command may be thought of as

complementary functions. The telemetry subsystem transmits
information about the satellite to the earth station, while the
command subsystem receives command signals from the earth
station, often in response to telemetered information. The
command subsystem demodulates and, if necessary, decodes the
command signals and routes these to the appropriate equipment
needed to execute the necessary action.

Thus attitude changes may be made, communication

transponders switched in and out of circuits, antennas redirected,
and station-keeping maneuvers carried out on command. It is
clearly important to prevent unauthorized commands from being
received and decoded, and for this reason, the command signals
are often encrypted.

Encrypt is derived from a Greek word kryptein, meaning

to hide, and represents the process of concealing the command
signals in a secure code. This differs from the normal process of
38 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

encoding which converts characters in the command signal into a

code suitable for transmission.

Tracking of the satellite is accomplished by having the

satellite transmit beacon signals which are received at the TT&C
earth stations.

Tracking is obviously important during the transfer and

drift orbital phases of the satellite launch. Once it is on station,
the position of a geostationary satellite will tend to be shifted as a
result of the various disturbing forces, as described previously.

Therefore, it is necessary to be able to track the satellite’s

movement and send correction signals as required.

2.6.1 Transponders

A transponder is the series of interconnected units which forms a

single communications channel between the receive and transmit
antennas in a communications satellite.

Some of the units utilized by a transponder in a given

channel may be common to a number of transponders. Thus,
although reference may be made to a specific transponder, this
must be thought of as an equipment channel rather than a single
item of equipment.

Before describing in detail the various units of a

transponder, the overall frequency arrangement of a typical
C-band communications satellite will be examined briefly. The
bandwidth allocated for C-band service is 500 MHz, and this is
divided into subbands, one transponder.

A typical transponder bandwidth is 36 MHz, and

allowing for a 4-MHz guardband between transponders, 12 such
transponders can be accommodated in the 500-MHz bandwidth.
2.6. TT&C SUBSYSTEM 39

Figure 2.7: Satellite control system.

By making use of polarization isolation, this number

can be doubled. Polarization isolation refers to the fact that
carriers, which may be on the same frequency but with opposite
senses of polarization, can be isolated from one another by
receiving antennas matched to the incoming polarization.

With linear polarization, vertically and horizontally

polarized carriers can be separated in this way, and with circular
polarization, left-hand circular and right-hand circular
polarizations can be separated.

Because the carriers with opposite senses of polarization

may overlap in frequency, this technique is referred to as
frequency reuse. Figure 2.8 shows part of the frequency and
polarization plan for a C-band communications satellite.
40 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.8: Section of an uplink frequency and polarization plan. Numbers refer
to frequency in megahertz.

Frequency reuse also may be achieved with spot-beam

antennas, and these may be combined with polarization reuse to
provide an effective bandwidth of 2000 MHz from the actual
bandwidth of 500 MHz.

For one of the polarization groups, Fig. 2.8 shows the

channeling scheme for the 12 transponders in more detail. The
incoming, or uplink, frequency range is 5.925 to 6.425 GHz.
2.6. TT&C SUBSYSTEM 41

The frequency conversion shifts the carriers to the

downlink frequency band, which is also 500 MHz wide,
extending from 3.7 to 4.2 GHz. At this point the signals are
channelized into frequency bands which represent the individual
transponder bandwidths.

2.6.2 The wideband receiver

The wideband receiver is shown in more detail in Fig. 2.9. A

duplicate receiver is provided so that if one fails, the other is
automatically switched in. The combination is referred to as a
redundant receiver, meaning that although two are provided,
only one is in use at a given time.

The first stage in the receiver is a low-noise amplifier

(LNA). This amplifier adds little noise to the carrier being
amplified, and at the same time it provides sufficient
amplification for the carrier to override the higher noise level
present in the following mixer stage.

Figure 2.9: Satellite transponder channels

42 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.10: Satellite wideband receiver.

In a well-designed receiver, the equivalent noise

temperature referred to the LNA input is basically that of the
LNA alone. The equivalent noise temperature of a satellite
receiver may be on the order of a few hundred kelvins.

The LNA feeds into a mixer stage, which also requires a

local oscillator (LO) signal for the frequency-conversion process.
With advances in field-effect transistor (FET) technology, FET
amplifiers, which offer equal or better performance, are now
available for both bands. Diode mixer stages are used.

The amplifier following the mixer may utilize bipolar

junction transistors (BJTs) at 4 GHz and FETs at 12 GHz, or FETs
may in fact be used in both bands.

2.6.3 The input demultiplexer

The input demultiplexer separates the broadband input, covering

the frequency range 3.7 to 4.2 GHz, into the transponder frequency
channels.

This provides greater frequency separation between

2.6. TT&C SUBSYSTEM 43

adjacent channels in a group, which reduces adjacent channel

interference.

The output from the receiver is fed to a power splitter,

which in turn feeds the two separate chains of circulators.
44 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN
2.6. TT&C SUBSYSTEM 45

The full broadband signal is transmitted along each

chain, and the channelizing is achieved by means of channel
filters con- nected to each circulator.

Each filter has a bandwidth of 36 MHz and is tuned to

the appropriate center frequency, as shown in Fig. 2.11.

Although there are considerable losses in the

demultiplexer, these are easily made up in the overall gain for the
transponder channels.

2.6.4 The power amplifier

The fixed attenuation is needed to balance out variations in the

input attenuation so that each transpon- der channel has the
same nominal attenuation, the necessary adjust- ments being
made during assembly.

The variable attenuation is needed to set the level as

required for different types of service (an example being the
requirement for input power backoff discussed later). Because
this variable attenuator adjustment is an operational requirement,
it must be under the control of the ground TT&C station.

Traveling-wave tube amplifiers (TWTAs) are widely

used in transpon- ders to provide the final output power required
to the transmit antenna. Figure 2.12 shows the schematic of a
traveling wave tube (TWT) and its power supplies.

In the TWT, an electron-beam gun assembly consisting

of a heater, a cathode, and focusing electrodes is used to form an
elec- tron beam. A magnetic field is required to confine the beam
to travel along the inside of a wire helix.
46 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.12: Satellite TWTA

It used in ground stations, the magnetic field can be

provided by means of a solenoid and dc power supply. The
comparatively large size and high power consumption of
solenoids make them unsuitable for use aboard satellites, and
lower-power TWTs are used which employ permanent- magnet
focusing. The wave actually will travel around the helical path at
close to the speed of light, but it is the axial component of wave
velocity which interacts with the electron beam.

This component is less than the velocity of light

approximately in the ratio of helix pitch to circumference.
Because of this effective reduction in phase velocity, the helix is
referred to as a slowwave structure. The advantage of the TWT
over other types of tube amplifiers is that it can provide
amplification over a very wide bandwidth. Input levels to the
TWT must be carefully controlled, however, to minimize the
effects of certain forms of distortion.

The worst of these result from the nonlinear transfer

characteristic of the TWT, illustrated in Fig. 2.13
2.7. SATELLITE UPLINK AND DOWNLINK ANALYSIS AND DESIGN 47

Figure 2.13: Power transfer characteristics of a TWT. The saturation point is

used as 0-dB reference for both input and output.

At low-input powers, the output-input power

relationship is linear; that is, a given decibel change in input
power will produce the same decibel change in output power. At
higher power inputs, the output power saturates, the point of
maximum power output being known as the saturation point.

The saturation point is a very convenient reference point,

and input and output quantities are usually referred to it. The
linear region of the TWT is defined as the region bound by the
thermal noise limit at the low end and by what is termed the 1-dB
compression point at the upper end. This is the point where the
actual transfer curve drops.

2.7 Satellite uplink and downlink Analysis and

Design

2.7.1 Introduction

This section describes how the link-power budget calculations

are made. These calculations basically relate two quantities, the
transmit power and the receive power, and show in detail how
the difference between these two powers is accounted for.
48 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Link-budget calculations are usually made using decibel

or decilog quantities. These are explained in App. G. In this text
[square] brackets are used to denote decibel quantities using the
basic power definition.

Where no ambiguity arises regarding the units, the

abbreviation dB is used. For example, Boltzmann’s constant is
given as 228.6 dB, although, strictly speaking, this should be
given as 228.6 deci logs relative to 1 J/K.

2.7.2 Equivalent Isotropic Radiated Power

A key parameter in link-budget calculations is the equivalent

isotropic radiated power, conventionally denoted as EIRP. From
Eqs, the maximum power flux density at some distance r from a
transmitting antenna of gain Gi
GP
Pr = (2.1)
4π 2
An isotropic radiator with an input power equal to GPS would
produce the same flux density. Hence, this product is referred to
as the EIRP, or EIRP is often expressed in decibels relative to 1 W,
or dBW. Let PS be in watts; then [ EIRP] = [ PS] × [ G ] dB ,where
[ PS] is also in dBW and [ G ] is in dB.

2.7.3 Transmission Losses

The [EIRP] may be thought of as the power input to one end of the
transmission link, and the problem is to find the power received
at the other end. Losses will occur along the way, some of which
are constant.

Other losses can only be estimated from statistical data,

and some of these are dependent on weather conditions,
especially on rainfall.
2.7. SATELLITE UPLINK AND DOWNLINK ANALYSIS AND DESIGN 49

The first step in the calculations is to determine the losses

for clear- weather or clear-sky conditions. These calculations take
into account the losses, including those calculated on a statistical
basis, which do not vary significantly with time. Losses which
are weather-related, and other losses which fluctuate with time,
are then allowed for by introducing appropriate fade margins into
the transmission equation.

Free-space transmission

As a first step in the loss calculations, the power loss resulting

from the spreading of the signal in space must be determined.

Feeder losses

Losses will occur in the connection between the receive antenna

and the receiver proper. Such losses will occur in the connecting
waveguides, filters, and couplers. These will be denoted by RFL,
or [RFL] dB, for receiver feeder losses.

Antenna misalignment losses

When a satellite link is established, the ideal situation is to have

the earth station and satellite antennas aligned for maximum gain,
as shown in Fig. There are two possible sources of off-axis loss,
one at the satellite and one at the earth station, as shown in Fig
2.14.
50 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.14: (a) Satellite and earth-station antennas aligned for maximum gain;
(b) earth station situated on a given satellite “footprint,” and earth-station
antenna misaligned.

The off-axis loss at the satellite is taken into account by

designing the link for operation on the actual satellite antenna
contour; this is described in more detail in later sections. The
off-axis loss at the earth station is referred to as the antenna
pointing loss. Antenna pointing losses are usually only a few
tenths of a decibel.

In addition to pointing losses, losses may result at the

antenna from misalignment of the polarization direction . The
polarization misalign- ment losses are usually small, and it will
be assumed that the antenna misalignment losses, denoted by
[AML], include both pointing and polar- ization losses resulting
from antenna misalignment.

2.8 The Link-Power Budget Equation

Now that the losses for the link have been identified, the power
at the receiver, which is the power output of the link, may be
calculated simply as [EIRP] [LOSSES] [GR], where the last
quantity is the receiver antenna gain. Note carefully that decibel
addition must be used.
2.9. AMPLIFIER NOISE TEMPERATURE 51

The major source of loss in any ground-satellite link is

the free-space spreading loss [FSL], the basic link-power budget
equation taking into account this loss only. However, the other
losses also must be taken into account, and these are simply added
to [FSL]. The losses for clear-sky conditions are

[ LOSSES] = [ FSL] + [ RFL] + [ AML] + [ AA] − [ PL] (2.2)

equation for the received power is then

[ PR] = [ EIRP] × [ GR] − [ LOSSES] (2.3)

where [ PR] received power, dBW

[ EIRP] → equivalent isotropic radiated power, dBW
[ FSL] → free-space spreading loss, dB
[ RFL] →receiver feeder loss, dB
[ AML] →antenna misalignment loss, dB
[ AA] → atmospheric absorption loss, dB
[ PL] →polarization mismatch loss,dB

2.9 Amplifier noise temperature

Consider first the noise representation of the antenna and the low
noise amplifier (LNA) shown in Fig. 2.15.

The available power gain of the amplifier is denoted as

G, and the noise power output, as Pno .
52 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.15: LNA Amplifier gain

For the moment we will work with the noise power per
unit bandwidth, which is simply noise energy in joules as shown
by Eq.

The input noise energy coming from the antenna is -

N0,ant = kTant (2.4)

2.10 The Uplink

The uplink of a satellite circuit is the one in which the earth station
is transmitting the signal and the satellite is receiving it specifically
that the uplink is being considered.

C
= [ EIRP] − [ Losses] + [k] (2.5)
N

In this Eq the values to be used are the earth station EIRP,

the satellite receiver feeder losses, and satellite receiver G/T. The
free-space loss and other losses which are frequency-dependent
are calculated for the uplink frequency.
2.11. DOWNLINK 53

2.10.1 Input backoff

Number of carriers are present simultaneously in a TWTA, the

operating point must be backed off to a linear portion of the
transfer characteristic to reduce the effects of inter modulation
distortion. Such multiple carrier operation occurs with frequency-
division multiple access (FDMA). The point to be made here is that
backoff (BO) must be allowed for in the link- budget calculations.

