Satellite Communication-Revised 3[1].5.09

8/4/2019 Satellite Communication-Revised 3[1].5.09

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Satellite Communication


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Dr. P r adip K r. DattaRetd . Head, Dept . of PhysicsP r esidency College,Kolkata


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I nt r oductionToday the whole world has become a global village.This has

been possible due to the tremendous advancement of electronic communication systems, particularly the satellitecommunication systems. Today, satellites, like clocks,telephones, and computers, are commonplace tools of technology. They help us navigate, communicate, monitor

the environment, and forecast weather. Appropriately, theword satellite means an "attendant."Within only 50 years after the launch of the first satellite ,

the Sputnik-1, in 1957,equipped with on-board radio-transmitters by the Soviet Union there has been a

revolution in communication .Space communication showed phenomenal growth in the1970s, and will continue to grow for some years to come.The growth has been so rapid that there is now danger of overcrowding the geo-stationary orbit.


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Introduction (Contd)

The idea of communication through satellite, in particular with a synchronous satellite was conceived by the famous science fiction writer Arthur C. Clarke.In October 1945 Clarke published an article titled

Extra-terrestrial Relays in the British magazineW ireless W orld . The article described thefundamentals behind the deployment of artificialsatellites in geo-stationary orbits for the purpose of

relaying radio signals. Thus Arthur C. Clarke is oftenquoted as being the inventor of the communicationssatellite.


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Contd

A satellite orbiting at a distance of 36000 Km from the Earthwould be geo-stationary , i.e. would have an angular orbitalvelocity equal to the Earth's own orbital velocity. It wouldthus appear to remain stationary relative to the Earth if placed in an equatorial orbit. It could receive and relaysignals from most of the hemisphere.


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This is aconsequence of Kepler's law that theperiod of rotation T of a satellite around theEarth can be easilycalculated.We see that as theorbit increases in

radius, the angular velocity reduces, untilit is coincident withthe Earth's at a

radius of 36000Km .

r mv

r GMm

2

2!

21)/( r GM v !

GM r

T 232 T


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Contd .In principle, threegeo-stationarysatellites spaced 120degrees apart canprovide completecoverage of theEarth's surface, withsome overlap if messages could berelayed betweensatellites.TheTELSTAR I and II Satellites launched in1962 and 1963established the basisof modern satellitecommunication


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Co ponents of an artificial satellite are

communication capabilities with Eartha power sourcea control system to accomplish its mission

Communications antennae, radio receivers andtransmitters enable the satellite to communicatewith one or more ground stations, called

command centers. Messages sent to the satellitefrom a ground station are "uplinked"; messagestransmitted from the satellite to Earth are

"downlinked."


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W hy Satellites fo r Communications

The greater amount of information required to transmittelevision pictures required that they operate at much higher frequencies than radio stations. A typical television stationwould operate at a frequency of 175 MHz. As a result,television signals would not propagate the way radio signalsdid.

Both radio and television frequency signals can propagatedirectly from transmitter to receiver. But it is more or less

limited to line of sight communication. For long distanceradio communication a signal is reflected back by theionosphere. The higher frequency television signalspenetrates the ionosphere and is not reflected. This isshown in the next slide.


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Radio Signals Reflect Off the I onosphe r e; TV Signals Do Not


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ContdConsequently, television reception was a "line-of-

sight" phenomenon, and television broadcasts werelimited to a range of 20 or 30 miles or perhaps acrossthe continent by coaxial cable. Transatlanticbroadcasts were totally out the question.

The need for transatlantic radio, T.V. and telephonewas increasing rapidly. So the existing communications

capabilities were simply not able to handle all of therequirements. The newly developed artificial satellitesseemed to offer the potential for satisfying many of these needs.


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U .S. Military MILSTAR Communication Satellite


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W hat is a Communication Satellite?

A c ommuni c ations satellite is an artificialsatellite stationed in space for the purposes of telecommunications.They carry microwavereceiving and transmitting equipments relayingsignals form one point on earth to other points.The use of artificial satellites to providecommunications links between various points onEarth . Communications satellites relay voice,video, and data signals between widely separatedfixed locations (e.g., between the switching officesof two different national telephone networks),between a fixed location and numerous small fixedlocations.


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A dvantages of satellite communication

The laying and maintenance of intercontinental cable is

difficult and expensive.The heavy usage of intercontinental traffic makes the satellite

commercially attractive.Satellites can cover large areas of the Earth. This is

particularly useful for sparsely populated areas.For fixed services, communications satellites provide a

technology complementary to that of fiber optic submarinecommunication cables. They are also used for mobileapplications such as communications to ships and planes, for which application of other technologies, such as cable, areimpractical or impossible.


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B r ief Histo r y

The first satellite equipped with on-board radio-transmitterswas the Soviet Sputnik 1, launched in 1957.The first American satellite to relay communications was

Project SCORE in 1958. It carried a tape recorder which wouldrecord messages as it passed over an originating station and

then rebroadcast them as it passed over the destination.However, it appeared only briefly every 90 minutes - a seriousimpediment to real communications.

In 1960, the simplest communications satellite ever conceived

was launched.It was called ECHO , because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio

and TV signals transmitted to the satellite would be reflectedback to earth and could be received by any station within viewof the satellite.


