UNIT-2 Satellite sub systems ppt topics.pdf

This pdf contains satellite subsystem ppts

1. Satellite Communication
SatelliteSubsystems
Mr.P.Krishna ReddyM.Tech.(Ph.D.)
Assistant Professor,
DeptofECE,
DIET.

2. UNITII
SATELLITESUBSYSTEMS[1] :
Attitudeandorbitcontrolsystem,telemetry,
tracking,Commandandmonitoring,powersystems,
communicationsubsystems,SatelliteantennaEquipment
reliabilityandSpacequalification.

3. •ASatelliteIsMade UpofSixMajorSubsystems:
⚫Attitude&orbitcontrolsystem(AOCS)
⚫TelemetryTrackingCommandandmonitoringSystem
(TTC&M)
⚫Powersupplysystem
⚫Communicationsubsystems
⚫SPACECRAFTANTENNAS
SatelliteSubsystems:

4. AOCS
(Attitude&orbitcontrolsystem)
Attitudeandorbitcontrolisusedtocontrolorbitofthe
satellite,besideshelpingmaintainstabilizationanditsposition.
➢Theattitudeandorbitofasatellitemust becontrolled sothat
thesatellite’santennaspointtowardtheearth.
➢Satellitemaydeviatefromitsorbitduetothegravitational
forcesfromthesun,moonandotherplanets..
➢Theseforceschangescyclicallyovera24-hourperiod,since
thesatellitemovesaroundtheearth.

5. AtGEOorbitaltitudethemoon’sgravitational
forceisabout twice asstrongas thesun’s
▪Moonorbitisinclinedtotheequatorialplaneby
approximately5 degrees
▪Theplaneoftheearth’srotationaroundthesunis
inclinedto23degreestotheequatorialplane
▪Netgravitationalforceonthesatellitetendsto
changetheinclinationofthesatelliteapproximately
0.86degreesperyearfromtheequatorialplane.
▪LEOsatellitesarelesseffectedbythisgravitationalpull
fromthesunandmoon

6. Itmayalsodeviatefromitsorbitbecauseofotherreasonslike:
➢Irregularitiesintheearthsgravitationalfields
➢Solarpressurefromthesun
➢Variationsintheearthsmagneticfield
▪Attheequatortherearebulgesofabout65matlongitudes15
0
W165
0
E,withtheresultthatthespacecraftexperiencestowardthe
stablepointinGEOatlongitudes105
0
westand75
0
East.
▪Tomaintainaccuratestationkeeping,thespacecraftmustbe
periodicallyacceleratedintheoppositedirectiontotheforcesacting
onit.Thisisdonebyfiringrocketmotorsorthrusters,atperiodic
intervals.ItcanbycontrolledbyTTC&Msystems.

7. Thecontrolcanbeeffectedfromthesatelliteit
selfandfromground.ThisAOC(Attitudeandorbit
controlsystem)subsystemconsistsoffourmajor
parts.
➢1.Attitudecontrolsystem
➢2.Orbitcontrolsystem
➢3.Sensors
➢4.Propulsionsystem

8. Attitudecontrolsystem
Twowaystomakethesatellitestablewhenisinorbit
andweightless.
➢Spinnersatellites--Satellitecanberotatedatarate
between30and100rpmto creategyroscopicaction,
whichmaintainthespinaxisinthesamedirection;sucha
spacecraftarecalledSpinners.
➢Three-axis stabilized satellites –The rotation of the
spacecraftabouteachaxiscanbecommandedfromearth
byincreaseordecreasetheappropriatemomentum
speedbyusingthreemomentumwheels.Thisiscalled
three-axisstabilizedsatellites.

