Electronic Journal of the ASA - January 1992 (Part F)
Larry Klaes
EJASA, Vol. 3, No. 6, January 1992
THE ELECTRONIC JOURNAL OF
THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
Volume 3, Number 6F - January 1992
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TABLE OF CONTENTS
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* ASA Membership and Article Submission Information
* The Search for Extraterrestrial Intelligence (SETI) in
the Optical Spectrum, Part F - Dr. Stuart A. Kingsley
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ASA MEMBERSHIP INFORMATION
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EJASA, Vol. 3, No. 6, January 1992
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EJASA, Vol. 3, No. 6, January 1992
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Optical SETI debate.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley Copyright (c) 1992 *
* Consultant *
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* Volunteer, SETI Group, Ohio State. *
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Version: 1.00
File: EJASAV3.N06
THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC
January 1992 - Vol. 3, No. 6F
Copyright (c) 1992 - ASA
EJASA, Vol. 3, No. 6, January 1992
THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (SETI)
IN THE OPTICAL SPECTRUM - PART F
Optical SETI Revisited and the Amateur Approach
by
Dr. Stuart A. Kingsley
FIBERDYNE OPTOELECTRONICS
545 Northview Drive
Columbus, Ohio 43209
United States
EJASA, Vol. 3, No. 6, January 1992
Page 71
APPENDIX A
THEORY AND SPECIMEN CALCULATIONS
The Drake Equation:
Fundamental to all SETI approaches is the belief that there are a
reasonable number of technological civilizations out there who might be
trying to communicate with us.
The following formula for the number of technological civilizations in
the galaxy is a modified form of the one devised in 1961 by Frank Drake
[2-3] of Cornell (also President of the SETI Institute) and is known
as the famous "Drake Equation": [13,25]
N = R*.fp.ne.fl.fi.fc.L (1)
where R* = number of stars in the Milky Way galaxy (400 X 10^9),
fp = fraction of stars that have planetary systems (0.1),
ne = average number of planets in such star systems that can
support life (1),
fl = fraction of planets on which life actually occurs (0.1),
fi = fraction of such planets which intelligent life arises
(0.01),
fc = fraction of intelligent beings knowing how to communicate
with other civilizations (0.1),
L = average lifetime (fraction of the age of its star) of such
technical civilizations (0.001).
Substituting what some might say are conservative values given in
parentheses for the entire Milky Way galaxy:
N = 4,000
Thus, there could be a minimum 4,000 worlds for us to detect in our
galaxy. If there were only 4,000 technical civilizations within a
galaxy that is 100,000 light years in diameter, then the probability
of detecting ETI signals is likely to be small. However, many SETI
scientists and exobiologists give more optimistic values for these
parameters, and thus yield higher values for N. If fp, fl, fi, fc,
and L are significantly higher, our galaxy would be teeming with
intelligent technical civilizations. If we assume that the average
lifetime of a star is 10 billion years, then a value of L = 0.001
implies that civilizations can last 10 million years. Clearly, there
is a substantial degree of uncertainty in the value of L.
Within 1,000 light years of Sol there are 10 million stars, of which
1 million are solar-type. Thus, taking a more optimistic value for
"N", the SETI community reasons that there is a significant chance of
detecting an ETI signal if we "look" out to 1,000 light years, assuming
of course, that we are tuned to the correct frequencies. The issue of
the correct frequencies to search is at the heart of this paper.
EJASA, Vol. 3, No. 6, January 1992
Page 72
Apparent Stellar and Signal Magnitudes:
The relationship between Apparent Stellar Magnitude (m) [88-90] and the
brightness or intensity of a solar-type star (or a laser operating at or
near the peak of the photopic response) may be expressed in the form:
m = -[19 + (2.5).log(Ir)] (2)
where Ir = received intensity (W/m^2).
