G. R. Harp Expanding the Scope of SETI at the Allen Telescope Array Gerry Harp, B. Wilcox, J....

G. R. Harp

Expanding the Scope of SETI at the Allen Telescope Array

Gerry Harp, B. Wilcox, J. Arbunich, P. Backus, R. Ackermann, J. Jordan, T. Kilsdonk, Samantha Blair,

J. C. Tarter and ATA Team

Search for Extraterrestrial IntelligenceExo-technology, Exo-civilizations

G. R. Harp

• They have a limited power budget.– They have to spend some power getting our

attention and some more to send info.

Tasks for ET

• Get our attention (help us locate them).– ET will send us a signal that is easily

recognized as artificial.

• Tell us something.– Sending any signal costs money (power).– They will encode info s.t. we can interpret it.

G. R. Harp

How do we know signal is artificial?

• Uncertainty principle argument?

G. R. Harp

• Uncertainty principle for signals of unknown bandwidth present in wave fields:

Heisenburg

• As soon as we measure the signal (convert to power), the wavefunction collapses and the uncertainty goes away:

PERIOD! Nyquist, Shannon

• We can’t classify signals as artificial or natural based on uncertainty principle, all measured signals have equal “uncertainty” = 0.

Uncertainty Principle is a Tautology For Sampled Signals

1t f

1 t f

G. R. Harp

For Example

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Measured Frequency Power Spectrum

Po

we

r (1

0^

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tts)

• Measurement time = t = 1s.

• What is signal bandwidth f?

1 s 1 Hz = 1!

t f

G. R. Harp

0

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Hey! Don’t use a degenerate example!

Measured Frequency Power Spectrum

Po

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tts)

• Measurement time = 1s

• Signal f = 2Hz• 2 Hz in 1 Hz

resolution -> Signal has short coherence time, t = 0.5 s.

1!t f You can’t beat Nyquist!

G. R. Harp

0

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PeakyBroad

Natural Law of the UniverseWell known but never identified.

• Natural signals can be broad or peaky

• But they always look like noise.

Measure the time variation of small bandwidth here.

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G. R. Harp

An Unfortunate Natural Law of the Universe

• Natural signals always look like noise.• This is a natural outcome of the central

limit theorem (Rice).• Simply put: If there are N independent

sources contributing to your measurement, then the signal converges to Gaussian noise as N

• It doesn’t matter what the spectrum of a single source is!

G. R. Harp


• Natural signals always look like noise.

• Counter example – pulsars.

• Why? Because N = 1. Central limit theorem does not apply.

Vela Pulsar

G. R. Harp

Counter example – blazars. Why? Because N ~1.


Simulaton

NASA

G. R. Harp


• Uncertainy principle argument?• Shannon’s theorem – Allows us to quantify the

amount of information signals contain.– A signal that transmits maximal informaion on channel

always looks like Gaussian White Noise.– Just like natural sources!– To recognized signal as artificial it must be redundant.– Corrolary: Signals containing no redundant information

are impossible to decode (w/o key).– Signals must contain redundancy or we have no hope

of understanding the message.

G. R. Harp

Required properties for ID of ETI Signal

• Must be noticeably artificial– Must contain less than the maximum information

content that could be conveyed Redundant!

• Must be persistent– Not their problem, but ours. We can’t be sure of

direction of signal arrival unless we can make many, many measurements.

G. R. Harp

2 Correlators + MIRIAD100 kHz to 3 kHz resolution, 100

MHz BW

2 (3) BeamformersReal time, time series voltages, 70

MHz BWPoints synthetic beam anywhere,

supports nulls

SETI spectrometerTypically 0.7 Hz resolution, .25 – 1

Hz / second drift rates

• Conventional SETI strategies: – Focus attention on a small region of

sky, look for ~1Hz signals– Extraordinary efficiency

• Unconventional SETI strategies– a “sky survey” covering large areas

of the sky with modest sensitivity– a “targeted search” looking for

signals with large autocorrelation (time slip spectroscopy).

