Earth Station Subsystem

earth station Subsystems

1. Earth Station
Subsystems
By
Ms. Madhuri N. Sachane

2. Earth Station
•Introduction
•The role of an Earth station
•Types of Earth station
•Earth station subsystems
•Earth station figure-of-merit
•Services offered by Earth stations

3. Earth Station
•Theearthsegmentofsatellitecommunicationsystem
mainlyconsistsofthreeearthstations.
•Therearethreecategoriesofearthstations
•Transmitandreceivetype:Theearthstationcantransmit
signalforsatelliteandreceivesignalfromsatellite.
•Thesestationsareusedfortwowaycommunicationlike
telephony,datatransfer.
•Receiveonlytype:Theseearthstationscanonlyreceivethe
signalfromasatellite.
•MostlyusedinCATVsystems.
•Transmitonlytype:heretheearthstationscantransmit
signalonlytowardssatellite.
•Mostlyusedindatacollectionsystems.

4. Earth Station
•Block Diagram of Earth Station
•DesigningofanEarthstationdependsnotonlyonthe
locationofearthstationbutalsoonsomeotherfactors.
•Thedependingfactorsaretypeofserviceproviding,frequency
bandsutilization,transmitter,receiverandantenna
characteristics.

5. Earth Station
•Block Diagram of Earth Station
•Digitalinformationintheformofbinarydigitsfromterrestrial
networksentersearthstationandisthenprocessed(filtered,
multiplexed,formattedetc.)bythebasebandequipment.
•Theencoderperformserrorcorrectioncodingtoreducethe
errorrate,byintroducingextradigitsintodigitalstream
generatedbythebasebandequipment.
•Theextradigitscarryinformation.Thepresenceofnoiseand
non-idealnatureofanycommunicationchannelproduceserror
rateisestablishedabovewhichthereceivedinformationisnot
stable.
•Thefunctionofthemodulatoristoacceptthesymbolstream
fromtheencoderanduseittomodulateanintermediate
frequency(I.F)carrier.

6. Earth Station
•Block Diagram of Earth Station
•Insatellitecommunication,I.Fcarrierfrequencyischosen
at70MHzforcommunicationusinga36MHztransponder
bandwidthandat140MHzforatransponderbandwidthof
54or72MHz.
•TheI.Fisneededbecauseitisdifficulttodesigna
modulatorthatworksattheuplinkfrequencyof6GHz(or
14GHz)directly.
•ThemodulatedI.Fcarrierisfedtotheup-converterand
frequency-translatedtotheuplinkr-ffrequency.
•ThismodulatedR.Fcarrieristhenamplifiedbythehigh
poweramplifier(HPA)toasuitablelevelfortransmission
andradiationbytheantennatothesatellite.

7. Earth Station
•Block Diagram of Earth Station
•Onthereceiveside,theearthstationantennareceivesthe
low-levelmodulatedR.Fcarrierinthedownlinkfrequency
spectrum.
•Thelownoiseamplifier(LNA)isusedtoamplifytheweak
receivedsignalsandimprovethesignaltoNoiseratio
(SNR).Theerrorraterequirementscanbemetmoreeasily.
•R.FistobereconvertedtoI.Fat70or140MHzbecauseitis
easierdesignademodulationtoworkatthesefrequencies
than4or12GHz.
•Thedemodulatorestimatewhichofthepossiblesymbols
wastransmittedbasedonobservationofthereceivedif
carrier.

8. Earth Station
•Block Diagram of Earth Station
•Thedecoderperformsafunctionoppositethatofthe
encoder.Becausethesequenceofsymbolsrecoveredbythe
demodulatormaycontainerrors.
•Thedecodermustusetheuniquenessoftheredundant
digitsintroducedbytheencodertocorrecttheerrorsand
recoverinformation-bearingdigits.
•Theinformationstreamisfedtothebase-bandequipment
forprocessingfordeliverytotheterrestrialnetwork.
•Thetrackingequipmentstrackthesatelliteandalignthe
beamtowardsittofacilitatecommunication.