The threshold for unaided eye visibility (dark sky) is m = +6. As
mentioned above, this expression may also be used to estimate the
approximate visibility of a laser, i.e., the apparent signal magnitude,
if its wavelength is not too far removed from the peak of the low-
intensity visual response at 500 nm. Here are several intensities and
corresponding magnitudes as a function of range R. We note that the
Sun's total output (EIRP) = 3.90 X 10^26 watts:
At R = 1 A.U. (1.496 X 10^11 m):
Ir = 1.39 kW/m^2
m = -26.8
Thus the solar flux density at normal incidence just outside Earth's
atmosphere is 1.39 kW/m^2.
At R = 10 L.Y. (9.461 X 10^16 m):
Ir = 3.48 X 10^-9 W/m^2
m = +2.2
At R = 100 L.Y. (9.461 X 10^17 m):
Ir = 3.48 X 10^-11 W/m^2
m = +7.2*
At R = 1,000 L.Y. (9.461 X 10^18 m):
Ir = 3.48 X 10^-13 W/m^2
m = +12.2*
* Not visible to the unaided eye.
In Table 2 (Page 22), Apparent Magnitudes are quoted for stars,
extrasolar planets, and ETI transmitters on the basis of the visual
brightness or intensity of each object acting alone. Because the
reason for quoting the Apparent Magnitudes is to demonstrate that
relatively strong laser transmitters are still "visually" weak, the
Apparent Magnitudes are only given for the visible wavelength.
EJASA, Vol. 3, No. 6, January 1992
Page 73
Planckian Starlight Background:
For observations at night, the background Nb may be taken as the
Planckian (black body) starlight continuum level (Npl). [88-90] With
no allowance for the Fraunhofer dark line absorption or bright line
emission, the non-polarized spectral energy density is given by:
2.PI.h.f^3r^2
Npl = ----------------------- W/m^2.Hz (3)
c^2[e^(h.f/k.T) - 1]R^2
where h = Planck's constant (6.63 X 10^-34 J.s),
c = velocity of light (3 X 10^8 m/s),
Wl = wavelength (656 nm),
f = frequency (c/Wl = 4.57 X 10^14 Hz),
k = Boltzmann's constant (1.38 X 10^-23 J/K),
T = temperature (5778 K),
r = radius of star (6.96 X 10^8 m),
R = distance of receiver (10 L.Y. = 9.461 X 10^16 m).
At R= 1 A.U.:
Npl = 2.19 X 10^-12 W/m^2.Hz
At R = 10 L.Y.:
Npl = 5.47 X 10^-24 W/m^2.Hz
Full Width Half Maximum (FWHM) Angular Beamwidth:
For the purposes of this part of the analysis, we have assumed a fully
(uniformly) illuminated circular aperture and not a beam with a
Gaussian intensity profile, as might be obtained from a laser with a
single transverse TEMoo mode. The diffraction limited half-power
(-3dB) beamwidth is given by: [66,85]
(58.5).Wl
FWHM Beamwidth = --------- degrees (4)
d
where Wl = wavelength,
d = diameter (aperture) of telescope.
For d = 10 m (professional telescope) and Wl = 656 nm:
FWHM Beamwidth = 0.0138 arc seconds
For d = 0.30 m (amateur telescope) and Wl = 656 nm:
FWHM Beamwidth = 0.461 arc seconds
EJASA, Vol. 3, No. 6, January 1992
Page 74
Full Width Half Maximum (FWHM) Diameter:
The diffraction limited far-field half-power (-3 dB) beam diameter is
given by:
(1.02).Wl.R
FWHM Diameter = ----------- meters (5)
d
At R = 10 L.Y.:
FWHM Diameter = 6.33 X 10^9 m = 0.0423 A.U.
Gaussian Beamwidth:
If a laser is used to illuminate a transmitting telescope, and if the
aperture is greater than 4wo, theory gives the far-field 1/e^2 beam
diffraction angle as:
(115).Wl
Gaussian Beamwidth = -------- degrees (6)
PI.wo
where wo = the TEMoo mode waist radius of the Gaussian beam.