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TELST A R

Telstar was the firstactive, direct relaycommunicationssatellite launchedon July 10, 1962. It

was placed in anelliptical orbit(completed onceevery 2 hours and37 minutes),rotating at a 45angle above theequator.


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Classification of Satellites

Satellites may be classified according to the regioncovered. Also it can be classified as active or passivedepending on whether they act as a simple reflector or not.

A passive satellite consist of a large reflector which simply

reflects the signal received from an earth station so that itreaches another earth station. An active satellite receives the signal from an earthstation, amplifies it by the active electronic circuits downcoverts the frequency and transmit the signal.


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Domestic- Confined to the country

Regional involves two or more countries

Global- are intercontinental

Also classified as : fixed, mobile, point-to-point,broadcasting, commercial, military, experimental,etc.


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B asic Communications SatelliteComponents

Every communications satellite in its simplest form involvesthe transmission of information from an originating groundstation to the satellite (the uplink), followed by aretransmission of the information from the satellite back to theground (the downlink). The downlink may either be to a select

number of ground stations or it may be broadcast to everyonein a large area. Hence the satellite must have a receiver and areceive antenna, a transmitter and a transmit antenna, somemethod for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of theelectronics. The exact nature of these components will differ,depending on the orbit and the system architecture, but everycommunications satellite must have these basic components.This is illustrated in the next slide.


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Choice of Frequency

T he choice of freq. is very important in case of satellitecommunication. Since all satellites are well above the

ionosphere t he freq. must be such that the signal will freely passthrough the ionosphere and will also not be attenuated in thetroposphere. Hence microwaves are used.Further, microwave is required to handle wide- band signalsencountered in present-day communication.The possible frequency bands are shown in the next slide.


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F r equency B andsBand Downlink(MHz) Uplink(MHz)

UHF- Military 250- 270 292-312

C- Band-

commercial

3700-4200 5925- 6425

X- band- Military 7250-7750 7900-8400

Ku-band-commercial

117000-12200 14000-14500

Ka-band-commercial

177000-18200 27500-30000

Ku-band-Military 202000-21200 43500-45500


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The upper limit of frequencies for long distancecommunication through satellites is decided by thelosses caused by absorption by precipitation particles( rain, fog, hail & snow), and by molecules, dust,smokes, etc. in the troposphere and is about 6 GHz.The loss in the troposphere is a function of frequencyand angles of elevation. If the ground station islocated in areas with low precipitation rates,the upper limit of freq. is fixed by the absorption by water vapour

and oxygen molecules in the atmosphere.Considering all these, freq. used are 3- 6 GHz.

Choice of Frequency ( Contd)


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F r equency A llocation

Rain attenuation is not seriousSky noise is low at 4 GHz6/4 band offer fewest propagation problems

Up link freq. higher because losses tend to begreater at higher frequencies. Easier to increasepower at earth station.

Co-ordination of satellite systems is carried out byInternational Telecommunication Union( AtGeneva)


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Bands

C-Band (3.7 - 4.2 GHz) - Satellites operating in this bandcan be spaced as close as two degrees apart in space,and normally carry 24 transponders operating at 10 to 17watts each. Typical receive antennas are 6 to 7.5 feet indiameter. More than 250 channels of video and 75 audioservices are available today from more than 20 C-Bandsatellites over North America. Virtually every cableprogramming service is delivered via C-Band.

Ku Band (11.7 - 12.2 GHz) - Satellites operating in thisband can be spaced as closely as two degrees apart inspace, and carry from 12 to 24 transponders that operateat a wide range of powers from 20 to 120 watts each.Typical receive antennas are three to six feet in diameter.


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Bands-Contd.

More than 20 FSS Ku-Band satellites are in operation over North America today, including several "hybrid" satelliteswhich carry both C-Band and Ku-Band.

Ku-Band (12.2 - 12.7 GHz) - Satellites operating in thisband are spaced nine degrees apart in space, and normallycarry 16 transponders that operate at powers in excess of 100 watts. Typical receive antennas are 18 inches in

diameter.


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F r eq . Reuse

Polarization discrimination is used. Adjacenttransponder channels are assigned alternatepolarization. Transponder channels aredivided into two groups A & B Example:Downlink for Gr.A- horizontally polarized,for Gr. B vertically polarized. This increasesbandwidth. If there is any overlap intransponder bandwidths, interference is

prevented due to different polarization. Theuse of polarization to increase availablebandwidth is called frequency reuse.


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OR BI TS Although an infinite no. of orbits are possible,only a few are used for communication. Choiceof orbit determines transmission path loss anddelay time, earth coverage area, time duringwhich it is visible from a given area.Different orbits serve different purposes. Eachhas its own advantages and disadvantages.There are several types of orbits:Polar

Sun SynchronousGeo-synchronous / Geo-stationaryInclined elliptical (Next slide)Polar circular


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N ear Polar Orbits

Th e more correct term would be near polarorbits. Th ese orbits h ave an inclination near 90degrees. Average h eig h t 800- 1000 km. Th isallows t h e satellite to see virtually every part ofth e Eart h as t h e Eart h rotates underneat h it. Ittakes approximately 90 minutes for t h e satelliteto complete one orbit. Th ese satellites h avemany uses suc h as measuring ozoneconcentrations in t h e stratosp h ere or measuringtemperatures in t h e atmosp h ere, pollutionmonitoring,etc..

