Digital Satelite Communications Tutorial

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HTEL 103DIGITAL SATELLITE

COMMUNICATIONS


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Lecture 1

INTRODUCTION

History of the evolution of satellites began inOctober 1945 when a famous science fictionwriter Arthur Clark proposed the feasibility ofestablishing a communication satellite in a

geostationary orbit. In his article he discussedhow a geostationary orbit satellite would lookstatic to an observer on Earth within thesatellites coverage, thus providing an

uninterrupted communication service across theglobe. This marked the beginning of the satelliteera.


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Space era started in 4 October 1957 with the launching of the 1-startificial satellite Sputnik-1 by Soviet Union from BaikonurCosmodrome. It orbited Earth once every 96 minutes for 92 days.Sputnik-2 was launched on 3 November 1957 and carried a dogcalled Laika the first living creature to orbit Earth. Sputnik-3

launched in 15 May 1958. The 1-st satellite Telstar-1 was launched on 10 July 1962 by AT &T

the American telecommunications giant.

In 1965, the 1-st commercial geostationary satellite INTELSAT 1(Early Bird) and the 1-st Soviet communication satellite of theMOLNYA series was launched.

Since then, countless satellites have been placed into earth orbit.These are: communication satellites, weather forecasting satellites,Earth observation satellites, navigational satellites and militarysatellites.


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What is a satellite?

A satellite in general is any natural or artificial

body moving around a celestial body such as

planets and stars. In this case we are referring

only to artificial satellites orbiting the planetEarth. These satellites are put into the desired

orbit and have payloads depending upon the

intended application. A satellite while in theorbit performs its designated role throughout

its lifetime.


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Satellite system

The satellite system is composed of the following:

The space segment contains one or several active and sparesatellites organised in a constellation.

The control segment consists of all ground facilities for the controland monitoring of the satellites, also TTC ( tracking, telemetry and

command) stations and for the management of the traffic and theassociated resources onboard the satellite.

The ground segment consists of all the traffic earth stations.Depending on the type of service considered , these stations can beof different size.

Figure 1.1 gives an overview of a satellite communication system andillustrates its interfacing with terrestrial entities.

Table 1.1 gives examples of traffic Earth stations.


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Table 1.1 services from different types

of traffic earth stationType of service Type of earth station Typical size (m)

Point-to-point Gateway, hub 2-10

VSAT 1-2

Broadcast/multicast Feeder station 1-5

VSAT 0,5-1,0

Collect VSAT 0,1-1,0

Hub 2-10

Mobile Handset, portable, mobile 0,1-0,8

Gateway 2-10


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Earth stations

They are classified as:

User stations, such as handsets, portables, mobilestations and VSATs (very small aperture

terminals), which allow customer a direct accessto the space segment.

Interface stations (gateways),which interconnectthe space segment to a terrestrial network.

Service stations such as hub/feeder stationswhich collect/distribute information from/to userstations via the space segment [see figure 1.1].


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Cont..

Communications between users are set up through userterminals which consist of equipment such astelephone sets, fax machines and computers that areconnected to the terrestrial network or to the user

stations, or are part of the user station. The path froma source user terminal to a destination user terminal isa simplex connection. There are two basic schemes:single connection per carrier (SCPC), where themodulated carrier supports one connection only, and

multiple connections per carrier (MCPC), where themodulated carrier supports several time or frequencymultiplexed connections. Interactivity between twousers requires a duplex connections.


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A connection between a service provider and

user goes through a hub or feeder station. A

connection from a gateway, hub or feeder

station to a user terminal is called a forwardconnection. The reverse connection is the

return connection. Both connections entail an

uplink and a downlink, and possibly one ormore inter satellite links.


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A link between a transmitting and a receivingterminal consists of a radio or optical modulatedcarrier. The performance of the transmittingequipment is measured by its effective isotropic

radiated power (EIRP), which is the power fed tothe antenna multiplied by the gain of the antennain the considered direction. The performance ofthe receiving equipment is measured by G/T, the

ratio of the antenna receive gain, G, in theconsidered direction and the system noisetemperature, T. G/T is called the receivers figureof merit.


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The types of link shown in figure 1.1 are:

The uplinks from the earth stations to thesatellites

The downlinks from the satellite to the earthstations

The inter satellite links, between the satellites.

Uplinks and downlinks consist of radio frequency

modulated carriers, while inter satellite links canbe either radio frequency or optical. Carriers aremodulated by baseband signals conveyinginformation for communications purposes.


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The link performance can be measured by the

ratio of the received carrier power, C, to the

noise power spectral density, N [C/N].

Another important parameter for the designof a link is the bandwidth, B, occupied by the

carrier. This bandwidth depends on the

information data rate, the channel codingrate, and the type of modulation used to

modulate the carrier.


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The space segment

The satellite consists of the payload and the

platform. The payload consists of the receiving

and transmitting antennas and all the

electronic equipment which supports thetransmission of the carriers.

They are two types payload and they are shown

in figure 1.2.


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Transparent payload

In a transparent payload carriers are power amplified andfrequency down converted. Power gain is the order of100-130 dB. Frequency conversion is required toincrease the isolation between the receiving input and

the transmitting output. The overall bandwidth is splitinto several sub bands. The carriers in each sub bandamplified by a dedicated power amplifier.

The amplifying chain associated with each sub band iscalled a satellite channel or transponder. The

bandwidth splitting is achieved by the set of filterscalled the input multiplexer (IMUX). Power amplifiedcarriers are recombined in the output multiplexer(OMUX).


