Satellite CommunicationCol John Keesee
Satellite CommunicationsArchitecture Identify RequirementsSpecify ArchitecturesDetermine Link Data RatesDesign & Size each linkDocument your rationale
Definition UplinksDownlinksCrosslinksRelaysTT & C LaunchphaseSensorsatelliteCrossover orIntersatellitelinksRelaysatelliteMission , Telemetryand ControlSatelliteGround stationThe communications architecture consists of satellites and ground stations interconnectedwith communications links. (Adapted from SMAD.)
Architectures:Defined by Satellite-Ground Geometry Store & ForwardGeostationaryMolniyaGeostationary/Crosslink LEO/ CrosslinkAdapted from SMAD.
Architectures:Defined by Function System Function– Tracking Telemetry & Command– Data Collection– Data Relay Satellite Design– Onboard Processing– Autonomous Satellite Control– Network Management
Communications Architecture:Selection Criteria OrbitRF SpectrumData RateDuty FactorLink AvailabilityLink Access TimeThreat
Advantages of DigitalCommunication Less distortion and interferenceEasy to regenerateLow error ratesMultiple streams can be easily multiplexedinto a single stream Security Drift free, miniature, low power hardware
Tracking Telemetry & Control Telemetry– Voltages, currents, temperatures, accelerations, valve and relay states Commanding– Low data rate– Store, verify, 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 NetworkIntelsat/ COMSATTDRS - Tracking and Data Relay Satellite
Data Collection MissionSw VnDR ( pushbroom )bX YBitsPixels Samples / SecondDR (imager )pixel sampleduty cycleAdapted from SMAD.
Variable DefinitionsChart 9Variable DefinitionUnitsDRData RateBits/secondSWSwath WidthMetersXAcross track pixelMetersdimensionGround track velocity Meters/secondVnYbAlong track pixeldimensionBits/pixelMetersBits
Reducing the Data Rate Increase the Duty CycleCollect only above-threshold dataAmplitude changes onlyData compression
Link Design Process1. Define Requirements for each link2. Design Each Link––––––Select frequencySelect modulation & codingApply antenna size & beam width constraintsEstimate atmospheric, rain attenuationEstimate received noise, interference powerCalculate required antenna gain & transmitter power3. Size the Payload– Payload antenna configuration, size & mass– Estimate transmitter mass & power– Estimate payload mass & power
Link EquationEbNoP Ll Gt Ls La Grk Ts REnergy/bit to noise-density ratio
Variable DefinitionsChart 12Variable DefinitionUnitsEbEnergy per bit Watt-secondsNoNoise spectral Watts/hertzdensityPTransmitterWattspowerLlLine lossUnits dBdBdBGtdbLsTransmitterantenna gainSpace lossdBWdBDB
Variable DefinitionsChart 12 h lossReceiver gainBoltzmannconstantSystem noisetemperatureData rateUnitsJ/KKBits/secondUnits (dB)dBdBdBW/(Hz-K)
Power Flux DensityWfPLlGt La24SS(EIRP) La24SSEIRP - Effective Isotropic Radiated Power
Variable Definitionsfor Chart 16VariableWfSEIRPDefinitionPower fluxdensityPath nits (dB)DBW
Received PowerCWf S Dr 2K42GrSDr K 4S() 24OSpace LossCPLl Gt La Dr 2 K216S22S Dr KO2LsO 2()4S SEIRP * Ls * La * Gr
Variable DefinitionsChart tennadiameterAntennaefficiencyWavelengthSpace lossUnitsWUnits (dB)mdBm