9. SpinStabilization
⚫Entirespacecraft
rotatesaboutvertical
axis
⚫Spinningsensorsand
payloads
⚫Cylindricalgeometry
andsolararrays

10. ⚫Satellitecanberotatedataratebetween30and100
rpm.Thisspinprovidesapowerfulgyroscopicaction
tomaintainthespinaxisinthecorrectdirection.
⚫Suchsatelliteconsistsofcylindricaldrumcoveredby
solarcellsandtherocketmotors
⚫Thetranspondersaremountedonthetopofthe
drum.
⚫Itisdrivenbyanelectricmotorsintheappropriate
directiontothatofthedrum,sothattheantennas
remainspointingtowardstheearth.
⚫Thisoppositemotioniscalleddespun.
⚫The despunsectioniskeptstationarybycounter
rotationprovidedbysmallgasjetsmountedonthe
peripheryofthedrum.

12. DualSpinStabilization
⚫Uppersectiondoesnotrotate
(de-spun)
⚫Lowersectionrotatesto
providegyroscopicstability
⚫Uppersectionmayrotate
slightlyorintermittentlyto
pointpayloads
⚫Cylindricalgeometryandsolar
arrays

13. Three-axisBodyStabilization
Asatellitecanrotateaboutthe
threeaxistermedasyaw,rollandpitch.
Whenasatelliteisstabilizationabout
theseaxesbyusingthreemomentum
wheels,itiscalledasthreeaxisbody
stabilization
⚫Advantages
◦Node-spinrequiredforpayloads
◦Accuratepointing
⚫Disadvantages
◦Complex
◦Addedmass

15. Twotypesofmotorsusedonsatellites.
➢Traditionalbipropellantthruster
▪BipropellantsusedareMono-methylHydrazine
and Nitrogentetraoxide
▪Theyarehypogolic,i.e.,theyignitesimultaneouslyon
contactwithoutanycatalystorheater
➢Arcjetsorionthrusters
◦Highvoltageisusedtoaccelerateions
▪FuelstoredinGEOsatelliteisusedfortwopurposes
➢Apogeekickmotor(AKM)thatinjectsthesatelliteinto
itsfinalorbit
➢Maintainthesatelliteinthatorbitoveritslifetime.

16. Sensors
⚫Forattitudecontrol twotypesofsensors
areused
1.Earth
sensor 2.Sun
sensor

17. Earthsensor
⚫It is apassive infrared device, operating in 14-16 um
wavelength.
⚫Itsensestheinfraredrayscomingfromthehorizon.
⚫Thereisasharptemperaturedifferencebetweenthe
spaceandearth’shorizon,asspaceiscoolandearthis
warm.
⚫Twoearthsensorsareused,positioned5
0northand5
0
southofthespinaxis.

18. Cont…..
⚫Whenthespinaxisofthesatelliteiscorrectly
maintainedtheoutputofnorthandsouthsensorsare
inphase,otherwisetheyareinoutofphase.
⚫Thephasedifferencepulsesaresenttotheearth
stationandtheymeasureearthaspectangle.

19. Sunsensor
⚫Itoperatesinthevisiblespectrumandusea
photocellfordetectingsolarradiations.
⚫Therearetwosolarsensors,oneparalleltothe
spinaxisandtheothercanted35
0
.
⚫Pulsesfromthesensorsendto theearth
stationtodetermineSolaraspectangle.

22. Example

23. Example

24. Orbitcontrol system

26. Telemetry,Tracking,Commandand
monitoringSystem(TTC&MSystem)
FunctionofTTC&M:Providesthemeansofmonitoringand
controllingthesatelliteoperations.
Telemetry:asystemthatreliablyandtransparentlyconveys
measurementinformationfromaremotelylocateddata
generatingsourcetouserslocatedinspaceoronEarth.
Tracking:asystemthatobservesandcollectsdatatoplotthe
movingpathofanobject.
Command:asystembywhichcontrolisestablishedand
maintained.

