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EJASA - Part 4
PROJECT CYCLOPS
In this paper, many references are made to the Project Cyclops [5]
study and the effect that it has had on SETI thinking over the past
two decades. Table 1 is taken from this report, which illustrates this
author's view that Cyclops has been at least partially responsible for
the lack of interest in the optical approach to SETI after the early
1970's.
The first column A is the most revealing in this comparison table,
in that it models an ETI transmitter at the Nd:YAG (Neodymium: Yttrium-
Aluminum-Garnet) laser wavelength of 1,060 nm, that has an aperture of
22.5 cm! As can be seen, in the Cyclops analysis, the onus for
detecting a strong signal has been placed at the receiver end of the
system, where by definition, the technology available would be far
inferior to that at the transmitter. The resulting huge multi-mirror
receiving telescope system is thus incredibly expensive.
The performance of the 1.06 um (1.06 microns) and 10.6 um systems
modelled in the Cyclops study have been severely compromised by
restricting the transmitters and receivers to ground-based operation
within terrestrial-type atmosphere, and limiting beamwidths to
one second of arc. As previously mentioned, the atmospheric coherence
cell size (ro) is about 20 cm (8") at Wl = 0.5 um, and is proportional
to Wl^(6/5). The A infrared systems are essentially state-of-the-art
for 1971. The B infrared systems are futuristic for 1971. If we
assume that the 1 ns pulses have a repetition rate of one per second in
the case of the first 1.06 um Nd:YAG system (Optical System A), the
average power is only a modest 1 kW. One does wonder though, what a
peak power of 1 Terrawatt (1,000 GW) would do to a 22.5 cm diameter
transmitting mirror, or the air contained within the telescope!
SETI COMPARISONS
This paper describes two basic types of Optical SETI receiver; the
Professional (coherent) heterodyne system and the Amateur (incoherent)
photon-counting system. However, there is no reason why a professional
receiver could not use photon-counting, and vice versa, why an amateur
Page 19
==========================================================================
Table 1 Project Cyclops comparison scenarios
==========================================================================
OPTICAL INFRARED MICROWAVE
--------------------------------------------------------------------------
PARAMETER A B A B A B
Wavelength 1.06 um 1.06 um 10.6 um 10.6 um 3 cm 3 cm
==========================================================================
TRANSMITTER
==========================================================================
Antenna Diameter 2.25 m 2.25 m 100 m 3 km*
--------------------------------------------------------------------------
No. Of Elements 1 1 1 1 1 900
Element Diameter 2.25 m 2.25 m 100 m 100 m
Antenna Gain 4.4x10^11 4.4x10^11 4.4x10^11 4.4x10^11 1.1x10^8 9.8x10^10
Peak or CW Power, W 10^12 10^5 10^5 10^5 10^5 10^5
Modulation Pulse Pulse Pulse PSK PSK PSK
Pulse, s 10^-9 1 1 1 1 1
Energy per Bit, J 10^3 10^5 10^5 10^5 10^5 10^5
EIRP, W 4.4x10^23 4.4x10^16 4.4x10^16 4.4x10^16 1.1x10^13 9.9x10^15
Beamwidth 1" 1" 1" 1" 64" 1"
==========================================================================
RECEIVER
==========================================================================
Antenna Diameter 100 m 100 m 100 m 2.25 m 100 m 3 km*
--------------------------------------------------------------------------
No. Of Elements 400 400 1975 1 1 900
Element Diameter 5 m 5 m 2.25 m 2.25 m 100 m 100 m
Atmosphere Tran. 0.7 0.7 0.5 0.5 1 1
Quantum Effic. 0.4 0.1 0.2 0.2 0.9 0.9
Solar Background 1.2x10^-3 36 1.7x10^-3 6x10^-7 ----- -----
Noise Temp., K 13,600 13,600 1360 1360 20 20
RF Bandwidth 1 GHz 3 MHz 3 kHz 1 Hz 1 Hz 1 Hz
Detection Method Photon Photon Sq. Law Synch. Synch. Synch.
Range Limit (L.Y.) 26 24 22 41 500 450,000
State Of The Art? ? No ? No Yes Yes
All Weather? No No No No Yes Yes
==========================================================================
* Array spread out to 6.4 km diameter to avoid vignetting.