For a compromise aperture diameter d = 2wo, where a little diffraction
will occur and produce some sidelobe energy, the (1/e^2) diffraction
angle of the main lobe of a 10-meter telescope is given by:
Gaussian Beamwidth = 0.0172 arc seconds
The corresponding (1/e^2) Gaussian beam diameter at the target is:
Gaussian Diameter = 0.0527 A.U.
This is not that different to the previous case for a fully-illuminated
aperture (no amplitude taper apodization).
Rayleigh Range:
For a Gaussian beam, the Rayleigh or near-field range of a diffraction
limited single or multi-aperture (array) telescope is given by:
PI.wo^2
Ray = ------- (7)
Wl
EJASA, Vol. 3, No. 6, January 1992
Page 75
At the Rayleigh range Ray, the beam diameter has expanded by a factor of
1.414. As the distance increases beyond the Rayleigh range, the beam
diameter becomes proportional to distance, and the inverse square law
applies to the beam intensity.
Considering our 10-meter diameter transmitting telescope with a
Gaussian beam, and a compromise aperture diameter d = 2wo.
For wo = 5 m and Wl = 656 nm:
Ray = 1.2 X 10^8 m
= 0.0008 A.U.
Now consider an array that has a width of 10 km.
For wo = 5 km and Wl = 656 nm:
Ray = 1.2 X 10^14 m
= 800 A.U.
Finally, consider a Mercury-size planetary phased-array as conjectured
by Dr. John Rather. [56]
For a wo = 2,439 km and Wl = 656 nm:
Ray = 2.8 X 10^19 m
= 3,000 L.Y.
With such a huge array, the inverse square law does not apply over
considerable distances. The Rayleigh range can stretch out over 3,000
light years, so that the flux density is essentially undiminished by
distance, accept for any interstellar absorption effects. Of course,
the implication that a pencil beam (celestial searchlight) some
3,500 km in diameter, i.e, of planetary diameter, could be landed on
a desired planet 10 lights years away, let alone 3,000 light years,
somewhat stretches even this author's imagination!
Polar Response:
The Polar Response (PR) or Directivity of a transmitting or receiving
telescope with a single fully illuminated circular aperture, with no
amplitude taper (apodization), is given by: [85]
[2.J1{(PI.d/Wl).sin(PHI)}]^2
PR = ---------------------------- (8)
[(PI.d/Wl).sin(PHI)]^2
EJASA, Vol. 3, No. 6, January 1992
Page 76
where J1 = Bessel Function of the first kind,
d = diameter (aperture) of telescope,
Wl = wavelength,
PHI = angular separation.
For the 10-meter diameter telescope at 656 nm, the first sidelobe is
located at 0.022 arc seconds from the main lobe, and the response is
17.6 dB down. The second sidelobe occurs at 0.036 arc seconds from the
main lobe, and response is 23.8 dB down.
In a diffraction limited space-based telescope system, where the angle
PHI between the image of the transmitter and star is >= FWHM/2 (-3 dB
half width half maximum), the Planckian suppression, ignoring
scattering within the telescope, is given by:
8
Suppression Factor >= 10.Log[-------------------------] dB (9)
PI.{(PI.d/Wl).sin(PHI)}^3
Equ. 9 essentially shows that the suppression factor is inversely
proportional to the telescope's aperture raised to the third power.
For a transmitter at 10 light years, located 1 A.U. from its star, and
centered on the main lobe of the receiver, the maximum angular
separation of the star is 0.275 arcseconds. Using the parameters for
the 10-meter diameter 656 nm telescope which has a FWHM beamwidth of
0.0138 arc seconds, we find that the condition PHI >= FWHM/2 is more
than satisfied, and the minimum suppression factor for the Planckian
starlight continuum is:
Suppression = 50 dB
This value is added to the Signal-To-Planckian Ratio (SPR) to arrive at
the effective SPR when a large telescope is diffraction limited, and
viewing a nearby star system at right angles to the star's plane of
ecliptic (Table 2, Line 23, Page 22). The suppression factor can be
larger than predicted by Equ. 9 (up to a limit set by scattering and
secondary mirror diffraction) if the star's image happens to be situated
in a response null. However, scattering effects and non-ideal optics
will set a limit to this suppression factor to between 40 and 50 dB.
