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EJASA - Part 3 Page 9
INTRODUCTION
This paper suggests that the modern Search for Extraterrestrial
Intelligence (SETI) [1-45,86], which was initiated by Cocconi, Morrison
[1,13], and Drake (Project Ozma) [2,3,13] is being conducted in the
wrong part of the electromagnetic spectrum, i.e., that SETI receivers
are presently "tuned to the wrong frequencies". This paper revisits a
subject first discussed by Schwartz and Townes [46-47] thirty years ago
and subsequently investigated by the late Shvartsman [48,50,54],
Connes [49], Zuckerman [52], Betz [53,57] and Beskin [58]. Dr. John
Rather (NASA-HQ) also considers that Optical SETI has much to commend
it. [56] According to the modern broader definition of the word
"optical", the wavelength region embraced covers the region between
350 nm in the ultra-violet, and far-infrared wavelengths greater than
300,000 nm (millimeter-waves start at 1 million nanometers).
Our Milky Way galaxy contains about 400 billion stars. We assume,
as does most of the SETI community, that at any time there are perhaps
thousands or tens of thousands of technical civilizations (the Drake
Equation, Page 71, Equ. 1) [2-39] within our own galaxy. There
should be at least a reasonable chance that at any time, one such
civilization might be signalling in our direction from within a sphere
several thousand light years in radius. The volume of space within a
sphere of two thousand light years in diameter contains about ten
million stars, one million of which may be capable of supporting life.
The sign of a mature technical civilization is not to waste power
over empty space, but to use refined signalling techniques in
preference to brute force. Although some authors have suggested that
optical ETI signals would appear in the form of bright flashing points
of light, this author thinks it very unlikely. The idea that such
signals will be like heliographs or semaphores, sending out intense
beams at Morse Code rates, is not one that should be seriously contem-
plated. As will be shown, there is no need to modulate the entire
output of a star in order to be detected across the galaxy. [20,33]
Just as on this planet, where there are a variety of communication
techniques employed, depending on distance, bandwidth, and techno-
logies available, there is no reason to assume that there is only one
universal communication frequency or spectral regime employed by Extra-
terrestrial Intelligences (ETIs). Different applications and environ-
ments will lead to the optimization of different technologies, so that
there may be many "magic wavelengths or frequencies". For example,
because of the huge distances and lower propagation losses, radio waves
may be better for communication between galaxies.
If the reader does not believe that advanced extraterrestrial
technical civilizations would have the wherewithal to aim tight
optical beams into neighboring stars, then they need read no further.
In correspondence with the author, Dr. Bernard Oliver, Deputy Director
of NASA's SETI Office, has put it very strongly that ETIs would not
have this capability. This viewpoint has dominated SETI rationale for
several decades, and in the author's opinion, is somewhat responsible
for the "bad press" that the optical approach has received.
Page 10
It is the author's view that the capability to target tight
optical beams is probably much easier to achieve than developing
relativistic or near-relativistic spacecraft. The same large optical
antenna array capability which would allow ETIs to produce narrow
transmitter beams would also allow them to "view" planets orbiting
nearby stars. Over millennia they will have developed catalogs for the
stars in their vicinity, with full details of each star's planetary
system. For them, the ballistic skills (point ahead targeting)
required to land photons on a designated target, over the equivalent of
twice the light time distance, will be relatively trivial. This is not
to discount the possibility that ETIs may send out space probes to
nearby planetary systems to gather information directly.
There is a concept inherent in the conventional SETI rationale
which might best be termed "Signpost SETI". This says, that the
signals we are looking for in the microwave spectrum, may only be
monochromatic/semi-monochromatic beacons or acquisition carriers, and
that the main transmission channels for extraterrestrials are
elsewhere. If this is the case, we might find a narrow-band modulated
microwave signal that tells us to tune to some place in the optical
regime, and perhaps provide the "Rosetta Stone" for decoding the
wideband optical channel. However, it is not clear why extra-
terrestrials would spectrally separate these signals into two different
wavelength regimes. Both the semi-monochromatic beacon and the main
wideband transmission channel could be side-by-side in the optical
spectrum (see Figure 1 below). Indeed, there would be good signal
processing reasons (advantages) for using what we terrenes would call a
"pilot-tone technique", particularly for reception within an atmosphere
(see Page 83 for a theoretical description of this technique).