ATA - 42 Dishes1’ pointing (at night, 2.5’ day)Wide FOV = 53’ @ 4 GHz0.7mm RMS Surfaces (at night,

~3 mm day)0.5 – 11.2 GHz4 x 100 MHz bands

Shostak

G. R. Harp

ATA is a Unique Telescope

New Search Spaces

• ATA has multiple beamformers, nulling technology to discriminate RFI.

G. R. Harp

x

Beam

Multiple BF + Nulling

• Simultaneously measure signal at one point (beam 1) while excluding signal from that direction in beam 2, beam 3, etc.

• Anticoincidence rejects signals that appear in both beams (must be RFI)

FOV

x Null

Beam

FOV

Null

G. R. Harp

Shostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center.

Kassim, et al.

Ongoing Searches at ATA

Gal. Center = 26000 LY. Survey detects transmitters with 20,000 x EIRP (transmitted power) of Arecibo planetary radar at the galactic center.Give or take 10 dB.

G. R. HarpShostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3.


Pike and Drake, 1964

Cygnus X3 Region

Distance to Cyg X3 is 1.5x distance to GC. Detects transmitters with 50% of the sensitivity of galactic survey (GC).

G. R. HarpShostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3. Exoplanet Search: Targeted survey, have observed 146 stars with planets in the Waterhole band, ongoing.


Kalas, et al.

Most are between 10-1000 LY. 1000 LY ~ 700x sensitivty of GC.10 LY = 104x more improvement.

G. R. Harp

Shostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3. Exoplanet Search: Targeted survey, have observed 146 stars with planets in the Waterhole band, ongoing.HabCat Search: Targeted survey, Turnbull and Tarter compiled list of stars that could be suitable hosts for habitable planets.


HabCat Catalog: Turnbull and Tarter

Habitable Zone, where astrophysics might permit life.

17,000 stars, 4-55000 LY4 LY – 20,000x better than GC search55000 LY = 25% sensitvity of GC

G. R. HarpShostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3. Exoplanet Search: Targeted survey, have observed 146 stars with planets in the Waterhole band, ongoing.HabCat Search: Targeted survey, Turnbull and Tarter compiled list of stars that could be suitable hosts for habitable planets. PiHI Search 1: Targeted survey, 100 HabCat stars near magic frequency of 4.462 GHz ( times the HI line frequency of 1.421 GHz). Almost virgin frequency territory. After Sagan.


Harp et al.

30 Polar Cap Sources always up

Choose ~100 Habcat stars within 200 LY of earth and in polar cap. 100 LY = 70,000 x more sensitivity than GCThis is a little unfair – GC measures millions of stars at once, PiHI measures 1.

G. R. Harp

PiHI Targeted Search• Last summer, Bethany Wilcox – dynamite!• No SI surveys have ever gone this high.• “Contact” Frequency = HI = 4462 MHz• 94 target stars chosen from HabCat.• F9-G7 (sun-like) stars within ~62 pc. • Stage 1: 30 MHz BW, 0.7 Hz resolution, 200 sec.

observations, ½ Hz / sec drift rate• 30 MHz BW is large enough to encompass any

proper motion (statistical analysis of catalog)• Stage 2: “Pi Hole” ( times waterhole) 2010

G. R. Harp

PiHI Targeted Search• All targets were declination > 60° so that

observations could be done at any time, day or night.

• Easy to schedule.

• Daytime obs havesun in sidelobes.

• Choice of sphericalcap reduced solarinterference.

G. R. Harp

PiHI Targeted Results

(We have a very effective mechanism for identifying RFI)

(^_^);

G. R. Harp

PiHI Targeted Results

• Every 200 second observation, ~2000 signals would pass the power threshold.

– Of those signals, ~800 would be immediately dismissed as recent RFI or zero-drift.

– Almost all of the remainder failed the null beam test.

– ~1 would be classified as 'confirm‘– Prelude would re-observe to see if the signal

persisted. If so, proceed to off test.• Ultimately 64 signals got to “first off” test. None of

these survived.

G. R. Harp

One that Got Away

Large Pulse Observed

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Pow

er (a

rb.) .

