SATELLITE COMMUNICATION

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Communication Satellites

Module-4

Introduction

• Satellite telecommunication stands out as the most prominent one among other applications of satellites, both in terms of application potential and the number of satellites launched in each category.
• The application areas of communication satellites mainly include

television broadcasting, international telephony and data communication services.

• Communication satellites act as repeater stations that provide either

point-to-point, point-to-multipoint or multipoint interactive services.

Communication-related Applications of Satellites

• Telecommunication satellites provide a varied range of services mainly including television broadcasting, international telephony and data communication services, most of these services being multipurpose in nature.
• Traditionally, satellite applications included television broadcasting and fixed and mobile telephony services but now newer dimensions are being added to the spectrum of the satellite applications with the advent of services like the internet and multimedia.

Communication-related Applications of Satellites

• Satellites are facing tough competition from terrestrial networks in general, with fibre optics in particular.
• Satellite TV refers to the use of satellites for relaying TV programmes from a point where they originate to a large geographical area.
• GEO satellites in point-to-multipoint configuration are employed for

satellite TV applications.

• There are primarily two types of satellite television distribution systems, namely the television receive-only (TVRO) and the direct broadcasting satellite (DBS) systems.

Communication-related Applications of Satellites

• Communication satellites can be GEO satellites or a constellation of LEO, MEO or HEO (highly elliptical orbit) satellites.
• GEO satellites maintain a key role in setting up national programmes and distributing traditional services such as television or more novel services such as access to the internet.
• New trends in mobile communication have led to the development of constellations of non-GEO satellites in the LEO, MEO and HEO.

Geostationary Satellite Communication Systems

• The geostationary orbit has been the preferred orbit for satellite communication systems and provides most of the revenue for satellite system operators.
• The first geostationary communication satellite, named Early Bird (Intelsat 1) was launched by INTELSAT in 1965.
• Commercial satellites launched in the 1970s and 1980s were all

geostationary satellites.

• These satellites were used for international, regional and domestic telephone and video distribution services.

Geostationary Satellite Communication Systems

• Some of the important geostationary satellite missions include Intelsat, Inmarsat, Telstar, Asiasat, Arabsat, Galaxy, GE, Superbird, Eutelsat, Astra, Palapa and so on.
• New trends in the field of satellite communication include the launch of satellites in non-geostationary orbits for some specialized applications.
• The most important applications of geostationary communication satellites in the current scenario include DTH satellite television broadcasting services and VSAT services.

Non-geostationary Satellite Communication Systems

• Non-geostationary satellite communication systems are emerging to provide mobile communication services as well as other services like messaging, video, fax and data communication.
• Constellations of satellites orbiting in LEO or MEO orbits can provide

global mobile communication services.

• The cost of building such a constellation of satellites is huge as compared to having a geostationary satellite.
• Therefore, these systems have not made great progress and are still in the developmental stage.
• IRIDIUM, Orbcomm, Globalstar and ICO systems are some of the non-

geostationary satellite communication systems.

Frequency Bands

• Satellite communication employs electromagnetic waves for transmission of information between Earth and space.
• The bands of interest for satellite communications lie above 100MHz including the VHF, UHF, L, S, C, X, Ku and Ka bands.

Payloads

• Transponder is the key payload of any communication satellite. A brief outline on the basic satellite communication link set-up would not be out of place.
• Basic elements of a satellite communication system include the ground segment and the space segment.
• The ground segment comprises the transmitting and the receiving Earth stations together with their associated instruments, antennae, electronic circuits, etc.

Payloads

• These Earth stations provide access to the space segment by transmitting and receiving information from the satellite, interconnect users with one another and with the terrestrial network.
• The space segment comprises one or more satellites, which act as repeater stations providing point-to-point, point-to-multipoint or multipoint interactive services.

Basic elements of a satellite communication system

Satellite versus Terrestrial Networks

• Satellites initially conceived to provide support services to terrestrial communication networks, have made a great deal of progress in the last fifty years.
• Satellites have established themselves as a pioneering element of communication networks.
• With the advances made in the field of terrestrial communication network technology, like the advent of fibre optic technology, satellites are facing tough competition from the terrestrial networks.
• When compared with each other, both satellites as well as terrestrial networks have certain advantages and disadvantages w.r.t. each other.