9. Earth Station Subsystems
•Therearefourmajorsubsystemsthatarepresentinany
earthstation.Thosearetransmitter,receiver,antennaand
trackingsubsystem.
•Thegroundstation,orearthstation,istheterrestrialbaseof
thesystem.
•Thegroundstationcommunicateswiththesatellitetocarry
outthedesignatedmission.
•Theearthstationconsistsofmajorsubsystems:
–Thetransmitsubsystem
–Thereceivesubsystem
–Theantennasubsystem
–Thetrackingsubsystem
–Powersubsystem

10. Earth Station Subsystems
•Transmitter Subsystems
•Theuplinkisthetransmittingsubsystemoftheearthstation.
•Itconsistsofalltheelectronicequipmentthattakesthesignal
tobetransmitted,amplifiesit,andsendsittotheantenna.
•Inacommunicationsystem,thesignalstobesenttothe
satellitemightbeTVprograms,multipletelephonecalls,or
digitaldatafromacomputer.
•Signalsmodulateacarrier,areamplified,andsenttoan
antennaviawaveguides,combiners,anddiplexers.

11. Earth Station Subsystems
•Transmit Ground Control Equipment
•Thetransmitsubsystembeginswiththebasebandsignals,
whicharefirstfedtoamultiplexer,ifmultiplesignalsaretobe
carriedbyasingletransponder.
•Themultiplexeroutputisthenfedtoamodulator.
•Inanalogsystems,awidebandfrequencymodulatorisnormally
used.
•Indigitalsystems,analogsignalsarefirstdigitizedwithPCM
converters.
•Theresultingserialdigitaloutputisthenusedtomodulatea
QPSKmodulator

12. Earth Station Subsystems
•Receive Subsystems
•Thedownlinkisthereceivesubsystemoftheearthstation.
•Itusuallyconsistsofverylownoisepreamplifiersthattakethe
smallsignalreceivedfromthesatelliteandamplifyittoalevel
suitableforfurtherprocessing.
•Thesignalisthendemodulatedandsentontootherpartsofthe
communicationsystem.
•ThereceivesubsystemconsistsoftheLNA,downconverters,
andrelatedcomponents.
•Thepurposeofthereceivesubsystemistoamplifythedownlink
satellitesignalandtranslateittoasuitableintermediate
frequency.
•TheIFsignalisthendemodulatedandde-multiplexedas
necessarytogeneratetheoriginalbasebandsignals.

13. Earth Station Subsystems
•Receiver Ground Control Equipment
•Thereceivergroundcontrolequipment(GCE)consistsof
oneormoreracksofequipmentusedfordemodulatingandde-
multiplexingthereceivedsignals.
•Thedownconvertersprovideinitialchannelizationby
transponder,andthedemodulatorsandde-multiplexing
equipmentprocessthe70-MHzIFsignalintotheoriginal
basebandsignals.
•Otherintermediatesignalsmaybedevelopedasrequiredbythe
application.

14. Earth Station Subsystems
•Antenna subsystem
•ThemajorpartsofEarthstationAntennaarefeed
systemandAntennareflector.
•Thesetwopartscombinedtogetherradiatesorreceives
electromagneticwaves.
•Sincethefeedsystemobeysreciprocitytheorem,theearth
stationantennasaresuitableforbothtransmittingand
receivingelectromagneticwaves.
•Parabolicreflectorsareusedasthemainantennain
earthstations.
•Thegainofthesereflectorsishigh.
•Theyhavetheabilityoffocusingaparallelbeamintoapoint
atthefocus,wherethefeedsystemislocated.

15. Earth Station Subsystems
•Tracking Subsystem
•TheTrackingsubsystemkeepstrackwiththesatelliteand
makesurethatthebeamcomestowardsitinordertoestablish
thecommunication.
•TheTrackingsystempresentintheearthstationperforms
mainlytwofunctions.
•Thosearesatelliteacquisitionandtrackingofsatellite.
•Thistrackingcanbedoneinoneofthefollowingways.
•Thoseareautomatictracking,manualtracking&program
tracking.