Suppose that the saturation flux density for single-carrier

operation is known. Input BO will be specified for multiple-carrier
operation, referred to the single carrier saturation level. The earth-
station EIRP will have to be reduced by the specified BO, resulting
in an uplink value of -

[ EIRP]U = [ EIRPS]U + [ BO]i (2.6)

2.10.2 The earth station HPA

The earth station HPA has to supply the radiated power plus the
transmit feeder losses, denoted here by TFL, or [TFL] dB. These
include waveguide, filter, and coupler losses between the HPA
output and the transmit antenna.

The earth station itself may have to transmit multiple

carriers, and its output also will require back off, denoted by
[BO]HPA. The earth station HPA must be rated for a saturation
power output given by -

[ PHPA,sat ] = [ PHPA ] + [ BO] HPA (2.7)

2.11 Downlink

The downlink of a satellite circuit is the one in which the satellite

is transmitting the signal and the earth station is receiving it.
54 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Equation can be applied to the downlink, but subscript D will be

used to denote specifically that the downlink is being considered.
Thus Eq. becomes-

C
= [ EIRP] − [ Losses] + [k] (2.8)
N

In Eq. the values to be used are the satellite EIRP, the

earth- station receiver feeder losses, and the earth-station receiver
G/T. The free space and other losses are calculated for the
downlink frequency. The resulting carrier-to-noise density ratio
given by Eq. 2.8 is that which appears at the detector of the earth
station receiver.

2.11.1 Output back-off

Where input BO is employed as described in a corresponding

output BO must be allowed for in the satellite EIRP. As the curve
of Fig. 2.16 shows, output BO is not linearly related to input BO.
A rule of thumb, frequently used, is to take the output BO as the
point on the curve which is 5 dB below the extrapolated linear
portion, as shown in Fig. 2.17. Since the linear portion gives a
1 : 1 change in decibels, the relationship between input and
output BO is [ BO]0 [ BO]i 5 dB. For example, with an input BO of
[ BO]i 11 dB, the corresponding output BO is [ BO]0 .
2.11. DOWNLINK 55

Figure 2.16: Input and output backoff relationship for the satellite traveling-
wave-tube amplifier; [ BO]i [ BO]0 5 dB.

2.11.2 Effects of Rain

In the C band and, more especially, the Ku band, rainfall is the

most significant cause of signal fading. Rainfall results in
attenuation of radio waves by scattering and by absorption of
energy from the wave.

Rain attenuation increases with increasing frequency

and is worse in the Ku band compared with the C band.

This produces a depolarization of the wave; in effect, the

wave becomes elliptically polarized. This is true for both linear
and circular polar- izations, and the effect seems to be much worse
for circular polarization (Freeman, 1981).

The C/N0 ratio for the downlink alone, not counting the
PNU contribution, is PR /PND , and the combined C/N0 ratio at the
ground receiver is
56 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

Figure 2.17: (a) Combined uplink and downlink; (b) power flow diagram

The reason for this reciprocal of the sum of the

reciprocals method is that a single signal power is being
transferred through the system, while the various noise powers,
which are present are additive. Similar reasoning applies to the
carrier-to-noise ratio, C/N.

2.12 Inter modulation and interference

Intermodulation interference is the undesired combining of

several signals in a nonlinear device, producing new, unwanted
frequencies, which can cause interference in adjacent receivers
located at repeater sites.

Not all interference is a result of intermodulation

distortion. It can come from co-channel interference, atmospheric
conditions as well as man-made noise generated by medical,
welding and heating equipment.

Most intermodulation occurs in a transmitter’s nonlinear

power amplifier (PA). The next most common mixing point is in
the front end of a receiver. Usually it occurs in the unprotected
2.13. PROPAGATION CHARACTERISTICS AND FREQUENCY CONSIDERATIONS57

first mixer of older model radios or in some cases an overdriven

RF front-end amp.

Intermodulation can also be produced in rusty or

corroded tower joints, guy wires, turnbuckles and anchor rods or
any nearby metallic object, which can act as a nonlinear
”mixer/rectifier” device.

2.13 Propagation Characteristics and Frequency

considerations

2.13.1 Introduction

A number of factors resulting from changes in the atmosphere

have to be taken into account when designing a satellite
communications system in order to avoid impairment of the
wanted signal.

Generally, a margin in the required carrier-to-noise ratio

is incorporated to accommodate such effects.

2.13.2 Radio Noise

Radio noise emitted by matter is used as a source of information

in radio-astronomy and in remote sensing. Noise of a thermal
origin has a continuous spectrum, but several other radiation
mechanisms cause the emission to have a spectral-line structure.
Atoms and molecules are distinguished by their different spectral
lines.

For other services such as satellite communications

noise is a limiting factor for the receiving system; generally, it is
inappropriate to use receiving systems with noise temperatures
which are much less than those specified by the minimum
58 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

external noise.

From about 30 MHz to about 1 GHz cosmic noise

predominates over atmospheric noise except during local
thunderstorms, but will generally be exceeded by man-made
noise in populated areas.

In the bands of strong gaseous absorption, the noise

temperature reaches maximum values of some 290 K. At times,
precipitation will also increase the noise temperature at
frequencies above 5 GHz.

Figure 2.18 gives an indication of sky noise at various

elevation angles and frequencies.

Figure 2.18: Sky-Noise Temperature for Clear Air

2.14 System reliability and design lifetime

2.14.1 System reliability

Satellites are designed to operate dependably throughout their

operational life, usually a number of years.

This is achieved through stringent quality control and

testing of parts and subsystems before they are used in the
construction of the satellite.
2.14. SYSTEM RELIABILITY AND DESIGN LIFETIME 59

Redundancy of key components is often built in so that

if a particular part or subassembly fails, another can perform its
functions.

In addition, hardware and software on the satellite are

often designed so that ground controllers can reconfigure the
satellite to work around a part that has failed.

2.14.2 Design lifetime

”The Milstar constellation has demonstrated exceptional

reliability and capability, providing vital protected
communications to the warfighter,” said Kevin Bilger, vice
president and general manager, Global Communications
Systems, Lockheed Martin Space Systems in Sunnyvale.

”Milstar’s robust system offers our nation worldwide

connectivity with flexible, dependable and highly secure satellite
communications.”

The five-satellite Milstar constellation has surpassed 63

years of combined successful operations, and provides a
protected, global communication network for the joint forces of
the U.S. military. In addition, it can transmit voice, data, and
imagery, and offers video teleconferencing capabilities.

The system is the principal survivable, endurable

communications structure that the President, the Secretary of
Defense and the Commander, U.S. Strategic Command use to
maintain positive command and control of the nation’s strategic
forces.

In addition to this 10-year milestone for Flight-5, each of

the first two Milstar satellites have been on orbit for over 16 years
– far exceeding their 10-year design life.

The next-generation Lockheed Martin-built Advanced

60 CHAPTER 2. SPACE SEGMENT AND SATELLITE LINK DESIGN

EHF satellites, joining the Milstar constellation, provide five

times faster data rates and twice as many connections, permitting
transmission of strategic and tactical military communications,
such as real-time video, battlefield maps and targeting data.
Advanced EHF satellites are designed to be fully interoperable
and backward compatible with Milstar.

Headquartered in Bethesda, Md., Lockheed Martin is a

global security company that employs about 123,000 people
worldwide and is principally engaged in the research, design,
development, manufacture, integration and sustainment of
advanced technology systems, products and services. The
Corporation’s net sales for 2011 were 46.5 billion dollar.
Chapter 3

Satellite Access

3.1 Modulation and Multiplexing: Voice, Data,

Video

Communications satellites are used to carry telephone, video,

and data signals, and can use both analog and digital modulation
techniques.

Modulation:
Modification of a carrier’s parameters (amplitude, frequency,
phase, or a combination of them) in dependence on the symbol to
be sent.

Multiplexing:
Task of multiplexing is to assign space, time, frequency, and code
to each communication channel with a minimum of interference
and a maximum of medium utilization Communication channel
refers to an association of sender(s) and receiver(s) that want to
exchange data One of several constellations of a carrier’s
parameters defined by the used modulation scheme.

61
62 CHAPTER 3. SATELLITE ACCESS

3.1.1 Voice, Data, Video

The modulation and multiplexing techniques that were used at

this time were analog, adapted from the technology developed
for The change to digital voice signals made it easier for
long-distance.

Figure 3.1: Modulation and Multiplexing: Voice/Data/Video

Communication carriers to mix digital data and

telephone Fiber-optic Cable Transmission Standards System Bit
rate (Mbps) 64- kbps Voice channel capacity Stuffing bits and
words are added to the satellite data stream as needed to fill
empty bit and word spaces.

Primarily for video provided that a satellite link’s overall

carrier-to-noise but in to older receiving equipment at System and
Satellite Specification Ku-band satellite parameters.
3.2. ANALOG – DIGITAL TRANSMISSION SYSTEM 63

3.1.2 Modulation And Multiplexing

In analog television (TV) transmission by satellite, the baseband

video signal and one or two audio subcarriers constitute a
composite video signal.

Digital modulation is obviously the modulation of

choice for transmitting digital data are digitized analog signals
may conveniently share a channel with digital data, allowing a
link to carry a varying mix of voice and data traffic.

Digital signals from different channels are interleaved

for transmission through time division multiplexing TDM carry
any type of traffic at the bent pipe transponder that can carry
voice, video, or data as the marketplace demands.

Hybrid multiple access schemes can use time division

multiplexing of baseband channels which are then modulate.

3.2 Analog – digital transmission system

3.2.1 Analog vs. Digital Transmission

Compare at two levels:

1 Data - continuous (audio) vs. discrete (text)

2 Signaling - continuously varying electromagnetic wave vs.

sequence of voltage pulses.

Also Transmission—transmit without regard to signal

content vs. being concerned with signal content. Difference in
how attenuation is handled, but not focus on this.Seeing a shift
towards digital transmission despite large analog base. Why?
64 CHAPTER 3. SATELLITE ACCESS

Figure 3.2: Basic communication systems

1 Improving digital technology

2 Data integrity. Repeaters take out cumulative problems in

transmission. Can thus transmit longer distances.

3 Easier to multiplex large channel capacities with digital

4 Easy to apply encryption to digital data

5 Better integration if all signals are in one form. Can integrate

voice, video and digital data.

3.2.2 Digital Data/Analog Signals

Must convert digital data to analog signal such device is a modem

to translate between bit-serial and modulated carrier signals? To
send digital data using analog technology, the sender generates
a carrier signal at some continuous tone (e.g. 1-2 kHz in phone
circuits) that looks like a sine wave. The following techniques are
used to encode digital data into analog signals.
3.2. ANALOG – DIGITAL TRANSMISSION SYSTEM 65

Figure 3.3: Digital /Analog Transmitter & receiver

Resulting bandwidth is centered on the carrier

frequency.

1 Amplitude-shift modulation (keying): vary the amplitude

(e.g. voltage) of the signal. Used to transmit digital data over
optical fiber.

2 Frequency-shift modulation: two (or more tones) are used,

which are near the carrier frequency. Used in a full-duplex
modem (signals in both directions).
3 Phase-shift modulation: systematically shift the carrier wave
at uniformly spaced intervals.

For instance, the wave could be shifted by 45, 135, 225,

315 degree at each timing mark. In this case, each timing interval
carries 2 bits of information.

Why not shift by 0, 90, 180, 270? Shifting zero degrees

means no shift, and an extended set of no shifts leads to clock
synchronization difficulties.

Frequency division multiplexing (FDM): Divide the

frequency spectrum into smaller subchannels, giving each user
exclusive use of a subchannel (e.g., radio and TV). One problem
with FDM is that a user is given all of the frequency to use, and if
the user has no data to send, bandwidth is wasted — it cannot be
used by another user.
66 CHAPTER 3. SATELLITE ACCESS

Time division multiplexing (TDM): Use time slicing to

give each user the full bandwidth, but for only a fraction of a
second at a time (analogous to time sharing in operating
systems). Again, if the user doesn’t have data to sent during his
timeslice, the bandwidth is not used (e.g., wasted).

Statistical multiplexing: Allocate bandwidth to

arriving packets on demand. This leads to the most efficient use
of channel bandwidth because it only carries useful data.That is,
channel bandwidth is allocated to packets that are waiting for
transmission, and a user generating no packets doesn’t use any of
the channel resources.

3.3 Digital Video Broadcasting (DVB)

1 Digital Video Broadcasting (DVB) has become the synonym

for digital television and for data broadcasting world-wide.

