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Satellite communication

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CONTENTS

(iii)

Foreword … … … … … … … … … … (i)

Summary of Changes … … … … … … … … … (ii)

Contents … … … … … … … … … … … (iii)

Introduction … … … … … … … … … … … 1

Chapter 1 Basics of Satellite Communication … … … … 3

Chapter 2 System Elements … … … … … … … … 11

Chapter 3 Satellite Network Architecture … … … … … 21

Chapter 4 INMARSAT … … … … … … … … … 29

Resources … … … … … … … … … 39

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introduction

How to Use this Booklet

To obtain the full benefit of this booklet you must work through it in the correct sequence. DON’T skip some of the sections because you think you know them, because they look too easy, or because they look too difficult.

Add your own notes where you think it necessary. If you strike a problem, read the notes again. The resources section below gives suggested further reading and explains how you can get further reading and explains how you can get further help.

When you think you have fully understood the topic, and you have competed all the required actions, you can then tackle the self-test questions. Try to answer these questions without reference to the notes.

To help you navigate around this booklet and quickly find the various sections, symbols are placed in the margins. These symbols indicate the following:-

Important information, which must be read and remembered An action which you must undertake to improve your knowledge of that topic on your own ship. Series of self-test questions, so that you can test your own understanding of the topic.

Reading of topics to get details of the topics.

Indicates Resources.

Indicates areas where learning can be enhanced by Group Activities.

Action

The actions in this handbook must be done carefully. They are designed to

strengthen your knowledge and understanding of the topic as it applies to your own ship. Some may require you to list the equipment on your ship, or to sketch equipment aboard your ship. Don’t do this from memory, go and look at

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the equipment properly, and make sure you understand what each part of that piece of equipment does.

Self-Test Questions

At the conclusion of each topic, you will find some questions. These are for self-testing purposes only, they are not part of your formal assessment. Some are brief True/False questions or Fill in the Blanks while some may require more detailed answers. Think carefully before answering these questions, each is designed to emphasize a particular point, as well as to test your overall knowledge of the topic.

DON’T skip these questions. It is better to make a mistake and learn from that mistake, than to think you know all the answers.

Resources

Most of the information you will require is included in this booklet, however further reading will expand and improve your knowledge of the subjects.

Details of such books are indicated at the end of the Handbook

Perhaps your greatest resource is the knowledge and expertise of faculty teaching you. Don’t be afraid to ask them about the subjects we are dealing with here, and discuss any problems you may have in the understanding of these topics.

Learning can best be improved by doing activities in a group and

then discussing the same to find out the things that were done right or wrong.

The activities that can be undertaken in a group are indicated by this icon.

Group Activities

Good Luck with your progress

Through this Handbook and with

your final assessment Of topics

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1.1 A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: Natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth).

1.2 There are many natural satellites in the solar system, with almost every

planet having at least one moon. Saturn, for example, has at least 53 natural

satellites.

1.3 Artificial satellites, however, did not become a reality until the mid-20th

century. The first artificial satellite was Sputnik, a Russian football-size space

probe that lifted off on Oct 4, 1957.

1.4 Satellites being above the earth can cover a larger area. The information

to be transmitted from a mobile user should be correctly received by a satellite

and forwarded to one of the earth stations. This puts the limitation that only Line

of Sight (LOS) communication is possible.

1.5 Satellites can be classified by their functions. Satellites are launched into space to do a specific job. The type of satellite that is launched to monitor cloud patterns for a weather station will be different than a satellite launched to send television signals. The satellite must be designed specifically to fulfill its function. The application areas of Satellite systems are:

1.6 Traditionally

1.6.1 Meteorological satellites 1.6.2 Radio and TV broadcast satellites

1.6.3 Military satellites

1.6.4 Satellites for navigation and localisation (e.g. GPS)

BASICS OF SATELLITE

COMMUNICATIONS

Types and Uses of Satellites

Description of a Satellite

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1.7 Telecommunications

1.7.1 Global telephone connections

1.7.2 Backbone for global networks

1.7.3 Connections for communication in remote places

1.7.4 Global mobile communication

1.8 Satellites have several unique characteristics which make them particularly useful for remote sensing of the Earth's surface. Communications satellite is a microwave repeater station that permits two or more users with appropriate earth stations to deliver or exchange information in various forms.

1.9 The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the capability and objective of the sensor(s) they carry. Orbit selection can vary in terms of altitude (their height above the Earth's surface) and their orientation and rotation relative to the Earth.

Fig 1.1 - Orbits of Different Satellites

Satellite Orbits

Basic Characteristics of Satellite

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Fig 1.2 - Satellites in GEO

1.10 Low Earth Orbit (LEO). LEO systems employ satellites at altitudes ranging from 500 to 1,000 km. Over that range, the orbit period is between 1.6 and 1.8 hours, the higher orbit resulting in a slightly longer period of revolution. The principal advantage of LEO satellites is the shorter range that the radio signal has to traverse, requiring less power and minimizing propagation delay. Their short orbital period produces relatively brief durations when a given satellite can serve a particular user.