27. TTC&M

29. TelemetryModes

32. Tracking

33. Command

35. ⚫Decoderreproducecommandmessageandproduce
lock/enableandclocksignal.
⚫Commandlogicvalidatesthecommand.
1.defaultistorejectifanyuncertaintyofvalidity
2.drivesappropriateinterfacecircuitry.
⚫CommanddecoderdecodesdetectsPCMencodingand
outputsbinarystreamsinNRZformat
⚫Thefunctionofinterfacecircuitryarelatchingrelays
withtelesales,pulsecommands,levelcommandsand
serialandparalleldatacommands.

38. Powersupplysystem
⚫Incommunicationsatellitetheelectricpowersupply
systemprovideselectricalenergyrequiredtorunthe
spacecraft,whenthesatelliteisinstationaryorbit.
⚫Thissubsystemisfurtherdividedintothreeparts
1.Solararray.
2.Battery.
3.Powersupplycontrolcircuit.
Solarcellsarephotovoltaiccellsthatconvertssolarradiationinto
electricityandthenitisconvertedtotherequiredvoltagelevelin
apowersupply.
Theconversionefficiencyofsolarcellisaround12%to15%and
typicalsatellitesolarcellsgenerate1to2KWofpower.

39. Powersystems-1
Powersystems-2

40. Powersystems-3

41. Power
⚫Provides,stores,distributes,andcontrolselectricalpower.
⚫Needpowerfor(basicallyeverything)communications,
computers, scientific instruments, environ. control and life
support,thermalcontrol,andevenforpropulsion(tostart
therocketengine).
⚫Solararray:sunlight➔electricalpower
◦max.efficiency=17%(231W/m
2ofarray)
◦degradedueto radiationdamage0.5%/year
◦bestformissionslessthanMars’dist.fromSun
⚫RadioisotopeThermoelectricGenerator
(RTG):nucleardecay➔heat➔electricalpower
◦max.efficiency=8%(lotsofwasteheat!)
◦bestformissionsto outerplanets
◦politicalproblems(protestsaboutlaunching
238PuO
2)
⚫Batteries–goodforafewhours,thenrecharge

42. Power
⚫DynamicPowerSources
◦LikepowerplantsonEarth.
⚫FuelCells
◦Think oftheseasrefillablebatteries.
◦TheSpaceShuttleuseshydrogen-oxygenfuel
cells.
⚫Thedesignishighlydependenton:
◦SpaceEnvironment(thermal,radiation)
◦Shadowing
◦MissionLife

43. Communicationsubsystems
⚫Communication:asystemthatenablingthetransferof
informationfromonepointtoanother.
⚫Transmitsdatatogroundortorelaysatellite(e.g.
TDRS)
⚫Receivescommandsfromgroundorrelaysatellite

44. RepeatersandTransponders
A transponder consists of BPF to select the particular channel’s band
offrequency,a downconverter& anoutputamplifier.

45. Typesofpayloads/Transponders
therearetwo typesoftransponders.Thoseare
•Bentpipetranspondersand
•Regenerativetransponders.
BentPipeTransponders:
Bentpipetransponderreceivesmicrowavefrequencysignal.Itconverts
thefrequencyofinputsignaltoRF frequencyandthenamplifiesit.
Bentpipetransponderis alsocalledasrepeaterandconventional
transponder.Itissuitableforbothanalogand digitalsignals.

46. RegenerativeTransponders
RegenerativetransponderperformsthefunctionsofBentpipe
transponder.i.e.,frequencytranslationandamplification.In
additiontothesetwofunctions,Regenerativetransponderalso
performsthedemodulationofRFcarriertobaseband,regeneration
ofsignalsandmodulation.
RegenerativetransponderisalsocalledasProcessingtransponder.
Itissuitableonlyfordigitalsignals.Themainadvantagesof
RegenerativetranspondersareimprovementinSignaltoNoise
Ratio(SNR)andhavemoreflexibilityinimplementation.