Ep(t) * Signal Modulation
* Bandwidth
* <------------->
* ------------- Es(t)
* | |
Beacon * | Signal |
or * | (Main |
Pilot-Tone * | Channel) |
* | |
----------------------------------------------------------->
fp fs Optical Frequency
Figure 1 -
Signpost SETI or pilot-tone system. The beacon or pilot-tone carrier
is at frequency fp and has an electric-field amplitude Ep(t), while the
information signal with amplitude Es(t) is intensity, polarization, or
frequency-modulated onto a signal carrier at frequency fs. The
frequency separation (fs-fp) may be several MHz to several GHz,
depending upon the signal modulation bandwidth, and other factors, and
fp may be above fs. The ETI beacon or pilot-tone might also contain a
simple very low bandwidth intensity or polarization modulation
providing the Rosetta Stone for decoding the main channel.
Page 11
Such techniques can reduce the effect of transmitter and local-
oscillator laser phase-noise and correct for phase-noise and wavefront
distortion produced by Earth's atmosphere, allowing more efficient
reception with large heterodyning telescopes, i.e., reduced signal
fading and improved mean SNR. [81-82,84] The coherence cell size (ro)
at visible wavelengths (Wl) is approximately 20 cm (8"), and is
proportional to (Wl)^1.2. In the infrared at 10,600 nm, ro can be as
large as eight meters. At the best astronomical observatories in the
world, the spectral power in atmospheric turbulence is confined below
30 to 50 Hz.
Clearly, this pilot-tone technique could be used for free-space
optical communications between space and Earth with some advantage. It
also reduces the differential Doppler Shift and Chirp (Drift) by the
ratio (fs-fp)/fs; a ratio which can be of the order of 10^-8. Note
that the wideband optical signal might use spread-spectrum techniques,
so that the signal energy density might be too low to be detectable.
Without the "key" to unlock the pseudo-random sequence, we might
mistake the main signal channel for an excess amount of random noise.
There is something quite philosophically appealing about the pilot-
tone technique. It satisfies the conventional SETI rationale for the
need of a "Signpost", while at the same time provides the means for
more efficiently detecting the main wideband ETI channel from within a
planetary atmosphere.
THE MICROWAVE OBSERVING PROJECT (MOP)
From time to time, references will be made to NASA's Microwave
Observing Project, otherwise known by the acronym MOP. The objectives
of this program are summarized as follows:
Project Goal: To carry out a search for microwave signals of
extraterrestrial intelligent origin.
Project Objectives:
1. To use existing large radio telescopes, e.g. Arecibo, to carry
out a Targeted Search of about 800 nearby solar-type stars with
high spectral resolution of 1 Hz, and sensitivity in the region
of 5 X 10^-27 to 1.4 X 10^-25 W/m^2, over the frequency range
from 1 to 3 GHz. (Ames Research Center)
2. To use the 34-meter telescopes of NASA's Deep Space Network
(DSN) to carry out a Sky Survey that will examine the whole sky
at a moderate spectral resolution of 30 Hz, and sensitivity
2 X 10^-23 to 2 X 10^-22 W/m^2) over the frequency range from
1 to 10 GHz. (Jet Propulsion Laboratory - JPL)
Duration: 1990 to 1999
Cost: $12.1 million for starters, $100 million over ten years.
Page 12
As will be indicated later, the author would like to add (and has
recommended this to the SETI Institute) that a third objective be
added to this program, to run concurrently with the previous:
3. To solicit the help of dedicated groups of amateur astronomers
and coordinate their activities to conduct with their ground-
based optical telescopes, a low-sensitivity Targeted Search of
about 800 nearby solar-type stars with spectral resolution
< 1 nm, and sensitivity 10^-16 W/m^2. For selected wavelength
bands in the visible and near-infrared wavelength range (350 nm
to 1,200 nm).
Page 13
ASSUMPTION OF INEPTITUDE
Unfortunately, despite declarations to the contrary, many SETI
activists have been very anthropocentric and have in the main assumed
that ETIs are technically inept. The "Assumption of (Technical)
Ineptitude" (private discussions between the author and Clive Goodall),
not to be confused with the "Assumption of Mediocrity" [5-39] applied
to our own emerging technical civilization, has caused a gross under-
estimate of the technical prowess of ETIs, e.g., their capability to
aim very high-power tight beams into the life zones of nearby stars.