2 DVB services have recently been introduced in Europe, in

North- and South America, in Asia, Africa and Australia.

3 This article aims at describing what DVB is all about and at

introducing some of the technical background of a technology
that makes possible the broadcasting.
3.4. MULTIPLE ACCESS TECHNIQUES 67

Figure 3.4: Digital Video Broadcasting systems

3.4 Multiple Access Techniques

1 The transmission from the BS in the downlink can be heard

by each and every mobile user in the cell, and is referred as
broadcasting. Transmission from the mobile users in the
uplink to the BS is many-toone, and is referred to as multiple
access.
2 Multiple access schemes to allow many users to share
simultaneously a finite amount of radio spectrum resources.
(a) Should not result in severe degradation in the
performance of the system as compared to a single user
scenario.
(b) Approaches can be broadly grouped into two categories:
narrowband and wideband.
3 Multiple Accessing Techniques : with possible conflict and
conflict- free
(a) Random access
(b) Frequency division multiple access (FDMA)
(c) Time division multiple access (TDMA)
68 CHAPTER 3. SATELLITE ACCESS

(d) Spread spectrum multiple access (SSMA) : an example is

Code division multiple access (CDMA)
(e) Space division multiple access (SDMA)

Duplexing:

1 For voice or data communications, must assure two way

communication (duplexing, it is possible to talk and listen
simultaneously). Duplexing may be done using frequency or
time domain techniques.

• Forward (downlink) band provides traffic from the BS to

the mobile
• Reverse (uplink) band provides traffic from the mobile to
the BS.

3.4.1 Frequency division duplexing (FDD)

1 Provides two distinct bands of frequencies for every user, one

for downlink and one for uplink.

2 A large interval between these frequency bands must be

allowed so that interference is minimized.

Figure 3.5: Frequency Separation

3.4. MULTIPLE ACCESS TECHNIQUES 69

3.4.2 Time division duplexing (TDD)

1 In TDD communications, both directions of transmission use

one contiguous frequency allocation, but two separate time
slots to provide both a forward and reverse link.

2 Because transmission from mobile to BS and from BS to

mobile alternates in time, this scheme is also known as “ping
pong”.

3 As a consequence of the use of the same frequency band, the

communication quality in both directions is the same. This is
different from FDD.

Figure 3.6: Time Slot

3.4.3 FDMA

1 In FDMA, each user is allocated a unique frequency band or

channel. During the period of the call, no other user can share
the same frequency band.
70 CHAPTER 3. SATELLITE ACCESS

Figure 3.7: FDMA Channels

2 All channels in a cell are available to all the mobiles. Channel

assignment is carried out on a first-come first- served basis.
The number of channels, given a frequency spectrum BT ,
depends on the modulation technique (hence Bw or Bc ) and
the guard bands between the channels 2Bguard .

3 These guard bands allow for imperfect filters and oscillators

and can be used to minimize adjacent channel interference.

4 FDMA is usually implemented in narrowband systems.

Figure 3.8: FDMA/FDD/TDD

3.4. MULTIPLE ACCESS TECHNIQUES 71

Nonlinear effects in FDMA:

1 In a FDMA system, many channels share the same antenna at

the BS. The power amplifiers or the power combiners, when
operated at or near saturation are nonlinear.

2 The nonlinear ties generate inter-modulation frequencies.

3 Undesirable harmonics generated outside the mobile radio

band cause interference to adjacent services.

4 Undesirable harmonics present inside the band cause

interference to other users in the mobile system.

3.4.4 TDMA

1 TDMA systems divide the channel time into frames. Each

frame is further partitioned into time slots. In each slot only
one user is allowed to either transmit or receive.
2 Unlike FDMA, only digital data and digital modulation must
be used.
3 Each user occupies a cyclically repeating time slot, so a
channel may be thought of as a particular time slot of every
frame, where N time slots comprise a frame.

Figure 3.9: TDMA Channels

72 CHAPTER 3. SATELLITE ACCESS

Features :

1 Multiple channels per carrier or RF channels.

2 Burst transmission since channels are used on a time sharing

basis. Transmitter can be turned off during idle periods.

3 Narrow or wide bandwidth – depends on factors such as

modulation scheme, number of voice channels per carrier
channel.

4 High ISI – Higher transmission symbol rate, hence resulting

in high ISI. Adaptive equalizer required.

Figure 3.10: TDMA Channels time slot

5 A guard time between the two time slots must be allowed in

order to avoid interference, especially in the uplink
direction. All mobiles should synchronize with BS to
minimize interference.

6 Efficient power utilization : FDMA systems require a 3- to

6-dB power back off in order to compensate for
inter-modulation effects.

7 Efficient handoff : TDMA systems can take advantage of the

fact that the transmitter is switched off during idle time slots
3.4. MULTIPLE ACCESS TECHNIQUES 73

to improve the handoff procedure. An enhanced link control,

such as that provided by mobile assisted handoff (MAHO)
can be carried out by a subscriber by listening to neighboring
base station during the idle slot of the TDMA frame.

8 Efficiency of TDMA

9 Efficiency of TDMA is a measure of the percentage of bits per

frame which contain transmitted data. The transmitted data
include source and channel coding bits.
bT − bOH
ηf = × 100 (3.1)
bT

10 bOH includes all overhead bits such as preamble, guard bits,

etc.

3.4.5 Code Division Multiple Access (CDMA)

1 Spreading signal (code) consists of chips

(a) Has Chip period and and hence, chip rate
(b) Spreading signal use a pseudo-noise (PN) sequence (a
pseudo-random sequence)
(c) PN sequence is called a codeword
(d) Each user has its own cordword
(e) Codewords are orthogonal. (low autocorrelation)
(f) Chip rate is oder of magnitude larger than the symbol
rate.
2 The receiver correlator distinguishes the senders signal by
examining the wideband signal with the same
time-synchronized spreading code

3 The sent signal is recovered by despreading process at the

receiver.
74 CHAPTER 3. SATELLITE ACCESS

CDMA Advantages:

1 Low power spectral density.

(a) Signal is spread over a larger frequency band

(b) Other systems suffer less from the transmitter

2 Interference limited operation

(a) All frequency spectrum is used.

3 Privacy : The codeword is known only between the sender

and receiver. Hence other users can not decode the messages
that are in transit.

4 Reduction of multipath affects by using a larger spectrum

CDMA Data:

Figure 3.11: CDMA Channels transmission

DSSS Transmitter:
3.4. MULTIPLE ACCESS TECHNIQUES 75

Figure 3.12: CDMA Transmitter

r
2Es
Sss (t) = m(t) p(t) cos(2π f c t + θ ) (3.2)
Ts

DSSS Receiver :

Figure 3.13: CDMA Receiver

r
2Es
S1 ( t ) = m(t) cos(2π f c t + θ ) (3.3)
Ts

1 FDMA/CDMA : Available wideband spectrum is frequency

divided into number narrowband radio channels. CDMA is
employed inside each channel.
76 CHAPTER 3. SATELLITE ACCESS

2 DS/FHMA : The signals are spread using spreading codes

(direct sequence signals are obtained), but these signal are
not transmitted over a constant carrier frequency; they are
transmitted over a frequency hopping carrier frequency.

3 Time Division CDMA (TCDMA)

(a) Each cell is using a different spreading code (CDMA
employed between cells) that is conveyed to the mobiles
in its range.
(b) Inside each cell (inside a CDMA channel), TDMA is
employed to multiplex multiple users.

4 Time Division Frequency Hopping

(a) At each time slot, the user is hopped to a new frequency
according to a pseudo-random hopping sequence.
(b) Employed in severe co-interference and multi-path
environments.

Bluetooth and GSM are using this technique.

1 A large number of independently steered high-gain beams

can be formed without any resulting degradation in SNR
ratio.
2 Beams can be assigned to individual users, thereby assuring
that all links operate with maximum gain.

3 Adaptive beam forming can be easily implemented to

improve the system capacity by suppressing co channel
interference.

Advantage of CDMA

1 It is recognized that CDMA’s capacity gains over TDMA

3.5. CHANNEL ALLOCATION SCHEMES 77

2 FDMA are entirely due to Its tighter, dynamic control over

the use of the power domain.

3 Choosing a new non-orthogonal PN sequence a CDMA

system does not encounter the difficulties of choosing a
spare carrier frequency or time slot to carry a Traffic Channel

4 Ensure that interference will not be too great if it begins to

transmit -that there is still enough space left in the power
domain.

Disadvantages of CDMA:

1 Satellite transponders are channelized too narrowly for

roadband CDMA, which is the most attractive form of
CDMA.
2 Power control cannot be as tight as it is in a terrestrial system
because of long round-trip delay.

3.5 Channel allocation schemes

In radio resource management for wireless and cellular network,

channel allocation schemes are required to allocate bandwidth
and communication channels to base stations, access points and
terminal equipment.

The objective is to achieve maximum system spectral

efficiency in bit/s/Hz/site by means of frequency reuse, but still
assure a certain grade of service by avoiding co-channel
interference and adjacent channel interference among nearby
cells or networks that share the bandwidth. There are two types
of strategies that are followed:-

1 Fixed: FCA, fixed channel allocation: Manually assigned by

the network operator
78 CHAPTER 3. SATELLITE ACCESS

2 Dynamic:

(a) DCA, dynamic channel allocation,

(b) DFS, dynamic frequency selection
(c) Spread spectrum

3.5.1 FCA

In Fixed Channel Allocation or Fixed Channel Assignment

(FCA) each cell is given a predetermined set of frequency
channels.

FCA requires manual frequency planning, which is an

arduous task in TDMA and FDMA based systems, since such
systems are highly sensitive to co-channel interference from
nearby cells that are reusing the same channel.

This results in traffic congestion and some calls being lost

when traffic gets heavy in some cells, and idle capacity in other
cells.

3.5.2 DCA and DFS

Dynamic Frequency Selection (DFS) may be applied in wireless

networks with several adjacent non-centrally controlled access
points.

A more efficient way of channel allocation would be

Dynamic Channel Allocation or Dynamic Channel Assignment
(DCA) in which voice channel are not allocated to cell
permanently, instead for every call request base station request
channel from MSC.
3.6. SPREAD SPECTRUM 79

3.6 Spread spectrum

Spread spectrum can be considered as an alternative to complex

DCA algorithms. Spread spectrum avoids co-channel interference
between adjacent cells, since the probability that users in nearby
cells use the same spreading code is insignificant.

Thus the frequency channel allocation problem is relaxed

in cellular networks based on a combination of Spread spectrum
and FDMA, for example IS95 and 3G systems.

In packet based data communication services, the

communication is bursty and the traffic load rapidly changing.
For high system spectrum efficiency, DCA should be performed
on a packet-by-packet basis.

Examples of algorithms for packet-by-packet DCA are

Dynamic Packet Assignment (DPA), Dynamic Single Frequency
Networks (DSFN) and Packet and resource plan scheduling
(PARPS).

3.6.1 Spread spectrum Techniques

1 In telecommunication and radio communication,

spread-spectrum techniques are methods by which a signal
(e.g. an electrical, electromagnetic, or acoustic signal)
generated with a particular bandwidth is deliberately spread
in the frequency domain, resulting in a signal with a wider
bandwidth.

2 These techniques are used for a variety of reasons, including

the establishment of secure communications, increasing
resistance to natural interference, noise and jamming, to
prevent detection, and to limit power flux density (e.g. in
satellite downlinks).
80 CHAPTER 3. SATELLITE ACCESS

3 Spread-spectrum telecommunications this is a technique in

which a telecommunication signal is transmitted on a
bandwidth considerably larger than the frequency content of
the original information.

4 Spread-spectrum telecommunications is a signal structuring

technique that employs direct sequence, frequency hopping,
or a hybrid of these, which can be used for multiple access
and/or multiple functions.

5 Frequency-hopping spread spectrum (FHSS),

direct-sequence spread spectrum (DSSS), time-hopping
spread spectrum (THSS), chirp spread spectrum (CSS).

6 Techniques known since the 1940s and used in military

communication systems since the 1950s ”spread” a radio
signal over a wide frequency range several magnitudes
higher than minimum requirement.

7 Resistance to jamming (interference). DS (direct sequence) is

good at resisting continuous-time narrowband jamming,
while FH (frequency hopping) is better at resisting pulse
jamming.

8 Resistance to fading. The high bandwidth occupied by

spread spectrum signals offer some frequency diversity, i.e.
it is unlikely that the signal will encounter severe multipath
fading over its whole bandwidth, and in other cases the
signal can be detected using e.g. a Rake receiver.

9 Multiple access capability, known as code-division multiple

access (CDMA) or code-division multiplexing (CDM).
Multiple users can transmit simultaneously in the same
frequency band as long as they use different spreading
codes.
3.7. COMPRESSION – ENCRYPTION 81

3.7 Compression – Encryption

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

goes through an MPEG encoder, which converts the programming
to MPEG-4 video of the correct size and format for the satellite
receiver in your house.