Not observed in follow-up observations. Dang It!

Requirement of persistence is a pain in the neck.

Futher study showed this signal appeared in both beams. (;_;)

Exactly the right place.

G. R. Harp

Nothing for a dynamic young scientist to do.

• Conventional SETI observing system is ~100% automated.

Push into a different search space.

G. R. Harp

Dish Size ~ 1 / FOV

Aperture Size ~ 1 / Resolution

Aperture Size = Dish SizeResolution = FOV

What new directions can we take?To length scales describe interferometer.

42 ATA Dishes <> One ~40 m Single Dish

G. R. Harp

Differences between Interferometer and Filled Aperture Telescopes

Radio Image of Moon (ALMA)

Single pixel of ATA gets data only from black dotATA can get signal from anywhere in blue circle

ATA FOV At 7 GHz

ResolutionElement

Point SourceTransmitter

For single dish, get one data pointAll the signal in the green circleTransmitter signal corrupted by 100 x more noise from the Moon.

ATA better distinguishes point source emitters from background.ATA can image many 1000’s of points at once.

EquivalentSingle-DishFOV andResolutionElement

G. R. Harp


New Search Spaces


• Interferometer can see multiple points on sky at once.– Opens the door to

signals with Optical Angular Momentum

– Nothing to do with polarization (spin) of signal!

G. R. Harp

Optical Angular Momentumand its generalizations

June 2005Two Signals, images to send.

Imaged with ATA350 interferometer

+ =

ET applies 10 ħ OAM beforesumming.

Extra infolooks like structure.Repeats.

Same data with 10 ħ removed.

G. R. Harp

Problem with OAM

• First, OAM is not so special, just one of number of bases for expressing E field.

• In order to see OAM (or …), must be able to resolve dark from light on RHS.

• For source at 100 pc, need multiple transmitters spaced at 0.5 pc, received in perfect phase at receiver.

• Because of ISM, this is impossible.

= |0,0> = |1,1> + |1,-1>

Point SourceOAM = 0 ħ

Point SourceOAM = 1 ħ

G. R. Harp

18-May-2008

How could I have been so naїve?

G. R. Harp


New Search Spaces


• Interferometer can see multiple points on sky at once.– Use correlator in conventional mode.– Look for narrowband point sources in

images that might be beacons– Cover large area of sky at once

G. R. HarpShostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3. Exoplanet Search: Targeted survey, have observed 146 stars with planets in the Waterhole band, ongoing.HabCat Search: Targeted survey, Turnbull and Tarter compiled list of stars that could be suitable hosts for habitable planets. PiHI Search 1: Targeted survey, 100 HabCat stars near magic frequency of 4.462 MHz ( times the HI line frequency of 1.421 GHz). Almost virgin frequency territory. After Sagan.PiHI Search 2: Sky survey using ATA correlator. Novel scheme runs commensally with targeted survey.


Harp, Wilcox, et al.

4 detection of 100 kHz signalat 4455.3 MHz

G. R. Harp

PiHI Sky Survey

• “Contact” Frequency = HI = 4462 MHz

• 94 regions chosen centered on HabCats.

• Stage 1: 100 MHz BW, 100 or 3 kHz resolution, 2 minute observations

• All targets were declination > 60° so that observations could be done at any time, day or night.

G. R. Harp

Frequency Space Double Difference Method

• Each correlator dump has ~824 images arranged by frequency (edge images are corrupted by aliasing)

• Each contains 100 kHz (or 3 kHz) of image data. • From these we calculate 824 numerical second derivative

images• – (image1) + 2*(image2) – (image3) = Double Diff

=– – + 2*

=– – + 2*

G. R. Harp

4555.1 MHz 4555.2 MHz 4555.3 MHz 4555.4 MHz 4555.5 MHz

Second derivative feature at 4

Frequency Space Double Difference Method

• Typical candidate signal• Double difference causes single feature to show up

in 3 adjacent channels

• Noise level determined empirically from DD image background (resistant to RFI)

G. R. Harp

Frequency Search Results Double Difference Method

• 14 targets had peak values with an SNR greater than 4.