1.11 Medium Earth Orbit (MEO). The altitude of a MEO is around at 6000-20000 kms above earth’s surface (a period of about 6 hours). Between 2,000 and 8,000 km, there is an inhospitable environment for electronic components. For altitudes in the range of 8,000 to 10,000 km, a MEO satellite has a much longer period and thus tends to ‘‘hang’’ over a given region on the Earth for a few hours. Transmission distance and propagation delay are greater than for LEO but still significantly less than for GEO.

1.12 Geo-synchronous Earth Orbit (GEO). A system of three satellites in GEO each separated by 120 degrees of longitude, as shown in Figure 1.2, can receive and send radio signals over almost all the inhabited portions of the globe.

The small regions send radio signals over almost all the inhabited portions of the globe. The small regions around the North and South Poles - above 81° N and below 81° S - are not covered. A given GEO satellite has a coverage region, illustrated by the shaded oval, within which Earth stations can communicate with and be linked by the satellite. The range from user to satellite is a minimum of 36,000 km. 1.13 In GEO, there are two classes of orbits that have a 24-hour period. A geosynchronous orbit could be elliptical or inclined with respect to the equator (or both), as follows:-

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1.13.1 GEO. The special case of an equatorial 24-hour circular orbit, in

which the satellite appears to remain over a point on the ground (which is

on the equator at the same longitude where the satellite is maintained), is

called Geostationary. A GEO satellite would not require ground antennas

that track the satellite

1.13.2 GEO with Inclined Orbit. A 24-hour circular geosynchronous

orbit that is inclined with respect to the equator is not GEO because the

satellite appears to move relative to the fixed point on the Earth, while an

inclined geosynchronous orbit satellite might.

1.14 HEO (Highly Elliptical Orbit). Other non-GEO orbits have been used at

various times, such as the highly elliptical Earth orbit (HEO) to allow coverage of

northern latitudes.

1.15 Characteristics of Satellites. Characteristics of GEO and Low Earth

Orbit (LEO) Satellites (Figure 1.3) are discussed below:-

1.15.1 Orbit inclined and 24-hour period of revolution. GEO satellite is

its ability to provide coverage of an entire hemisphere at one time.

1.15.2 Ideal system but is not stable, the inclination changes over a

period of time. The inclination is corrected by sending correcting signals

from earth.

1.15.3 Low Earth Orbit (LEO) Satellites. LEO satellites appear to

move past a point on the Earth. The Iridium mobile satellite system

employs Low Earth Orbit, in which satellites are at an altitude of

approximately 780 km and each passes a given user in only a few

minutes. The advantage to using a non-GEO satellite network is that the

range to the user is shorter hence power required is less.

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Fig 1.3 - GEO and LEO Satellites

1.16 Life of Satellites. Satellites are designed to last only about 15 years in

orbit, because of the practical inability to service a satellite in GEO and change

consumables (fuel, battery cells, and degraded or failed components). Non-GEO

satellites at altitudes below about 1,500 km are subject to atmospheric drag and a

harsh radiation environment and are likely to require replacement after 10 years

of operation.

1.16.1 Availability of Satellites. Approximately half of the current

commercial GEO satellites are owned and operated by a few major

companies that make the investment and sell the service to regional and

global markets. As the original global operator and now the largest

provider, Intelsat is able to serve users in nearly every country. Other

large operators include SES, Eutelsat, Loral Skynet, Telesat, INMARSAT,

and JSAT.

1.17 Advantages of Satellite Communications. Advantages of

communication through Satellites are as follows:-

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1.17.1 Mobile/Wireless Communication, Independent of Location.

Any users with an appropriate Earth station can employ a satellite as long

as they are within its footprint.

1.17.2 Wide Area Coverage. Country, Continent, or Globe.

1.17.3 Wide Bandwidth Available Throughout. Frequency spectrum

availability for satellites is quite good, and satellite users have enjoyed

ample bandwidth, principally for fixed services. Most of the satellites in

GEO employ microwave frequencies generally between 3.5 and 6.5 GHz

(C-band) and between 10.5 and 14.5 GHz (Ku-band).

1.17.4 Independence from Terrestrial Infrastructure. By providing a

repeater station in space, a satellite creates an independent microwave

relay for ground-based radio stations Rapid Installation of Ground

Networks.

1.17.5 Rapid Installation of Ground Networks. Once the satellite (or

satellite constellation, in the case of a non-GEO system) is operational,

individual Earth stations in the ground segment can be activated quickly in

response to demand for services.

1.17.6 Low Cost per Added Site. Along with rapid installation, the

cost of constructing a single site on earth can be quite modest.