Data taken from Table 5-3, page 50, July 1973 revised edition
(CR 114445) of the Project Cyclops design study of a system for
detecting extraterrestrial life. [5] This study was prepared under
Stanford/NASA/Ames Research Center 1971 summer faculty fellowship
program in engineering systems design. Note that at the time the
Cyclops study was done, the field of "optoelectronics" (photonics) had
not yet really begun. Thus, what the Cyclops study called "Optical" is
really a superset of both "near-infrared", and "infrared". In this
Optical SETI paper, "optical" covers the entire spectrum from ultra-
violet to the far-infrared. The near-infrared 1.06 um ETI transmitter
for the Optical System A is only 22.5 cm in diameter, and is modelled
to be putting out 1 kW pulses of 1 ns duration, with a peak power of
one trillion watts and corresponding peak EIRP of 4.4 X 10^23 W!
Page 20
receiver could not use heterodyne detection. The definition adopted
here is one based purely on performance and cost grounds.
We now continue with the comparisons between various type of
professional heterodyning SETI systems as tabulated in Table 2
(Page 22). It should be noted that while the microwave system in this
table is based on a 100-meter diameter dish, the microwave system
modelled in Figure 4 (Page 28) is based on a 300-meter diameter
Arecibo-type dish. The 100-meter diameter dish system of Table 2,
corresponds to the Microwave System A modelled in the Cyclops study
(Table 1, Page 19), each dish being one of up to nine hundred similar
dishes making up the Cyclops array.
The infrared telescope system is very similar to ones previously
modelled by Townes, Betz, and Zuckerman. [46-47,51-53,57] Note that by
increasing the 10,600 nm infrared transmitting and receiving
telescopes' diameters to twenty meters, the SNR (CNR) obtained can be
increased to the same value (34 dB) indicated for the 656 nm visible
system (Table 2, Line 26). Since the Carbon Dioxide (CO2) laser is
very efficient, coherent, and CO2 is likely to be readily available
where life becomes established, 10,600 nm may be considered a "magic
optical wavelength". [46-58] This wavelength is also capable of
propagating with little attenuation across substantial portions of the
Milky Way galaxy. The beam divergence is such as to make the targeting
of nearby stars easier. There is also an approximately sixty percent
atmospheric window at this wavelength.
All these telescopes, save for the Cyclops Array (Table 1) [5], may
be considered as "puny" for an Advanced Technical Civilization (ATC),
but are representative of state-of-the-art terrene technology,
technology available either now or within the next decade. The results
are based on "perfect" space-based systems (save for the daylight
background factor), so in practice, several dB may have to be taken off
the calculated SNR to account for imperfections, and atmospheric
absorption and turbulence, if ground-based. Because optical heterodyne
receivers are proposed for the professional optical systems, Planckian
starlight and daylight have no effect on ground-based system
performance if the local-oscillator power per pixel (per photodetector)
is a lot greater than the background power. Large ground-based optical
telescopes would likely use adaptive deformable mirror and laser guide-
star technology for removing the "twinkle" from the star and
transmitter's image. [68-70] The performance of such telescopes should
exceed the theoretical performance of the HST. [59-62] This technology
may be available within five years, and will be described in more
detail later.
The "pilot-tone" technique briefly described on Page 10, used in
conjunction with a photodetector array, might allow the implementation
of a Maximal Ratio Predetection Diversity receiver. This leads to a
very simple adaptive receiver which could be operated both during the
day and night. As previously indicated, a more detailed description of
how this operates may be found in Appendix A (Page 83). It should be
kept in mind that getting a "perfect" image of a star and/or an ETI
transmitter is a more rigorous pursuit than just collecting all the
Page 21
photons emitted by the ETI transmitter, wherever they fall within the
photodiode array area.
Table 2 (Page 22) summarizes the salient points of the comparison
between different electromagnetic communications technologies as
applied to SETI, using heterodyning telescopes. [71-79] A preferred
wavelength, not shown in this table, might be 1,060 nm, corresponding
to the Nd:YAG transitions in the near-infrared. The corresponding SNR
for a 10-meter diameter 1,060 nm system is 32.1 dB.
Given a modest extension to our technology over the next century,
such wideband terrene interstellar links should become feasible,
though they would use digital modulation and compression techniques
to reduce the required bandwidth and enhance the SNR. The apparent
visual intensity of the 1 GW transmitter, the power output of a
typical Twentieth Century terrene power station, would rise from an
apparent magnitude of +22.7 to +7.7. This is still below unaided human
eye visibility (sixth magnitude) even if not obscured by the light of
its star, and amounts to only 0.62% of the star's visual intensity (not
corrected for wavelength). This result demonstrates that references in
the literature to the fact that such signals have never been seen by
the unaided eye, or detected in low-resolution spectrographs, proves
nothing about whether ETIs are transmitting in the visible spectrum.
