New Search Spaces for

Radio SETI

G. R. (Gerry) Harp

April 22, 2011

Acknowledgement: Speaker is grateful to SETI Institute for support of the work described here. This work was also supported by Paul G. Allen Family Foundation, National

Science Foundation (0540599 and AST-08326), NASA (NNG05GM93G), and others.

1

Search for Extraterrestrial Intelligence

Beginning of EM field search Drake 1960, using radio wave detection.

Schwartz and Townes 1961 Suggestion to look for laser radiation in optical.

Fast forward 2011 Radio and Optical SETI searches are abundant (though each is relatively small)

Many nice introductions to (radio) SETI and OSETI can be found in book:Communication with Extraterrestrial Intelligence, Ed. by Douglas A. Vakoch, SUNY Press, 2011. Available at Amazon ($35).

Ill be making many references to this book, especially Ch. 4 by Harp et al.

Caveats: Chapter authors receive no royalties from book, Im not a spokesman for the SETI Institute, only for myself.

2

Motivating SETI

Exoplanets, Extremophiles, and SETI

(Jill C. Tarter, Ch. 1)

We find planets almost everywhere

~10% of stars observed are known to have planets,

higher than anyone anticipated 10 years agohigher than anyone anticipated 10 years ago

Probably the ratio is higher

We find life almost everywhere we look on Earth

Bio-signatures of simple life: Spectroscopy of

Exo-planetary atmospheres

Techno-signatures indicate intelligent life

3

Challenge for Interstellar Radio Communication

(Ch. 6, Blair et al. probes this deeply)

Ionized Interstellar Medium (ISM)

Space is almost empty, but tiny density of ionized

hydrogen (free electrons and protons) messes

with attempts to communicate

Left: A 50 ns pulse is emitted from transmitter located near Proxima Centauri (4 LY).

Right: The pulse as received by a detector on Earth. The pulse has been dispersed by

the ISM. Stars farther away will be further weakened and broadened.

0%

20%

40%

60%

80%

100%

-2 -1 0 1 2

Pu

lse

Po

we

r

Time (microseconds)

Transmitted Pulse

0%

1%

2%

3%

4%

5%

-2 -1 0 1 2

Pu

lse

Po

we

r

Time (microseconds)

Received Pulse

4

Non Obvious Aspects

Circularly polarized sine waves are not affected by ISM

Recognized only later, this is a good reason to keep doing sine wave search.

Sine waves are affected by relative acceleration between transmitter and receiver

Remember: Earth rotates about axis, about sun

Ambiguous? If we discover a sine wave we know Ambiguous? If we discover a sine wave we know: Either signal is from ETI or it is new sort of natural source.

Sine wave conveys zero information rate

Information must be conveyed in a separate signal

Scientists are incredibly tolerant. Though this is not conventional wisdom, I havent been fired. Yet. (^_^);;

5

SETI is Evidently Technology Driven

We look for technologies similar to our own 1961 - Started with radio carrier waves (think AM

radio)

1965 - Fast Fourier Transform algorithm. Cheap high speed computing makes this the dominant strategy for SETI even today.

2002 Branched to optical only after we developed 2002 - Branched to optical only after we developed lasers of our own that would outshine the host star and easily detected (Ch. 9 12)

2007 - SETI Institute commences observations with an interferometer radio telescope

2008 - Sophisticated communication protocols developed for satellite communication is driver of new ideas in radio SETI look for radio signals with information and built-in error correction (Ch. 4)

6

SETI is Evidently Technology Driven

Funding cycle is very conservative

We begin substantive searches for technologies only after they are available on earth

Anthropomorphize much?

SHOCK! The hidden reason that radio SETI is still dominated by narrow band (sine wave) searches is the Fast Fourier Transform: No other search algorithm can compete for speed.

This is an anthropomorphic factor (bad). Why should ET be limited by compute cycles at our end?

Sine waves used to seem obvious choice.

Pot calls kettle black: We propose new search spaces. At the heart of new searches is FFT.

7

GPS Communication

We dont use amplitude modulated (AM)

radio to talk to GPS satellites.

Phase modulation is less error prone:

GPS uses Binary Phase Shift Keying, Gold Code

error correctionerror correction

Start with a

1023 bit Gold Code: +1, -1

First 25 values:

8

GPS Communication

Multiply Gold Code with Carrier Wave

Gold Code

Product with

carrier wave

Note phase flips9

GPS Communication

Concatenate 20 copies of modulated carrier, this is 1 bit of information containing the value 1. Invert the sequence and you have a 0.