1.17.7 Uniform Service Characteristics.

1.17.8 Total Service from a Single Provider.

Now that you have been familiarized with

the topic, let’s see how much you have

learnt! Answer the following without

referring to the chapter

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Self-Test Questions

1. A television (TV) transmission is an example of Simplex transmission. (True/False)

2. The frequency band used by most satellites is SHF. (True/False)

3. The optimum working frequency for satellite systems lies between 2 GHz and 12 GHz. (True/False)

4. A synchronous satellite orbits the earth once in 24 hrs. (True/False)

5. Geostationary satellites are located at a height of 36000 km from earth’s surface. (True/False)

6. Geostationary satellite follow Elliptical path. (True/False)

7. Geostationary satellite are generally put in equatorial orbit and domestic satellite in polar orbit. (True/False)

8. The main advantage of satellite communication is High reliability. (True/False)

9. For global communication, the minimum number of satellites needed is 4. (True/False) 10. There are two orbits for a GEO satellite. (True/False)

1. In a communication satellite, the equipment which provides the connecting link between the satellites’ transmit & receive antennas is referred to as the ___________.

2. Primary component of uplink section of satellite is _____________.

3. Area least effectively covered by geostationary satellites ____________ region.

4. The signal from a satellite is normally aimed at a specific area called the _________.

5. MEO satellites are located at altitudes between ________ to ________ kms.

6. LEO satellites are normally below an altitude of ________ km.

Part A - True & False

Part B - Fill in the Blanks

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7. A GEO is at the ________ orbit and revolves in phase with Earth. 8. A system of three satellites in GEO each separated by ________ degrees of longitude can receive and send radio signals over almost all the inhabited portions of the globe.

9. The principal advantage of ______ satellites is the shorter range that the radio signal has to traverse, requiring less power and minimizing propagation delay.

10. Transmission distance and propagation delay are greater than for ______ but still significantly less than for ________.

11. What are the advantages of satellite communications?

12. List the various types of satellite orbits?

13. Why is the life of GEO satellites more than other satellites?

14. What is the difference between Geostationary and Geosynchronous satellites?

Part C - Short Answers

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2.1 Implementation of a communication satellite system is a major undertaking. This Chapter describes the system in terms of two major parts, the space segment and the ground segment.

2.2 The Space Segment (depicted in Fig 2.1) consists of the following:-

SYSTEM ELEMENTS OF A

COMMUNICATION SATELLITE

Fig 2.1 - Space Segment

Space Segment

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2.2.1 Satellite Itself.

2.2.2 Means for launching satellite.

2.2.3 Electrical Power System is a combination of solar cells and storage

batteries is the prime power source for a Satellite.

2.2.4 Mechanical Structure.

2.2.5 Communication Transponder.

2.2.6 Communication Antennas.

2.2.7 Attitude and orbit control system.

2.3 Providing a telecommunication service via satellite involves more than launching a satellite and installing Earth stations. Shown in Figure 2.2 is an Iridium Satellite.

Fig 2.2 - Iridium Satellite and its Parts

Overall System

Fig 2.4 – RAM

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2.4 The ground segment is not a single, homogeneous entity but rather a diverse collection of facilities, users and applications, as shown below in Figure 2.3.

2.5 Satellite Tracking and Control. The Tracking, Telemetry, and Command (TT&C) station (or stations) establishes a control and monitoring link with the satellite. Precise tracking data are collected periodically via the ground antenna to allow the pinpointing of the satellite’s position and the planning of on-orbit position corrections. That is because any orbit tends to distort and shift with respect to a fixed point in space due to varying gravitational forces from the non-spherical Earth and the pull of the sun and moon. 2.6 Communications Segment. The ground segment provides access to the satellite repeater from Earth stations to meet communications needs of users. A typical ground communications segment is illustrated in Figure 2.4; a single satellite is shown to indicate that the links are established through its repeater rather than directly from Earth station to Earth station. Incidentally, Earth station is an internationally accepted term that includes satellite communication stations located on the ground, in the air (on airplanes), or on the sea (on ships).

Fig 2.3 - Ground and User Segments

Ground Segment

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2.7 Microwave frequencies extend from 1 GHz to 30 GHz. Some of its important properties are listed below:-

2.7.1 Line-of-sight propagation through space and the atmosphere.

2.7.2 Blockage by dense media, like hills, tree trunks, solid buildings, metal walls, and at higher frequencies, heavy rain.

2.7.3 Wide bandwidths, compared to the lower frequency bands High Frequency (HF) shortwave radio, Very High Frequency (VHF) TV, Frequency Modulation (FM) radio, and Ultrahigh Frequency (UHF) land mobile radio.

2.7.4 Compact antennas typically using metal reflectors, which can focus energy in a desired direction (thus providing gain over an antenna with broad or omnidirectional radiation properties).

2.7.5 Transmission through metal waveguide structures as opposed to wire conductors.

2.7.6 Somewhat reduced efficiency of power amplification, which is the ratio of radio frequency (RF) power out divided by the direct current (dc) power in.