Contd..


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Sun Synch r onous O rb itsThese orbits allows a satellite to pass over asection of the Earth at the same time of day.Since there are 365 days in a year and 360degrees in a circle, it means that the satellite hasto shift its orbit by approximately one degree per day. These satellites orbit at an altitude between700 to 800 km. These satellites use the fact sincethe Earth is not perfectly round These orbits areused for satellites that need a constant amount of sunlight. Satellites that take pictures of the Earthwould work best with bright sunlight, whilesatellites that measure long wave radiation wouldwork best in complete darkness. (Next slide)


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Also known as geo-stationary orbits, satellites in these orbitscircle the Earth at the same rate as the Earth spins. TheEarth actually takes 23 hours, 56 minutes, and 4.09seconds to make one full revolution. So based on Kepler'sLaws of Planetary Motion, this would put the satellite atapproximately 35,790 km above the Earth.

Geosynchronous orbits allow the satellite to observe almosta full hemisphere of the Earth. These satellites are used tostudy large scale phenomenon such as hurricanes, or cyclones. These orbits are also used for communicationsatellites. The disadvantage of this type of orbit is that sincethese satellites are very far away, they have poor resolution.The other disadvantage is that these satellites have troublemonitoring activities near the poles.

G eo-synch r onous O rb its


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Contd .

A satellite in a geo-stationary orbit appears to be in a fixedposition to an earth-based observer. A geo-stationary satelliterevolves around the earth at a constant speed once per dayover the equator.

The geo-stationary orbit is useful for communicationsapplications because ground based antennas, which must bedirected toward the satellite, can operate effectively without theneed for expensive equipment to track the satellites motion.Especially for applications that require a large number of ground antennas (such as direct TV distribution), the savings inground equipment can more than justify the extra cost andonboard complexity of lifting a satellite into the relatively highgeo-stationary orbit. (Next slide)


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I nclined highly elliptical o rb its

Geostationary satellites are constrained tooperate above the equator. As a consequence,they are not always suitable for providingservices at high latitudes. So, Inclined highly

elliptical orbits are used where communication isdesired at high altitudes.(Example: Molniyasatellites). It provides coverage of the polar regions. It is used where coverage of more

remote region of the country is required. Theapogee is arranged to occur over the regionrequiring most coverage, since the velocity isleast at the apogee( acc. to Keplers Law).


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Low-Ea r th-o rb it It is typically a circular orbit about 400 km above theearths surface. Its period is about 90 minutes.

Because of their low altitude, the satellites are visiblefrom within a radius of roughly 1000 km from the sub-satellite point. In addition, satellites in low earth orbitchange their position relative to the ground positionquickly. So even for local applications, a large

number of satellites are needed if the missionrequires uninterrupted connectivity.These satellitesare less expensive to position in space than geo-stationary satellites.Because of their closer proximityto the ground, require lower signal strength (as it is

inversely proportional to the square of the distance).So there is a trade off between the number of satellites and their cost. In addition, there areimportant differences in the onboard and groundequipment needed to support the two types of

missions.


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satellite constellation

A group of satellites working in concert thus isknown as a satellite constellation. Two suchconstellations which were intended for provision for hand held telephony, primarily toremote areas, were the Iridium andGlobalstar. The Iridium system has 66satellites. Another LEO satellite constellation,with backing from Microsoft entrepreneur

Paul Allen, was to have as many as 720satellites.


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Spacing between adjacent satellites inspace is 2 degree along the equatorialarc and the earth station antennas aredesigned to accommodate this spacing


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General Structure of satellite communication system

It consists of a satellite that links many earthstations.The user is connected to the earth stationsthrough terrestrial link, which may be a telephoneswitch or a dedicated link.The signal generated bythe user is processed and transmitted to the

satellite at the earth station.The satellite acts as alarge repeater station in space that receives themodulated carriers in its up-link frequencyspectrum from all the earth stations, amplifies and

re-transmits back to earth in the down-link freq.Spectrum.At the receiving station the signal isprocessed to get back the original signal, which isthen sent to the user through a terrestrial network.


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Satellite

Earth Station

Terrestrial System

User

Earth Station

Terrestrial System

User


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Satellite system

A satellite communication system can be broadlydivided into two segments, a ground segment and aspace-segment. Th e space system includes Satellite.Satellite system consist of t h e following systems.

Power supply:The primary electrical power foroperating electronic equipment is obtained fromsolar cells. Individual cells can generate smallamounts of power, and therefore array of cells inseries-parallel connection are required. To maintainservice during an eclipse, storage batteries mustbe provided.


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Satellite system(Contd.)

TT& C subsystem :T he telemetry, tracking, and command (TT& C) subsystem performs

several routine functions abroad a spacecraft.T

elemetry could beinterpreted as "measurement at a distance". Specifically, it refers tothe overall operation of generating an electrical signal proportionalto the quantity being measured (such as such as temperatures and

power supply voltages, stored fuel pressure,etc. ) and encoding andtransmitting this to a distant station (I.e., is one of the earth stations )Command systems receives instructions from ground station anddecodes the instruction and sends commends to other systems as per the instruction.