Encoding works in conjunction with compression to

analyze each video frame and eliminate redundant or irrelevant
data and extrapolate information from other frames. This process
reduces the overall size of the file. Each frame can be encoded in
one of three ways:

1 As an intraframe, which contains the complete image data for

that frame. This method provides the least compression.
2 As a predicted frame, which contains just enough
information to tell the satellite receiver how to display the
frame based on the most recently displayed intraframe or
predicted frame.
3 As a bidirectional frame, which displays information from the
surrounding intraframe or predicted frames. Using data from
the closest surrounding frames, the receiver interpolates the
position and color of each pixel.

This process occasionally produces artifacts – glitches in

the video image. One artifact is macroblocking, in which the fluid
picture temporarily dissolves into blocks. Macroblocking is often
mistakenly called pixilating, a technically incorrect term which
has been accepted as slang for this annoying artifact.

There really are pixels on your TV screen, but they’re

too small for your human eye to perceive them individually –
they’re tiny squares of video data that make up the image you
see. (For more information about pixels and perception, see How
TV Works.)
82 CHAPTER 3. SATELLITE ACCESS

The rate of compression depends on the nature of the

programming. If the encoder is converting a newscast, it can use
a lot more predicted frames because most of the scene stays the
same from one frame to the next.

In more fast-paced programming, things change very

quickly from one frame to the next, so the encoder has to create
more intraframes. As a result, a newscast generally compresses to
a smaller size than something like a car race.

3.7.1 Encryption and Transmission

After the video is compressed, the provider encrypts it to keep

people from accessing it for free. Encryption scrambles the digital
data in such a way that it can only be decrypted (converted back
into usable data) if the receiver has the correct decryption
algorithm and security keys.

Once the signal is compressed and encrypted, the

broadcast center beams it directly to one of its satellites. The
satellite picks up the signal with an onboard dish, amplifies the
signal and uses another dish to beam the signal back to Earth,
where viewers can pick it up.

3.7.2 Video and Audio Compression

Video and Audio files are very large beasts. Unless we develop
and maintain very high bandwidth networks (Gigabytes per
second or more) we have to compress to data.

Relying on higher bandwidths is not a good option –

M25 Syndrome: Traffic needs ever increases and will adapt to
swamp current limit whatever this is. As we will compression
becomes part of the representation or coding scheme which have
become popular audio, image and video formats.
3.7. COMPRESSION – ENCRYPTION 83

We will first study basic compression algorithms and

then go on to study some actual coding formats.

Figure 3.14: coding scheme

What is Compression?

Compression basically employs redundancy in the data:

1 Temporal - in 1D data, 1D signals, Audio etc.

2 Spatial - correlation between neighbouring pixels or data

items
3 Spectral - correlation between colour or luminescence
components. This uses the frequency domain to exploit
relationships between frequency of change in data.

4 psycho-visual - exploit perceptual properties of the human

visual system.

Compression can be categorised in two broad ways:

1 Lossless Compression :
where data is compressed and can be reconstituted
84 CHAPTER 3. SATELLITE ACCESS

(uncompressed) without loss of detail or information. These

are referred to as bit-preserving or reversible compression
systems also.

2 Lossy Compression :
where the aim is to obtain the best possible fidelity for a given
bit-rate or minimizing the bit-rate to achieve a given fidelity
measure. Video and audio compression techniques are most
suited to this form of compression.

If an image is compressed it clearly needs to

uncompressed (decoded) before it can viewed/listened to. Some
processing of data may be possible in encoded form however.
Lossless compression frequently involves some form of entropy
encoding and are based in information theoretic techniques.

Lossy compression use source encoding techniques that

may involve transform encoding, differential encoding or vector
quantization.

3.7.3 MPEG Standards

All MPEG standards exist to promote system interoperability

among your computer, television and handheld video and audio
devices. They are:

1 MPEG-1: the original standard for encoding and decoding

streaming video and audio files.

2 MPEG-2: the standard for digital television, this compresses

files for transmission of high-quality video.

3 MPEG-4: the standard for compressing high-definition video

into smaller scale files that stream to computers, cell phones
and PDAs (personal digital assistants).
3.8. ENCRYPTION 85

4 MPEG-21: also referred to as the Multimedia Framework.

The standard that interprets what digital content to provide
to which individual user so that media plays flawlessly
under any language, machine or user conditions.

Figure 3.15: MPEG scheme

3.8 Encryption

It is the most effective way to achieve data security. To read an

encrypted file, you must have access to a secret key or password
that enables you to decrypt it. Unencrypted data is called plain
text ; encrypted data is referred to as cipher text.
86 CHAPTER 3. SATELLITE ACCESS

Figure 3.16: Encryption methods

3.8.1 Symmetric key encryption

In symmetric-key schemes, the encryption and decryption keys

are the same. Thus communicating parties must have the same
key before they can achieve secret communication.

In public-key encryption schemes, the encryption key is

published for anyone to use and encrypt messages. However, only
the receiving party has access to the decryption key that enables
messages to be read.

Figure 3.17: General block diagram Encryption methods

Decryption: It is the process of taking encoded or

3.8. ENCRYPTION 87

encrypted text or other data and converting it back into text that
you or the computer are able to read and understand.

This term could be used to describe a method of

un-encrypting the data manually or with un-encrypting the data
using the proper codes or keys.

Data may be encrypted to make it difficult for someone

to steal the information. Some companies also encrypt data for
general protection of company data and trade secrets. If this data
needs to be viewable, it may require decryption.
88 CHAPTER 3. SATELLITE ACCESS
Chapter 4

Earth Segment

4.1 Earth Station Technology

The earth segment of a satellite communications system consists

of the transmit and receive earth stations. The simplest of these are
the home TV receive-only (TVRO) systems, and the most complex
are the terminal stations used for international communications
networks. Also included in the earth segment are those stations
which are on ships at sea, and commercial and military land and
aeronautical mobile stations.

As mentioned in earth stations that are used for logistic

sup- port of satellites, such as providing the telemetry, tracking,
and command (TT&C) functions, are considered as part of the
space segment.

4.1.1 Terrestrial Interface

Earth station is a vital element in any satellite communication

network. The function of an earth station is to receive
information from or transmit information to, the satellite network
in the most cost-effective and reliable manner while retaining the
desired signal quality. The design of earth station configuration

89
90 CHAPTER 4. EARTH SEGMENT

depends upon many factors and its location. But it is

fundamentally governed by its Location which are listed below,

1 In land

2 On a ship at sea

3 Onboard aircraft

The factors are -

1 Type of services

2 Frequency bands used

3 Function of the transmitter

4 Function of the receiver

5 Antenna characteristics

4.1.2 Transmitter and Receiver

Any earth station consists of four major subsystems -

1 Transmitter

2 Receiver

3 Antenna

4 Tracking equipment

Two other important subsystems are -

1 Terrestrial interface equipment

4.1. EARTH STATION TECHNOLOGY 91

2 Power supply

The earth station depends on the following parameters -

1 Transmitter power

2 Choice of frequency

3 Gain of antenna

4 Antenna efficiency

5 Antenna pointing accuracy

6 Noise temperature

The functional elements of a basic digital earth station

are shown in the below figure 4.1.

Figure 4.1: Transmitter- Receiver

Digital information in the form of binary digits from

terrestrial networks enters earth station and is then processed
(filtered, multiplexed, formatted etc.) by the base band
equipment.
92 CHAPTER 4. EARTH SEGMENT

1 The encoder performs error correction coding to reduce the

error rate, by introducing extra digits into digital stream
generated by the base band equipment. The extra digits
carry information.

2 In satellite communication, I.F carrier frequency is chosen at

70 MHz for communication using a 36 MHz transponder
bandwidth and at 140 MHz for a transponder bandwidth of
54 or 72 MHz.

3 On the receive side, the earth station antenna receives the

low-level modulated R.F carrier in the downlink frequency
spectrum.

4 The low noise amplifier (LNA) is used to amplify the weak

received signals and improve the signal to Noise ratio (SNR).
The error rate requirements can be met more easily.

5 R.F is to be reconverted to I.F at 70 or 140 MHz because it

is easier design a demodulation to work at these frequencies
than 4 or 12 GHz.

6 The demodulator estimate which of the possible symbols was

transmitted based on observation of the received if carrier.

7 The decoder performs a function opposite that of the

encoder. Because the sequence of symbols recovered by the
demodulator may contain errors, the decoder must use the
uniqueness of the redundant digits introduced by the
encoder to correct the errors and recover
information-bearing digits.

8 The information stream is fed to the base-band equipment for

processing for delivery to the terrestrial network.

9 The tracking equipments track the satellite and align the

beam towards it to facilitate communication.
4.2. ANTENNA SYSTEMS 93

4.1.3 Earth Station Tracking System

Tracking is essential when the satellite drift, as seen by an earth

station antenna is a significant fraction of an earth station’s
antenna beam width. An earth station’s tracking system is
required to perform some of the functions such as -

1 Satellite acquisition

2 Automatic tracking

3 Manual tracking

4 Program tracking.

4.2 Antenna Systems

The antenna system consist of -

1 Feed System

2 Antenna Reflector

3 Mount

4 Antenna tracking System

4.2.1 Feed System

The feed along with the reflector is the radiating/receiving

element of electromagnetic waves. The reciprocity property of
the feed element makes the earth station antenna system suitable
for transmission and reception of electromagnetic waves.
94 CHAPTER 4. EARTH SEGMENT

The way the waves coming in and going out is called

feed configuration Earth Station feed systems most commonly
used in satellite communication are:

1 Axi-Symmetric Configuration

2 Asymmetric Configuration

3 Axi-Symmetric Configuration

In an axi-symmetric configuration the antenna axes are

symmetrical with respect to the reflector ,which results in a
relatively simple mechanical structure and antenna mount.

1 Primary Feed :
In primary, feed is located at the focal point of the parabolic
reflector. Many dishes use only a single bounce, with
incoming waves reflecting off the dish surface to the focus in
front of the dish, where the antenna is located. when the
dish is used to transmit ,the transmitting antenna at the
focus beams waves toward the dish, bouncing them off to
space. This is the simplest arrangement.

2 Cassegrain :
Many dishes have the waves make more than one bounce
.This is generally called as folded systems. The advantage is
that the whole dish and feed system is more compact. There
are several folded configurations, but all have at least one
secondary reflector also called a sub reflector, located out in
front of the dish to redirect the waves.
A common dual reflector antenna called Cassegrain has a
convex sub reflector positioned in front of the main dish,
closer to the dish than the focus. This sub reflector bounces
back the waves back toward a feed located on the main
dish’s center, sometimes behind a hole at the center of the
main dish. Sometimes there are even more sub reflectors
4.2. ANTENNA SYSTEMS 95

behind the dish to direct the waves to the fed for

convenience or compactness.

3 Gregorian :
This system has a concave secondary reflector located just
beyond the primary focus. This also bounces the waves back
toward the dish.

Asymmetric Configuration

1 Offset or Off-axis feed :

The performance of tan axi-symmetric configuration is
affected by the blockage of the aperture by the feed and the
sub reflector assembly. The result is a reduction in the
antenna efficiency and an increase in the side lobe levels. The
asymmetric configuration can remove this limitation..This is
achieved by offsetting the mounting arrangement of the feed
so that it does not obstruct the main beam.As a result, the
efficiency and side lobe level performance are improved.

4.2.2 Antenna Reflector

Mostly parabolic reflectors are used as the main antenna for the
earth stations because of the high gain available from the reflector
and the ability of focusing a parallel beam into a point at the
focus where the feed,i.e., the receiving/radiating element is
located .For large antenna system more than one reflector
surfaces may be used in as in the cassegrain antenna system.

Earth stations are also classified on the basis of services

for example:

1 Two way TV ,Telephony and data

2 Two way TV
96 CHAPTER 4. EARTH SEGMENT

3 TV receive only and two way telephony and data

4 Two way data

From the classifications it is obvious that the technology

of earth station will vary considerably on the performance and the
service requirements of earth station.

For mechanical design of parabolic reflector the

following parameters are required to be considered:

1 Size of the reflector

2 Focal Length /diameter ratio

3 RMS error of main and sub reflector

4 Pointing and tracking accuracies

5 Speed and acceleration

6 Type of mount

7 Coverage Requirement

The size of the reflector depends on transmit and

receive gain requirement and beamwidth of the antenna.Gain is
directly proportional to the antenna diameter whereas the
beamwidth is inversely proportional to the antenna diameter .for
high inclination angle of the satellite ,the tracking of the earth
station becomes necessary when the beamwidth is too narrow.