Fig 2.4 - Ground Communications Segment

Microwave Frequencies - Satellite

Communication

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2.5 Most useful RF frequencies lie in the 300-MHz and 300,000-MHz range,

although lower frequencies (longer wavelengths) are attractive for certain

applications. Some segments of the band can be transmitted with greater ease

(e.g., can propagate through the atmosphere and physical media with less loss

and temporary fading) than others.

2.6 Also, differing levels of natural and man-made noise interfere with the

transfer. Figure 2.5 indicates the absorption by air at various frequencies. We see

that above about 30 GHz, propagation grows in difficulty due to higher levels of

atmospheric attenuation effects. Not shown in the figure is that manmade noise

becomes a significant detriment below about 1 GHz.

2.7 Noise Window. The optimum piece of the spectrum for space-to-Earth

applications lies between about 1 and 4 GHz, which we call the noise window for

microwave transmission. Above about 12 GHz, there is a significant amount of

signal absorption by the atmosphere. There is a point of near total absorption at

66 GHz, due to resonance by oxygen. Those frequencies cannot be used for

Earth-to-space paths but can be applied for inter-satellite links.

2.8 Rain Noise. The most detrimental atmospheric effect above 4 GHz is rain

attenuation, which is a decrease of signal level due to absorption of microwave

energy by water droplets in a rainstorm. Due to the relationship between the size

of droplets relative to the wavelength of the radio signal, microwave energy at

higher frequencies is absorbed more heavily than that at lower frequencies. Rain

Microwave Frequencies - Satellite

Communication

Fig 2.5 - Absorption of Frequencies

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attenuation is a serious problem in tropical regions of the world with heavy

thunderstorm activity, as those storms contain intense rain cells.

2.9 Bands of Interest. The frequency bands of interest for satellites lie above

100 MHz, where we find the VHF, UHF, and super high frequency (SHF) bands.

The SHF range has been broken down further by common usage into sub-bands

with letter designations, the familiar L-, S-, C-, X-, Ku-, and Ka-bands being

included (figure 2.6). Generally, Ku-band and those below it are the most popular

because of the relative low cost of available equipment and the more favorable

propagation characteristics.

2.10 Uplink and Downlink. A typical satellite band is divided into separate

halves, one for ground-to-space links (the uplink) and one for space-to-ground

links (the downlink). That separation is reflected in the design of the satellite

microwave repeater to minimize the chance of downlink signals being re-received

and thereby jamming the operation of the satellite. Uplink frequency bands

allocated by the ITU are typically slightly above the corresponding downlink

frequency band, to take advantage of the fact that it is easier to generate RF

power within an Earth station than it is onboard a satellite.

Fig 2.5 - Absorption of Frequencies

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2.11 Satellite application of the band between about 400 MHz and 1,000 MHz.

That range already is highly contested for terrestrial wireless applications, notably

cellular; mobile data; and various radio systems used for government and

emergency services. There are advantages to using longer wavelengths to bend

around obstacles and to penetrate nonmetallic structures. That is why some of the

little LEO systems have chosen these bands for a variety of low-speed data

services.

2.12 These frequencies have greater bandwidth and more stable propagation

under most conditions than do frequencies below 1 GHz. The L- and S-bands are

particularly effective for providing rapid communications by way of mobile and

transportable Earth stations. These have the capacity of thousand to tens of

thousands of channels. That can be adequate for high-value mobile telephone or

data communications or one-way mobile broadcasting. A number of S-band

satellites are being used to broadcast audio to vehicles, with as many as 100

different channels of music, talk radio, and driver information etc.

2.13 GPS. The Global Positioning Satellite (GPS) system operates in a

segment of L-band and employs non-GEO satellites at approximately 26,500-km

altitude. Each satellite transmits in the same bandwidth using a different CDMA

spreading code.

2.14 INMARSAT. Larger parabolic reflector antennas are practical for L-band

ship-to-shore satellite communications. These satellites are operated by Inmarsat,

the global provider of international maritime communications.

2.15 The most popular microwave bands exploited for satellite communication

are the C-, X-, and Ku-bands. These bands, which lie between 3 and 15 GHz,

offer substantially more bandwidth than L- and S-bands. C-band maintains its

popularity because it is the mainstay of such industries as cable TV.

2.16 C-Band. C-band was the first part of the microwave spectrum to be used

extensively for commercial satellite communication. Another common designation

we use is 6/4 GHz, the uplink/downlink frequencies. The principal benefit of C-

band is the low level of natural and man-made noise coupled with a very small

degree of attenuation in heavy rain.

VHF and UHF Ranges

Microwave Bands: L and S

Microwave Bands - C, Ku and Ka

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2.17 Ka Band. Government and military satellite communications systems in

the United States and some other countries employ X-band and, on a limited

basis, Ka-band. With an uplink range of 7.90 to 8.40 GHz and a downlink range of

7.25 to 7.75 GHz, X-band is used extensively for military long-haul

communications links, much as C-band is used on a commercial basis.