Antenna Gain:
The gain of a uniformly illuminated antenna is given by: [5,71,85]
4.PI.At
G = ------- (10)
Wl^2
where At = area of transmitting telescope mirror (78.5 m^2).
EJASA, Vol. 3, No. 6, January 1992
Page 77
For a 10-meter diameter telescope at 656 nm:
G = 2.3 X 10^15
= 153.6 dB
Effective Isotropic Radiated Power (EIRP):
The Effective Isotropic Radiated Power [5,8,85] is given by:
EIRP = G.Pt Watts (11)
where Pt = transmitter power (W).
For Pt = 1 GW:
EIRP = 2.29 X 10^24 W
Received Signal Intensity:
The received signal intensity just outside Earth's atmosphere is:
EIRP
Ir = -------- (12)
4.PI.R^2
where EIRP = effective isotropic radiated power (W),
R = range (10 L.Y. = 9.461 X 10^16 m).
At a range of ten light years, a 1 GW transmitter EIRP = 2.29 X 10^24 W
produces an intensity (Ir) just outside our atmosphere of
2.04 X 10^-11 W/m^2. For a perfect space-based 10-meter diameter
telescope, the received signal power (Pr) is 1.6 nW.
Received Signal Power:
From Equs. 10, 11, and 12, and because the receiving aperture area
At = PI.D^2/4, we may write the "perfect" received signal for the
symmetrical telescope system in the simple form:
PI^2.D^4
Pr = Pt.----------- (13)
16.R^2.Wl^2
It can be clearly seen from the above, that the received power is
proportional to D^4 and inversely proportional to Wl^2. Thus, beamed
optical links, particularly those operating in the visible spectrum,
have the potential for tremendous throughputs.
EJASA, Vol. 3, No. 6, January 1992
Page 78
A slightly simpler form of this expression has been used by Albert Betz
in his recent CO2 paper. [57] To a close approximation, Equ. 13 may be
further simplified to:
D^4
Pr = Pt.-------- (14)
R^2.Wl^2
A more conservative analysis for ground-based observatories, would take
into account atmospheric transmission losses, aperture blocking, and
spectrometer efficiency in the case of an incoherent receiver. For a
a ground-based telescope, the optical power reaching the photodetector
is given by:
Pr = Ir.Tr.Ae.Ar.SE (15)
where Ir = intensity just outside atmosphere (2.04 X 10^-11 W/m^2),
Tr = atmospheric transmission (0.4 for visible, 0.6 for CO2),
Ae = antenna efficiency (0.7),
Ar = antenna aperture area (0.0707 m^2),
SE = spectrometer efficiency (0.5).
For a 30-cm diameter (12-inch) visible telescope, and the above
parameter values (1 GW, 10 m transmitter, EIRP = 2.29 X 10^24 W,
Ir = 2.04 X 10^-11 W/m^2), the received visible signal:
Prv = 2 X 10^-13 W (-127 dBW)
For a 30-cm diameter (12-inch) CO2 telescope, and the above parameter
values (1 GW, 10 m transmitter, EIRP = 8.78 X 10^21 W,
Ir = 7.81 X 10^-14 W/m^2), the received infrared signal:
Pri = 1.2 X 10^-15 W (-149 dBW)
Daylight Background:
The sky background radiation power detected per pixel, is given by:
Pb = (PI.THETA^2.Ae.Ar.SE/4).Bo.N(Wl) W (16)
where THETA = diffraction limited beamwidth (5.34 X 10^-6 radians),
Bo = optical bandpass (0.143 nm),
N(Wl) = spectral radiance (W/m^2.sr.nm).