The gain of the antenna is given by -

η4πAe f f
Gain = (4.1)
λ2

Where Ae f f is the aperture, λ is wave length and η is

efficiency of antenna system.
4.2. ANTENNA SYSTEMS 97

For a parabolic antenna with circular aperture diameter

D, the gain of the antenna is :

πD2
  
η4π
Gain = (4.2)
λ2 4
 2
πD
=η (4.3)
λ

The overall efficiency of the antenna is the net product of

various factors such as -

1 Cross Polarization

2 Spill over

3 Diffraction

4 Blockage

5 Surface accuracy

6 Phase error

7 Illumination

In the design of feed ,the ratio of focal length F to the

diameter of the reflector D of the antenna system control the
maximum angle subtended by the reflector surface on the focal
point. Larger the F/D ratio larger is the aperture illumination
efficiency and lower the cross polarization.
98 CHAPTER 4. EARTH SEGMENT

Figure 4.2: Antenna Sub System

4.2.3 Antenna Mount

Type of antenna mount is determined mainly by the coverage

requirement and tracking requirements of the antenna systems.
Different types of mounts used for earth station antenna are:

1 The Azimuth –elevation mount :

This mount consists of a primary vertical axis. Rotation
around this axis controls the azimuth angle. The horizontal
axis is mounted over the primary axis, providing the
elevation angle control.

2 The X-Y mount

It consists of a horizontal primary axis (X-axis) and a
secondary axis (Y axis) and at right angles to it. Movement
around these axes provides necessary steering.

4.2.4 Antenna Tracking System

Tracking is essential when the satellite drift, as seen by an earth

station antenna is a significant fraction of an earth station’s
antenna beam width.

An earth station’s tracking system is required to perform

4.3. RECEIVE-ONLY HOME TV SYSTEMS 99

some of the functions such as -

1 Satellite acquisition

2 Automatic tracking

3 Manual tracking

4 Program tracking.

Recent Tracking Techniques:

There have been some interesting recent developments in auto-
track techniques which can potentially provide high accuracies at
a low cost.

In one proposed technique the sequential lobing

technique has been implemented by using rapid electronic
switching of a s single beam which effectively approximates
simultaneous lobbing.

4.3 Receive-Only Home TV Systems

Planned broadcasting directly to home TV receivers takes place in

the Ku (12-GHz) band. This service is known as direct broadcast
satellite (DBS) service.

There is some variation in the frequency bands assigned

to different geographic regions. In the Americas, for example, the
down- link band is 12.2 to 12.7 GHz.

The comparatively large satellite receiving dishes

[ranging in diameter from about 1.83 m (6 ft) to about 3-m (10 ft)
in some locations], which may be seen in some “backyards” are
used to receive downlink TV signals at C band (4 GHz).

Originally such downlink signals were never intended

for home reception but for network relay to commercial TV outlets
100 CHAPTER 4. EARTH SEGMENT

(VHF and UHF TV broadcast stations and cable TV “head-end”

studios).

4.3.1 The Indoor unit

Equipment is now marketed for home reception of C-band

signals, and some manufacturers provide dual C-band/Ku-band
equipment. A single mesh type reflector may be used which
focuses the signals into a dual feed- horn, which has two separate
outputs, one for the C-band signals and onefor the Ku-band
signals.

Much of television programming originates as first

generation signals, also known as master broadcast quality
signals.

These are transmitted via satellite in the C band to the

network head- end stations, where they are retransmitted as
compressed digital signals to cable and direct broadcast satellite
providers.

1 Another of the advantages, claimed for home C-band

systems, is the larger number of satellites available for
reception compared to what is available for direct broadcast
satellite systems.

2 Although many of the C-band transmissions are scrambled,

there are free channels that can be received, and what are
termed “wild feeds.”

3 These are also free, but unannounced programs, of which

details can be found in advance from various publications
and Internet sources.

4 C-band users can also subscribe to pay TV channels, and

another advantage claimed is that subscription services are
4.3. RECEIVE-ONLY HOME TV SYSTEMS 101

cheaper than DBS or cable because of the multiple-source

programming available.

5 The most widely advertised receiving system for C-band

system appears to be 4DTV manufactured by Motorola.

This enables reception of:

1 Free, analog signals and “wild feeds”

2 VideoCipher ll plus subscription services

3 Free DigiCipher 2 services

4 Subscription DigiCipher 2 services

102 CHAPTER 4. EARTH SEGMENT

Figure 4.3: TVRO System block diagrams

4.3.2 The Outdoor Unit

This consists of a receiving antenna feeding directly into a

low-noise amplifier/converter combination. A parabolic reflector
is generally used, with the receiving horn mounted at the focus.
A common design is to have the focus directly in front of the
reflector, but for better interference rejection, an offset feed may
4.3. RECEIVE-ONLY HOME TV SYSTEMS 103

be used as shown.

Comparing the gain of a 3-m dish at 4 GHz with a 1-

m dish at 12 GHz, the ratio D/l equals 40 in each case, so the
gains will be about equal. Although the free-space losses are much
higher at 12 GHz compared with 4 GHz.

The downlink frequency band of 12.2 to 12.7 GHz spans

a range of 500 MHz, which accommodates 32 TV/FM channels,
each of which is 24-MHz wide. Obviously, some overlap occurs
between channels, but these are alternately polarized left-hand
circular (LHC) and right-hand circular (RHC) or
vertical/horizontal, to reduce interference to accept- able levels.
This is referred to as polarization interleaving. A polarizer that
may be switched to the desired polarization from the indoor
control unit is required at the receiving horn.

The receiving horn feeds into a low-noise converter

(LNC) or possibly a combination unit consisting of a low-noise
amplifier (LNA) followed by a converter.

The combination is referred to as an LNB, for low-noise

block. The LNB provides gain for the broadband 12-GHz signal
and then converts the signal to a lower frequency range so that a
low-cost coaxial cable can be used as feeder to the indoor unit.

The signal fed to the indoor unit is normally a wideband

signal cov- ering the range 950 to 1450 MHz. This is amplified
and passed to a tracking filter which selects the desired channel,
as shown in Fig.

As previously mentioned, polarization interleaving is

used, and only half the 32 channels will be present at the input of
the indoor unit for any one setting of the antenna polarizer. This
eases the job of the tracking filter, since alternate channels are
well separated in frequency.

The selected channel is again down converted, this time

104 CHAPTER 4. EARTH SEGMENT

from the 950- to 1450-MHz range to a fixed intermediate

frequency, usually 70 MHz although other values in the very high
frequency (VHF) range are also used.

The 70-MHz amplifier amplifies the signal up to the

levels required for demodulation. A major difference between
DBS TV and conventional TV is that with DBS, frequency
modulation is used, whereas with conventional TV, amplitude
modulation in the form of vestigial single side- band (VSSB) is
used.

The 70-MHz, FM intermediate frequency (IF) carrier

therefore must be demodulated, and the baseband information
used to generate a VSSB signal which is fed into one of the
VHF/UHF channels of a standard TV set.

4.4 Master Antenna TV System

A master antenna TV (MATV) system is used to provide

reception of DBS TV/FM channels to a small group of users, for
example, to the tenants in an apartment building. It consists of a
single outdoor unit (antenna and LNA/C) feeding a number of
indoor units, as shown in Fig. 4.4

It is basically similar to the home system already

described, but with each user having access to all the channels
independently of the other users. The advantage is that only one
outdoor unit is required, but as shown, separate LNA/Cs and
feeder cables are required for each sense of polarization.

Compared with the single- user system, a larger antenna

is also required (2- to 3-m diameter) in order to maintain a good
signal-to-noise ratio at all the indoor units.

Where more than a few subscribers are involved, the

distribution system used is similar to the community antenna
4.5. COMMUNITY ANTENNA TV SYSTEM 105

(CATV) system described in the following section.

Figure 4.4: CATV System block diagrams

4.5 Community Antenna TV System

The CATV system employs a single outdoor unit, with separate

feeds available for each sense of polarization, like the MATV
system, so that all channels are made available simultaneously at
the indoor receiver.
106 CHAPTER 4. EARTH SEGMENT

Instead of having a separate receiver for each user, all

the carriers are demodulated in a common receiver-filter system,
as shown in Fig. The channels are then combined into a standard
multiplexed signal for transmission over cable to the subscribers.

In remote areas where a cable distribution system may

not be installed, the signal can be rebroadcast from a low-power
VHF TV transmitter.

Figure 4.5shows a remote TV station which employs an

8-m (26.2-ft) antenna for reception of the satellite TV signal in the
C band.

Figure 4.5: One possible arrangement for the indoor unit of a community
antenna TV (CATV) system.

With the CATV system, local programming material

also may be distributed to subscribers, an option which is not
permitted in the MATV system.
4.6. TEST EQUIPMENT MEASUREMENTS ON G/T, C/NO, EIRP 107

4.6 Test Equipment Measurements on G/T, C/No,

EIRP

Measurement of G/T of small antennas is easily and simply

measured using the spectrum analyser method. For antennas
with a diameter of less than 4.5 meters it is not normally
necessary to point off from the satellite.

A step in frequency would be required into one of the

satellite transponder guard bands.

However antennas with a G/T sufficiently large to

enable the station to see the transponder noise floor either a step
in frequency into one of the satellite transponder guard bands
and/or in azimuth movement would be required.

The test signal can be provided from an SES WORLD

SKIES beacon.

Figure 4.6: One possible arrangement for Measurement of G/T

108 CHAPTER 4. EARTH SEGMENT

Procedure :

1 Set up the test equipment as shown below. Allow half an

hour to warm up, and then calibrate in accordance with the
manufacturer’s procedures.

2 Adjust the centre frequency of your spectrum analyzer to

receive the SES WORLD SKIES beacon (data to be provided
on the satellite used for testing)

3 Carefully peak the antenna pointing and adjust the polarizer

by nulling the cross polarized signal. You cannot adjust
polarization when using the circularly polarized SES
WORLD SKIES beacon.

4 Configure the spectrum analyser as follows:

Centre Frequency: Adjust for beacon or test signal frequency
(to be advised).
Use marker to peak and marker to centre functions.

• Frequency Span: 100 KHz

• Resolution Bandwidth: 1 KHz
• Video Bandwidth: 10 Hz (or sufficiently small to limit
noise variance)
• Scale: 5 dB/div
• Sweep Time: Automatic
• Attenuator Adjust to ensure linear operation. Adjust to
provide the ”Noise floor delta” described in steps 7 and
8.

5 To insure the best measurement accuracy during the

following steps, adjust the spectrum analyser amplitude
(reference level) so that the measured signal, carrier or noise,
is approximately one division below the top line of the
spectrum analyser display.
4.6. TEST EQUIPMENT MEASUREMENTS ON G/T, C/NO, EIRP 109

6 Record the frequency and frequency offset of the test signal

from the nominal frequency: For example, assume the
nominal test frequency is 11750 MHz but the spectrum
analyser shows the peak at 11749 MHz. The frequency offset
in this case is -1 MHz.

7 Change the spectrum analyser centre frequency as specified

by SES WORLD SKIES so that the measurement is performed
in a transponder guard band so that only system noise power
of the earth station and no satellite signals are received. Set
the spectrum analyser frequency as follows:

Centre Frequency = Noise slot frequency provided by the PMOC

8 Disconnect the input cable to the spectrum analyser and

confirm that the noise floor drops by at least 15 dB but no
more than 25dB. This confirms that the spectrum analyser’s
noise contribution has an insignificant effect on the
measurement. An input attenuation value allowing a ”Noise
floor Delta” in excess of 25 dB may cause overloading of the
spectrum analyser input.

9 Reconnect the input cable to the spectrum analyser.

10 Activate the display line on the spectrum analyser.

11 Carefully adjust the display line to the noise level shown on

the spectrum analyser. Record the display line level.

12 Adjust the spectrum analyser centre frequency to the test

carrier frequency recorded in step (e).

13 Carefully adjust the display line to the peak level of the test
carrier on the spectrum analyser. Record the display line
level.
110 CHAPTER 4. EARTH SEGMENT

14 Determine the difference in reference levels between steps (l)

and (j) which is the (C + N )/N.

15 Change the (C + N )/N to C/N by the following conversion:

This step is not necessary if the (C + N )/N ratio is more than
20 dB because the resulting correction is less than 0.1 dB.
C+ N
   
C N
= 10 log10 10 10 −1 dB (4.4)
N

16 Calculate the carrier to noise power density ratio (C/No)

using:
   
C C
= − 2.5 + 10 log10 ( RBW × SAcorr ) dB (4.5)
No N

The 2.5 dB figure corrects the noise power value measured

by the log converters in the spectrum analyser to a true RMS
power level, and the SAcorr factor takes into account the actual
resolution filter bandwidth.