2.18 Ku Band.

2.18.1 More Reliable Transmission. The Ku-Band SATCOM system

uses Ku-Band radio frequency which offers higher bandwidth for

data transmission. The Ku-Band is also less susceptible to

interference from other electronic transmissions such as television

broadcasts and civilian mobile wireless networks.

2.18.2 Quick Deployment. Depending on operational requirements, the

Ku-Band SATCOM system can be deployed in two configurations;

first, the system can be housed in enclosed shelters, and mounted

on 5-Ton Transport trucks. The system, featuring a smaller

SATCOM dish, can also be housed in portable containers which

operate independently of vehicle platforms. On average, for both

configurations, setting up and establishing connectivity can be

completed in less than 20 minutes.

2.18.3 Allows Incorporation of New Technologies. The Ku-Band

SATCOM system features an Internet Protocol (IP) based

architecture, which is compatible with a variety of commercially

available expertise, technologies and equipment. This translates to

a greater potential for the incorporation of new technologies and

lower maintenance costs.

Now that you have been familiarized with

the topic, let’s see how much you have

learnt! Answer the following without

referring to the chapter

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Self-Test Questions

1. A satellite crosslink means Earth-Satellite Link. (True/False)

2. A satellite in geo-synchronous orbit is a distance of 36000 miles from the equator of the earth. (True/False)

3. Point-to-point microwave link is the best option to provide large connectivity. (True/False)

4. Polar orbiting Satellites orbit the earth in such a way as to cover the earth from east & west. (True/False)

5. Es’hailSat 1 is a MEO satellite. (True/False)

6. Satellite communication use Microwaves. (True/False)

7. GPS satellites are MEO satellites. (True/False)

8. In the C band and the Ku band, rainfall is the most significant cause of Signal fading. (True/False)

9. The uplink frequency of satellite is same as the downlink frequency. (True/False)

10. MEO satellites have a larger coverage area than LEO satellites. (True/False)

11. One GEO satellite has a coverage of 50% of area of the planet. (True/False)

12. The Ku-Band SATCOM system uses Ku-Band radio frequency which offers higher bandwidth for data transmission. (True/False)

13. These bands, which lie between 4 and 15 GHz, offer substantially more bandwidth than L- and S-bands. (True/False)

14. The most popular microwave bands exploited for satellite communication are the C-, X-, and Ku-bands. (True/False)

15. The frequency bands of interest for satellites lie above 400 MHz, where we find the VHF, UHF, and super high frequency bands. (True/False)

Part A - True & False

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1. The path of a satellite around a planet is called its _________________.

2. A satellite that merely relays the uplink carrier as a downlink is called a ______________________or a ________________________.

3. The connection between Receive and Transmit antennas is provided by the

_____________. 4. Ground Segment comprises of a network of_______________,

________________ and ___________________. 5. Communication between two earth stations not visible to the same satellite is

achieved by ______________.

6. The optimum piece of the spectrum for space-to-Earth applications lies between about 1 and 4 GHz, which we call the _____________ for microwave transmission.

7. A typical satellite band is divided into separate halves, one for ground-to-space links called _________and one for space-to-ground links called _________.

1. List the main parts of the Space segment.

2. List the parts of the Ground segment.

3. What is rain fade?

4. Why is uplink frequency higher than downlink frequency?

Part B - Fill in the Blanks

Part C - Short Answers

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3.1 Satellite networks are capable of interfacing with terrestrial networks at

high data rates and also provide networking access to a variety of users directly.

The property which a satellite network provides via links between users is called

connectivity. The three generic forms of connectivity are point-to-point, point-to-

multipoint, and multipoint interactive, shown in Figures 3.1, 3.2, and 3.3,

respectively. The first uses of satellites were for point-to-point links between fixed

pairs of Earth stations.

3.2 Point to Point. As shown in Figure 3.1, communication from one station

to the other is on a dedicated path over the same satellite. Therefore, two links

are needed to allow simultaneous communication in both directions. That is how

the typical telephone conversation or interactive data link functions.

SATELLITE NETWORK

ARCHITECTURE

General Features of Satellite Communication

Fig 3.1 - Point to Point

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3.3 Point to Multi-Point. As shown in Figure 3.2, a single uplink station can

transmit a continuous stream of information to all receiving points within the

coverage area.

3.4 Multi-Point to Point. Adding a transmit capability to each receive

terminal is a technique to convert the broadcast system into a multipoint

interactive network, shown

Fig 3.2 - Point to Multi-point

Fig 3.3 - Multi-point to Point

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in Figure 3.3. Each remote site employs a type of Earth station called a very small

aperture terminal (VSAT). The broadcast half of the link transmits bulk information

to all receiving points. Those points, in turn, can transmit their individual requests

or replies back to the originating point over the same satellite.