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The transparent payload belongs to a single beamsatellite where each transmit and receive antennagenerates one beam only. One could also considermultiple beam antennas. The payload would then

have as many inputs/outputs asupbeams/downbeams. Routing of carriers fromone upbeam to a given downbeam implies eitherrouting through different satellite channels,

transponder hopping, depending on the selecteduplink or on-board switching with transparent onboard processing.


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Regenerative payload

In a multiple beam regenerative payload the uplinkcarriers are demodulated. The availability of thebaseband signals allows on-board processing and

routing of information from upbeam to adownbeam through on-board switching atbaseband. The frequency conversion is achievedby modulating on-board generated carriers at

downlink frequency. The modulated carriers arethen power amplified and delivered to thedestination downbeam.


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Figure 1.3 illustrates a multiple beam satellite

antenna and its associated coverage areas.

Each beam defines a beam coverage area

(footprint) on the earth surface. Theaggregate beam coverage areas define the

multibeam antenna coverage area. A given

satellite may have several multiple beamantennas, and their combined coverage

defines the satellite coverage area.


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Figure 1.4 shows the concept of instantaneoussystem coverage and long term coverage.

The instantaneous system coverage consists

of the aggregation at a given time of thecoverage areas of the individual satellitesparticipating in the constellation.

The long term coverage is the area on theearth scanned over time by the antennas ofthe satellites in the constellation.


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The coverage area should encompass the servicezone, which corresponds to the geographicalregion where the stations are installed. For real-time services, the instantaneous system

coverage should at any time have a footprintcovering the service zone, while for non-realtime services, it should have long term coverageof the service zone.

The platform consists of all the subsystems whichpermit the payload to operate. Table 1.2 liststhese subsystems.


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Table 1.2 Platform subsystem

Subsystem Principal functions Characteristics

Attitude and orbit control

(AOCS)

Attitude stabilisation

Orbit determination

Accuracy

Propulsion Provision of velocity

increments

Specific impulse, mass of

propellant

Electric power supply Provision of electric energy Power, voltage stability

Telemetry, tracking and

command (TTC)

Exchange of housekeeping

information

Number of channels,

security of

communications

Thermal control

Structure

Temperature maintenance

Equipment support

Dissipation capability

Rigidity, lightness


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To ensure a service with a specified availability, a satellitecommunication system must make use of several satellites.A satellite can cease to be available due to a failure orbecause it has reached the end of its lifetime.

Reliability is a measure of the probability of a breakdownand depends on the reliability of the equipment

The lifetime is conditioned by the ability to maintain thesatellite on station in the nominal attitude, and depends onthe quantity of fuel available for the propulsion system andattitude and orbit control. In a system, provision isgenerally made for an operational satellite, a backupsatellite in orbit and a backup satellite on the ground. Thereliability of the system will involve not only the reliabilityof each satellites but also the reliability of launching.


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The ground segment

Consists of all the earth stations. These are oftenconnected to the end-users terminal by a terrestrialnetwork or, in the case of small stations (VSAT) directlyconnected to the end users terminal. Stations are

distinguished by their size which varies according tothe volume of traffic to be carried on the satellite linkand the type of traffic( telephone, television, data). Thelargest are equipped with antennas of 30 m diameter (INTELSAT). The smallest have 0,6 m antennas or even

smaller (0,1m) antennas. Some stations both transmitand receive. Others are receive-only (RCVO) stations.

Figure 1.5 shows the typical architecture of an earthstation for both transmission and reception.


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Lecture 2

SATELLITE ORBITSThe orbit is the trajectory followed by the satellite. The

trajectory is within a plane and shaped as an ellipse with amaximum extension at the apogee and a minimum at theperigee. The satellite moves more slowly in its trajectory asthe distance from the earth increases.

The most favourable orbits are as follows: Elliptical orbits inclined at an angle of 64 degree with

respect to the equatorial plane. It enables the satellite tocover regions of high latitude for a large fraction of theorbital period as it passes to the apogee. This type of orbithas been adopted by MOLNYA satellites with period of 12hours.


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Figure 1.6 shows the geometry of the orbit. The satelliteremains above the regions located under the apogeefor a time interval of 8 hours. Continuous coverage canbe ensured with 3 phased satellites on different orbits.

Circular low earth orbits (LEO). The altitude of thesatellite is constant and equal to several hundreds ofkm. The period is 1,5 hour. With near 90 degreeinclination this orbit guarantees a world wide long

term coverage as a result of the combined motion ofthe satellite and earth rotation , as shown in Figure 1.7.


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This type of orbit is chosen for observation satellites (SPOT,GLOBALSTAR, ECCO).

Circular medium earth orbits (MEO), also named intermediatecircular orbits (ICO). Their altitude 10000 km and an inclination 50degree. The period is 6 hours. With constellations of 10-15

satellites, a continuous coverage of the world is allowing worldwidereal-time communications (IMMARSAT).

Circular orbits with 0 inclination (equatorial orbits). The mostpopular is the geostationary satellite orbit. The period is equal tothe rotation of the earth. The satellite thus appears as a point fixedin the sky and ensures continuous operation as a radio relay in real

time for the area of visibility of the satellite ( 43% of the earthssurface).

Hybrid systems include combinations of circular and elliptical orbits(ELLIPSO).


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The choice of orbit depends on the nature of the mission, theacceptable interference and the performance of the launchers:

The extent and latitude of the area to be covered.

The elevation angle.

Transmission duration and delay.

Interference The performance of launchers.

The geostationary satellite is the most popular. At the present timethere are around 600 geostationary satellites in operation withinthe 360 degree of the whole orbital arc.

QUESTION: how many geostationary satellites are in operation today?


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Orbits and related issues

In order to understand the satellites motion aroundthe earth there is a need of discussing thefollowing aspects.

