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UT AST 309L - Communication

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1Communication, 2.Recognizing the MessageDistinguishing from natural “signals”:Expect: Variation with time, narrow band(small range of freq.)Not random noiseIf not random, it is artificial (ETI or Human)Examples of natural signals that might have been ETI1. Pulsars (LGM)2. OH MasersBoth are random noise (no coded information)Crucial2Coding the MessageChange the signal with time1. Amplitude modulation (AM)AM RadiotCoding the Message2. Frequency Modulation (FM Radio)tsidebandscarrierν3Coding the Messagehttp://www.chem.tamu.edu/rgroup/north/FM.htmlAnalog vs. Digital1. Analog - need accurate amplifiers, etc.to avoid distortione.g. radios, tv, records, analog tapes2. Digital “digitize” signalRepresent by Base 2 NumberBase 10 Base 20 01 12 103 114 100……4Analog vs. DigitalSend one digit at a time so electronics just need toDistinguish 1 from 0Can use 2 very different voltagesNeed fast digital electronicse.g. CD’s, DVDs, Computers, Digital Tapes, DigitalTV, …Decoding the MessageAssume DigitalRepeat to Establish Pattern5Image? 1 dimension (string of bits)2 dimensionsRows + columnsMake product of # rows + # of columnseach a prime numbere.g., 23 × 73 = 1679 so 23 rows, 73 columnsor vice versaSemanticsCan we understand the message?6Search Strategies• Basic Problem: where to look?• Possible Scenarios– Powerful, omnidirectional beacons• Implies very advanced civilization• Seeking to attract attention of new civilizations– Nearby, not so advanced, broadcasting tous• Unlikely– Detect leakage radiation7Leakage Radiation• Various sources– TV, radio, …• Repeatable pattern due to Earth rotation– Defense radars• Most powerful, but won’t repeat8World Television Transmitters9Television Leakage (kW)1200 300 Stars “illuminated”45 ly 25 lyDistance from Sun 1940 1990107 Watts10The Cosmic HaystackStrong signals, unknown origin⇒ Small telescope, short t , cover skyWeak signals, nearby stars⇒ Large telescope, longer t , only starsSensitivityS ∝ D-2 t-1/2 want small SLarge telescopeLong time perdirectionDirectionLarge number ofdirections Small beamConflictFrequencyLarge frequency rangeBut narrow channelsLots of channels⇓⇓Cosmic Haystacklog sensitivitylog ν (GHz)log numbertargets (directions)1112Channel number1 2 3 4 5 6 7 8 9 10νbandwidthfrequency range coveredD θ = λ/D1D/2θ = 2 θ1λ/DThe Cosmic HaystackStrong signals, unknown origin Sky Survey⇒ Small telescope, short t , cover skyWeak signals, nearby stars Targeted Search⇒ Large telescope, longer t , only starsSensitivityS ∝ D-2 t-1/2 want small SLarge telescopeLong time perdirectionDirectionLarge number ofdirections Small beamConflictFrequencyLarge frequency rangeBut narrow channelsLots of channels⇓⇓13Targeted Search vs Sky Survey102128106104101010101021-271010-2510-2310-21ν (GHz)Targets or DirectionsSensitivity (W m )-2Sky SurveyTargeted Search10Some Searches for ETI# of stars2602All sky244800All SkyTelescope size(m)2691263003434Frequency(MHz)1420142014201200-3000+ selected νUp to 25 GHz1000 - 10,000+ selected νNamesOzma(Frank Drake)Ozma II(Zuckerman & Palmer)Meta(Horowitz; Planetary Soc.;Spielberg)NASA searchDiscrete source madeAll sky SurveyYear1960197219851992(?)Oct. 12, 1992[ 8 million channels ][ 10 million channels +? ]2 million in 1992 ; ~ 16 million in 199614Some Searches10212108106104101010101021-271010-2510-21ν (GHz )Targets or DirectionsSensitivity (W m )-2Ozma II22 GHz(water)Ohio State-2310Previous Searches15SERENDIP - SETI@home• Latest version:SERENDIP IVUses ARECIBO telescope while regular obs. going onν = 1420 MHz5 × 10–25 W m-2very sensitiveData analyzed by screen saverson millions of PC’s SETI@HOMEReport on Project METAMegachannel Extra Terrestrial AssayHorowitz & Sagan, 1993, Astrophysical Journal, 415, 218.5 years of searching at 1.420 GHz8 × 106 channels channel width: 0.05 Hzcoverage: 400 kHzCovered sky 3 times 1.7 × 10–23 W m–237 candidate events: narrow-band, apparently not interferenceBut none repeated8 signals truly hard to explain as noiseProbably electronic “glitches”But some tendency to lie in plane of galaxy ⇒ extraterrestrial ?Nothing convincing yet.16BETASuccessor to META 2.5 × 108 channels0.5 Hz channel widthCovers 1.4 - 1.7 GHz in 8 stepsSensitivity: 2 × 10-22 W m-2Started 1995, suspended in Spring 1999 (antenna blew off mount!)repairs underway17NASA SearchMicrowave Observing Program (MOP) Main improvement: frequency coverage2 parts:1. All sky survey - JPL - runTelescopes of modest 34-m diameterCalifornia, Australia, …Cover 1 - 10 GHz2 × 106 channels 16 × 106 channels ( ~ 1996)Channel width: 20 HzTo begin Oct. 12, 1992beganendedrevived?Coverage: 40 MHz , 320 MHzright and left circular polarizationSensitivity: only spend a few sec. per direction ⇒ strong signal(Arecibo Planetary Radar) out to 25 lyTimespan: 6 years to cover sky once182. Targeted search - Ames - run ( ~ 800 Nearest ( < 75 ly) stars like Sun)Largest telescopes available:Arecibo 300 m (244 stars)+ Australia, France, …Cover: 1 - 3 GHz16 × 106 channelsChannel width: 1 HzCoverage: 10 MHzright and left circular polarizationSensitivity: ~ 103 sec. per star⇒ 10–27 W m-2 Ptrans = 10–27 W m-2 · 4π d2(m)d(m) ~ 1016 d(ly)Ptrans ~ 106 d2(ly) = 1 M Watt at 1 lye.g. 100 Mega Watts at d = 10 lyDefense radars to ~ 1000 ly19HR 515820Project PhoenixSETI Institute (- minus NASA $$)Private Funding (Packard of HP)+ …Relocate to Australia 64 - m telescope1.2 - 3.0 GHz , 28 × 106 channels1 Hz channel widthTargeted search sensitivity ~ 1 × 10-26 W m-2~ 200 stars like Sun, no binaries, t > 3 × 109 yrWithin 150 l y observe each for 5 min(eventually 1000 stars)Underway Feb. 2, 1995Can detect 1 Mega Watt if beamed to us bysimilar size telescopeImmediate followup by second telescopeNo ETI found in first run (sp 95)Webpage: http://www.seti-inst.eduUsed various other telescopes, including AreciboNo civilizations found yet.21Amateur ProjectsBAMBI (Bob and Mike’s Big Investment)3.7 - 4.2 GHz Sky surveySETI League project ARGUSUse Satellite TV Dishes (~ 100) as of 20011.4 - 1.7 GHz Channel width: 1 HzSens. ~ 1 × 10-21 W m-2Goal is 5000 sitesAim for continuous sky coverageAllen Telescope Array (ATA)Under construction SETI Institute, UC


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UT AST 309L - Communication

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