Link Equation ConcludedCREbenergy/bitNonoise spectral densityNBtotal received noise powerreceiver noise bandwidthNo k TsEbNoN/BP u Ll u Gt u La u Gr u Lsk Ts R
Link Equation in dBEbNoP Ll Gt Ls La Gr 228 .6 10 log Ts 10 log REIRP Ls La Gr 228 .6 10 log Ts 10 log RCNoCNRIPGrEIRP Ls La 228.6TsGrEIRP Ls La 228.6 10log BTsEb Gr 228.6 10log RNo Ts(Received isentropicpower)
Gain in dBGrGS 2 Dr 2KO2fcO20log S 20log D 20 log f 10log K 20log c (dB) 159.59 20log D 20log f 10log K (dB)
Beamwidth21f DTGLTT [degrees]27,000T 2f [GHz]D [m]Antenna gain12(e / T )2 (dB)Offset beam loss
Space loss in dBLs§ O · 2 (ratio) 4S S ¹ Ls 147.55- 20 log S - 20 log f (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 filtersTr(1 L)TLPoPiF is a figureof merit fora receiver Low noise amplifierTrTsF 1 290KTant§ 1 L · § F 1· r To L To L r ¹ r ¹
Variable DefinitionsChart 21VariableTrLTPoPIFToDefinitionReceiver noisetemperaturePower ratioComponent temperatureOutput powerInput powerNoise figureReference temperature(usually 290 K)UnitsKKWWK
Modulation Modulation modifies an RF Carrier signalso that it contains input signal rization
Modulation Techniques BPSK - Binary Phase Shift KeyingQPSK - Quadriphased Phase Shift KeyingFSK - Frequency Shift KeyingMFSK - Multiple FSKDPSK - Differential Shift Keying
Bit Error Rate Primary Figure of Merit for Digital LinkPerformance Energy/bit (Eb) must exceed the noisespectral density (No) to achieve a requiredBER
Coding Forward Error Correction sends additionaldata to help detect and correct errors.–––––Reduces the Eb/No requirementReduces required transmitter powerReduces antenna sizeIncreases marginIncreases data rate and bandwidth
Convolutional Coding withViterbi Decoding Extra bits sent with each block of data bits Receiver examines string of bits, generatespossible code sequences, selects mostlikely Shannon limit Eb/No -1.6 dB Double coding necessary on deep spaceprobes
Attenuation Atmosphere absorbs some frequencies Divide zenith attenuation bysin(elevation angle) Oxygen absorption at 60 GHz Scintillation disrupts below 200 MHz
Rain and Cloud Attenuation Crane model for world’s climatic dataImportant above 10 GHzWorst for elevation angles 20 degreesRain reduces availability
Rain and Cloud AttenuationAdapted from SMAD.
SATCOM Frequencies ces“regular” cellular(Land Mobile Radio)800 Mhzinmarsat, odyssey,iridium, globalstar900ALL CAPS Fixed Satellite Service (FSS)small case Mobile Satellite Service (MSS)/Personal Comm Services yssey(gateway .5Com’l.KCom’l.KuCom’l.C20.217.3SPACEWAY, CYBERSTAR, ASTROLINKTELEDESICiridium, odyssey (gateway links)40GHzKKuVKa75GHz225 MhzDSCSGPSAF / FLTSATCOMUFOL1:L2:1227.6 Mhz 1575.42MilitaryUHFBand7.25Downlink500 MHzSATCOM users aresecondary in UHF:subject to interferencefrom terrestrial users7.75DSCS400 Mhz1.7611.8427.92.2002.290Freq at Risk: Int’l &US Commercialencroachment7Uplink500 MHz820.2Downlink1 GHz20.2GBSUplink1 GHz21.2 30UplinkUplink291 GHz31MILSTARACTS8.4Heavy orbital/terrestrialcongestion: much coordinationwith terrestrial users neededGovernmentGovernment/ /MilitaryMilitarySATCOMSATCOMServicesServices1 GHz19.2USGovernmentX-BandGovernmentS-Band (SGLS)MILSTAR,GBSACTSDownlink30432 GHzMilitary EHF (44/20)45