Keplerian orbits are named after Kepler who

established, in 17 century, that the trajectories ofplanets around the sun were ellipses and notcombinations of circular movements as had beenthought since the time of Pythagoras. Keplerian

movement is the relative movement of two pointbodies under the sole influence of theirNewtonian attractions.


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Keplers laws

The planets move in a plane; the orbitsdescribed are ellipses with the sun at onefocus (1602).

The vector from the sun to the planet sweepsequal areas in equal times (1605).

The ratio of the square of the period T ofrevolution of a planet around the sun to thecube of the semi-major axis a of the ellipse isthe same for all planets (1618).


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Newtons law

Newton extended the work of Kepler and, in 1667,discovered the universal law of gravitation. This lawstates:

Two bodies of mass m and M attract each other with a

force which is proportional to their masses andinversely proportional to the square of the distance rbetween them:

F = GM m/r

where G= 6,672 x 10 m kg s is the universalgravitation constant,

M = 5,974 x 10 kg mass of the earth.

GM = = 3,986 x 10 m/s.


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From the universal of gravitation and using the

work of Galileo, Newton proved Keplers laws

and identified the assumptions ( the problem

of two spherical and homogeneous bodies).He also modified these laws by introducing

the concept of orbit perturbations to take

account of actual movements.


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Relative movement of two point

bodies

The movement of satellites around the earth observesKeplers laws to a 1-st approximation. The proof resultsfrom Newtons law and the following assumptions:

The mass m of the satellite is small with respect to the

mass M of the earth which is assumed to be sphericaland homogeneous.

Movement occurs in free space; the only bodiespresent are the satellite and the earth.

The actual movement must take account of the fact thatthe earth is neither spherical nor homogeneous, theattraction of the sun and moon and other perturbingforces.


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Keplers laws treat the relative movement of two

bodies by applying Newtons law. It is

convenient to consider the body of greater

mass to be fixed, with the other movingaround it ( as the force of attraction is the

same for the two bodies, the resulting

acceleration is much greater for the body oflow mass than for the other).


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Perturbations of the orbit

Movement of the satellite in its orbit is determined by the forcesacting on the centre of mass. With the Keplerian hypotheses , thereis only the attraction of a central, spherical and homogeneous bodywhich defines a conservative field of forces.

Perturbations of the orbit are the result of various forces which are

exerted on the satellite other than the force of attraction of thecentral, spherical and homogenous body. These forces consist of:

The contribution of the non-spherical components of terrestrialattraction.

The attraction of the sun and moon.

Solar radiation pressure.

Aerodynamic drag.

Motor thrust.


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Lecture 3

SATELLITE FREQUENCY BANDS

Radio regulations are necessary to ensure an efficient andeconomical use of the radio-spectrum by terrestrial andsatellite communications systems. While so doing, thesovereign right of each state to regulate itstelecommunications must be preserved. It is the role of the

International Telecommunications Union (ITU) to promote,coordinate and harmonise the efforts of its members.

ITU, a United Nations organ, operates under the conventionadopted by its member administrations . The ITU publishersthe Radio Regulations (RR), which are reviewed by the

delegates from ITU member administrations at periodicWorld/Regional Radio Conferences (WRC/RRC).


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From 1947 to 1993 the technical and operational matterswere administered by two committees: the CCIR(Comit Consultatif International desRadiocommunications) and the CCITT (ComitConsultatif International Tlgraphique etTlphonique). The International FrequencyRegistration Board (IFRB) was responsible for theexamination of frequency-use documentationsubmitted to the ITU by its member administrations, in

compliance with the Radio Regulations, and formaintaining the Master International FrequencyRegister (MIFR).


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Since 1994 the ITU has been reorganised into threesectors:

The Radiocommunications Sector (ITU-R) deals with allregulatory and technical matters that were previously

handled respectively by the IFRB and CCIR. The Telecommunications Standardisation Sector (ITU-T)

continues the work of the CCITT, and those studies bythe CCIR dealing with the interconnection ofradiocommunications systems with public networks.

The Development Sector (ITU-D) acts as a forum andan advisory structure for the harmonious developmentof communications in the world.


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The Radio-communications Regulations refer to the following spaceradio-communications services, defined as transmission and/orreception of radio waves for specific telecommunicationsapplications.

Fixed Satellite Service (FSS)

Mobile Satellite Service (MSS) Broadcasting Satellite Service (BSS)

Earth Exploration Satellite Service (EES)

Space Research Service (SRS)

Space Operation Service (SOS)

Radio-determination Satellite Service (RSS) Inter-Satellite Service (ISS)

Amateur Satellite Service (ASS)


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Frequency bands are allocated to the above radio-communications services to allow compatible use. Theallocated can be either exclusive for a given service, orshared among several services. Allocations refer to the

following division of the world into three regions: Region 1: Europe, Africa, the Middle East, the former

USSR.

Region 2: the Americas

Region 3: Asia and Oceania, except the Middle Eastand the former USSR .

Table 3.1 shows the frequency allocations.


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Table 3.1 Frequency allocations

Radiocommunications service Frequency bands for

uplink/downlink (GHz)

Used

terminology

(band)

Fixed satellite service (FCC) 6/4 C band

8/7 X band

14/12-11 Ku band

30/20 Ka band

50/40 V band

Mobile satellite service (MSS) 1.6/1.5 L band

30/20 Ka band

Broadcasting satellite service (BSS) 2/2,2

12

2,6/2,5

S band

Ku band

S band

L 4


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Lecture 4

EARTH STATIONS

An Earth station is a terrestrial terminal station mainlylocated on the Earths surface. It could be evenairborne or maritime. Those located on the Earthssurface could be fixed or mobile.

