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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    22.5 cm   22.5 cm    2.25 m    2.25 m    100 m     3 km*
--------------------------------------------------------------------------
No. Of Elements      1         1         1         1         1         900
Element Diameter     22.5 cm   22.5 cm   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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