The onus will be on them to transmit the strongest signal with their
stellar or nuclear-pumped orbital lasers.
It is humbling to remind ourselves that just one century ago, very
few people on this planet used electricity. We have come a long way in
a short time! Yet, in the space of one hundred years, we have been
able to send astronauts to the Moon, robot probes to other planets, and
deploy a large space telescope in Earth orbit. Despite the very
unfortunate technical problems that have plagued the 2.4-meter aperture
Hubble Space Telescope (HST), we should note that being representative
of state-of-the-art terrene technology, it has a designed angular
resolution of 0.043" and a designed pointing accuracy of 0.012". [59-62]
In 1961, just after the invention of the laser and only two years
following Cocconi and Morrison's [1] classic paper which initiated
modern SETI, Schwartz and Townes [46-47] (of laser fame) suggested that
in other societies, laser communications technology may have been
developed before microwave communications. From looking at the
development of technology during the Twentieth Century, it is probable
that the development of microwave and laser technology must occur
within a short time of each other. As Schwartz and Townes implied,
another society, having developed laser technology first, might
cultivate a SETI rationale which was based on the superiority of laser
communications over its radio frequency counterpart. It may only be a
historical accident that the science of SETI on this planet became so
dominated by radio astronomers.
Even Townes and his colleagues [46-47,51-53] have been somewhat
constrained in imagination by limiting beam divergences to be greater
than about one second of arc. A uniformly illuminated diffraction
limited ten-meter diameter carbon dioxide (CO2) transmitter has a FWHM
beamwidth equals 0.22 arc seconds (see Table 1, Page 19, and Table 2,
Line 5, Page 22), so that even this system has a beam that is slightly
too narrow by their definition. Note that more recently, Betz [57] has
reduced the technical limits on beam divergence to 0.1 arc seconds.
When we decide what might be technically feasible in one hundred, one
thousand, or ten thousand years, the only thing which should constrain
our imagination are the laws of physics as we presently know them. We
are reminded that mere decades ago, the idea of geosynchronous
communication satellites and men walking on the Moon was considered
science fiction by most people.
Although SETI is about the passive activity of listening for
signals, otherwise it would be (and was) called CETI (Communications
Page 14
With Extraterrestrial Intelligence), how close are we to being able to
transmit strong gigawatt-type optical signals across the galaxy? The
answer to this question is that we are now much closer in time to be
in a position to do this than we are to the Industrial Revolution.
This is practically no time at all on the Cosmic Time Scale. Perhaps
SETI is one way to take those Strategic Defense Initiative (SDI)
"swords" on both sides of the now defunct Iron Curtain and turn them
into CETI "plowshares"!
PROFESSIONAL OPTICAL SETI
In this paper, the model employed for the Professional Optical SETI
analysis is based on a very modest continuous wave (C.W.) transmitter
power of 1 kilowatt (1 kW) over a range of ten light years. As a
modelling convenience, it assumes symmetrical systems, i.e., that the
receiver aperture is identical to that of the transmitter. This
symmetrical modelling technique is one often adopted by previous
comparative analyses. In reality, because by definition Extra-
terrestrial Intelligences (ETIs) will be older and more technically
mature civilizations, if and when we do detect ETI, it will be found
that the alien transmitters are huge compared to our own puny receivers.
Figure 2 is a schematic diagram showing the most important features
of a heterodyning receiving system (Equs. 23, 32, and 34) suitable for
Professional Optical SETI. The optical pre-detection filter is not
really required for SETI activities because of the excellent background
noise rejection inherent in such systems. In practice, such a receiver
would at least be duplicated for the detection of two orthogonally-
polarized or circularly-polarized signal components.
This optical heterodyne receiver might well use a dye local-
oscillator laser that has very narrow linewidth (< 5 kHz), and which is
tunable across the entire visible and near-infrared regimes. The
intermediate frequency (I.F.) bandwidth of such a system might be as
high as 10 GHz. The output of each photodetector might be taken to a
single 10 GHz Multi-Channel Spectrum Analyzer (MCSA) which sequentially
samples all 16,384 photodetectors in the 128 X 128 pixel array, or
there might be one MCSA for every row or for every photodetector,
leading to substantial reductions in search time.