Simply put, a powerful communications signal is still weak compared to
a star's (integrated over wavelength) output radiated in our direction.
Table 2, Line 11 -
The reader is left to judge whether ATCs (ETIs) would have the
wherewithal to aim narrow optical beams over tens and hundreds of
light years and still be sure that their signal would strike a planet
orbiting within the targeted star's biosphere (zone of life). Perhaps
it is this assumption alone that is the key to the efficacy of the
optical approach to SETI. The option is available to defocus
(decollimate) the transmitted beam when targeting nearby stars. In
such a situation, the signal strength would be weakened (reduced EIRP)
for nearby target systems, but would remain relatively constant when
operated on more remote targets out to distances of several thousand
light years. It does not make sense to cripple, which is the result of
Dr. Bernard Oliver's approach, [5] the long-range performance of Extra-
terrestrial Intelligence (ETI) transmitters just because the beams
happen to be too narrow for nearby stars.
Clifford Singer [15] has described how superior ETI technical
prowess for transmitting microwave signals at certain preferred times
related to the targeted star's proper motion, can lead to an enhanced
transmission efficiency, making it more likely that the recipient
will be able to detect those signals. In a similar vein, Filippova and
others [55] have suggested that ETIs might make use of the moment of
opposition to ensure that a narrow optical beam aimed at a star would
be detectable at a target planet approaching opposition. Dr. John
Rather, in the August, 1991 issue of the JOURNAL OF THE BRITISH
INTERPLANETARY SOCIETY (JBIS) [56], describes huge Optical ETI
Page 22
=========================================================================
Table 2 Summary of SETI performance for (symmetrical) professional
heterodyne communication systems over a range of 10 light years.
=========================================================================
MICROWAVE SETI OPTICAL SETI
PARAMETER SINGLE DISH INFRARED VISIBLE
=========================================================================
1. Wavelength 0.20 m 10,600 nm 656 nm
2. Frequency, Hz 1.50 X 10^9 2.83 X 10^13 4.57 X 10^14
=========================================================================
TRANSMITTERS
-------------------------------------------------------------------------
3. Diameter, m 100 10 10
4. Gain, dB 63.9 129.4 153.6
5. FWHM Beamwidth, arcsecs. 421 0.223 0.0138
6. Power, kW 1 1 1
7. EIRP, W 2.47 X 10^9 8.78 X 10^15 2.29 X 10^18
=========================================================================
RECEIVERS
-------------------------------------------------------------------------
8. Diameter, m 100 10 10
9. Gain, dB 63.9 129.4 153.6
10. FWHM Beamwidth, arcsecs. 421 0.223 0.0138
11. FWHM Diameter, A.U. 1,290 0.684 0.0423
12. Intensity, W/m^2 2.19 X 10^-26 7.81 X 10^-20 2.04 X 10^-17
13. Signal, W 1.72 X 10^-22 6.13 X 10^-18 1.60 X 10^-15
14. Photon Count, s^-1 NA 163 2,640
15. Equivalent Magnitude NA NA +22.7
16. Quantum Efficiency NA 0.5 0.5
17. Effec. Noise Temp., K 10 2,719 43,900
18. Planckian, W/m^2.Hz* 8.80 X 10^-33 1.07 X 10^-25 2.74 X 10^-24
19. Star Stellar Magnitude NA NA +2.2
20. Relative Brightness, % NA NA 6.2 X 10^-7
21. Alien Planet Magnitude NA NA +24
22. SPR, dB* 64.0 55.7 65.7
23. Minimum SPR, dB* 64.0 69.5 115.7
24. Daylight, W/m^2.sr.nm NA 2 X 10^-3 1 X 10^-1
25. SDR, dB* NA 50.6 106.0
-------------------------------------------------------------------------
26. SNR, dB* 1.0 22.1 34.2
-------------------------------------------------------------------------
27. Radial Doppler, Hz 1.0 X 10^5 1.9 X 10^9 3.1 X 10^10
28. Orbital Doppler, Hz 1.5 X 10^5 2.8 X 10^9 4.6 X 10^10
29. Synchronous Chirp, Hz/s 1.1 X 10^0 2.1 X 10^4 3.4 X 10^5
30. Ground-Based Chirp, Hz/s 1.7 X 10^-1 3.2 X 10^3 5.1 X 10^4
31. Symbiotic Cost, $M 2 20 20
32. Ground-Based Cost, $M 200 200 200
33. Space-Based Cost, $M 100 10,000 10,000
=========================================================================
FWHM = Full Width Half Maximum (3 dB beamwidth).