For the incoherent optical systems, the pixel has a diffraction limited
field-of-view (FOV) corresponding to the Airy disk, i.e., (2.44)Wl/d
radians, where Wl = wavelength, and d is the aperture diameter. For
coherent systems, a smaller FOV is employed; that corresponding to the
FWHM response, i.e., (1.02)Wl/d radians. The latter pixel size is
smaller because of the requirement to reduce the amount of local-
EJASA, Vol. 3, No. 6, January 1992
Page 79
oscillator power that does not beat with the signal but only induces
excess quantum shot-noise.
At visible wavelengths:
N(Wl) = 0.01 W/cm^2.sr.micron [71]
= 0.1 W/m^2.sr.nm
N(f) = 1.43 X 10^-13 W/m^2.sr.Hz
The daytime sky background for a 30 cm (12") telescope at 656 nm (not
allowing for atmospheric distortion effects) with an optical bandpass
filter bandwidth Bo = 100 GHz (0.143 nm):
Pbv = 7.9 X 10^-15 W (-141 dBW)
The background is about 14 dB (Prv - Pbv) below the signal from the
1 GW transmitter which produces an EIRP = 2.29 X 10^24 W, and a flux of
2.04 X 10^-11 W/m^2 at a range of 10 light years. Thus, in this small
photon-counting receiver, the fluctuation noise from the daylight
background is 14 dB below that of the quantum shot-noise generated by
the signal. This has little effect on signal detectability. If a
polarizer is employed, Pb can be reduced by a further 3 dB. For a
perfect space-based 10 meter diameter visible telescope, the daylight
spectral density is about 4 X 10^-26 W/Hz (Figure 3, Page 17).
For infrared systems, the 300 K temperature of the atmosphere produces
a black body peak at approximately 10,600 nm, with a spectral radiance
given by:
N(Wl) = 0.0002 W/cm^2.sr.micron [71]
= 0.002 W/m^2.sr.nm
N(f) = 7.5 X 10^-13 W/m^2.sr.Hz
The sky background for a cooled 30 cm (12") telescope at 10,600 nm (not
allowing for atmospheric distortion effects) with a cooled 0.35 percent
optical bandpass filter bandwidth Bo = 100 GHz (37.5 nm):
Pbi = 1.1 X 10^-11 W (-110 dBW)
For an EIRP = 8.78 X 10^21 W and Ir = 7.81 X 10^-14 W/m^2, the
potential CO2 SNR is degraded by about 39 dB (Figure 6, Page 38)
because the background noise is 39 dB -(Pri - Pbi) above the quantum
shot noise. The infrared graph of Figure 6 is plotted to the same
scales as that of the Figure 8 (Page 44) visible graph, to make
comparisons easier, and the pages may be flicked back and forth to show
the differences more dramatically. We can clearly see that the
effective optical bandwidth must be substantially reduced if ETI signal
detectability at 10.6 microns is not to be impaired. Thus, only
heterodyning receivers, with effective optical bandwidths measured in
MHz and not GHz, are suitable for CO2 SETI within the atmosphere.
EJASA, Vol. 3, No. 6, January 1992
Page 80
Field Of View (FOV):
The relationship between the solid angle occupied by each star and the
area of the celestial sphere "occupied" by a typical star is:
A
OMEGAs = --- sr (17)
R^2
where A = area of the celestial sphere, i.e., 4.PI.R^2/N; N being the
number of stars being considered (10^6).
