13 Satellite Communication

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Satellite Communication

13SATELLITE

COMMUNICATION

Introduction

Satellite Communication is the outcome of the desire of man to achieve the concept ofglobal village. Penetration of frequencies beyond 30 Mega Hertz through ionosphereforce people to thin that if an ob!ect "#eflector$ could be placed in the space aboveionosphere then it could be possible to use complete spectrum for communicationpurpose.

History

%n an article published in &'ireless 'orld( in )*+, -rthur C.Clare foresa that it ouldbe possible to provide complete coverage of orld from !ust three satellites/ providedthat they could be placed at &eostationary orbit( i.e./ at an orbit 3,/1** 2M above theequator. or placing the satellite at this height/ the speed of satellite should be 3.01+m4sec. 5r ))/600 m4hour. 7his as indeed the pioneering concept on satellitecommunication even though the article as meant to be scientific fiction. Hoever/ therequired technology to put satellite in space as not available at that time and so thescientists and engineers did not tae this article by Clare seriously.

8SS# could master rocet technology and put &Sputni 9 %( the orld:s first satellite inspace in )*,1 from ;aionour cosmodromme in 2azahistan. 7his as the beginning ofsatellite era. Sputni


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Induction Course (General)

in the orld. -round the same time/ Molniya satellite as launched by 8SS# but it asa lo orbit active satellite ith an inclination of ==o. 7his orbit as found the best suitedto cover the northern part of 8SS# effectively and so this is preferred by the #ussianseven no.

%ntelsat


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solar day/ reference of rotation is sun$. Solar day has 6+ hours as the reference forrotation is sun. 7he earth is not remaining in the same location every day but isrevolving around the sun and thus advancing about one degree per day. %n a year/ itould have advanced 3=, degrees "about one full day$ in a year and this ould haveproduced one additional rotation of earth on its on aGis. 7hus there are 3==.6, siderealdays in a year as compared to 3=,.6, solar days. 7his correspondingly reduces the

sidereal day time from 6+ hours "solar day$ to 63 hours/ ,= minutes and +.) seconds.Since earth is actually maing one rotation on its on aGis in 63 hours/ ,= Minute and+.) seconds the geosynchronous satellite should also mae one revolution during thisperiod as to remain at same point hile looing from earth.

7o technologies responsible for the birth of Satellite Communication System are F/ -#%>S/ ?>5S-7/ S7-#B>7/ etc.

Circular oritsith zero inclination ">quatorial orbits$. 7he most popular is the

geo stationary satellite orbits L the satellite orbits around the earth at an altitudeof 3,1= m/ and in the same direction as the earth. 7he period is equal to thatof the rotation of the earth and in the same direction. 7he satellite thus appearsas a point fiGed in the sy and ensures continuous operation as a radio relay inreal time for the area of visibility of the satellite "+3I of the earth:s surface$.

Factors deciding the selection of Orit

7he choice of orbit depends on the nature of the mission/ the acceptable interferenceand the performance of the launchers

7he eGtent and latitude of the area to be covered.

7he elevation angle of earth stations.

S7%"7$ Publication 165 0064%C"$4600=


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7ransmission duration and delay.

%nterference

7he performance of launchers

S7%"7$ Publication 166 0064%C"$4600=


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T#$O System

Presently Aoordarshan is up lining its national/ metro and regional services to %BS-7B?-BA. 7he meridians or longitudes east of greenich are called eastmeridians and those lying in the est are called est meridians. 7he maGimum is )0degrees on both sides.

Fig. 2 ongitude as angular distance east Fig. 3 ongitudesor 4est of the Green,ich !eridian

A5imuth and "levation

or receiving a satisfactory signal from the satellite the dish antenna should be pointedtoards the satellite accurately. or that e need to no the azimuth and elevation of

a particular satellite from our place.

7he azimuth and elevation are angles hich specify the direction of a satellite from apoint on the earths surface. %n layman terms the azimuth is the east est movementand the elevation can be defined as the north south movement of the dish.

;oth the azimuth and elevation of a dish can be affected by three factors for geolliptical

a$ #ight hand circular "#HCP$ b$ ?eft half circular "?HCP$.

Synthesised T#$O System

- simplified bloc diagram of Synthesised 7J#5 System is given in ig.1.Parabolic Aish-ntenna "PA-$ is idely used to receive the satellite signals. ain of a PA- is directlyproportional to the square of frequency being received and its size. -fter reflection fromPA-/ signal strength is maGimum at the focus of PA-/ so feed is placed at its focus iththe help of a mounting plate. or the reception of S


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7he don


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7he % so obtained is suitably amplified. or this three or four stages of amplifier areprovided. -fter amplification the output signal level of ?B;C is C receiver is shon in figure *.

7he % is applied to a four7 miGer here a ?5frequency of ),00 to 6300 MHz is miGed so that an % of =00 MHz is produced.

7he local oscillator consists of to similar JC5s "voltage controlled oscillator$ oneoperating in the range of ),00 < )1+* MHz and the other in the range of )1,0 to 6300MHz. 7hey are controlled by a synthesiser %C. - sample of the ?5 frequency is taenand phase compared ith a stable reference crystal frequency of + MHz and error if any/

S7%"7$ Publication 172 0064%C"$4600=


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is then applied to the JC5 for frequency correction through a lo pass filter. 7hus theJC5 ors in a phase loced loop mode.

7he thumb heel sitches in the front panel generates a ;CA "binary coded decimal$signal and is applied to the >P#5M "erasable programmable read only memory$ %Cshich generates a corresponding divisors code and is then applied to the synthesiser %C

through shift registers hich convert parallel data into serial data. 7his code is used todivide the JC5 frequency so that the comparison frequency at the comparator end isalmost same.

7he =00 MHz % signal is passed through a band pass filter of 3= MHz bandidth. ")MHz for half transponder$. 7hen it is fed to a five stage % and -C amplifier. 7he gainach module taes the ,6 MHz %. -ny of the channels in the+1.,= MHz band can be selected by using the front panel thumb heel sitches.

Synthesizer is used as a variable local oscillator. %t consists of a JC5 and a P??. %n

P?? synthesizer a reference signal of 6.0+ MHz is generated using a crystal of 6.0+MHz.

Specification of Synthesized 7ranslator

). requency of operation "input$ +1


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Aemodulator specifications

). %nput frequency ,., Mhz

6. 5utptu level O* d;m across =00 ohms

3. Aistortion 6I

+. S4B ,0 d;

,. Aynamic range

13 Satellite Communication

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