Earth stations are generally categorised on the basis oftype of services or functions provided by them thoughthey may sometimes be classified according to the sizeof the dish antenna. Based on the type of serviceprovided they are classified into the following:

Fixed Satellite Services (FSS) Earth Stations Broadcast Satellite Services (BSS) Earth Stations

Mobile Satellite Service (MSS) Earth Stations


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Earth stations are also categorised as: Single function stations

Gateway stations

TeleportsThe general organisation of an earth station isshown in figure 4.1. It consists of an antenna sub-system, with an associated tracking system, a

transmitting section and a receiving section. Italso includes equipment to interface with theterrestrial network together with variousmonitoring and electricity supply installations.


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The antenna is generally common to

transmission and reception for reasons of cost

and bulk. Separation of transmission and

reception is achieved by means of a duplexer.Antennas are often capable of transmitting

and receiving on orthogonal polarisations (

circular or linear) in order to permit re-use offrequencies.


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The tracking system keeps the antenna pointing inthe direction of the satellite in spite of therelative movement of the satellite and thestation. Even in the case of a geostationarysatellite, orbital perturbations cause apparentdisplacements of the satellite which are,however, limited to the station-keeping box.

Furthermore the station can be installed on amobile vehicle whose location and direction varywith time.


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The performance required of the tracking system varies inaccordance with the characteristics of the antennabeam and the satellite orbit. For small antennas, thetracking system can be eliminated (fixed mounting) andthis enables reduced cost.

The size and the complexity of stations depend on theservice to be provided and the effective isotropicradiated power (EIRP) and the figure of merit of theearth station (G/T) of the satellite. The simplest

stations permit reception only and are equipped witha parabolic antenna which may have a diameter of lessthan 1m. The largest are 1-st built Intelsat Standard Astations with antennas of 32 m diameter.


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The characteristics which determine the radio-frequencyperformance of earth stations occur in the link budgetexpressions for the uplink and the downlink budget.

In the early years, international satellite communications

services were provided by international organisations.These organisations (now privatised) have definedvarious standards for earth stations operating inconnection with the satellites the operate. These

standards specify numerous parameters, e.g. the figureof merit G/T, for different services and applications.


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Satellite system standards

The characteristics of earth stations used in the INTELSATnetwork grouped into INTELSAT Earth Station Standards(IESS) modules (according to IESS -101, Rev 61).

The EUTELSAT Earth Stations Standards (EESS) arepublished by EUTELSAT to provide users with a common

source of reference for performance characteristicsrequired from earth stations and associated equipment foraccess to the EUTELSAT space segment and theestablishment of communication links.

IMMARSAT for mobile maritime telecommunication

services also has its own standards.


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The antenna subsystem

The characteristics required for an earth stationantenna are as follows:

High directivity, in the direction of the nominal

satellite position (for useful signals) Low directivity in other directions, in particular

that of nearby satellites to limit interference withother systems.

Antenna efficiency as high as possible for bothfrequency bands( uplink and downlinks) on whichthe antenna operates.


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High isolation between orthogonal

polarisations.

The lowest possible antenna noise

temperature.

Continuous pointing in the direction of the

satellite with the required accuracy.

Limitation of the effect of local meteorological

conditions on the overall performance.


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The radio-frequency subsystem

It contains: On the receiving side , low noise amplifying equipment

and equipment for routing the received carriers to thedemodulating channels.

On the transmitting side, equipment for coupling thetransmitters carriers and power amplifiers.

In each direction, frequency converters form the interfacewith the telecommunications subsystem whichoperates at intermediate frequency.

To satisfy the objectives of reliability and specifiedavailability, it is often necessary to back up the radio-frequency equipment of an earth station.


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Communication subsystems

The communication subsystem on the transmission sideconsists of equipment for converting baseband signalsto radio-frequency carriers for amplification. On thereception side, it converts the carriers at the output of

the low noise amplifier to baseband signals.The baseband signal may be either analogue or digital. In

the analogue case, it can be a telephone channel, amultiplex of telephone channels, a TV signal or a sound

programme. In the digital case, it is usually in the formof a bit stream which corresponds to one or a multiplexof voice channels or data packets.


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The functions on the receiving side are:

Conversion of the carrier frequency (RF) to anintermediate frequency (IF)

Filtering and equalisation of group propagation delay.

Carrier demodulation.

In the case of transmission using TDMA, it is necessary tore-establish a continuous digital stream from thepackets of the received frame. If TDMA is used on thetransmission side, it is necessary to group the bits ofthe baseband signal into packets which are inserted inthe proper time slots provided in the frame.


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The network interface subsystem

This is the interface between baseband signalsproduced by, or destined for, the communicationcommon equipment and baseband signals in theterrestrial network format. The main functionsare multiplexing (demultiplexing) of telephonechannels, which may include digital speechinterpolation (DSI) and channel multiplication(DCME), suppression (or cancellation) of echoesand various functions particular to single channeltransmission (SCPC).

Monitoring and control; Auxillary


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Monitoring and control; Auxillary

equipment

Its purpose is monitoring of correct operation and controlof the earth station.

The monitoring, alarm and control equipment has thefollowing purposes:

To provide operators with the necessary informationfor monitoring and controlling the station andmanaging the traffic.

To initiate alarms in case of incorrect operation or anincident affecting the main station equipment or the

link performance and permit identification of theequipment which is involved.

To permit the control of the station equipment.


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Monitoring and control functions can be provided

locally, in a centralised manner, or under the

control of a computer.