3.5 The basic elements in an end-to-end satellite communication link are

illustrated in Figure 3.4. In the figure are the transmitting Earth station, which

establishes the uplink path, a simplified satellite and its microwave repeater, the

downlink path, and a receiving Earth station. The entry and exit points to the

propagation medium are provided by the transmitting and receiving antennas.

Antennas convert electrical energy at microwave frequencies into electromagnetic

waves, and vice versa. A transmitting Earth station consists of equipment that

modulates the information to be sent on an RF signal called the carrier, translates

it to the appropriate frequency and amplifies it to a high-enough power level to

provide an adequate uplink.

3.6 Transmitting Station. The signal to be transmitted consists of information

in electrical form, such as one or more voice channels for telephone service,

digital data in the form of a high-speed bit stream, or a composite video signal

such as that delivered from, a video tape recorder. In modern satellite systems,

analog information forms like voice and TV are first digitized and then

compressed to reduce the required bandwidth. From that point, the link is digital in

nature. The transmitter has the following modules:-

3.6.1 Encoder and Modulator. 3.6.2 Frequency Conversion and RF Amplification.

3.6.3 High Power Amplification.

3.7 Receiving Station. The reverse process found in the receiving station is

illustrated in Figure 3.4. It has the following elements:-

3.7.1 Low Noise Amplifier.

3.7.2 Block Down Converter.

3.7.3 Demodulator.

3.8 Satellite Transponder. Transponder itself is a contraction of transmitter-

responder, originally referring to a single-frequency repeating device found on

Microwave Transmitters and Receivers

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aircraft. The purpose of the aircraft transponder is to add the identification of the

aircraft and actively enhance the power to be reflected back to the radar

transmitter. A satellite transponder is entirely different because it is more of a

transparent microwave relay channel, also taking into account the need to

translate the frequency from the uplink range to the downlink range. We can

better define a transponder by examining the two different payload configurations

shown in Figure 3.4.

3.9 Satellite Transponder. Transponder itself is a contraction of transmitter-

responder, originally referring to a single-frequency repeating device found on

aircraft. The purpose of the aircraft transponder is to add the identification of the

aircraft and actively enhance the power to be reflected back to the radar

transmitter. A satellite transponder is entirely different because it is more of a

transparent microwave relay channel, also taking into account the need to

translate the frequency from the uplink range to the downlink range. We can

better define a transponder by examining the two different payload configurations

shown in Figure 3.5.

3.9.1 Single Channel. The single-channel repeater (shown in Figure

3.5) does just what its name implies: provide a single channel of

transmission within the satellite. As shown at the top of the figure, the

entire uplink band is translated in the downconverter and applied to a

single power amplifier.

Fig 3.4 - Basic Elements of Satellite Communication

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3.9.2 Multi-Channels. The transponderized design (shown in Figure

3.5) breaks up the downlink range into individual frequency channels.

3.10 The basic concept of multiple access is to permit Earth stations to transmit

to the same satellite without interfering with one another. RF carriers can be

maintained separate in frequency, time or code. Hence, the use of FDMA, TDMA,

and CDMA, which are discussed in the following sections.

3.11 Frequency Division Multiple Access. The FDMA technique is traditional

in radio communications, since it relies on frequency separation between carriers.

All that is required is that the Earth stations transmit their traffic on different

microwave frequencies and that the modulation not cause the carrier bandwidths

to overlap.

3.12 Time Division Multiple Access. Earth station transmissions in a common

TDMA network are all on the same frequency, and each employs the full

bandwidth of the RF channel, which may consist of an entire transponder (full

transponder TDMA) or a segment of bandwidth within a transponder (narrowband

TDMA). Interference between transmissions, which are on the same frequency,

is prevented by synchronizing the transmission so they do not overlap in time.

Multiple Access

Fig 3.5 - Satellite Transponder

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3.13 Code Division Multiple Access. CDMA combines modulation and

multiple access to achieve a certain degree of information efficiency and

protection through the technique of spread spectrum communications. First

applied to guarantee security for military transmissions, it evolved into a multiple

access system that promises better bandwidth utilization and service quality in an

environment of spectral congestion and interference. The basic concept is to

separate or filter different signals from different users, not by using frequency or

time, but by the particular code that scrambles each transmission.

Fig 3.6 - Types of Multiple Access

Now that you have been familiarized with

the topic, let’s see how much you have

learnt! Answer the following without

referring to the chapter

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Self-Test Questions

1. Point-to-point microwave link is the best option to provide large connectivity. (True/False)

2. Satellite communication use Microwaves. (True/False)

3. Time Division Multiple Access is the normal downlink mode of transmission from hub to the VSATs. (True/False)

4. For secure communication we use CDMA method of multiple access. (True/False)

5. The signal to be transmitted consists of information in the electrical form, such as one or more voice channels for telephone service. (True/False)

6. A satellite transponder is entirely different because it is more of a transparent microwave relay channel. (True/False)

1. CDMA is also called ______________ ________________.

2. Military Services were the first to use __________ ____________ Multiple Access as it is very secure.

3. The regions covered by Inmarsat are ________________, _________________, ________________ and _________________.

4. Communication between two earth stations not visible to the same satellite is achieved by ______________.

5. In a Multipoint-to-Point network, transmission data rates are _____________, meaning that the hub transmits at a much higher data rate than each remote VSAT.