For several practical reasons, e.g., Doppler de-chirping, it is
likely that the alternative coherent detection technique called
"homodyne detection" (Equ. 33), which is essentially equivalent to a
heterodyne system with a zero I.F., would not be used for the frequency
search, though it might be employed after acquisition of an ETI signal.
One major reason why the SETI community generally discounts the
optical approach is the considerable amount of quantum noise generated
by optical photons. As we increase frequency, the number of photons
for a given flux intensity progressively falls, so that there is a
noise component associated with the statistics of photon arrival times,
which exceeds the thermal kT noise. If Bif is the electrical
bandwidth, it is assumed that sufficient photons arrive in the observa-
Page 15
I
----------<--------
|
-- Beamcombiner |
Signal Pr | | ------- |
-----------> | | -------> | . | ---->> ----- PIN Photodetector
-----------> | | -------> | . | ---->> / \ (One detector in a
-----------> | | -------> | . | ---->> ----- 128 X 128 array)
Background | | ------- |
Pb -- ^ ^ ^ ^ | -----
Optional | | | | | | |
Optical | | | | --->---| |---->
Bandpass Filter | | | | Po | |
| | | | -----
------------- Intermediate Frequency
| | Electrical Bandpass Filter
| Local | Bif
Po >> Pr | Oscillator |
| Laser |
| |
-------------
|
|
-----------------<----------------
Frequency Control
Figure 2 -
Coherent optical heterodyne receiver. The diagram shows just a single
photodetector, but in a large professional heterodyning telescope, a
focal-plane array of about 128 X 128 photodetectors would be used to
reduced the search time. This would also ensure that if a star is
centered on the array, the signal from an orbiting ETI transmitter
would fall on the same pixel or on an adjacent one within the array
area, depending on the distance of the star, the orbital distance and
position of the transmitter, and its plane of ecliptic. For each array
pixel (photodetector), the local-oscillator power Po >> the received
signal Pr to ensure quantum noise limited detection. A focussed local-
oscillator (L.O.) laser may be scanned across the photodetector array
in synchronism with the electronic sampling of the array. This would
avoid the requirement for a high power L.O., and would thus eliminate
heat dissipation problems in the array.
tion or measurement time 1/Bif, for Gaussian and Poisson statistics to
apply. In practice, this means that about ten photons have to be
detected during each measurement interval. For the photon-starved
situation at small and negative Carrier-To-Noise Ratios (CNRs), the
(analog) CNR values are somewhat meaningless.
The effective noise temperature (Equ. 30) of the 656 nm system
modelled in this paper is 43,900 Kelvin, considerably more than the
10 K of the microwave system. However, it is the potential high-gain
transmitting capability of optical antennas (Equ. 10) which can more
Page 16
than make up for this 36 dB reduction in sensitivity (36 dB increase in
the noise floor). As a reference performance criterion, it should be
noted that a symmetrical microwave system based on the 300-meter
diameter Arecibo radio telescope on the island of Puerto Rico, a 1 kW
transmitter and a 10 K system temperature, would produce a CNR of about
20 dB re 1 Hz (this is illustrated in Figure 4, Page 28).
For discussions about Professional Optical SETI heterodyne
receivers, we will often refer to the term Signal-To-Noise Ratio (SNR)
in a generic manner as a means of denoting signal detectability. In
such cases, what we really mean is CNR, as the measurement is taken at
the intermediate frequency (I.F.) before electrical demodulation
(detection) of the signal. In the material on Amateur Optical SETI
photon-counting receivers, we will be dealing with the post-detection
signal-to-noise ratio, so it is more accurately denoted by the term SNR.
Communication engineers know that it is often expedient to
normalize the CNR or SNR to a 1 Hz electrical bandwidth; a bandwidth
which is thought to be substantially smaller than the minimum bin
bandwidth required for actual SETI observations with Professional
Optical SETI receivers. This allows us to subtract 10 dB from the CNR
(SNR) for each decade increase in electrical bandwidth. For instance,
a CNR (SNR) of 94 dB re (with respect to) 1 Hz is equivalent to 19 dB
re 30 MHz, a figure arrived at by subtracting 10.log(30 X 10^6) from
94 dB. We shall be referencing these particular numbers again later.