With centralised or computer-aided management,it is possible to have a station without permanent

staff. Monitoring and control information can be

routed to a distant common network centre by

means of dedicated terrestrial lines or service

channels on the satellite links.


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Developed new systems opened the possibility oftelecommunication services in areas as businesscommunication, rural telecommunication, videodata distribution, Internet access, interactive

transfers and communication with mobiles.Many of these systems use small earth stations

which are installed on the user premises andprovide direct telephone links (rural

communication), data communications with verysmall aperture terminal (VSAT) on privatenetworks , Internet access, and video reception.

Lecture 5


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Lecture 5

THE COMMUNICATION PAYLOAD

The payload can be considered as the brain of thesatellite that performs its intended function. The basicpayload in the case of a communication satellite is atransponder, which acts as areceiver/amplifier/transponder. A transponder can beconsidered to be a microwave relay channel that alsoperforms the function of translation. A transponder is acombination of elements like sensitive high gainantennas for transmit-receive functions, a subsystem ofrepeaters, filters, frequency shifters, low noiseamplifiers (LNAs), frequency mixers and poweramplifiers


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The payload consists of two parts with well-definedinterfaces- the repeater and the antennas. Therepeater is the electronic equipment whichperforms a range of functions on the carriers

from the receiving antenna before deliveringthem to the transmitting antenna. The repeaterconsists of several channels( transponders) whichare individually dedicated to sub-bands withinthe overall payload frequency band. Thearchitecture differs from single-beam transparentrepeater, regenerative repeater and multi-beampayload.


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Functions of the payload

To capture the carriers transmitted, in a given frequency band andwith a given polarisation, by the earth stations of the network onthe surface of the earth and are seen from the satellite within anangle which determines the angular width of the satellite antennabeam.

To capture as little interference as possible.

To amplify the received carriers while limiting noise and distortionas much as possible.

To change the frequency of the carriers received on the uplinks tothat on the downlinks .

To provide the power required in a given frequency band at the

interface with the transmitting antenna. To radiate the carriers in a given frequency band and with a given

polarisation to a given region on the surface of the earth.


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Characteristic parameters

The transmitting and receiving frequency bands andpolarisations for the various repeater channels.

The transmit and receive coverages.

The effective isotropic radiated power (EIRP) or the flux

density achieved in a given region. The power flux density required at the satellite

receiving antenna in order to produce the performancespecified at the repeater channel output.

The figure of merit (G/T) of the receiving system in agiven region.

The reliability after N years for a specified number ofchannels in working order.


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Regenerative repeater

The specific equipment: demodulating and re-modulating equipment and the basebandsignal processing equipment. The signals

carried by a regenerative transponder aredigital. The specific equipment is designed toprocess digital signals.

Demodulation can be either coherent or

differential according to the digital modulationanticipated for the uplink.


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Multibeam antenna payload

It features several antenna beams which providecoverage of different service zones. As received onboard the satellite, the carriers appear at the outputsof one or more receiving antennas. The carriers at therepeater must be fed to the various transmittingantennas. Two basic configurations are possible:

Each receiving transmitting beam combinationconstitutes an independent network

The stations within different coverage regions belong

to a unique network and station-to-station connectionsmust be established between any pair of stationssituated in different service zones.


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Flexible payloads

They are reconfigurable in coverage, frequency

plan and routing, an efficient answer the

following needs:

Universal payload for in-orbit replacement ofany satellite with continuity of service

Reconfigurable payload to follow market

evolution

Standard payload for low cost and fast.

Lecture 6


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Lecture 6

THE PLATFORM

The organisation of a communications satellite platform is determinedby the following:

The requirements of the communications payload.

The nature and effects of the space environment

The performance of launchers and the constraints which they

impose.The communications mission conditions the design of the payload. The

platform is concerned this design results in requirements such asthe electrical power to be provided, the payload mass that can beaccommodated, the antenna pointing accuracy, the thermal powerto be extracted, the space required for equipment mounting, the

number of telemetry, tracking and command (TTC) channels and soon.


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The nature and effects of the space environmentaffect orbit control, subsystem organisation andthe choice of material and components.

A list of platform subsystems were given in Table

1.2 of Lecture 1. Three common characteristicsare not indicated but are essential and should beemphasised:

Minimum mass

Minimum consumption High reliability.


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Each subsystem is specified and designed for the

particular mission to be fulfilled, taking

account of these three criteria, the technology

used and the data characteristics of othersubsystems. The performance and

specification of a particular subsystem depend

on the presence of other subsystems and thisinfluences the interfaces between subsystems.


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Attitude control

The attitude of the satellite is represented with respect tothe yaw, roll and pitch axes of a local coordinate system(Fig 6.1). This coordinate system is centred on thecentre of mass of the satellite. The yaw axis points inthe direction of the centre of the earth. The role axis is

in plane of the orbit, perpendicular to the first andoriented in the direction of the velocity. The pitch axisis perpendicular to the other two and orientated insuch a way that the coordinate system is regular. In thenominal attitude configuration, the axes of thesatellite-fixed coordinate system are, in principle,aligned with the axes of the local coordinate system.


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Figure 6.1


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The attitude of the satellite is represented by theangles of rotation about the various axesbetween the local coordinate system and thesatellite-fixed coordinate system.

Maintaining attitude is fundamental for the satelliteto fulfil its function.

The role of attitude control usually consists ofmaintaining the mechanical axes in alignment

with a local coordinate system to an accuracydefined by the amplitude of rotation about eachof the axes.