LOCATIONS OF CURRENT & PROPOSED GEOSTATIONARY SATELLITESWITH 17.3-GHzNORTHERNTHRUHEMISPHERE20.2-GHz LEGEND13030160160 ACTS (PROPOSED)0170 AFRISAT (PROPOSED)180170 ARTEMIS (PROPOSED) SOUTH AFRICASAT MALTASAT ARABSAT (PROPOSED) ASIASAT (PROPOSED) ASTROLINK (PROPOSED) ECHOSTAR (PROPOSED) EUTELSAT MEGASAT (PROPOSED)(PROPOSED) SUPERBIRD (PROPOSED) THIACOM (PROPOSED)(OPERATIONAL/PROPOSED) EDRSS (PROPOSED) BSB (PROPOSED)(PROPOSED) EUROSKYWAY(PROPOSED) MORNINGSTAR (PROPOSED) MILLENIUM (PROPOSED) N STAR (PROPOSED) CANSAT (PROPOSED) TONGASAT TOR(PROPOSED)(OPERATIONAL/PROPOSED) TURKSAT (PROPOSED) GALAXY/SPACEWAY (PROPOSED) CHINASAT (PROPOSED) CYBERSTAR (PROPOSED) DACOMSAT ORION (PROPOSED) GE STAR (PROPOSED) HISPASAT (PROPOSED)(PROPOSED) INFOSAT (PROPOSED) DFS (OPERATIONAL/PROPOSED) INTELSAT-KA (PROPOSED) ITALSAT (PROPOSED) DIAMONDSAT PAKSAT (PROPOSED) PANAMSAT (PROPOSED)(PROPOSED)(PROPOSED) VISIONSTAR (PROPOSED) SAMSAT (PROPOSED) SARIT (PROPOSED) EASTSAT (PROPOSED) LUX\KA (PROPOSED) USCSID VIDEOSAT(PROPOSED) KASTAR (PROPOSED)(OPERATIONAL/PROPOSED) USASAT (PROPOSED) RADIOSAT (PROPOSED) DRTS (PROPOSED) KYPROS (PROPOSED) USABSS SKYSAT (PROPOSED) VOICESPAN (PROPOSED) YAMAL (PROPOSED)PROPERTY OF:JOINT SPECTRUM CENTERREVISED 6-27-96
Frequency Selection Drivers Spectrum availability and FCC allocationRelay/Ground Station frequencyAntenna sizeAtmospheric/Rain attenuationNoise temperatureModulation and coding
Communication PayloadAntennas ParabolicHelixHornPhased Arrays– Multiple beams– Hopping beams
Milstar Satellite Layout Weight:Length:10,000 lb51 ft (across payload)116 ft (across solar arrays)5,000 W22,500 miles geosynchronousTitan IV/Centaur upper stageArray Power:Orbit Altitude:Launch Vehicle:SPACECRAFT BUS Z X WING CROSSLINKANTENNASHFAGILEWSBUHFRCV XSHFECEHFAGILEEHFECHORIZONSENSORSREACTION WHEELASSEMBLIES (SCS)NSB #2MDR NULLINGANTENNASTHRUSTERSNSB #1 X WINGPAYLOAD(LDR)UHFXMTFLEXIBLE SUBSTRATESOLAR ARRAY PANELS-X WING CROSSLINKANTENNACROSSLINKTHRUSTERSMDR DUCAANTENNAS- Z- X-X WING PAYLOAD(MDR*, CROSSLINK)PROPELLANTTANKS
UPLINK:5 AGILES, 2 NARROW SPOTS,1 WIDE SPOT, 1 EARTH COVERAGE(Image removed due to copyright considerations.)(Image removed due to copyright considerations.)DOWNLINK:SINGLE DOWNLINK TIME-SHARED BY:1 AGILE, 2 NARROW SPOTS,1 WIDE SPOT, 1 EARTH COVERAGEUPLINK:2 NULLING SPOTS6 DISTRIBUTED USER COVERAGE(DUCs)DOWNLINK:SINGLE DOWNLINK TIME-SHARED BY:2 SPOTS AND 6 DUCs
Multiple Access Strategies FDMA - Frequency Division MultipleAccess TDMA - Time Division Multiple Access CDMA - Code Division Multiple Access– Phase Modulation plus pseudo-random noise
Antijam Techniques Spread SpectrumNarrow beamwidthsOn board processingNulling antennas
Special Topics Data security through encryptionSpatial, time and satellite diversityFrequency hoppingInterleaving
Why Compress Data Need to send more data than bandwidthaccommodates– Digital image files in particular are very large Bandwidth is limited by the link equationand international regulation Concept inseparable from data encoding
Early Development -- Huffmancodes Assign different number of bits to each possible symbolto minimize total number of bits– Example: Encode letters of alphabet– 26 symbols, each with equal chance of occurring 5bits/symbol(25 32 lowest power of 2 above 26)– If R occurs 50% of time, use fewer bits to encode R.