1 Astronomical Unit (A.U.) = 1.496 X 10^11 m.
1 Light Year (L.Y.) = 9.461 X 10^15 m = 63,239 A.U.
1 parsec (psc) = 3.26 L.Y.
Page 23
* Signal-To-Noise (SNR) and Signal-To-Planck/Daylight (SPR and SDR)
Ratios assume polarized starlight and background, with no Fraunhofer
dark-line suppression (typically 10 to 20 dB).
Signal-To-Noise Ratios (SNRs) in the galactic plane fall at the
rate of 20 dB per decade of range (see Equ. 38), out to approximately
one thousand light years in the visible regime, where attenuation by
gas and dust begins to become significant. The attenuation in the
visible, of 4 dB per three thousand light years (equivalent to a one
stellar magnitude reduction in brightness), drops significantly away
from the galactic plane.
The following numbers refer to the line numbers given in Table 2
and give a more detailed description of the parameters:
5. Full Width Half Maximum (FWHM) far-field beamwidth (Equ. 4).
8. The Cyclops Array proposed in 1971 consisted of nine hundred
100-meter diameter dishes (of the type modelled in the table)
covering an area 6.4 kilometers in diameter.
11. Full Width Half Maximum (FWHM) size of received beam (Equ. 5).
14. The rate at which photons are detected (Equ. 36).
15. Apparent visual magnitude of transmitter is not corrected for
visible wavelength (Equ. 2).
20. Relative brightness of transmitter in comparison to unpolarized
Planckian starlight from a G-type star (black-body at 5,800 K).
21. Apparent Stellar Magnitude of reflected Planckian starlight
from a Jupiter-size extrasolar planet. Note that if we want to
detect an extrasolar planet directly, it is easier to do so by
detecting its emitted heat in the infrared than by detecting
reflected light in the visible.
22. Signal-To-Planck Ratio (SPR) for a solar-type star at the
heterodyned I.F. frequency, assuming star and transmitter are
not separately resolved.
23. Minimum Signal-To-Planck Ratio (SPR) for a solar-type star at
the heterodyned I.F. frequency, assuming star and transmitter
are separately resolved (Equ. 9).
24. Background daylight sky radiance for ground-based visible and
infrared telescopes. For the latter, the 300 K temperature of
the atmosphere presents a relatively constant 24 hour/day
background.
25. Signal-To-Daylight Ratio (SDR) per pixel for diffraction-limited
ground-based visible and infrared telescopes.
Page 24
26. For convenience, SNRs (CNRs) are normalized to a 1 Hz
electrical bandwidth. The value for the microwave system is
given by Equ. 29. The values for the optical systems are given
by Equs. 32 and 34.
27. Typical Doppler Shift (+/-) due to line-of-sight relative
motions between stars at 20 km/s (Equ. 39).
28. Maximum local Doppler Shift (+/-) due to motion of transmitter/
receiver around solar-type star (1 A.U. orbit).
29. Maximum local Doppler Drift (+/-) for transmitter/receiver in
geosynchronous orbit around Earth-type planet (Equ. 40).
30. Maximum local Doppler Drift (+/-) for a ground-based equatorial
transmitter/receiver on an Earth-type planet.
31. Approximate ground-based receiver cost (millions), assuming
re-use or sharing of existing observatories in each hemisphere.
32. Approximate ground-based receiver cost (millions), assuming a
new dedicated (adaptive) telescope in each hemisphere.
33. Approximate receiver cost (millions) for a single space-based
telescope. A very conservative estimate has been used.
transmitting arrays which are of planetary size, sending out powerful
Free-Electron Laser beams to an enormous number of stars simultan-
eously. Huge arrays can provide an extended Rayleigh (near-field)
range so that the flux densities remain constant (the inverse square
law does not apply) out to considerable distances (Equ. 7, Page 74).
Table 2, Line 15 -
In this table, the apparent visual magnitude and brightness of a
star, planet, or transmitter, is given for comparison purposes, and is
defined only for visible wavelengths, since infrared light is
invisible. The apparent visual magnitude of the transmitter is
essentially independent of the optical detection bandwidth as long as
it is equal to or greater than the signal bandwidth, i.e., it is the
same for an optical bandwidth of 1 Hz, 1 MHz, or 1 THz; these band-
widths being much less than that of the human eye.
Table 2, Line 20 -
This shows the apparent visual intensity of the transmitter with
respect to the alien star (Equ. 2). If the 656 nm 1 kW transmitter
power is increased by six orders of magnitude to 1 GW, the received
signal will increase to 1.6 nW (2.6 X 10^9 photons detected per
second), and the Carrier-To-Noise Ratio (CNR) will increase to 94 dB.