Advantages of Satellites Over Terrestrial Networks

Satellites offer certain advantages over terrestrial networks. Some of the advantages are as follows:

1. Broadcast property – wide coverage area.
• Satellites by virtue of their very nature, are an ideal means of transmitting information over vast geographical areas.
• This broadcasting property of satellites is fully exploited in point-to- multipoint networks and multipoint interactive networks.
• The broadcasting property is one of the major plus points of satellites over terrestrial networks, which are not so well suited for broadcasting applications.

Advantages of Satellites Over Terrestrial Networks

2. Wide bandwidth – high transmission speeds and large transmission capacity.
• Over the years satellites have offered greater transmission bandwidths and hence more transmission capacity and speeds as compared to terrestrial networks.
• However with the introduction of fibre optic cables into terrestrial cable networks, they are now capable of providing transmission capabilities comparable to those of satellites.

Advantages of Satellites Over Terrestrial Networks

3. Geographical flexibility – independence of location.
• Unlike terrestrial networks, satellite networks are not restricted to any

particular configuration.

• Within their coverage area, satellite networks offer an infinite choice of routes and hence they can reach remote locations having rudimentary or nonexistent terrestrial networks.
• This feature of satellite networks makes them particularly attractive to Third World countries and countries having difficult geographical terrains and unevenly distributed populations.

Advantages of Satellites Over Terrestrial Networks

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4. Easy installation of ground stations.
• Once the satellite has been launched, installation and maintenance of

satellite Earth stations is much simpler than establishing a terrestrial infrastructure, which requires an extensive ground construction plan.

• This is particularly helpful in setting up temporary services.
• One fault on the terrestrial communication link can put the entire link

out of service, which is not the case with satellite networks.

5. Uniform service characteristics.
• Satellites provide a more or less uniform service within their coverage area, better

known as a ‘footprint’.

• This overcomes some of the problems related to the fragmentation of service that result from connecting network segments from various terrestrial telecommunication operators.

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• Immunity to natural disaster.
• Satellites are more immune to natural disaster such as floods,

earthquakes, storms, etc., as compared to Earth-based terrestrial networks.

7. Independence from terrestrial infrastructure.

• Satellites can render services directly to the users, without requiring a terrestrial interface.
• Direct-to-home television services, mobile satellite services and certain

configurations of VSAT networks are examples of such services.

• In general, C band satellites usually require terrestrial interfaces,

whereas Ku and Ka band systems need little or no terrestrial links.

8. Cost aspects – low cost per added site and distance insensitive costs.
• Satellites do not require a complex infrastructure at the ground level

hence the cost of constructing a receiving station is quite modest – more so in case of DTH and mobile receivers.

• The cost of satellite services is independent of the length of the transmission route, unlike the terrestrial networks where the cost of building and maintaining a communication facility is directly proportional to the distances involved.

Disadvantages of Satellites with Respect to Terrestrial Networks

For certain applications, satellites are at disadvantage with respect to terrestrial networks:

1. Transmission delay.
• Transmission delays of the order of a quarter of a second are involved in

transmission of signals from one Earth station to another via a geostationary satellite.

• It may be mentioned here that for satellite-based data communication services, the data communication protocols that require acknowledgement feedback further add to the delay.
• GEO satellites are not suited for certain applications like interactive media which require small transmission delays. Large transmission delays also have an adverse impact on the quality of voice communication and data transmission at high data rates.

2. Echo effects.
• The echo effect, in which the speaker hears his or her own voice, is more predominant in satellite-based telephone networks as compared to terrestrial networks.
• This is due to larger transmission delays involved in the case of satellites.
• With the development of new echo suppressors, satisfactory link quality has been provided in the case of single-hop GEO satellite networks.
• For double-hop GEO networks, the problem of echo still exists.

3. Launch cost of a satellite.

• Although the cost of a satellite ground station is less than that of terrestrial networks and the cost of satellite services are independent of the distances involved, the cost of launching a satellite is huge.

Satellite Telephony

• Satellites provide both long distance point-to-point trunk telephony

services as well as mobile telephony services, either to complement or to bypass terrestrial networks.

• Potential users of these services include international business travellers and people living in remote areas.
• Satellite telephones either allow the users to access the regular terrestrial telephone network or place the call through a satellite link.
• Satellite telephony networks employ point-to-point duplex satellite links enabling simultaneous communication in both the directions.
• Single GEO satellites or a constellation of LEO, MEO and GEO

satellites are used for providing telephony services.