Compression Algorithms Lossless compression– Ensures data recovered is exactly same as original data– Used for executable code, numeric data -- cannot toleratemistakes Lossy compression– Does not promise that data received is the same as data sent– Removes information that cannot later be restored– Used for still images, video, audio - Data contains more info thanhuman can perceive– Data may already contain errors/imperfections– Better compression ratios than Lossless (order of magnitude)
When does Compression PayOff? Compression/decompression algorithms involve timeconsuming computations Compression beneficial whenx / Bc x / (r Bn) x / BnWhere Bc data bandwidth through compress/decompressprocessBn network bandwidth for uncompressed datar average compression ratiox / Bn time to send x bytes of uncompressed datax / Bc x / (rBn) time to compress and send Simplified:Bc r / (r - 1) * Bn
Lossless CompressionAlgorithms Run Length Encoding Differential Pulse Code Modulation DPCM Dictionary-Based Methods
Run Length Encoding Replace consecutive occurrences of symbol with 1copy plus count of how many times symbol occurs:AAABBCDDDD 3A2B1C4D Can be used to compress digital imagery– Compare adjacent pixel values and encode only changes Scanned text can achieve 8-to-1 compression due tolarge white space Key compression algorithm used to transmit faxes Large homogeneous regions -- effective Small degree of local variation increases image bytesize– 2 bytes represent 1 symbol when not repeated
Differential Pulse CodeModulation - DPCM Represent differences between data– Output reference symbol– For each symbol in data, output difference between it andreference symbol: AAABBCDDDD - A0001123333 When differences are small, encode with fewer bits (2bits vs 8 bits) Takes advantage of fact that adjacent pixels are similar 1.5-to-1 Delta encoding encodes symbol as difference fromprevious one: AAABBCDDDD - A001011000. Works well when adjacent pixels are similar Can combine delta encoding and RLE
Dictionary-Based Methods Lempel-Ziv (LZ) most well known, used by Unixcompress command– Build dictionary of expected data strings– Replace strings with index to dictionary Example: "compression" (77-bits of 7-bit ASCII) hasindex 4978 (15 bits) in /usr/share/dict/words -- 5-to-1compression ratio How is the dictionary built?– A priori, static, tailored to data– Adaptively define based on contents of data. However,dictionary must be sent with data for proper decompression
Graphical Interchange Format(GIF) Variation of LZ algorithm used for digital images––––Reduce 24-bit color to 8-bit-colorStore colors in table which can be indexed by an 8-bit numberValue for each pixel replaced by appropriate indexRun LZ over result and create dictionary by identifying commonsequences of pixels If picture contains 256 colors, can achieve 10-to-1compression If picture contains 256 colors, Lossy! (e.g., naturalscenes)
Image Compression JPEG (Joint Photographic Experts Group) defines analgorithm and a format– Apply discrete cosine transform (DCT) to 8 x 8 block (transforminto spatial frequency domain). Lossless.– Low frequency gross features; high frequency detail– Quantize result, losing least significant info. Lossy– Encode result - RLE applied to coefficients. Lossless.
Color Images Three components used to represent each pixel - 3D– RGB - red, green, blue– YUV - luminance (Y) and two chrominance (U and V) To compress, each component is processed independentlyThree components used to represent each pixel - 3DJPEG can also compress multi-spectral imagesCompress 24-bit color images by 30-to-1 ratio– 24 bits - 8 bits (GIF) gives 3-to-1– 3D JPEG compression gives 10-to-1
Video Compressionx Moving Picture Experts Group (MPEG)x Succession of still images displayed at video rate– Each frame compressed using DCT technique (JPEG)– Interframe redundancy Typically, can achieve 90-to-1 ratio; 150-to-1 possible Involves expensive computation, typically done offline.
References Wertz, James R. and Wiley J.Larson, Space MissionAnalysis and Design, Microcosm Press, El Segundo CA1999, pg 533-586 Morgan and Gordon, Communication SatelliteHandbook, 1989 Peterson and Davie, on reserve in Barker Library f
Satellite Communication Col John Keesee. Satellite Communications Architecture Identify Requirements Specify Architectures Determine Link Data Rates Design & Size each link Document your rationale. Definition Uplinks Downlinks Crosslinks Relays TT & C Uplink Downlink Intersatellite links Relay satellite
the satellite output power to the maximum level, additional noise is introduced into the link from satellite to hub. Therefore, an accurate calculation of the SNR for the entire RTN link must consider: 1. the SNR of the terminal-to-satellite link 2. the SNR of the satellite-to-hub link When the output power of the satellite is at a maximum, SNR .
c. Satellite: Internet access provided through satellites orbiting the Earth. Satellite service requires a satellite Internet subscription from an Internet satellite service provider and a satellite dish. Carriers that provide satellite Internet service are DIRECTV, Dish Network, HughesNet, and Wildblue. d.
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Satellite Communication 1 In general terms, a satellite is a smaller object that revolves around a larger object in space. For example, moon is a natural satellite of earth. We know that Communication refers to the exchange (sharing) of information bet
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