Presentation on theme: "Talking to the Stars Deep Space Telecommunications"— Presentation transcript:
1 Talking to the Stars Deep Space Telecommunications James Lux, P.E.Spacecraft Telecommunications Equipment SectionJet Propulsion Laboratory29 Sep 2003, CL
2 Overview What is spacecraft telecom? What are the technical challenges?What’s different from the usual?How have we done it in the pastWhat’s going to happen in the future
3 A little about Jim New technologies Distributed Metrology and Control for Large Arrays“Adaptive Optics for RF”, with distributed computingDSP Scatterometer TestbedGeneral purpose DSP instead of custom hardwareAdvanced TransponderFPGA for NCO, de/modulation, de/codingSeawinds Calibration Ground Station (CGS)Measure time to ns, freq to Hz, pwr to 0.1dBTornadoes and projects in the garage
4 Tornadoes, Fire Whirls, Eclipses, High Voltage, Shrunken Coins, Robots! From top left, clockwise:Exploding Cadillac for a movie of the weekSmall Tesla coil1991 Total Eclipse in La Paz, Baja California8 foot high Fire vortex40 foot artificial tornado for Volvo (U.K.) commercialQuarter shrunk by electromagnetic forcesRoger the robot
5 Telecom-centric View of Spacecraft Design Telecom SubsystemCommand &Data HandlingSubsystemInstrumentTelemetryTranspondersRF TelemetryInstrumentCommandsPower AmpsRF CommandsAntennasPower SubsystemThere’s really a lot more to spacecraft design than shown here, however, we’re just talking about telecom.Power is really, really important for most deep space designs. Typically, you have a hundred watts DC, or so, to work with.(although, Prometheus will certainly change things, with 100 kWe available from the reactor)Attitude control is important, because that’s how we point the antenna, and it’s always a tradeoff between antenna gain and pointing.Solar PanelsPower ControlMechanicalThermalStructuralSubsystemsAttitudeControlBatteriesRadioisotopeThermal Generator
6 Some terminology Consultative Committee for Space Data Systems (red, green, blue books)Transponder = RadioHGA, MGA, LGA = High Gain Antenna, Medium… , Low…TWTA = Travelling Wave Tube AmplifierSSPA = Solid State Power Amplifier(tele)Commands = What we send to the spacecraft (uplink)Telemetry = What we get back from the spacecraft (downlink)Engineering, Housekeeping = what we need for operation and health monitoringScience Data = The raison d’être for the whole exerciseCCSDS – - all the specs can be downloaded as PDF files.
7 The Technical Challenges It’s a LONG way awayPath lossPointingLight timeWe have limited powerSolar panelsRadioisotope Thermal Generator (RTG)It takes forever to get there (and we hang out there a long time too!)Mars – 6-8 monthsOuter planetsJupiter (Galileo 6 yrs getting there, 7 yrs in orbit)Saturn (Cassini 7 yrs) (Voyager 26 yrs and still going!)Path loss of hundreds of dBPointing is a problem: you’ve got to find Earth, and high gain antennas have narrow beamwidths.Light time has a lot of weird side effects. For instance, you might be transmitting from a different DSN station than you receive from, because Earth has turned in the intervening time.Long life and reliability is a very important aspect. If you only have one widget, that MTBF of 50,000 hours starts looking awfully short.
8 Path Loss (Friis Equation) Loss (dB) = log(km) + 20 log(MHz)(Assumes Isotropic Antenna, which isn’t really fair!)Mars2 AU376E6 km172 dBJupiter 5AU750E6 km178 dBPluto40 AU5900E6 km195 dBS band (2.3 GHz)66 dB271277295X band (8 GHz)78 dB282288306Ka Band (32 GHz)90 dB294300318Isotropic antennas are scaled to the wavelength, so the “receiver capture area” gets smaller as the frequency gets higher. In real life, you’re constrained by a physical size of the antenna, more than some artificial “isotropic”.However, if you don’t know which direction to transmit (or receive), you do need to approach isotropic patterns.