• One target had a σ > 4 detection in both the 3 MHz and 100 MHz band.

• These 14 were re-observed with correlator and with conventional SETI instrument

• None of them were persistent. • [Dang that persistence clause!]

G. R. Harp


New Search Spaces


• Interferometer can see multiple points on sky at once.

• ATA has various fast dump modes. Can look for fast time-variable signals.

G. R. HarpShostak

Galactic Center Survey: Sky survey of 20 square degrees near the galactic center. Cygnus X-3 Survey: Sky survey of 4 sq. deg. near Cygnus. Includes the x-ray binary star Cygnus X-3. Exoplanet Search: Targeted survey, have observed 146 stars with planets in the Waterhole band, ongoing.HabCat Search: Targeted survey, Turnbull and Tarter compiled list of stars that could be suitable hosts for habitable planets. PiHI Search 1: Targeted survey, 100 HabCat stars near magic frequency of 4.462 MHz ( times the HI line frequency of 1.421 GHz). Almost virgin frequency territory. After Sagan.PiHI Search 2: Novel Sky survey using ATA correlator. Novel scheme runs commensally with targeted survey.Time slip spectroscopy: Targeted survey, prototype. Look for signals with narrow time-domain features. Esp. Autocorrelation spectra.


Harp, Ackermann, Arbunich

G. R. Harp

I was pretty proud for thinking of examining autocorrelation spectra

• But then: I was scooped again!

Dang!

G. R. Harp

Maximizing Information in Time Slip Spectra

• Transmitter sends arbitrary signal with arbitrary bandwidth, contains mucho information, e.g.Encyclopedia Galactica

• After a short delay (s – 10s) send second copy (can be overlaid on first signal)

• Very simple, very efficient algorithm discovers signal, same technology as conventional search

• Equal or near-equal sensitivity as conventional search

• Better yet, there is no ambiguity about where the information lies.

G. R. Harp

Time Slip Spectroscopy

We start with bright objects, masers, blazars.

Civilization pumps a maser with time dependent signal

Acts as “natural” amplifier for signal.

Idea of using a maser: J.M. Weisberg, et al.

G. R. Harp


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Delayed Signal

Assume time series contains information, sent multiple times. Could be simply repeating signal.Here we show a more interesting case.

Send signal once. Send signal again.

Receiver sees a superposition of the two signals.

G. R. Harp

Time Slip SpectroscopyTo IDENTIFY the signal, simply compute autocorrelation spectrum (trivial with FFT).

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Identify delayed signal using threshold

Once identified, deconvolve repetition to get good measure of information-containing signal.

G. R. Harp

Time Slip SpectroscopyWhat about ISM? Doesn’t that mess up wide bandwitdh signals? (Dispersion, incoherent scattering.)

YES!But both the original signal and time delayed signal are affected identically by ISM (provided time delay is not too long (e.g. hours).Therefore they will still correlate.

To decode signal will require more work, but at least we know where to start!

G. R. Harp


Proof of principle using ATA + Beamformer + Time capture.P

ow

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arb

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Time Delay (s)0 4883 9767 14648 19531 24414 29296

Autocorrelation spectrum of phase modulated GPS signal.

Downward trend of peaksis expected. Data are not normalized to # of samples.

G. R. Harp


Autocorrelation shows nothing special. We have ~3 such measurements so far.

The real thing, W3OH maser.

Future: Longer delays, faster.

G. R. Harp

Channelizer(forward)

Channelizer(forward)

Square(Power)

Channelizer(inverse)

Channelizer(inverse)


Same ol’ SETI detector as usual

• Deep FFT• Square Result• Inverse Deep FFT• Feed results to threshold detector• Exactly as fast as conventional SETI• ~2x as compute power.

Implementation uses same components as current SETI detector.

G. R. Harp

Who is the ATA?