4.PI
OMEGAs = ---- sr (18)
N
Let us assume that sky survey is done out to a distance of 1,000 light
years. This means that we are searching the entire celestial sphere
around the Sun with a radius of 1,000 light years. This sphere of
4.PI steradians (sr), contains about 10 million stars of which
approximately 1 million are solar-type. Assuming that for a sphere of
this size, these 1 million stars are distributed fairly uniformly:
OMEGAs = 1.26 X 10^-5 steradian
For small angles, the solid angle FOV OMEGAs and the linear angle FOV
THETAs, are related by:
PI.THETAs^2
OMEGAs = ----------- sr (19)
4
THETAs = 0.23 degrees
Array Field Of View:
Figure 10 shows the typical field-of-view (FOV) for a 10-meter
diameter telescope. It has a usable Telescope Field-Of-View of about
0.33 X 0.33 degrees. At 656 nm, the diffraction limited FOV for each
pixel, and based on the Rayleigh criterion (1.22)Wl/d radians, is
8 X 10^-8 radians (0.0165"). For a 128 X 128 diffraction limited
two-dimensional array, the array has a linear field-of-view =
1.02 X 10^-5 radians (2.1"). The corresponding array FOV is:
FOV = 2.1" X 2.1"
Thus, at any instant of time, the average number of stars in the
2.1" X 2.1" array field-of-view is approximately:
6.4 X 10^-6
EJASA, Vol. 3, No. 6, January 1992
Page 81
------------------------------
| |
| |
| |
| * |
| |
| |
| 2.1 arc seconds |
| -->o<-- |
| Array FOV |
| |
| |
| |
| |
| * 0.23 degrees * |
| <------------------> |
------------------------------
Telescope FOV = 0.33 degrees
<------------------------------>
Figure 10 -
Typical FOVs for a large optical telescope. The diagram (not to scale)
illustrates the fact that the optical telescope's array field-of-view
generally observes empty space; the array itself occupying just a small
fraction of the telescope's usable (focal plane) field-of-view.
Number Of Received Beams:
The number of directions resolved by a telescope (with a maximum off-
axis loss of 1 dB) is stated in the Cyclops report [5] as being given
approximately by:
Nd = 4.G (20)
where G = gain.
For a 10 meter diameter telescope at 656 nm, G = 2.3 X 10^15. Thus:
Nd = 9.2 X 10^15 beams
An alternative expression has been given [8] where Nd = G. In this
paper, for the purposes of roughly estimating the search time for an
All Sky Survey, Equ. 20 has been used. Nd has been taken as being
10^16 beams or directions.
EJASA, Vol. 3, No. 6, January 1992
Page 82
The Search Time
For the Targeted Search, the time to scan a single star with the
heterodyning array, is given by:
Inor.Npix.(fu-fl).Td
Ts = ---------------------- s (21)
Imin.Nmsca.Bmsca.Bbin
where Inor = normalized flux (8.12 X 10^-16 W/m^2),
Imin = minimum detectable flux (8.12 X 10^-16 W/m^2),
Npix = number of pixels (16,384 photodetectors),
Nmsca = number of parallel multi-channel spectrum analyzers
(MCSAs), {<= Npix} (1),
Bmsca = total bandwidth of MCSA (10 GHz),
Bbin = minimum MCSA bin bandwidth (100 kHz),
fu = upper optical frequency (8.57 X 10^14 Hz),
fl = lower optical frequency (4.29 X 10^14 Hz),
Td = dead time overhead factor per array scan (1.0).
The dead time overhead factor is >= 1, and for this estimate, has been
taken to be unity, i.e., implying zero overhead. The normalized flux
is defined as that flux level that causes the normalized CNR (SNR)
(dB re 1 Hz) to fall to 0 dB. Note that if the pilot-tone maximal
ratio predetection combining system described later is employed, the
number of pixels (Npix) is effectively reduced to unity. Also, the
number of receiver beams Nd is assumed relatively constant over the
band fu-fl. If we substitute the values given in parentheses into
Equ. (21), for the visible optical bandwidth between 350 nm and 700 nm,
and a minimum detectable flux level of about -150 dBW/m^2, we find
that:
Ts = 2 hours
The time to do an All Sky Survey of this type is increased by a factor
(10^16/16,384), so that Ts = 136 million years! If we wanted to store
all the data collected, the number of bits would be, to say the least,
astronomical. Clearly, we would need to be very selective in the wave-
lengths scanned. i.e., fu-fl would have to be very small, so that a
guess of the magic optical frequencies would be mandatory.