47. Fig. shows a typical single conversion bent pipe transponder used
onmanysatellitesfor6/4GHzband.
Thelocaloscillator isat2225MHzto providetheappropriate
shiftinfrequency from6GHzto4GHz.
Bandpassfilterafterthemixerremovesunwantedfrequencies.
Theoutput poweramplifierisusuallyasolidstatepower
amplifier(SSPA)unlessaveryhigh outputpowerisrequired.

48. ⚫Redundancyisprovidedforhigh-poweramplifiersineach
transponderbyincludingaspareTWTA(travellingWavetube
amplifier)orsolid-stateamplifier(SSPA)thatcanbeswitchedif
primarypoweramplifiersfail.
⚫Thelifetimeofhighpoweramplifier’sislimitedandtheyrepresent
leastreliablecomponentinmosttransponders.
⚫ProvidingaspareHPAineachtransponderincreasesthe
probabilitythatthesatellitewillreachtheendofitsworkinglife
withallitstranspondersworking.

51. ⚫Transpondersforuseinthe14/11-GHzbandsnormallyemploya
doublefrequencyconversionscheme
⚫Incoming14-GHzcarrieristranslatedtoanIFaround1GHz.
⚫Theamplification&filteringareperformedat1GHz&arelatively
high-levelcarrieristranslatedbackto11GHzforamplificationby
HPA.

54. Factorsaffectingpayloaddesign

55. SpacecraftAntennas

56. SatelliteAntennaTypes
Fourmaintypeofantennasareusedonsatellite.
Theseare
1.WireAntennas:Monopole&Dipole
2.HORN:Efficient,Low Gain,WideBeam
3.REFLECTOR:HighGain,NarrowBeam,Mayhave
tobedeployedinspace
4.PHASEDARRAY:Complex,Electronicallysteered
56

57. ⚫TheseareusedprimarilyatVHF
& UHF to provide
communicationsfortheTTC&M
systems.
⚫Theyarepositionedwithgreat
careonthebodyofthesatellite
inanattempttoprovideOmni
directionalcoverage.
⚫Anantennapatternisaplot
ofthefieldstrengthinfar
fieldonantenna.
⚫Itisusuallymeasuredin
decibels(dB).
WireAntennas:

58. HornAntennas
⚫Theseare
frequencies
usedatmicrowave
whenrelativelywide
globalbeamsarerequired,asfor
coverage.
⚫Ahornisaflaredsectionofwaveguide
thatprovidesanapertureseveral
wavelengthswideandagoodmatch
betweenthewaveguideimpedance
usedasfeedsfor
andfreespace.
⚫Hornsarealso
reflectors

59. ReflectorAntennas
⚫Themostoftenusedantennaforsatellitesystems,
particularlyforthoseoperatingabove10GHz,isthe
parabolicreflectorantenna.
⚫Theseareusuallyilluminatedbyoneormorehorns
andprovidealageraperturecomparedtohorn
antenna.
⚫Parabolicreflectorsofferamuchhighergainthanthat
achievablebythehornantennaalone.

60. ArrayAntennas:
⚫Arrayantennasarealsousedonsatellitestocreate
multiplebeamsfromasingleaperture.
⚫Multiplebeamsareformedbycombiningtheradiation
fromseveralsmallelementsmadeupofdipoles,horns
etc.
⚫IridiumandGlobalstarusedthesekindofantennasto
generateupto16beamsfromasingleaperturefor
theirLEOmobiletelephonesystems.
⚫Theseantennasgeneratehighgainoutputsignalswhich
arepreferredforlongdistancecommunication.

65. EquipmentReliability&Spacequalification:
⚫Oncesatelliteisingeostationaryorbit,thereislittlepossibilityofrepairing
componentsthatfailoraddingmorefuelforstationkeeping.
⚫Thecomponentthatmakeupthesatellitemustthereforehaveveryhigh
reliability.
⚫Twoapproachesareused:Spacequalificationofeverypartofthe
satellitetoensurethatithasalonglifeexpectancyinorbit&
redundancyofmost criticalcomponentstoprovidecontinuedoperation
whenonecomponentfails.