17 Calculate the G/T using the following:

   
G C
− − ( EIRPSC − Acorr ) + ( FSL + L a ) − 228.6 dB/K
T NO
(4.6)
where,
EIRPSC – Downlink EIRP measured by the PMOC (dBW)
Acorr – Aspect correction supplied by the PMOC (dB) FSL –
Free Space Loss to the AUT supplied by the PMOC (dB) L a –
Atmospheric attenuation supplied by the PMOC (dB)

18 Repeat the measurement several times to check consistency

of the result.
4.7. ANTENNA GAIN 111

4.7 Antenna Gain

Antenna gain is usually defined as the ratio of the power

produced by the antenna from a far-field source on the antenna’s
beam axis to the power produced by a hypothetical lossless
isotropic antenna, which is equally sensitive to signals from all
directions.

Figure 4.7: One possible arrangement for Measurement of Antenna Gain

Two direct methods of measuring the Rx gain can be

used; integration of the Rx sidelobe pattern or by determination
of the 3dB and 10dB beamwidths.

The use of pattern integration will produce the more

accurate results but would require the AUT to have a tracking
system. In both cases the test configurations for measuring Rx
gain are identical, and are illustrated in Figure 4.7.

In order to measure the Rx gain using pattern integration

the AUT measures the elevation and azimuth narrowband (±5o
corrected) sidelobe patterns.

The AUT then calculates the directive gain of the

112 CHAPTER 4. EARTH SEGMENT

antenna through integration of the sidelobe patterns. The Rx gain

is then determined by reducing the directive gain by the antenna
inefficiencies.

In order to measure the Rx gain using the beamwidth

method, the AUT measures the corrected azimuth and elevation
3dB/10dB beamwidths. From these results the Rx gain of the
antenna can be directly calculated using the formula below.
  
1 31000 91000
G = 10 log10 + − FLoss − R Loss
2 ( A Z3 )( El3 ) ( A Z10 )( El1 0)
(4.7)
where:
G is the effective antenna gain (dBi)
Az3 is the corrected azimuth 3dB beamwidth (o )
El3 is the elevation 3dB beamwidth (o )
Az10 is the corrected azimuth 10dB beamwidth (o )
El10 is the elevation 10dB beamwidth (o )
FLoss is the insertion loss of the feed (dB)
R Loss is the reduction in antenna gain due to reflector inaccuracies,
and is given by:

R Loss = 4.922998677(Sdev f )2 dB (4.8)

where: Sdev is the standard deviation of the actual reflector surface

(inches) f is the frequency (GHz)
Chapter 5

Satellite Applications

5.1 INTELSAT Series

INTELSAT stands for International Telecommunications Satellite.

The organization was created in 1964 and currently has over 140
member countries and more than 40 investing entities (see
http://www.intelsat.com/ for more details).

In July 2001 INTELSAT became a private company and

in May 2002 the company began providing end-to-end solutions
through a network of teleports, leased fiber, and points of presence
(PoPs) around the globe.

Starting with the Early Bird satellite in 1965, a succession

of satellites has been launched at intervals of a few years.

These satellites are in geostationary orbit, meaning that

they appear to be stationary in relation to the earth. At this point
it may be noted that geostationary satellites orbit in the earth’s
equatorial plane and their position is specified by their longitude.

For international traffic, INTELSAT covers three main

regions—the Atlantic Ocean Region (AOR), the Indian Ocean
Region (IOR), and the Pacific Ocean Region (POR) and what is
termed Intelsat America’s Region.

113
114 CHAPTER 5. SATELLITE APPLICATIONS

For the ocean regions the satellites are positioned in

geostationary orbit above the particular ocean, where they
provide a transoceanic telecommunications route. For example,
INTELSAT satellite 905 is positioned at 335.5o east longitude. The
INTELSAT VII-VII/A series was launched over a period from
October 1993 to June 1996. The construction is similar to that for
the V and VA/VB series, shown in Fig. in that the VII series has
solar sails rather than a cylindrical body.

The VII series was planned for service in the POR and
also for some of the less demanding services in the AOR. The
antenna beam coverage is appropriate for that of the POR. Figure
1.3 shows the antenna beam footprints for the C-band
hemispheric cover- age and zone coverage, as well as the spot
beam coverage possible with the Ku-band antennas (Lilly, 1990;
Sachdev et al., 1990). When used in the AOR, the VII series
satellite is inverted north for south (Lilly, 1990), minor
adjustments then being needed only to optimize the antenna pat-
terns for this region. The lifetime of these satellites ranges from
10 to 15 years depending on the launch vehicle.

Recent figures from the INTELSAT Web site give the

capacity for the INTELSAT VII as 18,000 two-way telephone
circuits and three TV channels; up to 90,000 two-way telephone
circuits can be achieved with the use of ”digital circuit
multiplication.”

The INTELSAT VII/A has a capacity of 22,500 two-way

telephone circuits and three TV channels; up to 112,500 two-way
tele- phone circuits can be achieved with the use of digital circuit
multipli- cation. As of May 1999, four satellites were in service
over the AOR, one in the IOR, and two in the POR.

The INTELSAT VIII-VII/A series of satellites was

launched over the period February 1997 to June 1998. Satellites in
this series have similar capacity as the VII/A series, and the
lifetime is 14 to 17 years.
5.2. INSAT 115

It is standard practice to have a spare satellite in orbit on

high reliability routes (which can carry preemptible traffic) and to
have a ground spare in case of launch failure.

Thus the cost for large international schemes can be

high; for example, series IX, described later, represents a total
investment of approximately 1 billion dollar.

Figure 5.1: Region of glob

5.2 INSAT

INSAT or the Indian National Satellite System is a series of

multipurpose geo-stationary satellites launched by ISRO to
satisfy the telecommunications, broadcasting, meteorology, and
search and rescue operations.

Commissioned in 1983, INSAT is the largest domestic

communication system in the Asia Pacific Region. It is a joint
venture of the Department of Space, Department of
Telecommunications, India Meteorological Department,

All India Radio and Doordarshan. The overall

coordination and management of INSAT system rests with the
Secretary-level INSAT Coordination Committee. INSAT satellites
provide transponders in various bands (C, S, Extended C and Ku)
116 CHAPTER 5. SATELLITE APPLICATIONS

to serve the television and communication needs of India. Some

of the satellites also have the Very High Resolution Radiometer
(VHRR), CCD cameras for metrological imaging.

The satellites also incorporate transponder(s) for

receiving distress alert signals for search and rescue missions in
the South Asian and Indian Ocean Region, as ISRO is a member
of the Cospas-Sarsat programme.

5.2.1 INSAT System

The Indian National Satellite (INSAT) System Was Commissioned

With The Launch Of INSAT-1B In August 1983 (INSAT-1A, The
First Satellite Was Launched In April 1982 But Could Not Fulfil
The Mission).

INSAT System Ushered In A Revolution In India’s

Television And Radio Broadcasting, Telecommunications And
Meteorological Sectors. It Enabled The Rapid Expansion Of TV
And Modern Telecommunication Facilities To Even The Remote
Areas And Off-Shore Islands.

5.2.2 Satellites In Service

Of The 24 Satellites Launched In The Course Of The INSAT

Program, 10 Are Still In Operation.INSAT-2E.

It Is The Last Of The Five Satellites In INSAT-2

SeriesPrateek . It Carries Seventeen C-Band And Lower Extended
C-Band Transponders Providing Zonal And Global Coverage
With An Effective Isotropic Radiated Power (EIRP) Of 36 Dbw.

It Also Carries A Very High Resolution Radiometer

(VHRR) With Imaging Capacity In The Visible (0.55-0.75 µm),
Thermal Infrared (10.5-12.5 µm) And Water Vapour (5.7-7.1 µm)
Channels And Provides 2 × 2 Km, 8 × 8 Km And 8 × 8 Km
5.2. INSAT 117

Ground Resolution Respectively.

INSAT-3A

: The Multipurpose Satellite, INSAT-3A, Was Launched By

Ariane In April 2003. It Is Located At 93.5 Degree East Longitude.
The Payloads On INSAT-3A Are As Follows:
12 Normal C-Band Transponders (9 Channels Provide Expanded
Coverage From Middle East To South East Asia With An EIRP Of
38 Dbw, 3 Channels Provide India Coverage With An EIRP Of 36
Dbw And 6 Extended C-Band Transponders Provide India
Coverage With An EIRP Of 36 Dbw).

A CCD Camera Provides 1x1 Km Ground Resolution, In

The Visible (0.63- 0.69 µm), Near Infrared (0.77-0.86 µm) And
Shortwave Infrared (1.55-1.70 µm) Bands.

INSAT-3D

Launched In July 2013, INSAT-3D Is Positioned At 82 Degree East

Longitude. INSAT-3D Payloads Include Imager, Sounder, Data
Relay Transponder And Search & Rescue Transponder. All The
Transponders Provide Coverage Over Large Part Of The Indian
Ocean Region Covering India, Bangladesh, Bhutan,Maldives,
Nepal, Seychelles, Sri Lanka And Tanzania For Rendering
Distress Alert Services.

INSAT-3E

Launched In September 2003, INSAT-3E Is Positioned At 55

Degree East Longitude And Carries 24 Normal C-Band
Transponders Provide An Edge Of Coverage EIRP Of 37 Dbw
Over India And 12 Extended C-Band Transponders Provide An
Edge Of Coverage EIRP Of 38 Dbw Over India.
118 CHAPTER 5. SATELLITE APPLICATIONS

KALPANA-1

KALPANA-1 Is An Exclusive Meteorological Satellite Launched

By PSLV In September 2002. It Carries Very High Resolution
Radiometer And DRT Payloads To Provide Meteorological
Services. It Is Located At 74 Degree East Longitude. Its First
Name Was METSAT. It Was Later Renamed As KALPANA- 1 To
Commemorate Kalpana Chawla.

Edusat

Configured For Audio-Visual Medium Employing Digital

Interactive Classroom Lessons And Multimedia Content,
EDUSAT Was Launched By GSLV In September 2004. Its
Transponders And Their Ground Coverage Are Specially
Configured To Cater To The Educational Requirements.

GSAT-2

Launched By The Second Flight Of GSLV In May 2003, GSAT-2 is

Located At 48 Degree East Longitude And Carries Four Normal C-
Band Transponders To Provide 36 Dbw EIRP With India Coverage,
Two Ku Band Transponders With 42 Dbw EIRP Over India And
An MSS Payload Similar To Those On INSAT-3B And INSAT-3C.
5.2. INSAT 119

INSAT-4 Series

Figure 5.2: INSAT 4A

INSAT-4A is positioned at 83 degree East longitude along with

INSAT-2E and INSAT-3B. It carries 12 Ku band 36 MHz
bandwidth transponders employing 140 W TWTAs to provide an
EIRP of 52 dBW at the edge of coverage polygon with footprint
covering Indian main land and 12 C-band 36 MHz bandwidth
transponders provide an EIRP of 39 dBW at the edge of coverage
with expanded radiation patterns encompassing Indian
geographical boundary, area beyond India in southeast and
northwest regions. Tata Sky, a joint venture between the TATA
Group and STAR uses INSAT-4A for distributing their DTH
service.

1 INSAT-4A

2 INSAT-4B

3 Glitch In INSAT 4B

4 China-Stuxnet Connection
120 CHAPTER 5. SATELLITE APPLICATIONS

5 INSAT-4CR

6 GSAT-8 / INSAT-4G

7 GSAT-12 /GSAT-10

5.3 VSAT

VSAT stands for very small aperture terminal system. This is the
distinguishing feature of a VSAT system, the earth-station
antennas being typically less than 2.4 m in diameter (Rana et al.,
1990). The trend is toward even smaller dishes, not more than 1.5
m in diameter (Hughes et al., 1993).

In this sense, the small TVRO terminals for direct

broadcast satellites could be labeled as VSATs, but the
appellation is usually reserved for private networks, mostly
providing two-way communications facilities.

Typical user groups include bank- ing and financial

institutions, airline and hotel booking agencies, and large retail
stores with geographically dispersed outlets.
5.3. VSAT 121

Figure 5.3: VSAT Block Diagrams

5.3.1 VSAT network

The basic structure of a VSAT network consists of a hub station

which provides a broadcast facility to all the VSATs in the network
and the VSATs themselves which access the satellite in some form
of multiple- access mode.

The hub station is operated by the service provider, and

it may be shared among a number of users, but of course, each
122 CHAPTER 5. SATELLITE APPLICATIONS

user organ- ization has exclusive access to its own VSAT network.

Time division mul- tiplex is the normal downlink mode

of transmission from hub to the VSATs, and the transmission can
be broadcast for reception by all the VSATs in a network, or
address coding can be used to direct messages to selected VSATs.

A form of demand assigned multiple access (DAMA) is

employed in some systems in which channel capacity is assigned
in response to the fluctuating demands of the VSATs in the
network.

Most VSAT systems operate in the Ku band, although

there are some Cband systems in existence (Rana et al., 1990).