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Satellite system (contd.)

Tracking:Tracking of the satellite is accomplishedby having the satellite transmit beacon signals

which are received at the TT&C earth stations. Ageo-stationary satellite will tend to shifted resultof the various distributing forces (to bediscussed later).So it is necessary to track thesatellites movements and send correction signals

as required. Satellite range is also required fortime to time. This can be determined bymeasurement of propagation delay of signalsspecially transmitted for ranging purposes.


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Some Te r minologiesT ransponder - transmitter- responder. It receives a signal,processes it and then re-transmits at a different freq.Modem - Mod ulation- Dem odulation. It is used as aninterface between analog and digital systems. Hasimportant role in computer comm. Networks and ISDNsystems.Codec - Cod ing- Dec oding. Codecs are used in digital TVsystems. It consists of a DAC & ADC ( Fig.3)A s c ending N ode - The point where the orbit crosses theequatorial plane going from South to North

In c lination - The angle from equatorial plane to theorbital plane at the ascending node.


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Ea r th station T his is the earth segment. T he ground station's job

is two-fold. In the c ase of an uplink, or transmittingstation, terrestrial data in the form of basebandsignals, is passed through a baseband pro c essor,an up c onverter, a high powered amplifier, andthrough a paraboli c dish antenna up to an orbiting

satellite. In the c ase of a downlink, or re c eivingstation, works in the reverse fashion as the uplink,ultimately c onverting signals re c eived through theparaboli c antenna to base band signal.


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Block diagram of an earth station that transmits to

and receives info. from a satellite is shown in thenext slide. The signal from the terrestrial systementers the earth station. After being processed (buffered, multiplexed, etc.) by base- bandequipment, the Encoder & Modulator converts the

signal to the up-link freq. It is then amplified anddirected to the appropriate polarization port of theantenna feed.

Critical components are installed redundantly withautomatic switch over in the event of failure for

uninterrupted operation Isolation of LNA from the high power transmitter is

of much concern in the design considerations of an earth station.


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Block diagram of an earth station

(

TRANSM ITTER

Antenn

RECE IVER

Encoder M odulator PCON VERTER

IGH POWER AM P

PolarisedD PLEXER

LOW NO ISE AM PDOWN CON VERTER D

E-MO D LATOR D

ECO DE


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Station Keeping

External forces (gravitational field of the moon, to a lesser extent that of Sun, variation of earths gravitational force withposition, meteorite bombardment, etc.) acting on the satellitealter its position and orientation w.r.t. earth (causes a drift inthe angle of inclination @ 0.85 /yr.) The drift is to be

constantly nullified so that the orbit remains stationary.However, perfect stationary is not possible. So a satellite isconstrained to remain within a window It is defined by anangular shift as seen from the centre of the earth around therequired normal position.Usually the window is an angular

shift of 0.1 in longitude i.e., 75 Km on the sphere containingthe geo-stationary satellite orbit.This is done by means of command controls from the earth. The control routine whichkeeps the satellite in position is called Station Keeping


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M ethod of Station Keeping

To minimize the propellant consumptionstation, the strategy has several steps.

1. The direction & speed of drift is determined.2. By extrapolation, prediction is made as to which day

the satellite will escape the window3. A few days before that date the true orbit is determined

using a new series of measurement. After thatcalculations are carried out regarding the date, theamplitude & direction of velocity increments required

to modify the orbital parameters.4. Appropriate thrusters are then fired and the effects of

correction are monitored.


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Attitude means satellites orientation in space. Attitudecontrol is necessary to keep the directional antennas of thesatellite pointing to desired regions of the earth. Even if station keeping is perfect, satellite may have motions aboutits C.M. This will disturb the pointing satellite antennasnarrow beam towards earth. So it is essential that thesatellite must be perfectly balanced. The dis-balancing iscaused by gravitational force of sun, moon, planets, solar pressure on the antennas, space-craft body, solar cells,

meteorite impacts, etc.These forces vary cyclicallythrough the day & cause wobbing of the spacecraft whichis to be damped out mechanically. This is done by Attitudecontrol/ stabilisation.

Attitude Control


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A ttitude Cont r ol-Contd .

A satellites attitude can be altered along one or

more of 3 axes: Roll, Pitch, Yaw axes ( next slide)Yaw axis is directed towards the earths centrePitch axis is normal to the orbital planeRoll axis is perpendicular to the other two.Geostationary satellites are stabilised in one of

two ways. Spin stabilisation can be utilised withsatellites that are cylindrical.Since the antennas are oriented to point to fixedregions on earth, the antenna platform must bede-spun at the same rate as the satellite spins sothat the antennas are constantly pointing towardsthe earth.

.


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irection of flight

own

Rollitch

Yaw


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Roll axis

Pitch axis

Yaw axisS

RPY axes for a geo stationary satellite. Roll axistangential to the orbit & lies along satellite velocity

vector


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G ene r al Link Design Equations

Let P T = average power assumed to be radiatedisotropicallyG T = maximum directivity gain of the transmitting antenna

G R = maximum directivity gain of the receiving antennaP R = power received by the receiving antennad = distance of the receiving antenna from the transmitting

antennaDirectivity gain G T = actual power density along tha main

axis of rotation of the antenna / which would be

produced by an isotropic antenna at the same dist. fedwith the same input power. This is defined as isotropic radiation is not physically possible and a directiveantenna is used.


