Satellite Communication - MIT OpenCourseWare

1 Satellite CommunicationCol John KeeseeSatellite Communications Architecture Identify Requirements Specify Architectures Determine Link Data Rates Design & Size each link Document your rationaleDefinition Uplinks Downlinks Crosslinks Relays TT & CUplinkDownlinkIntersatellitelinksRelay satelliteRelay satelliteRelay satelliteSensor satelliteSensor satelliteCrossover orIntersatellitelinksMission dataLaunchphaseTT&CTT&CSatelliteGround stationTT&CTracking, Telemetry and ControlThe communications architecture consists of satellites and ground stations interconnectedwith communications links.

2 (Adapted from SMAD.)Architectures:Defined by Satellite -Ground Geometry Store & Forward Geostationary Molniya Geostationary/ Crosslink LEO/ CrosslinkAdapted from :Defined by Function System Function Tracking Telemetry & Command Data Collection Data Relay Satellite Design Onboard Processing Autonomous Satellite Control Network ManagementCommunications Architecture: selection criteria Orbit RF Spectrum Data Rate Duty Factor Link Availability Link Access Time ThreatAdvantages of Digital Communication Less distortion and interference Easy to regenerate Low error rates Multiple streams can be easily multiplexed into a single stream Security Drift free, miniature, low power hardwareTracking Telemetry & Control Telemetry Voltages, currents, temperatures , accelerations, valve and relay states Commanding Low data rate Store, verify.

3 Execute or execute on time Programmable control Range or Range Rate Round trip delay yields range Doppler shift yields range rate Pseudo-random code Existing TT&C Systems AFSCN (SGLS) - AF Satellite Control Network (Space Ground Link System) NASA DSN - Deep Space Network Intelsat/ COMSAT TDRS - Tracking and Data Relay SatelliteData Collection MissioncycledutySecondSamplessamplePixel spixelBitsimagerDRbYVnXSwpushbroomDR_/)( )( Adapted from Definitions Chart 9 Variable DefinitionUnitsDR Data RateBits/secondSW Swath WidthMetersXAcross track pixel dimensionMetersVnGround track velocity Meters/secondY Along track pixeldimensionMetersbBits/pixelBitsReduc ing the Data Rate Increase the Duty Cycle Collect only above-threshold data Amplitude changes only Data compressionLink Design Process1.

4 Define Requirements for each link2. Design Each Link Select frequency Select modulation & coding Apply antenna size & beam width constraints Estimate atmospheric, rain attenuation Estimate received noise, interference power Calculate required antenna gain & transmitter power3. Size the Payload Payload antenna configuration, size & mass Estimate transmitter mass & power Estimate payload mass & powerLink EquationEbNo PLlGtLsLaGrkTsREnergy/bit to noise-density ratioVariable Definitions Chart 12 Variable Definition Units Units dBEbEnergy per bit Watt-seconds dBNoNoise spectral densityWatts/hertz dB P Transmitter powerWatts dBW LlLine loss dBGtTransmitterantenna gain db LsSpace loss DBVariable DefinitionsChart

5 12 continuedVariableDefinitionUnitsUnits (dB)LaTransmissionpath lossdBGrReceiver gaindBkBoltzmannconstantJ/KdBW/(Hz-K)TsS ystem noisetemperatureKRData rateBits/secondPower Flux DensityWf PLlGtLa4SS2 (EIRP)La4SS2 EIRP - Effective Isotropic Radiated PowerVariable Definitionsfor Chart 16 VariableDefinitionUnitsUnits (dB)WfPower fluxdensityW/m2 SPath lengthMEIRPE ffectiveIsentropicRadiatedPowerWDBWR eceived PowerC Wf SDr2K4 PLlGtLaDr2K16S2Gr (SDr2K4)4SO2 S2Dr2KO2 Space LossLs (O4SS)2C EIRP*Ls*La*GrVariable DefinitionsChart 18 VariableDefinitionUnitsUnits (dB)CReceivedpowerWDrReceiverantennadiam etermdBKAntennaefficiencyOWavelengthmLsS pace lossLink Equation ConcludedEb energy/bit CRNo noise spectral densityN total received noise powerB receiver noise bandwidthNo=kTs N/BEbNo PuLluGtuLauGruLsk Ts RLink Equation in dBEbNo P Ll Gt Ls La Gr 228.