5.3.2 Applications

1 Supermarket shops (tills, ATM machines, stock sale updates

and stock ordering).

2 Chemist shops - Shoppers Drug Mart - Pharmaprix.

Broadband direct to the home. e.g. Downloading MP3 audio
to audio players.

3 Broadband direct small business, office etc, sharing local use

with many PCs.

4 Internet access from on board ship Cruise ships with internet

cafes, commercial shipping communications.

5.4 Mobile satellite services

5.4. MOBILE SATELLITE SERVICES 123

5.4.1 GSM

Services and Architecture

If your work involves (or is likely to involve) some form of

wireless public communications, you are likely to encounter the
GSM standards. Initially developed to support a standardized
approach to digital cellular communications in Europe, the
’Global System for Mobile Communications’ (GSM) protocols are
rapidly being adopted to the next generation of wireless
telecommunications systems.

In the US, its main competition appears to be the

cellular TDMA systems based on the IS-54 standards. Since the
GSM systems consist of a wide range of components, standards,
and protocols.

The GSM and its companion standard DCS1800 (for the

UK, where the 900 MHz frequencies are not available for GSM)
have been developed over the last decade to allow cellular
communications systems to move beyond the limitations posed
by the older analog systems.

Analog system capacities are being stressed with more

users that can be effectively supported by the available frequency
allocations. Compatibility between types of systems had been
limited, if non-existent.

By using digital encoding techniques, more users can

share the same frequencies than had been available in the analog
systems. As compared to the digital cellular systems in the US
(CDMA [IS-95] and TDMA [IS-54]), the GSM market has had
impressive success. Estimates of the numbers of telephones run
from 7.5 million GSM phones to .5 million IS54 phones to .3
million for IS95.

GSM has gained in acceptance from its initial

124 CHAPTER 5. SATELLITE APPLICATIONS

beginnings in Europe to other parts of the world including

Australia, New Zealand, countries in the Middle East and the far
east. Beyond its use in cellular frequencies (900 MHz for GSM,
1800 MHz for DCS1800), portions of the GSM signaling protocols
are finding their way into the newly developing PCS and LEO
Satellite communications systems.

While the frequencies and link characteristics of these

systems differ from the standard GSM air interface, all of these
systems must deal with users roaming from one cell (or satellite
beam) to another, and bridge services to public communication
networks including the Public Switched Telephone Network
(PSTN), and public data networks (PDN).

The GSM architecture includes several subsystems:

Figure 5.4: GSM Block Diagrams

1 The Mobile Station (MS) – These digital telephones include

vehicle, portable and hand-held terminals. A device called
5.4. MOBILE SATELLITE SERVICES 125

the Subscriber Identity Module (SIM) that is basically a

smart-card provides custom information about users such as
the services they’ve subscribed to and their identification in
the network
2 The Base Station Sub-System (BSS) – The BSS is the
collection of devices that support the switching networks
radio interface. Major components of the BSS include the
Base Transceiver Station (BTS) that consists of the radio
modems and antenna equipment.
In OSI terms, the BTS provides the physical interface to the
MS where the BSC is responsible for the link layer services
to the MS. Logically the transcoding equipment is in the BTS,
however, an additional component.

3 The Network and Switching Sub-System (NSS) – The NSS

provides the switching between the GSM subsystem and
external networks along with the databases used for
additional subscriber and mobility management.
Major components in the NSS include the Mobile Services
Switching Center (MSC), Home and Visiting Location
Registers (HLR, VLR). The HLR and VLR databases are
interconnected through the telecomm standard Signaling
System 7 (SS7) control network.

4 The Operation Sub-System (OSS) – The OSS provides the

support functions responsible for the management of
network maintenance and services. Components of the OSS
are responsible for network operation and maintenance,
mobile equipment management, and subscription
management and charging.

Several channels are used in the air interface:

1 FCCH - the frequency correction channel - provides

frequency synchronization information in a burst
126 CHAPTER 5. SATELLITE APPLICATIONS

2 SCH - Synchronization Channel - shortly following the

FCCH burst (8 bits later), provides a reference to all slots on
a given frequency

3 PAGCH - Paging and Access Grant Channel used for the

transmission of paging information requesting the setup of a
call to a MS.

4 RACH - Random Access Channel - an inbound channel used

by the MS to request connections from the ground network.
Since this is used for the first access attempt by users of the
network, a random access scheme is used to aid in avoiding
collisions.

5 CBCH - Cell Broadcast Channel - used for infrequent

transmission of broadcasts by the ground network.

6 BCCH - Broadcast Control Channel - provides access status

information to the MS. The information provided on this
channel is used by the MS to determine whether or not to
request a transition to a new cell.

7 FACCH - Fast Associated Control Channel for the control of

handovers.

8 TCH/F - Traffic Channel, Full Rate for speech at 13 kbps or

data at 12, 6, or 3.6 kbps.

9 TCH/H - Traffic Channel, Half Rate for speech at 7 kbps, or

data at 6 or 3.6 kbps.

Mobility Management

One of the major features used in all classes of GSM networks

(cellular, PCS and Satellite) is the ability to support roaming
users. Through the control signaling network, the MSCs interact
to locate and connect to users throughout the network.
5.4. MOBILE SATELLITE SERVICES 127

”Location Registers” are included in the MSC databases

to assist in the role of determining how, and whether connections
are to be made to roaming users. Each user of a GSM MS is
assigned a Home Location Register (HLR) that is used to contain
the user’s location and subscribed services.

Difficulties facing the operators can include;

1 Remote/Rural Areas : To service remote areas, it is often

economically unfeasible to provide backhaul facilities (BTS
to BSC) via terrestrial lines (fiber/microwave).

2 Time to deploy : Terrestrial build-outs can take years to plan

and implement.

3 Areas of ’minor’ interest. These can include small isolated

centers such as tourist resorts, islands, mines, oil exploration
sites, hydro-electric facilities.

4 Temporary Coverage : Special events, even in urban areas,

can overload the existing infrastructure.

GSM service security

GSM was designed with a moderate level of service security.

GSM uses several cryptographic algorithms for security. The
A5/1, A5/2, and A5/3 stream ciphers are used for ensuring
over-the-air voice privacy.

GSM uses General Packet Radio Service (GPRS) for data

transmissions like browsing the web. The most commonly
deployed GPRS ciphers were publicly broken in 2011The
researchers revealed flaws in the commonly used GEA/1.
128 CHAPTER 5. SATELLITE APPLICATIONS

5.4.2 Global Positioning System (GPS)

The Global Positioning System (GPS) is a satellite based

navigation system that can be used to locate positions anywhere
on earth. Designed and operated by the U.S. Department of
Defense, it consists of satellites, control and monitor stations, and
receivers. GPS receivers take information transmitted from the
satellites and uses triangulation to calculate a user’s exact
location. GPS is used on incidents in a variety of ways, such as:

1 To determine position locations; for example, you need to

radio a helicopter pilot the coordinates of your position
location so the pilot can pick you up.

2 To navigate from one location to another; for example, you

need to travel from a lookout to the fire perimeter.

3 To create digitized maps; for example, you are assigned to

plot the fire perimeter and hot spots.

4 To determine distance between two points or how far you are

from another location.

Figure 5.5: GPS Block Diagrams

5.4. MOBILE SATELLITE SERVICES 129

The purpose of this chapter is to give a general

overview of the Global Positioning System, not to teach
proficiency in the use of a GPS receiver. To become proficient
with a specific GPS receiver, study the owner’s manual and
practice using the receiver.

The chapter starts with a general introduction on how

the global positioning system works. Then it discusses some
basics on using a GPS receiver.

Three Segments of GPS:

1 Space Segment — Satellites orbiting the earth

The space segment consists of 29 satellites circling the earth
every 12 hours at 12,000 miles in altitude. This high altitude
allows the signals to cover a greater area. The satellites are
arranged in their orbits so a GPS receiver on earth can receive
a signal from at least four satellites at any given time. Each
satellite contains several atomic clocks.

2 Control Segment — The control and monitoring stations

The control segment tracks the satellites and then provides
them with corrected orbital and time information. The
control segment consists of five unmanned monitor stations
and one Master Control Station. The five unmanned stations
monitor GPS satellite signals and then send that information
to the Master Control Station where anomalies are corrected
and sent back to the GPS satellites through ground antennas.

3 User Segment — The GPS receivers owned by civilians

and military
The user segment consists of the users and their GPS
receivers. The number of simultaneous users is limitless.
130 CHAPTER 5. SATELLITE APPLICATIONS

How GPS Determines a Position

The GPS receiver uses the following information to determine a

position.

1 Precise location of satellites

When a GPS receiver is first turned on, it downloads orbit
information from all the satellites called an almanac. This
process, the first time, can take as long as 12 minutes; but
once this information is downloaded, it is stored in the
receiver’s memory for future use.

2 Distance from each satellite

The GPS receiver calculates the distance from each satellite to
the receiver by using the distance formula: distance = velocity
x time. The receiver already knows the velocity, which is the
speed of a radio wave or 186,000 miles per second (the speed
of light).

3 Triangulation to determine position The receiver determines

position by using triangulation. When it receives signals from
at least three satellites the receiver should be able to calculate
its approximate position (a 2D position). The receiver needs
at least four or more satellites to calculate a more accurate 3D
position.

Using a GPS Receiver

There are several different models and types of GPS receivers.

Refer to the owner’s manual for your GPS receiver and practice
using it to become proficient.

When working on an incident with a GPS receiver it is

important to:

1 Always have a compass and a map.

5.5. INMARSAT 131

2 Have a GPS download cable.

3 Have extra batteries.

4 Know memory capacity of the GPS receiver to prevent loss of

data, decrease in accuracy of data,or other problems.

5 Use an external antennae whenever possible, especially

under tree canopy, in canyons, or while flying or driving.

6 Set up GPS receiver according to incident or agency standard

regulation; coordinate system.

7 Take notes that describe what you are saving in the receiver.

5.5 INMARSAT

Inmarsat - Indian Maritime SATellite is still the sole

IMO-mandated provider of satellite communications for the
GMDSS.

Inmarsat has constantly and consistently exceeded this

figure & Independently audited by IMSO and reported on to IMO.

Now Inmarsat commercial services use the same

satellites and network & Inmarsat A closes at midnight on 31
December 2007 Agreed by IMO – MSC/Circ.1076. Successful
closure programme almost concluded Overseen throughout by
IMSO.
132 CHAPTER 5. SATELLITE APPLICATIONS

Figure 5.6: INMARSAT Satellite Service

GMDSS services continue to be provided by:

• Inmarsat B, Inmarsat C/mini-C and Inmarsat Fleet F77

• Potential for GMDSS on FleetBroadband being assessed

1 The IMO Criteria for the Provision of Mobile Satellite

Communications Systems in the Global Maritime Distress
and Safety System (GMDSS)

2 Amendments were proposed; potentially to make it simpler

for other satellite systems to be approved The original
requirements remain and were approved by MSC 83

• No dilution of standards

3 Minor amendments only; replacement Resolution expected

to be approved by the IMO 25th Assembly

4 Inmarsat remains the sole, approved satcom provider for the

GMDSS
5.6. LEO 133

5.6 LEO

Low Earth Orbit satellites have a small area of coverage. They are
positioned in an orbit approximately 3000km from the surface of
the earth.

1 They complete one orbit every 90 minutes

2 The large majority of satellites are in low earth orbit

3 The Iridium system utilizes LEO satellites (780km high)

4 The satellite in LEO orbit is visible to a point on the earth for

a very short time

Figure 5.7: LEO, MEO & GEO range

5.7 MEO

Medium Earth Orbit satellites have orbital altitudes between

3,000 and 30,000 km. They are commonly used used in
navigation systems such as GPS
134 CHAPTER 5. SATELLITE APPLICATIONS

5.8 GEO

Geosynchronous (Geostationary) Earth Orbit satellites are

positioned over the equator. The orbital altitude is around
30,000-40,000 km.

1. There is only one geostationary orbit possible around the

earth.

1 Lying on the earth’s equatorial plane.

2 The satellite orbiting at the same speed as the rotational
speed of the earth on its axis.
3 They complete one orbit every 24 hours. This causes the
satellite to appear stationary with respect to a point on the
earth allowing one satellite to provide continual coverage
to a given area on the earth’s surface.
4 One GEO satellite can cover approximately 1/3 of the
world’s surface.