66. SpaceQualification:
distances,isaharsh⚫Outerspace,atgeostationaryorbit
environment.
⚫Thesunirradiatesthesatellitewith1.4kWofheatandlighton
eachsquaremeterofexposedsurface.
⚫Whensurfacesareinshadow,surfacetemperaturewillfall
towardabsolutezero.
⚫Electronicequipmentcan’toperateatsuchextremesof
temperature&heatedorcooledsothatit’stemp.stayswithin
therange0˚to75˚C.Thisrequiresathermalcontrolsystem.
⚫Thefirststageinensuringhighreliabilityinasatelliteisby
selectionandscreeningofeverycomponentused.
⚫Pastoperational&testexperienceofcomponentsindicates
whichcomponentscanbeexpectedtohavegoodreliability.
⚫Eachcomponentistestedindividuallytoensurethatitmeetsits
specification.Thisprocessisknownasqualitycontrolorquality
assurance&isvitalinbuildinganyequipmentthatistobe
reliable.

67. ⚫Onceindividualcomponentsandsubsystemshavebeenspacequalified,the
completesatellitemustbetestedasasystemtoensurethatitsmany
systemsarereliable.
⚫Whenasatelliteisdesigned,threeprototypemodelsareoftenbuildand
tested.
Mechanicalmodel-Itcontainsallthestructural&mechanicalpartsthat
willbeincludedinthesatellite&istestedtoensurethatallmovingparts
operatecorrectlyinvacuum,overawidetemperaturerange.Italso
subjectedtovibration&shocktesting.
Thermalmodel-Itcontainsallelectronicspackagesandothercomponents
thatmustbemaintainedatthecorrecttemperature.
Electricalmodel-containsalltheelectronicpartsofthesatelliteandis
testedforcorrectelectricalperformanceundertotalvacuumandawide
rangeoftemperatures.
⚫Testingcarriedoutonmodelsisdesignedtooverstressthesystemand
inducefailureinanyweakcomponents.
⚫Temperaturecyclingwillbecarriedoutto 10%beyondexpected
extremes;structuralloads50%abovethoseexpectedmaybeapplied.
⚫Electricalequipmentwillbesubjectedtoexcessvoltageandcurrentdrain
totestfor foodelectronicandthermalreliability.

68. Reliability:
⚫Weneedtobeabletocalculatereliabilityfortwo
reasons:Wewanttoknowwhattheprobabilityisthat
subsystemwillstillbeworkingafteragiventimeperiod,
&weneedtoprovideredundantcomponentsor
subsystemswhereprobabilityoffailureistoogreatto
beaccepted.
⚫Reliabilityisamathematicaltechniquewhichgivesan
ideaabouttheprobabilityoffailure.
a b

69. Reliability Cont..
⚫The reliabilityofacomponentcanbeexpressedin
termsofprobabilityoffailureaftertimet.
⚫Formostelectronicequipment,probabilityoffailureis
higheratbeginningoflife-theburn–inperiod.
⚫Asthecomponentages,failurebecomesmorelikely.
⚫Mathematicallyreliabilityofdeviceorsub-systemis
definedas,

70. Whereti=timeafterwhichithdevicefails.
⚫MTBFisrelatedtoaveragefailureby
Thefailurerate⅄isoftengivenasaveragefailurerate
per109hours.Intermsofreliabilityofsubsystemor
deviceitisgivenby
Theotherimprovementparameterinreliability
studyisthemeantimebeforefailure(MTBF)and
isgivenby

71. Redundancy:
⚫Byusingredundantcomponents,reliabilityofsubsystem
canbeincreased.
⚫Theredundantdevicecanincorporatewiththeexisting
operatingdevices/componentseitherinseriesorin
parallelormaybemixed.

72. END