6 6 10 logTs 10 logR EIR P Ls La Gr 228 .6 10 logTs 10 logRCNo EIRP Ls La GrTs EIRP Ls La GrTs 10 logBRIP EbNo GrTs 10 logR(Received isentropic power)Gain in dBGr S2Dr2KO2f cOG 20 logS 20 logD 20 logf 10 logK 20 logc (dB) 20 logD 20 logf 10 logK (dB)BeamwidthT [degrees]f [GHz]D [m]T 21f DG 27, 000T2LT 12(e/T)2 (dB)Antenna gainOffset beam lossSpace loss in dBLs O4SS 2 (ratio)Ls= - 20 logS- 20 logf (dB)System Noise Temperature- External to Antenna Galactic noise Clouds, rain in path Solar noise (in mainbeam or sidelobe) Earth (290K) Man-made noise Nearby objects Satellite structure(See SMAD Fig 13-7)System Noise Temperature- Internal to System Transmission lines and filtersFis a figure of merit for a receiverTr (1 L)

7 TL PoPi Low noise amplifierTr F 1 290 KTs Tant To1 LrLr ToF 1Lr Variable DefinitionsChart 21 VariableDefinitionUnitsTrReceiver noisetemperatureKLPower ratioTComponent temperature KPoOutput powerWPII nput powerWFNoise figureToReference temperature(usually 290 K)KModulation Modulation modifies an RF Carrier signal so that it contains input signal information Amplitude Frequency Phase PolarizationModulation Techniques BPSK - Binary Phase Shift Keying QPSK - Quadriphased Phase Shift Keying FSK - Frequency Shift Keying MFSK - Multiple FSK DPSK - Differential Shift KeyingBit Error Rate Primary Figure of Merit for Digital Link Performance Energy/bit (Eb) must exceed the noise spectral density (No)

8 To achieve a required BERC oding Forward Error Correction sends additional data to help detect and correct errors. Reduces the Eb/No requirement Reduces required transmitter power Reduces antenna size Increases margin Increases data rate and bandwidthConvolutional Coding with Viterbi Decoding Extra bits sent with each block of data bits Receiver examines string of bits, generates possible code sequences, selects most likely Shannon limit Eb/No= dB Double coding necessary on deep space probesAttenuation Atmosphere absorbs some frequencies Divide zenith attenuation bysin(elevation angle)

9 Oxygen absorption at 60 GHz Scintillation disrupts below 200 MHzRain and Cloud Attenuation Crane model for world s climatic data Important above 10 GHz Worst for elevation angles < 20 degrees Rain reduces availabilityRain and Cloud AttenuationAdapted from MHzGBSU plinkACTSD ownlinkCommercial SATCOM ServicesCommercial SATCOM ServicesGovernment / Military SATCOM ServicesGovernment / Military SATCOM ServicesVHFUHFLSXKVVHFEHFAF / FLTSATCOMUFOM ilitaryUHFBandGovernmentS-Band (SGLS)USGovernmentX-Band1 GHz8 GHz18 GHz40 GHz75 GHzDSCSU plink500 MHzMilitary EHF (44/20)inmarsat, odyssey,iridium, globalstarodyssey,inmarsat,globalstarINT ELSAT,inmarsatINTELSATTELEDESIC,COMERCIA L,iridium,odyssey(gateway links)SPACEWAY, CYBERSTAR, ASTROLINKTELEDESIC iridium, odyssey (gateway links)Com Frequencies UsageALL CAPS = FixedSatelliteService (FSS)small case = MobileSatelliteService (MSS)/PersonalCommServices (PCS) Mhz400 , users aresecondary in UHF.

10 Subject to interferencefrom terrestrial usersHeavy orbital/terrestrialcongestion: much coordinationwith terrestrial users needed1 GHz1 GHz1 GHz1 GHz2 Mhz900 regular cellular(Land Mobile Radio)Freq at Risk: Int l &US CommercialencroachmentGPSL2 MhzL1 HEMISPHERELEGENDLOCATIONS OF CURRENT & PROPOSED GEOSTATIONARY SATELLITESWITH THRU DOWNLINKS= SKYSAT (PROPOSED)PROPERTY OF:JOINT SPECTRUM CENTERREVISED 6-27-96= SAMSAT (PROPOSED)= EUROSKYWAY(PROPOSED)= SOUTH AFRICASAT(PROPOSED)= ARTEMIS (PROPOSED)= ASTROLINK (PROPOSED)= EDRSS (PROPOSED)= MORNINGSTAR