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Optical SETI Survey - Problems of Targeting Tight Beams (Part C)

Radobs 7C

 
Version 1.0
Preface:  Most of the questions in this section relate to the problem of
          aiming a very narrow beam over tens and hundreds of light years. 
          What we think will be the ability of advanced technical
          civilizations (ATCs) to do this, is perhaps the most important
          assumption to be made when comparing different interstellar
          communication techniques.  The benefits of the optical approach
          really hinge on this ability, so consider your response very
          carefully.  Read questions 2, 3 and 4 together before answering,
          so that you are aware of the differences.


1.   Just to give a flavor of what humans can do today, you should note that
     the specifications for the Hubble Space Telescope call for it being
     held on target to an accuracy of 0.012 arcseconds, which is pretty
     impressive as far as our own crude technologies go.  Would you expect
     that mankind would be able to do even better in time?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


2.   It is thought by some to be relatively easy for ATCs to precisely aim
     (control) beams which have widths smaller than 1 second of arc.  Do you
     agree?  If so, indicate what you think would be the narrowest beam that
     an ATC could bring to bear on a desired target.  This basic question
     relates only to the ability of actually being able to steer a tight
     beam onto a small target with high accuracy, and overcome any inherent
     problems associated with laser beam wander.  It does not relate to the
     problem of actually knowing where the target is because of the lack of
     visibility, relative motions and the light distance of the target. 
     These problems are addressed in the two following questions.

     (a) < 0.1 arcsecond
     (b) < 0.01 arcsecond
     (c) < 0.001 arcsecond
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO



3.   It is a part of this Optical SETI rationale that it should be possible
     for ATCs to transmit optical beams with beamwidth << 1 second of arc
     and hit targeted nearby stars (less than a few hundred light years
     away) with forward predictive targeting, even though the beam diameter
     at the target will be a smaller than 1 astronomical unit.  The problem
     here relates to the fact that ATCs must have a very accurate idea about
     the relative motion of the targeted star with respect to the
     transmitter if they are to "hit the bull's eye".  If the ATC's
     knowledge about the position of the target is still limited by the
     speed of light, then the problem amounts to being able to land a beam
     centrally on a star, which will have moved (transversely) from its last
     known position (when the photons left the transmitter) by the relative
     distance travelled by the star in twice the light time distance away. 
     Allowances would also have to be made for the aberration of light, and
     perhaps the gravitational bending of light by other stars close to the
     line-of-sight.  Considering these difficulties, do you think that ATCs
     could land such a narrow beam on a star?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


4.   It is also part of the rationale that it should be possible for ATCs to
     transmit optical beams with beamwidth << 1 second of arc and hit
     targeted planets in nearby star systems (less than a few hundred light
     years away) with forward predictive targeting, even though the beam
     diameter at the target will be smaller than 1 astronomical unit.  This
     is an even more stupendous feat for ATCs than just hitting a star with
     its beam.  They could have information about our planet, perhaps having
     actually visited here at some point in our history, or they may have
     sent out probes which have transmitted (by photons?) back information
     about us, or their knowledge may come via the more basic technique of
     actually being able to directly image the planets in the Solar System
     with their large space-based telescopes.  Simply put, if they have the
     optical telescope technology to transmit to us effectively, they also
     have the capability to "see" our planets.

     They might, of course, also know our position and motion by detection
     of radio frequency leakage, which would also tell them that there was
     intelligent life on this planet.  Just because we may have been
     detected by radio waves does not necessarily mean that they would use
     the same technology to signal us.  Do you agree that they could know
     about the positions of planets in nearby star systems and thus ensure
     that when the photons arrived at the targeted star, they passed through
     the star-system in a region of space occupied by the targeted planet?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


5.   Transmitted beams might be shaped with phased array elements to produce
     a fan-shape.  If the target planetary's plane of ecliptic is known to
     the aliens, and the line-of-sight is more or less along the plane of
     the ecliptic, then the energy density can be maintained to a
     significantly higher level by a fan-shaped beam, than if a circular-
     profile beam is employed which is as large as the entire planetary
     system.  In this way, the targeting problem is substantially reduced
     (one doesn't need to know the position of the planets in their orbits),
     and energy is not wasted outside the plane of the ecliptic.  For
     instance, the beamwidth might be 1 X 0.014 seconds of arc, such that at
     a range of 10 light years, the beam would have dimensions
     3.6 X 0.05 A.U.  This would decrease the SNR by about 18.5 dB, but make
     it much easier to simultaneously illuminate the entire biosphere of
     nearby stars.  Under such beaming techniques, a symmetrical 10 meter
     diameter visible light system has an SNR only about 5 dB less than is
     available from a symmetrical 300 meter diameter 1.5 GHz microwave
     system.  Is a fan-shaped beam a good approach for overcoming the
     targeting problem if the line-of-sight is near the target's planetary
     plane of the ecliptic?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