16. Earth Station Subsystems
•a)SatelliteAcquisition
•Beforecommunicationcanbeestablisheditisnecessaryto
acquireasatellite.
•Onemethodistoprogramtheantennatoperformascan
aroundthepredictedpositionofthesatellite.
•Theautomatictackingisswitchedonwhenthereceiversignal
strengthissufficienttolockthetrackingreceivertothe
beacon.
•b)AutomaticTracking
•Afteracquisitionasatelliteneedstobetrackedcontinuously.
•Thisfunctionisperformedbytheautomatictrackingsystem.
Auto-tacksystemsareclosed-loopcontrolsystemsandare
thereforehighlyaccurate.
•Thistrackingmodeisthepreferredconfigurationwhen
accuracyisthedominantcriterion.

17. Earth Station Subsystems
•c)Manual track:
•Toavoidatotallossofcommunicationduetoafailureinthe
trackingsystem,earthstationsgenerallyalsohavemanual
mode.
•Inthismodeanantennaismovedthroughmanualcommands.
•d)Program Track:
•Inthistrackingmodetheantennaisdriventothepredicted
satellitepositionbyacomputer.
•Thesatellitepositionpredictionsareusuallysuppliedbythe
satelliteoperators.
•Itmaybenotedthatsinceaprogramtracksystemisanopen-
loopcontrolsystem,itsaccuracyismainlygovernedbythe
accuracyofthepredictiondata.

18. Earth Station Subsystems
•Communicationsatellitestransmitabeaconwhichisused
byearthstationsfortracking.Thereceivedbeaconsignalis
fedintotheauto-trackreceiverwheretrackingcorrections
or,insomeauto-tracksystemsestimatedpositionsofthe
satellitearederived.
MAIN ELEMENTS OF A
SATELLITE TRACKING SYSTEM

19. Earth Station Subsystems
•Tracking Subsystem
•Inotherauto-tracktechniquesthefeedsystemprovidesthe
requiredcomponentsoferrorsignals.
•Theoutputsoftheauto-trackreceiversareprocessedandused
todriveeachaxisoftheantennatotheestimatedsatellite
position.
•Inthemanualmode,anoperatorsetsthedesiredanglesfor
eachaxisonacontrolconsole.
•Thispositioniscomparedwiththeactualantennaposition
obtainedthroughshaftencoders,andthedifferencesignalis
usedtodrivetheantenna.
•Intheprogramtrackmodethedesiredantennapositionis
obtainedfromacomputer.
•Thedifferenceinthedesiredantennapositionsconstitutesthe
errorandisusedtodrivetheantenna.

20. Earth Station Subsystems
•Telemetry and Control Subsystems
•Thetelemetryequipmentconsistsofareceiverandthe
recordersandindicatorsthatdisplaythetelemetrysignals.
•Thesignalmaybereceivedbythemainantennaora
separatetelemetryantenna.
•Aseparatereceiveronafrequencydifferentfromthatofthe
communicationchannelsisusedfortelemetrypurposes.
•Theelectronicequipmentusedtodealwiththeinformation
transmittedbacktotheearthstation.

21. Earth Station Subsystems
•Power Subsystems
•Mostearthstationsreceivetheirpowerfromthenormalac
mains.
•Standardpowersuppliesconverttheacpowertothedc
voltagesrequiredtooperateallsubsystems.
•Mostearthstationshavebackuppowersystemsthattakeover
ifanacpowerfailureoccurs.
•Thebackuppowersystemmayconsistofadieselengine
drivinganacgenerator,whichautomaticallystartswhenac
powerfails.
•Smallersystemsmayuseuninterruptiblepowersupplies
(UPS),whichderivetheirmainpowerfrombatteries.

22. Thank you