A bandwidth of "1 Hz" has a special significance to Microwave SETI
researchers. It is often the minimum bin bandwidth employed to analyze
the received signals as dispersion effects and Doppler chirp rates in
the low microwave region, i.e., around 1.5 GHz, would spread the most
monochromatic of signals to that order (Table 2, Line 30, Page 22)
shows the maximum equatorial ground-based chirp due to Earth's rotation
to be about 0.17 Hz/s). Thus, it is important to realize that for this
Optical SETI analysis, the 1 Hz bandwidth is used just for the con-
venience of normalizing the SNR. It does not imply anything about the
ideal electrical (I.F.) or post-detection bandwidth. Note that in this
study, it is generally assumed that the optical predetection bandwidth
is at least twice the electrical or post-detection bandwidth.
Although in Figure 2 we have indicated an optoelectronic front-end
array, it is possible that future developments in photonic computer
technology will allow for the employment of an all-optical receiver and
signal processing array.
In terms of mean transmitter power, it is useful to normalize
the different ETI transmitters to a basic unit of 1 kW. Again, this
implies no preconception about the actual powers available to ETIs,
which inevitably will be far in excess of this. The noise level
associated with the signal is assumed to be only that due to quantum
shot noise. For power-starved receiving condition, non-Poisson noise
at optical frequencies may actually raise the noise floor and degrade
the CNR. In the quantum (Poisson) limited detection case, for every
factor of ten that we increase the power, the CNR (SNR) will increase
by 10 dB. If the optical receiver is background or internally noise
Page 17
Relative Levels Per Pixel re 1 Hz
|
34 dB |___1 kW Signal *** 1.6 X 10^-15 W______
| *S*
| *I* CNR = 34 dB re 1 Hz
| *G*
0 dB |___Quantum Shot Noise_________*N*________6.3 X 10^-19 W/Hz___
| .A.
| .L.
| . .
-32 dB |___Planckian Continuum__ . . __4.0 X 10^-22 W/Hz___
| \ . . /
| \ . . /
-52 dB |___Fraunhofer Dark Line_ \___._.___/ __4.0 X 10^-24 W/Hz___
| <----------->
| H_alpha (656.2808 nm) Bandwidth = 0.402 nm = 280 GHz
-72 dB |___Day_Sky_______________________________4.0 X 10^-26 W/Hz___
|
|
|
|
|
-154 dB |___Night_Sky_____________________________2.5 X 10^-34 W/Hz___
|
| H_alpha
-------------------------------------------------------------
Wavelength or Frequency
Figure 3 -
Spectral levels at a range of ten light years, per diffraction limited
pixel. The normalized transmitter power is 1 kW at 656 nm, and various
noise sources for a space-based or adaptive ground-based heterodyne
observatory are indicated. Both the transmitter and receiver are of
10 meters aperture and are assumed perfect. Receiver quantum
efficiency equals 0.5. For convenience, the quantum noise level is
taken as a reference level from which the signal and other noise
sources are measured. Fraunhofer dark lines are typically 10 to 20 dB
below the Planckian continuum level.
limited, the CNR (SNR) will increase by 20 dB. Figure 3 is a graph of
signal and relative noise spectral levels for an imagined symmetrical
visible SETI system with heterodyne receiver (Equs. 32 and 34).
One of the main benefits from the optical approach is its ability
to sustain wideband communications over vast distances with very high
Effective Isotropic Radiated Powers (EIRPs), but using relatively small
apertures (Equ. 10). The latter attribute is particularly useful for
spacecraft applications. [63-66] The EIRP is the apparent power that
the transmitter would have to emit for a given received signal
intensity, if it was an isotropic radiator, i.e., if it radiated energy
uniformly in all directions, instead of confining the energy to a
narrow beam. It is given by the product of the antenna gain and
Page 18
transmitter power (Equ. 11). The 656 nm system has a Full Width Half
Maximum (FWHM) beamwidth of 0.014 arcseconds (Page 73), so that over
ten light years, the beam diameter has expanded to about 0.04
Astronomical Units (A.U.); roughly two percent of the diameter of
Earth's solar orbit (Page 74)!
For many years the author had been perplexed by the fact the optical
approach to SETI had been ignored. There was very much a feeling of
"What did the SETI community know that he did not?". Investigations
over the past eighteen months indicate that to a large extent, the
answer to this paradox was that the SETI community had simply refused
to believe in the possibility that ETIs could aim narrow beams, such as
the 0.04 A.U. dia. beam just described, and hit their targeted planet.
Continued
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