UCB and HCROBacker, DonBlitz, LeoBock, DouglasBower, GeoffCheng, CalvinCroft, Steve Dexter, MattEngargiola, GregFields, EdForster, RickGutierrez-Kraybill, ColbyHeiles, CarlHelfer, TamJorgensen, SusieKaufman, JeffKeating, Garrett (Karto)MacMahon, Dave

SETI InstituteAckermann, RobBackus, PeterBarott, BillyBradford, TuckerDavis, MikeDeBoer, DaveDreher, JohnHarp, GerryJordan, JaneKilsdonk, TomPierson, TomRandall, KarenRoss, JohnTarter, Jill

MinexCork, ChrisFleming, MattVitouchkine, Artyom

Student InternsImara, NiaChubb, Kelsey Adair, AaronNadler, ZacharyPearson, Ruth

Milgrome, OrenThornton, DougUrry, LynnVan Leuven, JoriVanourtryve, CassandraWelch, JackWerthimer, DanWilliams, Peter Wright, Mel

G. R. Harp

End

G. R. Harp

Generalization of OAM: Dual-beam Cross Correlation.

• OAM is only one basis. Infinite number of bases (representations) of light.

• Another one is to take two beams and correlate them coherently (astronomical data should never correlate)

• Can be done (RIGHT NOW!) with ordinary correlator.• Dang! I ought to publish this!

• BUT!• All such techniques rely on ability to resolve multiple

transmitters.• Transmitters must be light years apart and perfectly in

phase at the receiver.• This is literally impossible because of ISM.• We can expect never to receive phase-coherent multi-

angle signals unless we have super-long baseline interferometry and nearby transmitters.

G. R. Harp


• Uncertainy principle argument?• Shannon’s theorem -- the amount of information

they contain.– Define “channel” as bandwidth/time for signal.– A signal that transmits maximal informaion on channel

always looks like Gaussian White Noise.– Corrolary: Such signals contain no redundant

information they are impossible to decode w/o key– Signals must contain redundancy or we have no hope

of understanding the message.

G. R. Harp

How much information is contained in a signal?

2log 1S

C BN

Shannon’s TheoremAny signal that is easy to find

doesn’t contain much information.

Information carrying capacity of signal or “channel”, C

S = Average signal power in one sample

N = Average noise power in one sample

If B = 0, then there is no information (excepting…)

G. R. Harp

Senario 0

• ET uses all their power budget to send a sine wave “beacon” to get our attention.

Call a stranger on the phone and then not say anything. It’s a prank call!

G. R. Harp

Senario 1

• ET puts all their power into 2 discontinuous sine waves while alternating polarization.

• Detection signal to noise ratio reduced by – BWeff ~ inverse of bit rate.– Even small bit rate gives large degradation in SNR.– To discriminate against natural sources, bit rate

cannot exceed 200 bits per second. (Tarter, et al.) – Very little info may be exchanged.

LHCPRHCP

Signals are shown in sync, but ISM will cause time difference of arrival (e.g. delay the RCHP)

eff

1

BW 2

G. R. Harp

Detection Algorithm For Sine Wavescan be discontinous

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ET Sends Signal Noise Picked Up Along Way

Composite Picked Up At Receiver

= = S(t)

| Fourier Transform [S(t)] |2 Signal

Noise

G. R. Harp

• Advantages:– Super Easy! – FFT implementation of Fourier Transform is

fastest algorithm known to man.

• Very small information channel

• Problem: How do humans find the information-containing signal?– No way to point us toward information

Senarios 0 or 1

G. R. Harp

Senario 2

• ET puts all their power into a single signal containing 2 copies of arbitrary information– P = PAvg / 2

• Use other half to send us a modulated signal with simple encoding.– P = PAvg / 2

• Problem: How do humans find the information-containing signal?– No way to point us toward information– Information content must be small or we won’t find it

G. R. Harp

Example: Sine Wave

• Once you know the frequency, you know everything, can predict future values with perfect accuracy.

• Information content = 0 (or one very important bit)

• Payoff: Easily distinguished from random nature.

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G. R. Harp Expanding the Scope of SETI at the Allen Telescope Array Gerry Harp, B. Wilcox, J....

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Transcript of G. R. Harp Expanding the Scope of SETI at the Allen Telescope Array Gerry Harp, B. Wilcox, J....