Part A - True & False

Part B - Fill in the Blanks

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6. The multiple access technique suitable only for digital transmission is __________.

7. VSAT stands for ___________________________________.

1. Differentiate between the three point-to-point links.

2. What are the three forms of multiple access?

3. With the help of diagrams, explain the three generic forms of satellite

connectivity viz. point-to-point, point-to-multipoint, and multipoint-to-point.

4. With the help of a diagram explain TDMA?

Part C - Short Answers

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4.1 Introduction. International Maritime Satellite (INMARSAT) was

established in 1979 to serve the maritime industry by developing satellite

communications for ship distress, safety and management applications. Today it

operates a global satellite network for maritime, land and aeronautical users.

4.2 The Inmarsat communications system has three major components:-

4.2.1 The space segment.

4.2.2 The ground segment.

4.2.3 The mobile earth stations (MESs) for maritime and land operation

and aeronautical earth stations (AESs) for aircraft operation.

4.3 The space segment is provided by Inmarsat and consists of four

operational satellites, with back-up satellites in orbit and ready to be used if

necessary. The family of satellites includes:-

4.3.1 Older Inmarsat-2 Series. Launched between 1990 and 1992.

4.3.2 Inmarsat-3 Constellation. The third-generation Inmarsat-3

satellites have been in service since 1997.

4.3.2.1 Offer coverage using a global beam and spot beams.

4.3.2.2 Each Inmarsat-3 satellite is eight times more powerful

than an Inmarsat-2 satellite.

4.3.2.3 The flexibility offered by the Inmarsat-3 satellites makes

it possible to reallocate both RF power and bandwidth between the

global beam and spot beams, allowing a more efficient use of the

available spectrum.

4.3.2.4 Each Inmarsat-3 satellite also carries a navigation

transponder, designed to enhance the accuracy, availability and

integrity of the Global Positioning System (GPS) and Glonass.

Space Segment

INMARSAT

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4.4 Frequency and Configuration. Inmarsat’s network uses L-band

(1.5/1.6GHz) frequencies from the ship direction. Each satellite’s global beam

covers approximately one-third of the Earth’s surface (including land and sea)

from a geostationary orbit nearly 36,000 kilometres above the Equator. Figures

4.1 and 4.2 show the four satellites in space and their coverage areas, which

correspond to the four ocean regions:-

4.4.1 Atlantic Ocean Region-East (AOR-E).

4.4.2 Atlantic Ocean Region-West (AOR-W).

4.4.3 Indian Ocean Region (IOR).

4.4.4 Pacific Ocean Region (POR).

Fig 4.1 - Location of INMARSAT Satellites

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4.5 The ground segment comprises a network of the following:-

4.5.1 Land Earth Stations (LESs). There are more than 40 LESs

operated by land earth station operators.

4.5.1.1 Each land earth station operator provides a link between

the satellite network and the international telecommunication

network. An LES is capable of handling many calls to and from

MESs simultaneously, over the different Inmarsat networks.

4.5.1.2 LESs are owned by telecommunications operators which

act as land earth station operators and provide a wide range of

communications services to the MES user.

4.5.2 Network Co-Ordination Stations (NCSs) and Network

Operations Centre (NOC).

4.5.2.1 For each Inmarsat system and ocean region there is a

NCS which monitors and controls all communications.

Fig 4.2 - Location of INMARSAT Satellites

Ground Segment

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4.5.2.2 Each NCS communicates with the LES operators in its

ocean region, the other NCSs and the network operations centre

(NOC) located at Inmarsat’s London headquarters, making it

possible to transfer operational information throughout the system.

4.5.2.3 The NCSs are involved in setting up calls between an

MES and a LES operator.

4.6 A mobile earth station (MES) is a device installed on a ship (or on a fixed

installation in a maritime environment) to enable the user to communicate with

land-based subscribers via the Inmarsat satellites. Inmarsat does not manufacture

such equipment itself, but permits independent manufacturers to produce models.

4.7 An Inmarsat-C mobile earth station (MES) is a small and power-efficient

terminal which provides global communications, is inexpensive to purchase and

simple to install and use. The Inmarsat-C network can be used to send and

receive text or data messages only.

4.7.1 Equipment Used.

4.7.1.1 Data Terminal Equipment. Computer, Screen,

Keyboard and Printer.

4.7.1.2 Data Communication Terminal (DCT). The Inmarsat-C

DCT is a ‘satellite modem’ which provides an interface between the

MES and the satellite system using a transmitter, receiver and an

antenna.