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G ene r al Link Design Equations(Contd .)

T he receiver is characterised by the effective area (A eff ) of itsantenna and by the noise temp. of its N A. For the moment T will not be considered. Now,

i = power density in a wave at a distance d = T /(4 d2)

= power incident on the receiving antenna = i GT

= T GT /(4 d2)

So, R = . Aeff = Aeff . T GT /(4 d2)

= ( 2GR / 4 ). T GT /(4 d2) ( since Aeff =

2GR / 4 )

T herefore, R / T = G T GR 2/ (4 d) 2 (1)

T his is the fundamental eqn. in free space communication.

G l k ( d )


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Since f = 3x 108 m/s, R / T = G T GR x 0.57x 10-3/ (d2 f 2)

(f in MHz, d in Km )Or, (PR / PT )dB = (GT )dB + (GR )dB (32.5 + 20 log 10d + 20 log 10f )T he power attenuation in dB is dB = 10 log 10(PT / PR )Or, dB = (32 .5 + 20 log 10d + 20 log 10f ) - (GT )dB - (GR )dB

= FS - (GT )dB - (GR )dBFS is called transmission path loss or free space loss. It is the loss

resulting from the spreading of the wave as it propagate outwardfrom the source. It expresses the signal power attenuation betweentwo isotropic antennas.

T he product PT GT is called eirp of the transmitter and is a fig of merit of the transmitter.Eqn. (1) represents an idealised situation. In reality there arevariety of losses. So instead of FS we use = FS x A


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LA being the additional losses.

Thus, eqn. (1) becomes

PR / PT = G T GR /L = G T GR / L FS x L A ----- (2) Now, L A = L FT x AAI x A R x L PO L x L point X L FR W here, L FT = losses between the transmitter output & the

transmitting antenna (wiring, filters, duplexers, etc. )AAI = attenuation by the atmosphere & ionosphere

AR = attenuation due to precipitations & cloudsL

PO L = losses due to polarization mismatch between the

transmitting&

receiving antennaL point = losses due to antenna depointing

LFR = losses between the receiving antenna & the receiver output

(wiring, duplexers, etc. )


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G ene r al Link Design Equations(Contd .)So, Eqn. (2) can be written as (in dB )PR =eirp + G R - L FS - L FT - AAI- AR - L PO L - L point - L FR ------ (3) PR is referred to as the carrier power (C) in satellite communication

T he noise temp is an important parameter that governs the performance of a receiver and therefore in the design of satellitelinks. T he most important source of noise in the receiver is thermalagitation noise in its preamplifier. Noise power P N =K T B (T =receiver noise temp ). T he system noise temp (or effective inputnoise temp ) T S is defined as the noise temp of a noise sourcelocated at the input of a noiseless receiver which would produce thesame noise at the output of the receiver as the internal noise of theactual system.T he receiver has a RF Amp and IF Amp prior to its demodulator. If the overall RF & IF gains of the receiver is G, noise power at thedemodulator input is Pn = K T S BG.


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G ene r al Link Design Equations(Contd .)If P

r = Signal Power at the Input of the RF Section of the

Receiver, the Signal Power at the emodulator Input Is Pr .G.Hence,C/ N = Pr G /(K T SBG ) = Pr /(K T SB) ----- (4)T

S in eqn. (4) Represents Several Sources of Noise in the Receiver.If the RF Section, Mixer & IF Amp. Have Equiv. Noise T emp T rf ,T

m , T if Respectively and T heir Gains Be G r , G m, G if Respectively, T in Be the Input Noise T emp at the RF Section, theT otal Noise Power at the Output of the IF Amp. Is

Pn = G if K T if B + G if Gm K T mB + G if Gm G r KB (T rf + T in ) = G if Gm G r KB (T rf + T in + T m/G r + T rf / Gm G r )

In T erms of T S , Pn = G if Gm G r K T S B .So, T S = T rf + T in + T m/Gr + T rf / Gm G r

Beyond IF Stages Noise Contribution Is Negligible.

C/N d G/T R ti


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C/N and G/T Ratio

Using eqns. (1) ,(2) , (4) --- C/ N= [GT GR PT2/ (4 d)2]/(K T S BL A)

Now, noise power spectral density (W /Hz) N0 = . Hence, thecarrier power - to - noise power spectral density is(C/ N0 )dBHz = 10 log G T PT 20 log (4 d/ ) +10 log (GR / T S)

10 log L A 10 logK GR / T S ratio is called G /T ratio. It specifies the quality of an earthstation. C / N increases as G /T increases. At 4 GHz, freq andelevation angle has to be specified, since G R f

2 and T S dependson sky noise temp which increases if the elevation angle is less

than 100

. At higher freq. ( say 11 GHz ) the variation in G R / T S issmall.


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A t ospheric & ionospheric effects on link design

Atmospheric & ionospheric propagation are subject to absorption,diffraction, rotation of the plane of polarisation of the e.m.wave .T hese depend on path length, so more pronounced at smallelevation angles. T he lower layer of atmosphere cause absorptionand diffraction. T he troposphere (upper layer ) causes refraction and

depolarisation occurs as the e.m.wave traverse the ionosphere. A t ospheric attenuation is negligible below 10GHz. But there area few molecular resonance absorption peaks at 22 .2 GHz (water vapour molecules go into vibrational resonance ), at 6 0 GHz ( for oxygen molecules ). Atmospheric effect is small at 2 GHz to 10 GHz for higher elevation angles.