Total of 20460 bits 1 bit of informaon

redundancy allows error correction

If ETI uses this communication mode, we can ,lock on to the repeating Gold Code

We propose Autocorrelation as method for detecting these signals

AC is also good detector for amplitude modulated signals (whats old is new again) Pumping a pulsar with an external driving source

Examine AC of the square (absolute value) of signal

10

GPS Communication - Discovery

With telescope measure E(t), electric field amplitude versus time. If we are tuned right, well pick up GPS signal.

Compute E(t)E(t), first value of AC spectrum

Even though E(t) runs positive & negative, E(t)2 is always positive

E(t)2 = large number

Shift by 1 sample: Compute E(t)E(t+1)

Not necessarily large.

Shift by 2. Recompute. Shift by 3. Etc. If the values of E(t) ever repeat (like GPS signal) after

time ~ N samples, then the magnitude of E(t)E(t+ N) will suddenly become large again

11

GPS Communication - DiscoveryWe found it just as planned:

.)

Autocorrelation spectrum of

captured GPS signal. See a peak in

spectrum every 1 ms = repeat

time of Gold Code

Po

we

r (a

rb

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

Decay of peak values

with increasing delay is

expected (details!)

12

Improve Example to Make SETI Beacon

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)copy (can be overlaid on first signal)

Autocorrelation (very efficient algorithm)

discovers signal

There is no ambiguity about where the

information lies. Information is right there in

detected signal.

13

Twice Sent Message = Beacon

3

4

Signal

D l d Si l

Blue: Mock-up realization of generic signal (sampled

Gaussian white noise). Magenta: Same signal sent

second time after delay = 16 samples.

Receiver sees a superposition of the two signals

-3

-2

-1

0

1

2

30 46 62

Time (arb.)

Voltage (arb

.) Delayed Signal

Send signal once. Second copy with delay

N = 16 samples.

14

AC of Exemplary Signal

1500

2000

2500

3000

3500

wer

(arb

.)

Zero lag autocorrelation

= E(t)E(t) = large number

Detection of repeat signal at

N = 16 samples = E(t)E(t+16)

-500

0

500

1000

0 16 32 48 64 80 96 112 128

Time Delay (arb.)

Pow

Detail: Once identified, deconvolve repetition to get

good measure of information-containing signal.

15

Non Obvious Aspects

Autocorrelation (AC) detection of arbitrary

signal is not affected by ISM

Another reason why AC is proposed

AC detection is not affected by relative

acceleration between transmitter and receiveracceleration between transmitter and receiver

Forget: Earth rotates about axis, about sun

AC detection is more easily confounded by

background radiation, noise in receiver

New ideas every day: Symbol-wise AC, Morrison

et al. Improved sensitivity for subset of signals.

16

What Does AC See?

Discovered

repeating signal

with 1267.5

microsecond

delays.

In setiQuest data posted

online, weve observed

several such repeating

signals.

Correlation good

for 100 repeats!

This was shown

to be RFI in the

vicinity of ATA.

Harp, Ackermann, et al. 17

north

mid

How do we know it is RFI?

Signal is found to be strongest on horizon near 120

Azimuth. Tests using subarrays show that source is

closer to north end of array. Hence RFI.

south

Harp, Ackermann,

Astorga, et al. 18

Power vs. Time AC

Naturally occurring masers observed in setiQuest data.

Civilization pumps a maser with

time dependent signal

Maser acts as amplifier for signal.

Idea of using a maser: J.M.

Weisberg, et al.

A pulsating maser is an Amplitude

Modulated signal. If

modulation is repetitive, can be

observed with AC of the square

of measured signal.

No interesting signals on masers so far, but we can apply this

method to any setiQuest data we like. 19

What Does Power AC See?

Discovered amplitude

modulated repeating

signal with 10 ms

delays. Direction of

Galactic North Pole.

Correlation good

In setiQuest data posted

online, weve observed

several such repeating

signals.

Correlation good

for >20 repeats.

Very likely RFI. It is too

fishy that repeat is

close to a human-

referenced time of

exactly 10 ms. 20

Traditional Narrow Band

Search: A few Kepler

Objects of Interest (KOIs)

A few examples of weird signals from Conventional SETI search

(Narrowband) on setiQuest data.

Ackermann, Harp, et al.

21

Squiggle

Pulse

Ackermann, Harp, et al.

22

Broadband

Highly dispersed pulse

Ackermann, Harp, et al.

23