Mobile Earth Station

INMARSAT C

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4.8 Equipment

4.8.1 Directional Antenna. The narrow beam required for F77

applications requires a dish antenna which is in a fiberglass housing,

called a radome.

4.8.2 Antenna Stabilisation. It is essential that the antenna remains

pointed at the satellite through ship’s motions of roll, pitch and yawing. To

achieve this the antenna is stabilised and also given a feed from the Gyro.

4.8.3 Below Deck Equipment. The Below Deck equipment consists of

the actual satcom terminal, with a computer and peripherals, telephones,

fax machine and call alarms. Modulation takes place at VHF frequency

and gets up-converted to UHF transmit frequency.

Fig 4.3 - INMARSAT C Configuration

Fleet F77

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4.9 Fleet F77 supports increased use of Internet-based applications and data

services. Main features provided are:-

4.9.1 High-Speed Data Services

4.9.1.1 64 kbit/s Mobile ISDN.

4.9.1.2 56 kbit/s speech (64 kbit/s).

4.9.1.3 3.1 kHz audio.

4.9.1.4 Mobile Packet Data Service (MPDS).

4.9.2 Low-speed services

4.9.2.1 Voice 2.4 kbit/s.

4.9.2.2 Fax 2.4 kbit/s

Fig 4.4 - INMARSAT Fleet F77 Configuration

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Fig 4.5 - Broadband Global Area Network

4.10 The Broadband Global Area Network (BGAN) is a global satellite network

with telephony using portable terminals. The terminals are normally used to

connect a laptop computer to broadband Internet in remote locations, although as

long as line-of-sight to the satellite exists, the terminal can be used anywhere.

4.11 The value of BGAN terminals is that, unlike other satellite Internet services

which require bulky and heavy satellite dishes to connect, a BGAN terminal is

about the size of a laptop and thus can be carried easily. The network is provided

by Inmarsat and uses three geostationary satellites called I-4 to provide almost

global coverage.

4.12 BGAN is accessible via a range of small, lightweight satellite terminals,

which provide performance options to suit different operational needs. The

smallest terminals are designed to suit single users. The larger terminals offer a

WLAN capability and are particularly suitable for small teams that need to

establish a temporary office for an extended period. They are also suitable for

users requiring higher bandwidth to enable applications such as live broadcasting.

4.13 BGAN supports increased use of Internet-based applications and data

services. Main features provided are:-

4.13.1 High-speed data network.

4.13.2 Rates up to 492 kbps.

Broadband Global Area Network

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4.13.3 Worldwide coverage.

4.13.4 Guaranteed data rates.

4.13.5 Supports IP and circuit-switched applications for voice, file

transfer, video and fax.

4.13.6 Third generation GSM.

4.13.7 Packet-switched network.

4.13.8 IPv6 compatible.

Now that you have been familiarized with

the topic, let’s see how much you have

learnt! Answer the following without

referring to the chapter

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Self-Test Questions

1. The uplink frequency of satellite is same as the downlink frequency. (True/False)

2. MEO satellites have a larger coverage area than LEO satellites. (True/False)

3. One GEO satellite has a coverage of 50% of area of the planet. (True/False)

4. For secure communication we use CDMA method of multiple access. (True/False)

5. Pacific Ocean region is covered by two Inmarsat satellites. (True/False)

6. Inmarsat provides reliable coverage between latitudes 80° North and 80° South for both SOLAS and non-SOLAS vessels and available free of charge. (True/False)

7. Inmarsat stands for International Maritime Satellite Organisation. (True/False)

8. The “heart” of the Inmarsat system is the NCS which operates 24 hours a day, monitors, co-ordinates and controls the operational activities of all satellites in the network. (True/False)

9. Inmarsat is the only provider of GMDSS-approved satellite communications systems. (True/False)

10. Inmarsat C does not provide any storage capability for messages. (True/False)

11. Network Operations Centre (NOC) of INMARSAT is located at Inmarsat’s New York headquarters. (True/False)

Part A - True & False

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1. In Inmarsat Fleet 77, Fleet 55 and Fleet 33 systems, 77, 55 and 33 are ______________ of the antenna in cms.

2. Ground Segment of Inmarsat comprises of a network of_______________, ________________ and ___________________.

3. The regions covered by Inmarsat are ________________, _________________, ________________ and _________________.

4. Communication between two earth stations not visible to the same satellite is achieved by ______________.

5. The diameter of the antenna of Fleet 33 is ________ inches.

6. Inmarsat _____ has an important feature called send and store messages.

1. List the four ocean regions.

2. What are the fetaures of Fleet 77?

3. What are the parts of the Ground segment of Inmarsat?

4. What are the features of BGAN?

Part B - Fill in the Blanks

Part C - Short Answers

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RESOURCES

1. Introduction to Satellite Communication - Bruce R. Elbert.

2. INMARSAT Handbook Issue 4 - Published by INMARSAT