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Maintaining attitude requires two functions: A steering function causes the part of the satellite

which must be oriented toward the earth to turn aboutthe pitch in order to compensate for the apparent

movement of the earth with respect to the satellite. A stabilisation function compensates for the effects of

attitude-disturbing torques. The disturbing torques arecreated by gravitational forces, solar radiation pressure

and interaction between current loops and theterrestrial magnetic field.

d


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Attitude sensors

Sun sensors

Earth sensors

Star sensors

Inertial units

Radio-frequency sensors

Laser detectors

h l b


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The propulsion subsystem

Its role is mainly to generate forces which acton the centre of mass of the satellite. These

forces modify the satellite orbit, either to

ensure injection into a predetermined orbit orto control drift of the nominal orbit. It also

serves to produce torques to assist the

attitude control system. The forces generatedby the propulsion units are reaction forces

resulting from the expulsion of material.

f h


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Types of thrusters

Low power thrusters, which are used forattitude and orbit control.

Medium and high power thrusters, which are

used for orbit changes during the launchphase. Depending on the type of launcher

used, these thrusters form the apogee kick

motor (AKM) and the perigee kick motor(PKM).

Th h i i f h


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The characteristics of thrusters

Low thrust levels

A large number of operating cycles of limited

duration

A cumulative operating time of several

hundreds or thousand of hours.

Lifetime of greater than 15 years.

Ch i l l i


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Chemical propulsion

The principle of chemical propulsion consists ofgenerating gases at high temperature by chemical

combustion of liquid or solid propellants. These

gases are accelerated by the nozzle.Solid propellant motors are reserved for generating

velocity increments for initial injection into orbit.

These motors can be used once only and develop

large thrusts. The specific impulse obtained is of

the order of 295 s.

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Electric propulsion

It involves the use of an electrostatic orelectromagnetic field to accelerate and eject

ionised material. Electric propulsion is an

advanced technology in comparison withchemical propulsion. It is characterised by low

thrusts with a high specific impulse (1000 to

10 000s). Various electric propulsions havebeen developed: electrothermal, plasma and

ionic.

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The electric power supply

This subsystem consists of: A primary source of energy which converts

energy available in another form into electricalenergy ( for civil applications, it consists of a solar

generator) A secondary source of energy (battery) which is

substituted for the primary energy source whenthis cannot fulfil its function, for example in an

eclipse period Conditioning ( regulation and distribution) and

protection circuits.

Lecture 7


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SATELLITE APPLICATIONS

Since the launch of Sputnik-1 over 8000satellites have been launched for different

applications. Based on the intended

applications, the satellites broadly classified ascommunication, navigation, weather

forecasting, earth observation, scientific and

military satellites.

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Communication satellites

The application areas of communication satellites mainlyinclude TV broadcasting, international telephony anddata communication services. Communicationsatellites act as repeater stations that provide eitherpoint-to-point, point-to-multipoint or multipoint

interactive services.Satellite TV refers to the use of satellites for relaying TV

programmes from a point where they originate to alarge geographical area. GEO satellites in point-to-

multipoint configuration are employed for satellite TV.There are two types of satellite TV distributionsystems: the television receive only (TVRO) and thedirect broadcasting satellite (DBS) systems.

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Satellite telephony

In satellite telephony, satellites provide both longdistance point-to-point or trunk telephony services aswell as mobile telephony services, either tocomplement or to bypass the terrestrial networks.

Satellites also provide data communication servicesincluding data, broadcast and multimedia services suchas data collection and broadcasting, image and videotransfer, voice, internet, two-way computerinteractions and database inquiries. Satellites in this

case provide multipoint interactive connectivity,enabling the user terminals to exchange informationwith the central facility as well as other user terminals.

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Cont

Communication satellites can be GEO satellites or aconstellation of LEO, MEO or HEO (highly ellipticalorbit) satellites.

Geostationary satellites used in Intelsat, Immarsat,

Telstar, Asiasat, Arabsat, Galaxy, GE, Superbird, Astra,Eutelsat, Palapa and other satellite systems.

Non-geostationary satellite communication systems areemerging to provide mobile communication services,

messaging, video, fax and data communication.Examples are: IRIDIUM, Orbcomm, Globalstar and ICOsystems.

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Cont

Figure 7.1 shows a variety of satellite point-to-point telephonenetworks having either single user or shared multi user Earthstations.

Various steps in making a call through a satellite network are outlinedbelow. This is just a conceptual explanation, the actual procedure ismuch more complicated.

1. The user lifts the receiver. This sends a request to the local Earthstation, which in turn sends a service request to the masterstation.

2. If the master station is able to provide the satellite capacity, itsends a confirmation signal to the local Earth station, resulting in a

dial tone in the phone.3. The user dials the destination number, which is transferred to the

control station, which determines the destination Earth stationand signals in that a connection needs to be established.


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Figure 7.1

Satellite point-to-point telephone networks.

Cont


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Cont

4. The destination Earth station then signals thecalled party of the incoming call by ringing.

5. The satellite capacity is allocated to theconnection 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, indicating to the localEarth station to terminate the call.

In the case of a telephony network using satelliteconstellation, the call may involve connectionthrough multiple satellites and cross-links.

Domestic fixed satellite service (DFSS)


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Domestic fixed satellite service (DFSS)

Domestic C-band 4/6 GHz satellites fall into three categories based onthe markets they serve.

Cable satellites distribute TV programming to cable head ends tohomes equipped with backyard Television Receive Only (TVRO)dishes

Broadcast satellites distribute network programming to affiliates andsyndicated programming to affiliates and independent stations.

The 3-rd category is used for point-to-point transmission of video anddata signals.

Most new domestic satellites use the higher frequency Ku band forVSAT networks, broadcast TV, and digital audio entertainment

Mobile satellite service (MSS)


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Mobile satellite service (MSS)

This is a service for mobile subscribers using cell handsetscommunicating with LEO satellites.