Satellite Telephony

• Telephone satellite links generally employ circuit-switched systems offering a constant bit rate services, but only for the limited duration of the call.
• Sometimes dedicated or preassigned bandwidth services are used, in which the communication is maintained continuously for an extended period of time, for heavy telephone trucking applications.
• Some of the major satellite systems offering voice services are Intelsat, Eutelsat, Inmarsat, Globalstar, Iridium, ICO, Ellipso and Odyssey systems.

Satellite Telephony

Various steps in making a call through a satellite network are outlined below.

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• The user lifts the receiver when he or she wants to make a call.
• This sends a request to the local Earth station, which in turn sends a service request to the master station.
• If the master station is able to provide the satellite capacity, it sends a confirmation signal to the local Earth station, resulting in a dial tone in the telephone instrument.
• The user then dials the destination number, which is transferred to the control station, which determines the destination Earth station and signals it that a connection needs to be established.

Satellite Telephony

4. The destination Earth station then signals the called party of the

incoming call by ringing that telephone instrument.

5. The satellite capacity is allocated to the connection and the telephone link is established once the called party lifts the handset.
6. Once the conversation is over, the calling party hangs up the receiver, hence indicating to the local Earth station to terminate the call.

Satellite point-to-point telephone networks (PBX, private branch exchange; PSTN, public switched telephone network)

Satellite Television

• Satellite television is the most widely used and talked about application area of communication satellites.
• It accounts for about 75%of the satellite market for communication

services.

• Satellite television basically refers to the use of satellites for relaying TV programmes from a central broadcasting centre to a large geographical area.
• Satellites, by their very nature of covering a large geographical area, are perfectly suited for TV broadcasting applications.
• As an example, satellites like GE and Galaxy in the US, Astra and Hot Bird in Europe, INSAT in India and JCSAT (Japanese communications satellite) and Superbird in Japan are used for TV broadcasting applications.

Satellite Television

• The five Hot Bird satellites provide 900 TV channels and 560 radio stations to 24 million users in Europe.
• Other means of television broadcasting include terrestrial TV broadcasting and cable TV services.
• Satellites can provide TV transmission services either directly to the users or in conjunction with the cable and terrestrial broadcasting networks.

Satellite Television

A Typical Satellite TV Network

• Satellite television employs GEO satellites acting as point-to-

multipoint repeaters receiving a certain telecast from the transmission broadcasting centre and retransmitting the same after frequency translation to the cable TV operators, home dishes, etc., lying within the footprint of the satellite.

A Typical Satellite TV Network

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• Satellites can provide TV programmes either directly to the users (direct-to-home television) or indirectly with the help of cable networks or terrestrial broadcasting networks, where the satellite feeds the signal to a central operator who in turn transmits the programmes to the users either using cable networks or through terrestrial broadcasting.
• A typical satellite TV network, like any other satellite network, can be divided into two sections: the uplink section and the downlink section.

Uplink section of satellite TV networks

Satellite–Cable Television

• Cable TV refers to the use of coaxial and fibre optic

cables to connect each

house through a point-to- multipoint distribution network to the head end

distribution station.

• Cable TV, originally referred to as CATV (community antenna television) stood for a single head end serving a particular community, like various houses in a large building.

Satellite–Cable Television

• The present day cable TV system is more complex and involves a larger distribution area.
• The head ends receive programming channels from either a local broadcasting link or through satellites.
• The use of satellites to carry the programming channels to the cable

systems head ends is referred to as satellite–cable television.

• The head end in this case consists of various receive-only Earth stations with the capability of receiving telecast from two to six satellites.
• These Earth stations either have multiple receiving antennas or, a single dish antenna with multiple feeds, with each feed so aligned as to receive telecast from a different satellite.

Satellite cable television multiplexed

Satellite–Local Broadcast TV Network

• It is the same as the satellite–cable TV network except for the fact that here the satellite distributes programming to local terrestrial broadcasting stations instead of distributing it to the cable head end stations.
• The broadcasting stations use powerful antennas to transmit the received signals to various users within the line-of-sight (50–150 km) using UHF and VHF microwave bands.
• The users receive these TV signals using directional antennas like yagi antennas, reflector antennas or dipole antennas.