In a 30 MHz bandwidth this CNR will fall to 19 dB. This is more than
adequate to transmit a standard analog NTSC/PAL/SECAM F.M. video
Page 25
signal over 10 light years, though at a range of 100 light years the
CNR would fall to an unusable -1 dB (the F.M. threshold is typically
7 to 10 dB).
Table 2, Line 23 -
The Signal-To-Planck Ratio (SPR) on this line takes into account
the ability of large diffraction-limited optical telescopes to
spatially separate in the focal plane, the image of the transmitted
signal from the image of the aliens' star (Equs. 8 and 9). This leads
to the Signal-To-Planckian Ratio (SPR) being about 10 dB greater than
the Signal-To-Daylight Ratio (SDR). Clearly, even when the signal
source and Planckian noise (Equ. 3) are not optically separable, the
ratio of the signal to the Planckian background noise is much greater
than the quantum shot noise SNR, so it is not limiting on performance.
Contrary to statements in the literature [12], there may be no need
to select a laser wavelength to coincide with a Fraunhofer line if
optical heterodyne reception is assumed. This is really useful only
when incoherent optical detection techniques are employed (see the
later material on Amateur Optical SETI) with their relatively wideband
optical filters. However, it might be advisable to avoid bright
emission lines that rise substantially above the continuum level.
For an advanced technical society, a laser transmitting telescope
is only "slightly" more difficult to construct than a microwave
transmitting dish, though Isaac Asimov appeared to think otherwise in
the late 1970s. Towards the end of his 1979 book, EXTRATERRESTRIAL
CIVILIZATIONS [12] (page 263), Asimov says: "With laser light we come
closer to a practical signaling device than anything yet mentioned, but
even a laser signal originating from some planet would, at great
distances, be drowned out by the general light of the star the planet
circles." He goes on to say: "One possibility that has been suggested
is this: The spectra of Sun-type stars have numerous dark lines
representing missing photons - photons that have been preferentially
absorbed by specific atoms in the stars' atmospheres. Suppose a
planetary civilization sends out a strong laser beam at the precise
energy level of one of the prominent dark lines of the star's spectrum.
That would brighten that dark line...." Asimov went on to imply that a
laser system was complicated and that no civilization would be expected
to use the harder method if a simpler (microwave) method is available.
This erroneous idea that laser transmitters have to outshine stars
to be detectable has unfortunately been accepted by many in the SETI
community. Dr. Jill Tarter [24] (Chapter 14, SETI: THE FARTHEST
FRONTIER, Page 192) has said that "Any optical communications signal
coming from a planet circling a distant star would have to outshine the
star itself in order for us to detect it.". As we have seen, this is
simply not true. Indeed, as we shall show later, even small incoherent
receivers with optical bandwidths as large as 100 GHz can produce
electronically detectable signals at intensities considerably below
that of nearby stars. Note that this statement has nothing to do with
the assumed technical beaming prowess of ETIs, only that a visible
Page 26
wavelength signal strong enough for good communications, is still weak
compared to a star's visual brightness (intensity).
With optical heterodyne receivers, whose performance is essentially
independent of the optical pre-mixing bandwidth (the effective optical
bandwidth for background noise calculations is equal to the electrical
intermediate frequency bandwidth), there does not appear to be any
necessity to operate within a Fraunhofer dark absorption line in order
to avail ourselves of 10 to 20 dB of Planckian continuum noise
suppression. The "magic-wavelength" would thus be determined only by
the availability of highly efficient and coherent laser frequencies.
Table 2, Line 25 -
The high Signal-To-Daylight (background) ratio indicates that
Optical SETI is one of the few branches of optical astronomy, save for
solar astronomy, which can be conducted during daylight hours under a
clear, blue Earth sky. Since the background detected per diffraction
limited pixel is essentially independent of aperture, this ratio
(shown for 45 degrees to the zenith) is proportional to the receiving
telescope's aperture area, as is the quantum SNR. The Signal-To-
Nightlight ratio for ground-based observatories is some 80 dB greater.
Thus, it is suggested that Optical SETI observations with the
great optical telescopes of Earth could be conducted during daylight
hours while conventional astronomy is conducted at night. Also,
telescopes which have been decommissioned due to light pollution
effects might be brought back into service. A future symbiotic
relationship (sharing of facilities) between Optical SETI and
conventional astronomy, could allow Optical SETI to be conducted for
one-tenth the cost indicated on Line 32 for dedicated observatories,
i.e., for about twenty million dollars (United States currency).
Continued
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