9 Example Link Budgets Downlink dominates the design X band JupiterTelecommandTelemetryTx Power20 kW+73 dBm35 Watts+45 dBmTx Antenna(70 m)+77 dB(2 m)+46 dBPath Loss-288 dBRx AntennaRx Power-92 dBm-120 dBmRx kT noise(300K)-174 dBm/Hz(20K)-186 dBm/HzRx BW1kHz+30 dBHz100 kHz+50 dBHzSNR+52 dB!+16 dBDownlink dominates the designBut wait… are these assumptions reasonable?35W Tx PowerDC power avail?46 dBi for antenna?Surface figureAntenna efficiency2 m ok?300K receiver noise temp?100 kHz enough BW for data?Recall that the entire s/c may only have 100W DC power available. If your transmitter PA is 20% efficient, and you’ve only got 50W allocated to you, then you’re limited to 10W RF power.
10 What’s the Frequency? Protected spectrum Trend S > X > Ka band (more channels, more BW)Up and Down related by ratio for rangingS Up: Dn:X Up: Dn:Ka Up: Dn:
11 Transponders Phase locked Tx/Rx for ranging Bit/Command decoder CodingSDST – Small Deep Space TransponderTx SynRx SynStaloUSOPhase locked Tx/Rx for rangingBit/Command decoderMultiple BandsBit DemodLNA
12 Spacecraft Antennas Accomodation Deployment Pointing Fit in the launch vehicle shroud (few meter diameter)Fit on the spacecraftGimbals?DeploymentGalileo HGA didn’tPointingHigh gain is great, but you’ve got to point it to the Earth46 dB » 1º » 17 mrad (2 meter dish at X-band)
13 Power Amplifiers Phase Modulation (BPSK, QPSK) Power Amplifiers SSPAs & TWTAsEfficiency is real importantGD Xband SSPAThales X-band TWTTWTA = Travelling Wave Tube AmplifierComposed of TWT (the tube) and a High Voltage Power Supply (HVPS) (or Electronic Power Converter – EPC)Tubes have been around for decades.State of the art for tubes is steadily advancing, as people spend money to develop the next one.EPC is a challenge: High Voltage (8 kV) and thermal dissipation are challenges in space.Two flavors of tubes: conduction cooled (shown) and radiation cooled (big fins or a “can” that radiates to “cold space”)SSPAs are steadily gaining efficiency with new devices.GaAs, GaN, etc.Limited gain/power from one device means that SSPA design has a lot of work with (low-loss) power dividers and combiners.100W η: 50-70% 2-3 kg+EPC 30x5x5 cm17 W η: 29% 1.32kg 17.4x13.4x4.7 cm
14 Coding Coding gets you closer to the “Shannon Limit” Deep space telecom codes wind up in other industriesReed-SolomonTurbo codesEb/No = Energy in a bit divided by Noise density
16 So, now you want to build a deep space telecom system? You’re in for the long haul (5-10 years)You’re going to generate a lot of paper and go to a lot of meetingsIt’s a different environment out there!Mission/Quality Assurance is a very different animal in space than in consumer electronicsLong, long development schedules + long, long missionsPaper and meetings because spacecraft are very complex systems with lots of interactions and interfaces…and…Paper is important for traceability and mission assuranceQA for space concentrates on making sure QTY:1 works forever, as opposed to the consumer model of “yield management”
17 How can it take so long? Lots of steps in the process Lots of interaction/integration with other subsystemsContract to industryRFP10/05EM (Engineering Model)Pre Phase AFM (Flight Model)A12 Mos“Gleam in eye”10/039MosBATLOConcept Review10/05A/B is also called “formulation”C/D is “implementation”E is operationsRFP = Request for ProposalPMSR =PDR= Preliminary Design ReviewCDR = Critical Design ReviewATLO = Assembly Test and Launch Operations (final assy, attach to launch vehicle (LV), and light the fuse)C/D40 MosPMSR10/06ENASA commitsthe fundsPDR7/07CDR7/08Launch11/10Reach Mars9/11CY 03CY 04CY 05CY 06CY 07CY 08CY 09CY 10CY 11+
18 Some Odd Consequences of the Long Life Cycle Parts availabilityMission manager will want parts with “proven heritage” (i.e. they worked the last time)5 more years ‘til launchEngineer retentionYou’ll finish the telecom system a year or two before launchIt may take 5 years after launch to get there, then what if you have a question about how something works?Development toolsCompilers, in circuit emulators, etc.Keep those old databooks!Galileo used 1802 μP (until a week ago)
19 More Practicalities Our product is paper! Quote from a HRCR (Hardware Review and Certification Record) submittal document:“The documentation required for this submittal is not included due to its size. It is being supplied separately on a shipping pallet.”We are moving to electronic records, but, format compatibility is always a problem.Viewing a D-size sheet on your computer isn’t all that easy.