Satellite–Local Broadcast TV Network

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• A typical satellite–local broadcast network is shown in Figure
• A combination of both the satellite-cable TV and the satellite-local

broadcast TV networks is used for distributing TV programmes to the users.

• As an example, one of the possible configurations is where the satellite sends the signals to the local broadcasting stations, which in turn broadcast them to the cable operators.

Typical satellite local-broadcast TV network

Direct-to-Home Satellite Television

• Direct-to-home (DTH) satellite television refers to the direct reception of satellite TV programmes by the end users from the satellite through their own receiving antennas.
• DTH services can be broadly classified into two types, namely the television receive-only (TVRO) and the direct broadcasting satellite (DBS) services, depending upon the frequency band utilized and the size of the receiving antennas.
• TVRO systems operate in the C band whereas the DBS systems operate in the Ku band. In the present context, when DTH systems are mentioned

more often than not it refers to the DBS systems only.

Direct-to-home satellite television

Direct-to-Home Satellite Television

Satellite Radio

• A satellite providing high fidelity audio broadcast services to the broadcast radio stations is referred to as a satellite radio and is a major revolution in the field of radio systems.
• Sound quality is excellent in this case due to a wide audio bandwidth of 5–15 kHz and low noise provided over the satellite link.
• Satellite radio like the satellite TV employ GEO satellites and the network arrangement for the satellite radio is more or less identical to that used for TV broadcasting.

Satellite Radio

• Using point-to-multipoint connectivity, the audio signals from various music channels, news and sports centres are transmitted by the satellite to a conventional AM or FM radio station.
• The signal is then de-multiplexed and the local commercials and other information is added here in the same way as in a TV network and then sent to the users using terrestrial broadcasting topology.
• The satellite can also transmit the signal directly to the user’s radio sets.
• Some of the major providers of satellite radio services include Sirius and XM Radio of the USA.

Regional Satellite Systems

• One of the drawbacks of international satellite systems is that they are not optimized to the needs of the individual countries.
• The first step to meet the focused needs of the countries was to have a

regional system that provided services to countries on a regional basis rather than on a global basis. Regional satellite missions were established with the aim of strengthening the communication resources of the countries belonging to the same geographical area.

• Some of the regional satellite systems include Eutelsat, Arabsat, AsiaSat, Measat, ACeS (Asia cellular satellite), Thuraya, etc.
• EUTELSAT operates a fleet of satellites that provide communication services to Europe, the Middle East, Africa and large parts of the Asian and American continents.

Regional Satellite Systems

• Arabsat satellites provide satellite communication services to the Middle East, Africa and large parts of Europe.
• The Asia Satellite Telecommunications Company Limited (AsiaSat)

and Measat systems are Asia’s regional satellite operators, providing satellite services to the Asia Pacific region.

• ACeS is another satellite-based regional communication system providing services to Asia.
• It provides fully digital video, voice and data services throughout Asia.
• The Thuraya system provides mobile communication services to the

Middle East, North and Central Africa, Europe, Central Asia and the

Indian subcontinent.

National Satellite Systems

• National satellite systems, also referred to as domestic satellite systems, provide services to a particular country.
• National satellite systems were originally established by developed countries like the USA, USSR and Canada to serve their country’s population according to their specific needs.
• In addition to these developed countries, some developing nations like India, China, Japan, etc., also have their own national satellite systems. Some of the domestic satellite systems include Galaxy, Satcom, EchoStar and Telestar of the USA, Brasilsat of Brazil, INSATof India, Optus of Australia and Sinosat of China.

INSAT (Indian National Satellite)

• Owned by the Indian Department of Space, named the Indian Space

Research Organization (ISRO), INSAT is one of the largest domestic

communication satellite networks in the world, providing services in

the areas of telecommunications, television broadcasting, mobile satellite services and meteorology including disaster warning.

• INSAT is a joint venture of the Department of Space (DOS), Department of Telecommunications (DOT), Indian Meteorological Department (IMD), All India Radio (AIR) and Doordarshan.

INSAT (Indian National Satellite)

• Making a modest beginning with the launch of INSAT-1A in 1982,

the INSAT satellite programme has come a long way today.

• INSAT-1A belonged to the INSAT-1 series, further comprising INSAT-1B, 1C and 1D satellites.
• The INSAT-1 series was followed by INSAT-2 and INSAT-3 series of satellites. They were superceded by the INSAT-4 series of satellites.