Rain attenuation depends on freq., rainfall rate, diameter & distribution of rain drops. --- Negligible at 6 /4 GHz except for heavy rain.


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Co plete Link Design

A single complete link consists of 2 earth stations & a satellite. T heultimate info. quality received on earth depends on up - link, down -link and the satellite transponder.L et C u = signal power at the satellite transponder unit (up - link power )

GS, G T , G R = gains of transponder, transmitting antenna, receivingantenna, L = loss on down - link , N OU , N OD = noise power density atthe transponder input and earth station receiver input.T hen the useful carrier (signal ) power (C) and noise power spectraldensity ( N0) at the input of the earth station receiver areC = C u GS GT GR /L N0 = N OD + N OU GS GT GR /L

So, (C/ N0)T = C u / {N OU + N OD L /( GS GT GR ) }. For down - link only,signal power is C D = PT GT GR /L . If the transponder bandwidth beB and it radiates a const. power PT , its gain is G S = PT /( Cu + N OU B)


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Co plete Link Design(Contd)

Substituting for G S and re - arranging,(C/ N0)T = C u / {N OU + N OD (Cu + N OU B)/CD}

= [(C/ N0)U. (C/ N0)D ]/ [(C/ N0)D + (C/ N0)U +B]In most cases, (C/ N0)U and (C/ N0)D >> B

So, (C/ N0)-1

T = (C/ N0) 1

D + (C/ N0) 1

U------ (1)

Normally, (C/ N0) 1

U >> (C/ N0) 1

D . . In that case, (C/ N0)T (C/ N0)D T hus the complete link design depends on the quality of down - link and specially on (C/ N

0)

D Eqn (1) may be extended to include the effects of interferingsignals. If I = the noise power involved with the interfering signalsunder the bandwidth of the carrier, (C/ N) 1 Net = (C/ N)

1T + (C/I)

1T


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Other Attenuations

Attenuation by

Polarisation mismatch between transmitting &receiving antennas Antenna de-pointingLosses between the receiving antenna & receiver

input


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Powe r G ene r ation

Power into the transmitter will probably result inonly 100 or 150 watts of power being radiated. Thetransmitters are only 10 or 15% efficient There isno line from the power company to the satellite.

The satellite must generate all of its own power.For a communications satellite, that power usuallyis generated by large solar panels covered withsolar cells These convert sunlight into electricity.

Since there is a practical limit to the how big a solar panel can be, there is also a practical limit to theamount of power which can generated.


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Contd .

Unfortunately, transmitters are not very good at convertinginput power to radiated power so that 1000 watts of power into the transmitter will probably result in only 100 or 150watts of power being radiated. Thus the transmitters are only10 or 15% efficient. In practice the solar cells on the most"powerful" satellites generate only a few thousand watts of electrical power.

Satellites must also be prepared for those periods when the

sun is not visible This requires that the satellite have batterieson board which can supply the required power for thenecessary time and then recharge by the time of the nextperiod of eclipse


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A tmosphe r ic & I onosphe r ic Effects

Atmospheric & Ionospheric propagation are subject toabsorption, diffraction, refraction, rotation of the plane of polarisation of the e.m. wave.These effects depends onthe path length and so are more pronounced at smallelevation angles.

The lower layer of the atmosphere( troposphere) causesrefraction & de-polarisation is produced when radio wavestraverse the ionosphere.

Atmospheric attenuation is of no importance at freq.


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Rain A ttenuation

Rain attenuation depends on freq, rain fallrate, diameter & distribution of rain drops.

Negligible at 6/4 GHz except for heavy rain.

At 10 GHz. & above the atmospheric & rain

effects are more significant.

FD and TD


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From economic point of view several users can s h are asatellite. So satellite link will normally relay many signals froma single eart h station and t h e signals must not interfere wit h one anot h er.Hence, FDM is used for analog comm. and T DMfor digital communication. CDMA is also getting popular insatellite comm.FDM- T ransmits signals on diff. Frequencies.T DM th e signals enter t h e transponder at diff. T imes.Th e FDM sc h eme is s h own in t h e next slide. Eac h satelliteh as certain no. of transponders. Satellite receiver is wide-band, covers t h e entire range of up-link frequencies. Th e

6 GHz up-link freq.( 5.925 6.425 GHz) h as a bandwidt h of500 MHz to accommodate 12 c h annels. Th e wide-bandreceiver of t h e satellite will receive t h ese 12 c h annels. Eac h ch annel is assigned a band widt h of 36 MHz. wit h a 4 MHz.guard band so t h at t h e c h annels don t interfere wit h eac h oth er.


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36 MHz

Ch.1 2 3 11 12

Receiver 5.9 45 5.9 85 6. 025 6.385 6. 425ChannelT ransmitter 3.720 3.760 3.800 4.160 4.200

Channel

freq. (GHz )

FDM Scheme

Above t h e 12 th . c h annel t h ere is a 20 MHz. command andcontrol telemetry c h annel. Th e transponder converts t h efreq. to down-link freq. (3.72-4.20 GHz.)