6.   Dr. John Rather, of NASA Headquarters, has suggested that phased
     optical arrays could be constructed that produce annular rings of
     energy to match the biospheres of planetary systems.  In this way, the
     targeting problem is substantially reduced (one doesn't need to know
     the position of the planets in their orbits), and energy is not wasted
     in the center of the beam.  However, this only buys a few extra dB in
     signal strength and may not be worth the complication.  One would need
     to know the plane of the ecliptic.  It would also be possible to
     rapidly dither a small circular-profile beam to trace out an annular
     ring.  Do you think this annular ring beam is a reasonable idea?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


7.   If Optical SETI is restricted to far-infrared wavelengths, e.g., the
     CO2 wavelength of 10,600 nm, the diffraction limited beamwidth of a
     10 meter diameter transmitting telescope is 0.22 arcsecond, only
     slightly less than the approximate 1 arcsecond aiming limitation that
     has been suggested by some researchers.  Carbon Dioxide lasers are one
     of the most efficient continuous-wave lasers and most coherent lasers
     known to man, and CO2 is thought to be very common in planetary
     atmospheres.  The Carrier-To-Noise Ratio (CNR) from a symmetrical
     10 meter diameter telescope system at this wavelength, is only about
     12 dB less than from a similar size visible telescope system.  Also,
     the atmosphere is reasonably transparent at this wavelength, which may
     allow for ground-based observations.  Do you think that 10,600 nm
     should therefore be classified as a "magic wavelength"?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


8.   Even if the above targeting problem is thought to be too difficult for
     ATCs, it does not make sense to use too small a transmitting telescope
     to produce a relatively large beamwidth.  Rather, use the largest
     telescope that ATC technology can produce at a cost that is affordable,
     and defocus (decollimate) the beam for nearby stars only.  In this way,
     long range (greater than a few hundred light years) performance is not
     degraded.  The defocusing aspect would be under computer control during
     spatial/time-multiplexing of many targeted star-systems (see following
     question).  Do you agree with these sentiments?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


9.   Laser transmitting telescope technology makes it relatively easy (for
     ATCs) to rapidly spatially time multiplex their transmitted signal
     amongst many target receivers and star systems.  Do you agree?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


10.  Optical spatial/time-multiplexed communications may be a technique in
     day-to-day use by ATCs in order to keep in touch with their space
     probes, spaceships, space colonies, and perhaps other inhabited planets
     in their star system.  It is not unreasonable to suggest that the same
     technology would be used to signal emerging technical civilizations
     (ETCs).  Do you think that it is a strong possibility that ATCs will
     "spin-off" their conventional communications technology in an attempt
     to contact ETCs?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


11.  For an alien, if its worth sending your Cultural Encyclopedia or the
     Encyclopedia Galactica to an emerging technical civilization (us), then
     it is worth doing it properly at high data-rates, particularly if one
     is time-multiplexing many target star systems.  Do you agree with this
     philosophy?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


12.  If there is a Galactic Communications Network, it may rely on ETI
     relays spread out through the galaxy, for the Milky Way Galaxy to be
     linked in its entirety.  Each civilization would add to the message its
     own cultural history.  Thus, it would not be necessary to suppose very
     powerful transmitters that could directly send a strong signal over a
     distance equivalent to the diameter of the galaxy, i.e., about 100,000
     light years.  A relay system would be the most efficient way of linking
     the galaxy.  Just as we use repeaters in undersea communications, and
     terrestrial fiber-optic systems, so repeaters could be employed in
     interstellar communication systems.  Do you think intragalactic relays
     would have merit if intelligent life is relatively common throughout
     the galaxy?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


13.  The 1971/1972 study on Project Cyclops probably has much to do with the
     SETI lore that the optical approach is useless.  That study compared
     the performance of a 6.4 km diameter array consisting of 900 microwave
     dishes, each 100 meters in diameter, with an (asymmetric) near-infrared
     (1.06 micron Nd:YAG) system employing a transmitting telescope only
     22.5 cm in diameter - a telescope size smaller than what many
     terrestrial amateurs own.  The study did not consider space-based
     telescopes or adaptive telescope technology, which severely constrained
     the size of the optical telescopes.  Allowing for heat dissipation
     limitations for small transmitter apertures, the Effective Isotropic
     Radiated Power (EIRP) is proportional to the telescope diameter raised
     to the fourth power.  This has a tremendous effect on the signal-to-
     noise ratio (SNR), as an order of magnitude increase in diameter will
     increase the SNR by 40 dB.  On the receiver side, there could be an
     additional increase in SNR of between 20 to 40 dB when the receiving
     telescope size is increased by an order of magnitude, depending upon
     whether the SNR was previously severely limited by Planckian starlight. 
     Do you think it was unfair to suppose that ATCs would be so limited in
     their optical technology?
                                                                          YES
                                                                   DON'T KNOW
                                                                           NO


Score out of 13: YES        =
                 DON'T KNOW =
                 NO         =


December 27, 1990
RADOBS.7C
BBOARD No. 290


* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1990        *
* AMIEE, SMIEEE                                                           *
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