21 Space Environments Radiation Not something that commercial vendors usually care aboutRadiation tolerance/hardness is process dependentKinds of radiationTotal Ionizing Dose (TID)LEO – 25 kRad; Europa – 4 MRadSingle Event EffectsSEU (bit flips)SEGR (Gate rupture)LatchupLinear Energy Transfer (LET) 65 MeV/cmPrometheus adds something new: Neutrons!ShieldingAdds mass, scattering may make things worse etc.Design (Silicon on Insulator, TMR, etc.)Radiation susceptibility is more complex than you can imagine. Scattering, direction of travel, etc.
22 Space Environments Temperature Qualification vs Design vs TestTypical test range –45ºC to 75ºCThermal ManagementConduction Coolingno fans in space!Radiators, Heat pipes (Mass?)Heaters (survival, replacement)Space is very cold!Lots of modelingHigher efficiency designsDon’t generate heat in the first place
23 Space Environments Vacuum HV breakdownMultipactionLow pressure (e.g. Mars 5 Torr)Paschen minimumOutgassing & vacuum compatibilityMechanical issues (cold weld, lubes)Thermal managementRadiation & conduction: yes, convection: no
25 Mission Assurance (aka 5X) Good DesignDesign reviewsLots of analysis (Faults, Worst Case, Parts Stress)Good PartsParts selectionParts testingVerificationQualification TestingGood record keeping“Traceability to sand” – are the widgets we’re using the same as the ones we tested
26 Parts is NOT Parts Class “S” aka Grade 1 Class B+ aka Grade 2 (883B plus screening)Plastic Encapsulated Microcircuits (PEM)Inspectability!Traceabilitye.g. GIDEP alertsIf a given part fails for someone else, we can know if that part is in our system, and then we can determine if it’s going to cause a problem
27 Testing - Vibe and Shock Vibration and shockLaunch loadsPyro eventsTesting without breakingCassiniMER
28 The Future More networking Higher frequencies Higher data rates Not so much point to point “stovepipe”Higher frequenciesMore bandwidthOpticalHigher data ratesMore scienceMore functionality in the radioSoftware radios
29 Network designHistorically s/c to earthInterplanetary networks
30 Relay Orbiters Galileo & its probe DS-2 on ill fated Mars Polar OrbiterCassini & HuygensMRO, MGS, & future
31 New technologies FPGAs Optical Comm Reconfigurable in flight(but what if there’s a bug in the upload?)Upsets? Latchup? Power? Testability?Optical Comm100 MbpsAt least you have a telescope to see Earth (pointing!)Pushing the A/D closer to the antennaDirect IF conversionFast, low power, wide A/DsSSPAsNew topologies (Class E) give higher efficiencyIRFFE – self adjusting circuitsIRFFE = DARPA’s Intelligent RF Front Ends program from DARPA MTO
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