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Advances in Wideband SETI
and Implications for Radio Telescope Design

Dr Ian S. Morrison
24 October 2011


Resurgence in SETI
New generation radio telescopes and anticipation of the SKA Emergence of `wideband SETI' Successes in astrobiology, in particular exo-planet discoveries

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1235 Kepler Worlds
68 288 662

184

54 are `habitable'
Copyright © Jill Tarter, SETI Institute


SETI by Eavesdropping
Expect civilisations to have a short timespan for high power narrowband emissions

Emissions tend to become lower power, wider bandwidth and more noise-like as technology advances
Forgan & Nichol: eavesdropping with the SKA unlikely to succeed beyond 300 ly · Assumes pulsed radar: ranges even shorter for wideband communications emissions · Few habitable planets within this range

Eavesdropping is not a `percentage play'

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Galactic Habitable Zone
Present-Day Habitable Planet Density

Credit: Gowanlock, Patton & McConnell

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Deliberate Beacons
~260,000 stars within 250 ly of Earth ~106 times that in the whole galaxy Maximum star density (and habitable planet density) in the central bulge Good strategy to search the galactic centre, but only beacons detectable over s uc h r anges





Chances of detection are orders of magnitude higher if search for beacons near the galactic centre

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Beacons Appear as Transients
Galactic-scale beacons require large EIRPs Optimal way to achieve high EIRP is to split cost between antenna and power source (Benford et al)



High-gain antenna narrow beam Beam must scan to illuminate multiple targets (lighthouse analogy) Finite `dwell time' Possibly infrequent `revisit time'

on Earth a beacon will appear as a transient radio source

Credit: James Benford

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Frequency Range
Higher frequencies favoured for beacons because transmit antenna gain increases with frequency (for given area)

Terrestrial microwave window: ~1 ­ 10 GHz

50 GHz

Free-space microwave window: strong case for ~ 50 GH z

Credit: NASA (Philip Morrison, John Billingham, John W olfe)

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Narrowband versus Wideband
power spectral density

power spectral density

frequency

frequency

narrowband

wideband

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Narrowband SETI
Why has narrowband been favoured to date?
Assumed to be easier to generate at high powers Signal unaffected by dispersion from the ISM Highly sensitive receivers easier to construct

Concerns with narrowband
Not conclusively of technological origin Transmitted power concentrated in narrow bandwidth jammer Low information content (conveys just 1 bit: "you are not alone") · If many ETIs & beacons no incremental value in saying "you are not alone" Longer range (galactic scale) increases chance of being detected but high cost to build and operate. Why would ET invest in a beacon and not send information?
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Wideband SETI ­ benefits & challenges
Benefits
· · · · Conclusively of technological origin Robust to noise and interference Lower peak power density More scope to convey higher information content

Challenges
· Lower peak power density, so simple energy detection unlikely to succeed

·
·

More degrees of freedom in signal structure: harder to find a signal when you don't know what you're looking for
Wideband signals affected in a more complex way by Doppler and ISM degradations (dispersion, scattering), which further complicates discovery
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Wideband SETI ­ misconceptions
Wideband signals have too many degrees of freedom
We cannot hope to detect the signal unless we know the signal's structure and key parameters

Detection requires redundancy in the signal
There needs to be repetition of waveforms and/or information content

BOTH INCORRECT

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Detection of Unknown Wideband Signals
1. Matched Filtering
· Optimum but impractical without knowledge of signal form

2. Energy Detection
·
· · ·

Low sensitivity ­ useful only if signal well above noise
Regenerates discrete spectral lines ­ may give minor gain More generic than the FT but computationally very complex Generate detectable peaks with modulated and repetitive signals but poor sensitivity with randomly modulated signals Modified autocorrelation approach ­ more sensitive for randomly modulated signals
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3. Cyclic Spectral Analysis / Peak Regeneration (Gardner)

4. Karhunen-Loeve Transform (KLT)
5. Autocorrelation

6. Symbol-Wise Autocorrelation (SWAC)
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Symbol-Wise Autocorrelation (SWAC)
Correlate M successive adjacent symbol pairs, summing the modulus of each correlation score Repeat over a range of assumed symbol periods, looking for a strong peak

SWAC ( )




M

k0 n

n 1 k k0 (n 1)

yk y

k



D max SWAC ( )
1 , 2 ,k00,

^ TS arg max SWAC ( )
1 , 2 ,k00,



Do NOT need to know centre frequency, bandwidth (symbol/chip rate) or modulation type/alphabet

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Antipodal spread-spectrum BPSK
+1 0 -1

t

a1(t)
+1 0 -1

Ts

t a0(t)

Ts

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Example ­ antipodal spread-spectrum BPSK
2 sym/s, 1000 chips/sym BPSK, 50 second burst, no noise

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Example ­ antipodal spread-spectrum BPSK

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Example ­ antipodal spread-spectrum BPSK
2 sym/s, 1000 chips/sym BPSK, 50 second burst, -18 dB SNR

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Example ­ antipodal spread-spectrum BPSK

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Wideband SETI ­ telescope design implications
High-resolution time-domain sampling access to `raw' complex I/Q samples of beamformer output

Sampled bandwidth: minimum 1 MHz, preferably 10 to 100 MHz (selectable across the whole feed bandwidth)

Sample quantisation: minimum 8 bits, preferably 10 bits

Logging of samples to file ­ to support off-line processing

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Wideband SETI ­ Experiment Proposal
Project `STRAWBALE': Search for TRAnsient Wideband Beacons And Long-range Emissions

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Working Group Proposal
Current SETI in Australia:
ICRAR: VLBI SETI (using LBA) ACA/UNSW: Wideband SETI / SWAC (using ATA) UWS: Optical SETI Boonah: amateur optical & radio SETI

New working group to coordinate Australian SETI?
Opportunity to take a leading role in the overall planning of SETI for the SKA

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Summary
1. Focus on beacons emanating from the galactic centre 2. Such beacons can be expected to appear as transients 3. Such beacons can be expected to be wideband 4. Such beacons are more likely to be found at 10 GHz and higher 5. Design telescope back-ends to support detection of wideband transients 6. Australia well-placed to take a leading role in planning SETI fo r th e SKA
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Questions?

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