TDM


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T DM

TDM- Here diff. channels are assigned differenttime slots in the specified time intervals for transmission. It requires a perfect synchronisationbetween the multiplexing and de- multiplexingunits.The best example to understand the principle of TDM is the Bell T 1 24-channel system (Nextslide). It has 25 slots. Slot 0 consists of 1 bit andcarries synchronisation information.Other 24 slotscontain 8 bits and carry 24 telephone channels.


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Slots

8 bitst

193 bits

0 1 2 3 4 23 241 bit 8 bits

Bell T 1 24 - channel system

M ultiple A ccess Techniques


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M ultiple A ccess Techniques

Multiple Access Techniques allow interconnection

among many eart h stations simultaneously via satellite.Th e capacity of a satellite transponder is very h igh . It isunlikely for a single eart h station to utilise t h e fullcapacity. One eart h station can communicate with allother earth stations using the same satellites. Thecapacity of a satellite transponder is quite high (120Mbps.) and can handle about 3562 channels at 32Kbps. This much traffic is unlikely to be provided by asingle earth station. So, to have the best use of

transponders capacity it must be allocated to other earth stations. M ultiple A ccess allows the earth stations toaccess the transponder capacity allocated to the in anorderly fashion so that there may not be any chaos .


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A pplications

TelephonyThe first and historically the most important application for communication satellites is in international telephony. Fixed-point telephones relay calls to an earth station, where they arethen transmitted to a geostationary satellite. An analogous

path is then followed on the downlink. In contrast, mobiletelephones (to and from ships and airplanes) must be directlyconnected to equipment to uplink the signal to the satellite, aswell as being able to ensure satellite pointing in the presenceof disturbances, such as waves onboard a ship. Cellular phones used in urban areas do not make use of satellitecommunications. Instead they have access to a ground basedconstellation of receiving and retransmitting stations.


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A pplications- Contd

Satellite Television and radio

There are two types of satellites used for North Americantelevision and radio:Direct Broadcast Satellite (DBS), andFixed Service Satellite (FSS).

A direct broadcast satellite is a communications satellite thattransmits to small DBS satellite dishes (usually 18 to24 inches in diameter). Direct broadcast satellites generallyoperate in the upper portion of the K

uband. DBS technology

is used for DTH-oriented (Direct-To-Home) satellite TVservices.


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Fixed Service Satellites use the C band, and the lower portions of the K u bands. They are normally used for broadcast feeds to and from television networks and localaffiliate stations (such as program feeds for network andsyndicated programming), as well as being used for distancelearning by schools and universities, business television(BTV), video-conferencing, and general commercialtelecommunications. FSS satellites are also used to distributenational cable channels to cable TV headends.FSS satellite technology was also originally used for DTHsatellite TV from the late 1970s to the early 1990s in theUnited States in the form of TVRO (TeleVision Receive Only)receivers.

A BSS 601 model used for DTH television broadcasting


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A BSS 601 model used for DTH television broadcasting in Europe


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Diffe r ence b etween DB S & F SS

FSS satellites have a lower RF power outputthan the DBS satellite, requiring a much larger dish for reception (3 to 8 feet in diameter for K uband, and 12 feet on up for C band)

FSS satellites use linear polarization for eachof the transponders' RF input and output

whereas DBS satellites use circular polarization .

A pplications Contd


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Mobile satellite te c hnologiesInitially available for broadcast to stationary TV receivers, by2004 popular mobile direct broadcast applications made their

appearance with that arrival of two satellite radio systems inthe United States: Sirius and XM Satellite Radio Holdings.Special antennas have been used for mobile reception of

DBS television. Using GPS technology as a reference, theseantennas automatically re-aim to the satellite no matter where or how the vehicle (that the antenna is mounted on) issituated. These mobile satellite antennas are popular withsome recreational vehicle owners. Such mobile DBSantennas are also used by JetBlue Airways so that thepassengers can view on-board on LCD screens mounted inthe seats.

A li ti C td


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A pplications- Contd .A mature radio

OSCAR satellites have been designed specifically tocarry amateur radio traffic. Most such satellites operate asspaceborne repeaters, and are generally accessed byamateurs equipped with UHF or VHF radio equipment and

highly directional antennas such as Yagis or dishantennas. Due to the limitations of ground-based amateur equipment, most amateur satellites are launched into fairlylow Earth orbits, and are designed to deal with only alimited number of brief contacts at any given time. Somesatellites also provide data-forwarding services using the

AX.25 or similar protocols


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A pplications-Contd

In recent years, satellite communicationtechnology has been used as a means to

connect to theInternet via broadband dataconnections. This can be very useful for users

who are located in very remote areas, andcannot access a wireline broadband or dial up

connection.

FU TU RE


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FU T U RE

In the future, it is anticipated that these systems will

provide access to millions of users worldwide, using inexpensive interactive broadband terminals for a rangeof applications, whether mobile, fixed or broadcasting.This evolution is particularly visible in the broadcasting domain, with satcom systems like Eutelsat or SES-Astradirectly serving 20 million user dishes in Europe while at the same time expanding towards Asia and the U S.