They are commonly used in communication systems

2. Advantages:

1 Simple ground station tracking.

2 Nearly constant range
3 Very small frequency shift

3. Disadvantages:

1 Transmission delay of the order of 250 msec.

2 Large free space loss
3 No polar coverage

4. Satellite orbits in terms of the orbital height:

5.9. SATELLITE NAVIGATIONAL SYSTEM 135

5. According to distance from earth :

1 Geosynchronous Earth Orbit (GEO) ,

2 Medium Earth Orbit (MEO)
3 Low Earth Orbit (LEO)

Figure 5.8: LEO, MEO & GEO Orbits

5.9 Satellite Navigational System

Benefits :

1 Enhanced Safety

2 Increased Capacity

3 Reduced Delays

Advantage:

1 Increased Flight Efficiencies

2 Increased Schedule Predictability

3 Environmentally Beneficial Procedures

136 CHAPTER 5. SATELLITE APPLICATIONS

• Using ICAO GNSS Implementation Strategy and ICAO

Standards and Recommended Practices

• GPS Aviation Use Approved for Over a Decade

– Aircraft Based Augmentation Systems (ABAS) – (e.g.

RAIM)

• Space Based Augmentation System (SBAS) since 2003

– Wide Area Augmentation System (WAAS) augmenting

GPS

• Development of GNSS Ground Based Augmentation System

(GBAS) Continues

– Local Area Augmentation System (LAAS)

• GNSS is Cornerstone for National Airspace System

5.10 Direct Broadcast satellites (DBS)

Satellites provide broadcast transmissions in the fullest sense of

the word, because antenna footprints can be made to cover large
areas of the earth. The idea of using satellites to provide direct
transmissions into the home has been around for many years, and
the services pro- vided are known generally as direct broadcast
satellite (DBS) services.

Broadcast services include audio, television, and Internet

services.

5.10.1 Power Rating and Number of Transponders

Satellites primarily intended for DBS have a higher [EIRP] than for
the other categories, being in the range 51 to 60 dBW. At a Regional
Administrative Radio Council (RARC) meeting in 1983, the value
5.10. DIRECT BROADCAST SATELLITES (DBS) 137

established for DBS was 57 dBW (Mead,2000). Transponders are

rated by the power output of their high-power amplifiers.

Typically, a satellite may carry 32 transponders. If all 32

are in use, each will operate at the lower power rating of 120 W.

The available bandwidth (uplink and downlink) is seen

to be 500 MHz. A total number of 32 transponder channels, each
of bandwidth 24 MHz, can be accommodated.

The bandwidth is sometimes specified as 27 MHz, but

this includes a 3- MHz guardband allowance. Therefore, when
calculating bit-rate capacity, the 24 MHz value is used.

The total of 32 transponders requires the use of both

right- hand circular polarization (RHCP) and left-hand circular
polarization (LHCP) in order to permit frequency reuse, and
guard bands are inserted between channels of a given
polarization.

Figure 5.9: DBS Service

5.10.2 Bit Rates for Digital Television

The bit rate for digital television depends very much on the
picture format. One way of estimating the uncompressed bit rate
138 CHAPTER 5. SATELLITE APPLICATIONS

is to multiply the number of pixels in a frame by the number of

frames per second, and multiply this by the number of bits used
to encode each pixel.

5.10.3 MPEG Compression Standards

MPEG is a group within the International Standards Organization

and the International Electrochemical Commission (ISO/IEC) that
undertook the job of defining standards for the transmission and
storage of moving pictures and sound.

The MPEG standards currently available are MPEG-1,

MPEG-2, MPEG-4, and MPEG-7.

5.11 Direct to home Broadcast (DTH)

DTH stands for Direct-To-Home television. DTH is defined as the

reception of satellite programmes with a personal dish in an
individual home.

1 DTH Broadcasting to home TV receivers take place in the ku

band(12 GHz). This service is known as Direct To Home
service.
2 DTH services were first proposed in India in 1996.

3 Finally in 2000, DTH was allowed.

4 The new policy requires all operators to set up earth stations

in India within 12 months of getting a license. DTH licenses
in India will cost 2.14 million dollar and will be valid for 10
years.

Working principal of DTH is the satellite

communication. Broadcaster modulates the received signal and
5.11. DIRECT TO HOME BROADCAST (DTH) 139

transmit it to the satellite in KU Band and from satellite one can

receive signal by dish and set top box.

DTH Block Diagram

1 A DTH network consists of a broadcasting centre, satellites,

encoders, multiplexers, modulators and DTH receivers

2 The encoder converts the audio, video and data signals into
the digital format and the multiplexer mixes these signals.

It is used to provide the DTH service in high populated

area A Multi Switch is basically a box that contains signal
splitters and A/B switches. A outputs of group of DTH LNBs are
connected to the A and B inputs of the Multi Switch.

Figure 5.10: DTH Service

Advantage

1 DTH also offers digital quality signals which do not degrade

the picture or sound quality.

2 It also offers interactive channels and program guides with

140 CHAPTER 5. SATELLITE APPLICATIONS

customers having the choice to block out programming

which they consider undesirable

3 One of the great advantages of the cable industry has been the
ability to provide local channels, but this handicap has been
overcome by many DTH providers using other local channels
or local feeds.
4 The other advantage of DTH is the availability of satellite
broadcast in rural and semi-urban areas where cable is
difficult to install.

5.12 Digital audio broadcast (DAB)

DAB Project is an industry-led consortium of over 300 companies.

1 The DAB Project was launched on 10th September, 1993

2 In 1995 it was basically finished and became operational

3 There are several sub-standards of the DAB standard

• DAB-S (Satellite) - using QPSK – 40 Mb/s
• DAB-T (Terrestrial) - using QAM – 50 Mb/s
• DAB-C (Cable) - using OFDM – 24 Mb/s

4 These three sub-standards basically differ only in the

specifications to the physical representation, modulation,
transmission and reception of the signal.

5 The DAB stream consists of a series of fixed length packets

which make up a Transport Stream (TS). The packets support
‘streams’ or ‘data sections’.
6 Streams carry higher layer packets derived from an MPEG
stream & Data sections are blocks of data carrying signaling
and control data.
5.13. WORLDSPACE SERVICES 141

7 DAB is actually a support mechanism for MPEG.& One

MPEG stream needing higher instantaneous data can ‘steal’
capacity from another with spare capacity.

5.13 Worldspace services

WorldSpace (Nasdaq: WRSP) is the world’s only global media

and entertainment company positioned to offer a satellite radio
experience to consumers in more than 130 countries with five
billion people, driving 300 million cars. WorldSpace delivers the
latest tunes, trends and information from around the world and
around the corner.

WorldSpace subscribers benefit from a unique

combination of local programming, original WorldSpace content
and content from leading brands around the globe, including the
BBC, CNN, Virgin Radio, NDTV and RFI.

WorldSpace’s satellites cover two-thirds of the globe

with six beams. Each beam is capable of delivering up to 80
channels of high quality digital audio and multimedia
programming directly to WorldSpace Satellite Radios anytime
and virtually anywhere in its coverage area. WorldSpace is a
pioneer of satellite-based digital radio services (DARS) and was
instrumental in the development of the technology infrastructure
used today by XM Satellite Radio.

5.14 Business Television (BTV) - Adaptations for

Education

Business television (BTV) is the production and distribution, via

satellite, of video programs for closed user group audiences. It
often has two-way audio interaction component made through a
simple telephone line. It is being used by many industries
142 CHAPTER 5. SATELLITE APPLICATIONS

including brokerage firms, pizza houses, car dealers and delivery

services.

BTV is an increasingly popular method of information

delivery for corporations and institutions. Private networks,
account for about 70 percent of all BTV networks. It is estimated
that by the mid-1990s BTV has the potential to grow to a 1.6
billion dollar market in North America with more and more
Fortune 1,000 companies getting involved. The increase in use of
BTV has been dramatic.

Institution updates, news, training, meetings and other

events can be broadcast live to multiple locations. The expertise
of the best instructors can be delivered to thousands of people
without requiring trainers to go to the site. Information can be
disseminated to all employees at once, not just a few at a time.
Delivery to the workplace at low cost provides the access to
training that has been denied lower level employees. It may be
the key to re-training America’s work force.

Television has been used to deliver training and

information within businesses for more than 40 years. Its recent
growth began with the introduction of the video cassette in the
early 1970s. Even though most programming is produced for
video cassette distribution, business is using BTV to provide
efficient delivery of specialized programs via satellite.

The advent of smaller receiving stations - called very

small aperture terminals (VSATs) has made private
communication networks much more economical to operate.
BTV has a number of tangible benefits, such as reducing travel,
immediate delivery of time-critical messages, and eliminating
cassette duplication and distribution hassles.

The programming on BTV networks is extremely

cost-effective compared to seminar fees and downtime for travel.
It is an excellent way to get solid and current information very
5.15. GRAMSAT 143

fast. Some people prefer to attend seminars and conferences

where they can read, see, hear and ask questions in person. BTV
provides yet another piece of the education menu and is another
way to provide professional development.

A key advantage is that its format allows viewers to

interact with presenters by telephone, enabling viewers to
become a part of the program. The satellite effectively places
people in the same room, so that sales personnel in the field can
learn about new products at the same time.

Speed of transmission may well be the competitive edge

which some firms need as they introduce new products and
services. BTV enables employees in many locations to focus on
common problems or issues that might develop into crises
without quick communication and resolution.

BTV networks transmit information every business day

on a broad range of topics, and provide instructional courses on
various products, market trends, selling and motivation.
Networks give subscribers the tools to apply the information
they have to real world situations.

5.15 GRAMSAT

ISRO has come up with the concept of dedicated GRAMSAT

satellites, keeping in mind the urgent need to eradicate illiteracy
in the rural belt which is necessary for the all round development
of the nation.

This Gramsat satellite is carrying six to eight high

powered C-band transponders, which together with video
compression techniques can disseminate regional and cultural
specific audio-visual programmes of relevance in each of the
regional languages through rebroadcast mode on an ordinary TV
set.
144 CHAPTER 5. SATELLITE APPLICATIONS

The high power in C-band has enabled even remote

area viewers outside the reach of the TV transmitters to receive
programmers of their choice in a direct reception mode with a
simple .dish antenna.

The salient features of GRAMSAT projects are:

1 Its communications networks are at the state level connecting

the state capital to districts, blocks and enabling a reach to
villages.
2 It is also providing computer connectivity data broadcasting,
TVbroadcasting facilities having applications like e-
governance, development information, teleconferencing,
helping disaster management.
3 Providing rural-education broadcasting.

However, the Gramsat projects have an appropriate

combination of following activities.

1 Interactive training at district and block levels employing

suitable configuration
2 Broadcasting services for rural development

3 Computer interconnectivity and data exchange services

4 Tele-health and tele-medicine services.

5.16 Specialized services

5.16.1 Satellite-email services

The addition of Internet Access enables Astrium to act as an

Internet Service Provider (ISP) capable of offering Inmarsat users
a tailor-made Internet connection.
5.16. SPECIALIZED SERVICES 145

With Internet services added to our range of terrestrial

networks, you will no longer need to subscribe to a third party
for Internet access (available for Inmarsat A, B, M, mini-M, Fleet,
GAN, Regional BGAN & SWIFT networks).

We treat Internet in the same way as the other terrestrial

networks we provide, and thus offer unrestricted access to this
service. There is no timeconsuming log-on procedure, as users are
not required to submit a user-ID or password.

Description of E-mail Service:

Astrium’s E-Mail service allows Inmarsat users to send and
receive e-mail directly through the Internet without accessing a
public telephone network.

Features and Benefits :

1 No need to configure an e-mail client to access a Astrium e-

mail account.

2 Service optimized for use with low bandwidth Inmarsat

terminals

3 Filter e-mail by previewing the Inbox and deleting any

unwanted e-mails prior to downloading.

4 No surcharge or monthly subscription fees

5 Service billed according to standard airtime prices for

Inmarsat service used.

5.16.2 Video Conferencing (medium resolution)

Video conferencing technology can be used to provide the same

full, two way interactivity of satellite broadcast at much lower
cost. For Multi-Site meetings, video conferencing uses bridging
systems to connect each site to the others.
146 CHAPTER 5. SATELLITE APPLICATIONS

It is possible to configure a video conference bridge to

show all sites at the same time on a projection screen or monitor.
Or, as is more typical, a bridge can show just the site from which
a person is speaking or making a presentation.

The technology that makes interactive video

conferencing possible, compresses video and audio signals, thus
creating an image quality lower than that of satellite broadcasts.

5.16.3 Satellite Internet access

Satellite Internet access is Internet access provided through

communications satellites. Modern satellite Internet service is
typically provided to users through geostationary satellites that
can offer high data speeds, with newer satellites using Ka band to
achieve downstream data speeds up to 50 Mbps.

Satellite Internet generally relies on three primary

components: a satellite in geostationary orbit (sometimes referred
to as a geosynchronous Earth orbit, or GEO), a number of ground
stations known as gateways that relay Internet data to and from
the satellite via radio waves (microwave), and a VSAT
(very-small aperture terminal) dish antenna with a transceiver,
located at the subscriber’s premises.

Other components of a satellite Internet system include

a modem at the user end which links the user’s network with the
transceiver, and a centralized network operations center (NOC)
for monitoring the entire system.