1-st mobile service experiment was 1977 using NASAssatellite ATS-6. In 1982 1-st civilian mobile satelliteInmarsat was launched. Since then there has been many

other launches. The new 3-rd generation mobile satelliteservices are called Global Mobile Personal CommunicationServices (GMPCS).

GMPCS is a personal communication system providingtransnational, regional or global two-way voice, fax,

messaging, data and broadband multimedia services from aconstellation of satellites accessible with small and easilytransportable terminals. All these systems operate in the LKu and S bands.

Radiodetermination satellite services


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Radiodetermination satellite services

RDSS is a term used in the radio regulations to describe aservice that uses satellites for navigation purposes. The FCChas allocated radio spectrum in the L and S bands for theRDSS.

The U.S. and Russia each operate satellite services that

provide position location information. The U.S.Department of Defence (DOD) operates and maintains theGlobal Positioning System (GPS).

Russia operates Global Navigation Satellite System (GLONASS).Both systems operate on a similar principle of measuring

the time difference of arrival of radio transmissions fromseveral satellites with precisely known orbital element andaccurate clocks.

Cont


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Cont

The GPS constellation of 24 satellites in 8 planesis in figure 7.2 . This provides a minimum of 6

satellites in view at all times, anywhere on the

Earth.Table 7.1 highlights the features of the two

systems.

Fig 7 2


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Fig 7.2

Table 7 1


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Table 7.1

Position location determination using


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satellites

Determining the location of any GPS receiver on theearth requires a solution in three dimensions:latitude, longitude, altitude.

A minimum of 4 satellites is required to determine

these dimensions. GPS also provides a solutionfor velocity information.

Referring to figure 7.3, if the distance (x) is known,we can only determine that the receiver is

located anywhere on the sphere circumscribedaround satellite A. every point on sphere A is thesame distance from the satellite.

Cont


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Cont

If we can measure the distance (y) to the secondsatellite B, the location of the receiver can benarrowed to an area of the universe anywhere onthe circumference (a) formed by the intersection

of spheres A and B.The 3-rd satellite C, is needed to reduce theuncertainty to two points on circumference (a).This is sufficient for determining the receiverlocation because one of the points lies on theEarths surface and the other point lies in outerspace or deep in the Earth.

Cont


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Cont

The computers used in the GPS receiver have variousmethods for distinguishing the correct points from thefalse points. This three-satellite solution would besufficient if we had perfect synchronization of time

between the receiver and the satellite. To understandthe importance of time, it is necessary to examine howthe distances (range) to satellites A,B, and C can bemeasured.

GPS satellites use CDMA modulation using separate PNcodes for each satellite in the constellation. The samePN codes can also be generated in the GPS receiver.

Satellite TV


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Satellite TV

About 75% of the satellite market forcommunication services. Examples:

GE and Galaxy (US)

Astra and Hot Bird (Europe)

INSAT (India)JCSAT and Superbird (Japan).

The 5 Hot Bird satellites provide 900 TV channelsand 560 Radio stations to 24 mln users in Europe.Other means of TV broadcasting includeterrestrial TV broadcasting and cable TV services.

Cont


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Cont

Satellites can provide TV transmission serviceseither directly to the user or in conjunction withthe cable and terrestrial broadcasting networks.Satellite TV employs GEO satellites.

Digital Direct Broadcasting Satellite (DBS) TV offersusers with a lot of services like High Definition TV(HDTV), which is a high resolution digital TVservice, interactive programme watching in whichthe user can interact with the programme andcreate his own programme, do interactiveshopping, personal video recording.

Cont


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Cont

Other services offered include video-on-demand, in which the viewer can view at anymoment the programme of his choice, nearvideo-on-demand, in which the viewer canview the programme of his choice at a latterscheduled time, pay TV in which the viewercharged according to the programmes he

views. Another important service is a highspeed Internet connection through thesatellite TV link.

Cont


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Cont

Examples: DirectTV, Echostar, PrimeStar (USA)

TataSky, DishTV (India)

Star Choice (Canada)

They use only 8 orbital slots which are allocated.

Satellite radio


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Satellite radio

It provides high fidelity audio broadcast servicesto the broadcast radio stations. Sound quality

is excellent due to a wide bandwidth of 5-15

kHz and low noise. Satellite radio uses GEOsatellites. The satellite can also transmit the

signal directly to the users radio sets.

Examples: Sirus and XM Radio (USA).

Satellite data communication


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Satellite data communication

Data communication via satellites refers to theuse of satellites as a communication channel

to transmit data between two computers or

date processing facilities located at differentplaces. Data communication is provided by

GEO satellites or by constellation of LEO, MEO

or HEO satellites. Some of these satellites are

part of the GMPCS (global mobile personal

communication systems).

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Cont

GEO satellites provide broadcast, multicast andpoint-to-point unidirectional or biodirectional

data services through special networks called

VSAT networks.Satellites are broadly classified as: international,

regional and domestic systems.

For more information see:www.wiley.com/go/maini

International satellite systems
http://www.wiley.com/go/mainihttp://www.wiley.com/go/mainihttp://www.wiley.com/go/mainihttp://www.wiley.com/go/mainihttp://www.wiley.com/go/mainihttp://www.wiley.com/go/maini


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International satellite systems

Iridium Globalstar

Intelsat

Inmarsat

Intelsat Limited is the worlds largest commercialsatellite communications service provider.Originally, it was formed as the InternationalTelecommunication Satellite Organisation

(INTELSAT) in 1964 to own and manage aconstellation of GEO satellites that could provideinternational communication services.