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FU T U RE- Contd

The nature of future satellite communications systems willdepend on the demands of the marketplace (direct homedistribution of entertainment, data transfers betweenbusinesses, telephone traffic, cellular telephone traffic,etc.); the costs of manufacturing, launching, and operating

various satellite configurations; and the costs andcapabilities of competing systems - especially fiber opticcables, which can carry a huge number of telephoneconversations or television channels. In any case, however,several approaches are now being tested or discussed by

satellite system designers.


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FU T U RE- Contd .One approach, which is being tested experimentally, is the

"switchboard in the sky" concept. NASA's AdvancedCommunications Technology Satellite (ACTS) consists of arelatively large geosynchronous satellite with many uplinkbeams and many downlink beams, each of which covers arather small spot on the earth. However, many of the beamsare "steerable". That is, the beams can be moved to adifferent spot on the earth in a matter of milliseconds, so thatone beam provides uplink or downlink service to a number of locations. Moving the beams in a regular scheduled manner

allows the satellite to gather uplink traffic from a number of locations, store it on board, and then transmit it back to earthwhen a downlink beam comes to rest on the intendeddestination.

FU TU RE C d


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FU T U RE- Contd .The ACTS concept involves a single, rather complicated,

and expensive geo-synchronous satellite. An alternativeapproach is to deploy a "constellation" of low earth orbitingsatellites. By planning the orbits carefully, some number (perhaps as few as 20, perhaps as many as 250) of satellites could provide continuous contact with the entire

earth, including the poles.The ACTS concept involves a single, rather complicated,

and expensive geo-synchronous satellite. An alternativeapproach is to deploy a "constellation" of low earth orbiting

satellites. By planning the orbits carefully, some number (perhaps as few as 20, and as many as 250) of satellitescould provide continuous contact with the entire earth,including the poles.

FU TU RE C d


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FU T U RE- Contd

By providing relay links between satellites, it would be

possible to provide communications between any two pointson earth, even though the user might only be able to seeany one satellite for a few minutes every hour. Obviously,the success of such a system depends critically on the costof manufacturing and launching the satellites. It will benecessary to mass produce communications satellites, sothat they can turned out quickly and cheaply. This seems atruly ambitious goal since until now the averagecommunications satellite might require 6 months to 2 years

to manufacture

I di A i i i


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Indian ActivitiesIndian Spa c e ProgammeIndian space program had its genesis in the IndianNational Committee for Space Research, which wasestablished in 1962 as part of the Department of AtomicEnergy. It was initiated primarily for scientific purposes.However, since the inception, it was proven to bebeneficial for both civilian and military purposes.Especially, INSAT series of satellites made a major

impact on tele-communication scenario in India .Since the establishment of Experimental Earth Station in1967 the development of satellite communication hasbeen in constant advance.


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The prime objective of ISRO is to develop space technologyand its application to various national tasks. ISRO has

established two major space systems: INSAT for communication, television broadcasting and meteorologicalservices, and Indian Remote Sensing Satellites ( IRS) systemfor resources monitoring and management. ISRO hasdeveloped two satellite launch vehicles, PSLV and GSLV, toplace INSAT and IRS satellites in the required orbits. Despiteits limited resources, India has and is continuing to develop abroad-based space program with indigenous launch vehicles,satellites, control facilities, and data processing.


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Since 15 August 1969 the Indian Space ResearchOrganization ( ISRO) has controlled the management of space research and its utilization for peaceful purposes.In 1972 the Indian Government set up the SpaceCommission and entrusted a Department of Space(DOS) with responsibility for conducting the country'sspace activities. The ISRO is headquartered in

Bangalore and has operating units at twenty-two sitesthroughout the country that deal with space systems,propulsion, communications, telemetry and tracking,research, launches, and other facets of the space

program .

APPLE h 1 st I di i l i i


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APPLE was the 1 st Indian experimental communicationlaunched in 1981. It was placed in the geo-stationary orbit byan Indian developed kick motor. Its on- orbit life was 2yrs. Ithad 6/4 GHz transponders(C- band). It was totally designedand fabricated in India.

INSAT Satellite system is one of the largest domesticsatellite system in the world. It provides telecommunications,TV broadcasting, radio networking, meteorological services& disaster management services. Its transponder capacity isalso being used by other countries.

Th I di S P b di id d i j


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The Indian Space Program may be divided in two major categories. One is the Satellite Program and another is theLauncher Program. The launcher program involves thedesign, fabrication and launching of launch vehicles. Indiahas also developed a series of launch vehicles, after a longresearch and development through the painstaking ways of successes and failures. Present operational space systems

includeIndian National Satellite (

INSAT) for telecommunication, television broadcasting, meteorology

and disaster warning. It is one of the largest domestic

satellite system in the world .

Vikram Saravai Space Centre is the main establishment for all rocket & Launch Vehicle programmes.

ISRO lli C i d i h


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ISRO satellite Centre is engaged in thedevelopment of Satellite technology.

Space Application Centre is engaged in R & Drelated to space applications. It is alsoresponsible for operating the Delhi Earth Stn. for satellite communication.ISRO is working on the development of GSLVneeded for launching communication satellite inthe geo-synchronous orbit.


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T HA NKS


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Documents

Satellite Communication-Revised 3[1].5.09