Cont


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Cont

Intelsat was an inter-govermental consortium initiallyhaving 11 members. In 2001, it became a privatecompany and acquired PamAmSat in 2006. Today, it isthe worlds largest provider of fixed satellite services,

operating a fleet of more than 50 satellites.Intelsat VIII can handle more than 120 000 telephone

calls or 500 TV channels. In February 2007, Intelsatchanged the names of 16 of its satellites formerlyknown under the Intelsat Americas and PamAmSatseries to Galaxy and Intelsat series respectively.

Cont


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Cont

These satellites basically serve four regions: Atlantic Ocean Region (AOR)- covering North America, Central

America, South America, India, Africa and western portions ofEurope.

Indian Ocean Region (IOR)- covering Eastern Europe, Africa, India,South East Asia, Japan and Western Australia

Asia Pacific Region (AFR)- covering Eastern Europe, the former USSRand the regions from India to Japan and Australia.

Pacific Ocean Region (POR)- covering Southeast Asia to Australia,the Pacific and western regions of America and Canada.

These coverage regions overlap with each other providing truly global

services almost to every country.

Inmarsat


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Inmarsat

INMARSAT is an international organisation,currently having 85 member countries thatcontrol satellite systems in order to provideglobal mobile communication services. It wasestablished in 1979 to serve the maritimeindustry by providing satellite communicationservices for ship management and safety

applications. Now its applications expandedto providing land, mobile, aeronauticalcommunication services.

Cont


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Cont

Currently, more than125000 Inmarsat mobileterminals are in use. It began its operation in

1982 by leasing capacity from the MARISAT,

MARECS and INTELSAT satellites. Figure 7.2shows the network using Inmarsat satellites.


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Figure 7.2

Communication network using Inmarsat satellites

Iridium satellites


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Iridium satellites

It is global mobile communication system designedto offer voice communication services to pocket-sized telephones and data, fax and pagingservices to portable terminals, independent of

the users location in the world and of theavailability of traditional telecommunicationsnetworks. Iridium is expected to provide acellular-like service in areas where a terrestrialcellular service is unavailable or where the public

switched telephone network (PSTN) is not welldeveloped.

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The Iridium network can support 172 000simultaneous users, providing each of them with2,4 kbps fully duplex channels.

The space segment of the system comprises 66

active satellites and 14 spare back-up satellitesrevolving around Earth, in 6 LEO orbital planeshaving 11 satellites each, at an altitude of 780km. The system was originally supposed to have77 active satellites. Iridium element has anatomic number of 77. Each satellite is cross-linkedwith 4 other satellites.

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Figure 7.3 shows a typical communication set-up of the Iridium satellite constellation. Each

user of mobile service using single

subscription number is associated with agateway called the home gateway, which

maintains a record of its profile and location

and looks after its services. Each user also has

links with 2 satellites at a time.

Globalstar satellites


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Globalstar satellites

They belong to USA and provides global voice,data, fax and messaging service through a

constellation of 48 satellites. They orbit at an

altitude of 1410km and are inclined at anangle of 52 degree.

Regional satellites


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Regional satellites

EUTELSAT was formed 1977 to commission thedesign and construction of satellites and tomanage the operation of regional satellitecommunication services in Europe. Currently 24

satellites are operational on the GEO orbit,serving more than 150 countries and up to 90%of the worlds population. 20 satellites are fullyoperated by EUTELSAT, 4 are leased from

Telecom -2D, Telstar-12, Ekspress-A3 andEkspress-AM22.

Cont


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Atlantic Bird satellites provide video, IP and datacommunication services to Europe, the MiddleEast and North African markets. They have 4satellites.

Eutelsat series of satellites are Eutelsatscommunication satellites providingcommunication services to the Europeansubcontinent.

Eurobird satellites provide broadcasting andtelecommunication services primarily to theWestern and Central European region.

Cont


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Cont

Hot Bird satellites provide TV services to Europe ,North Africa and large parts of the Middle East. Italso provides radio and multimedia services overa wide coverage area. Currently 3 satellites are

operational. SESAT ( Siberia-Europe) satellites provide a wide

range of telecommunication services over a verylarge geographical coverage area that extendsfrom the Atlantic Ocean to Eastern Russia,including a large part of Siberia.

National satellites


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National satellites

USA satellites: Galaxy, Satcom, EchoStar,Telestar

Brazil: Brasilsat

India: INSAT

Australia:Optus

China: Sinosat

Future of Satcoms


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The future is towards launching more satelliteconstellations in low altitude orbits, designingcomplex satellite platforms with more on-boardpower, increased support to personal

communication services users, use of higherfrequency bands and shift from RF spectrum tolight quantum spectrum.

Key technology areas are: development of large-scale multi-beam antennas to allow intensivereuse of frequencies.

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Key technology areas are: Development of large-scale multi-beam antennas to

allow intensive reuse of frequencies,

Replace VSATs with USATs,

Development of signal processing algorithms toperform intelligent functions on-board the satelliteincluding signal regeneration,

Overcome the signal fading problem due to rain andallowing use of smaller antennas.

Flexible cross-link communication between satellites willbe developed to allow better distribution of trafficbetween the satellites .

Advanced concepts


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p

Satellite-to-submarine communication:Satellites can be used to communicate withmany submarines that are submerged in seawater at depths of 100m or so. See figure 7.4.

Interplanetary TV link: the set-up makes use ofa satellite orbiting around a planet with whichthe link has to be established and a satellite

orbiting in geostationary orbit around theEarth. View the figure 7.5


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Figure 7.4


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Figure 7.5

Documents

Digital Satelite Communications Tutorial