This rough optimistic search time estimate, shows that it would be
ridiculous to consider a Visible SETI All Sky Survey modelled on the
one being employed for the Microwave Observing Project (MOP). [40-45]
Optical Heterodyne Detection:
In an optical heterodyne receiver (Figure 2, Page 15), the signal
current I is proportional to the product of the signal electric field
and the local-oscillator electric field, and a difference or Inter-
mediate Frequency (I.F.) is produced because the photodetector is a
square-law device. [71-78,81-82] Let us see how this heterodyne beat
EJASA, Vol. 3, No. 6, January 1992
Page 83
signal is created. Consider two optical beams mixing on a photodiode
(square-law detector). Let the beams be given by:
Received signal beam electric-field component = Er.cos(wrt+phi),
Local-oscillator beam electric-field component = Eo.coswot.
The photodetector current is given by:
I = k(Er+Eo)^2 (22)
where k = a constant of proportionality relating the current respon-
sivity of the photodetector (Ri) to the electric-field.
I = k[Er.cos(wrt+phi)+Eo.coswot]^2
I = kEr^2.cos2(wrt+phi)+2kEr.Eo.cos(wrt+phi).coswot+kEo^2.cos2wot
I = 0.5kEr^2[1+cos2(wrt+phi)]
+ kEr.Eo[cos{(wr-wo)t+phi}]+kEr.Eo[cos{(wr+wo)t+phi)}]
+ 0.5kEo^2[1+cos2wot]
Rejecting all but the difference frequency term,
I = kEr.Eo[cos{(wr-wo)t+phi}] (23)
where (wr-wo)/(2.PI) = fr-fo = Bif, is the difference, beat or
intermediate frequency.
Thus, the signal detected is proportional to the product of the
received signal and local-oscillator electric-fields. In an optical
homodyne receiver, wo = wr, and the intermediate frequency is zero.
The optical mixing efficiency factor H, which is not indicated here
(Equ. 32 and 33) and accounts for wavefront distortion and beam
misalignment, is typically somewhat less than 50 percent.
Pilot-Tone Maximal Ratio Predetection Combining:
The pilot-tone technique has been previously applied to radio frequency
diversity receivers to overcome deep fades. [84] It has also been
employed by the author on multimode fiber homodyne and heterodyne
systems with a 4-quadrant photodetector acting as an optical space
diversity receiver. [81,82] The spatial incoherence of the radiation
pattern from a multimode optical fiber is very similar to that of a
free-space optical beam received by a large telescope within an
atmosphere.
The theory behind the terrene pilot-tone method is as follows, and
makes no specific assumption about modulation techniques employed by
ETIs, i.e., whether intensity, polarization, frequency or phase
modulation, analog or digital. With reference to Figure 1 (Page 10):
EJASA, Vol. 3, No. 6, January 1992
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Let the pilot-tone carrier at fp be given by:
Ep(t).sin[wpt+dphi] (24)
and the modulated information signal at fs be given by:
Es(t).sin[wst+phi(t)+dphi] (25)
where dphi = phase disturbance caused by the transmitter laser
(jitter) or Earth's atmosphere,
phi(t) = represents possible phase or frequency modulation.
The phase disturbances dphi, are essentially common to both the signal
and the pilot-tone, as they are almost identical optical frequencies
and travel the same optical path. However, dphi generally differs at
each photodetector.
-------
sin[(ws-wo)t+phi(t)+dphi]| | ----- cos[(ws-wp)t+phi(t)]
------------------------>| Mixer |-->| LPF |-------------------------->
1st I.F (1.1 GHz) | | ----- 2nd I.F (100 MHz)
-------
^ To Summer ------>
|
sin[(wp-wo)t+dphi] |
-----------------------------
2nd L.O. (1 GHz)
Figure 11 -
Maximal Ratio Precombining. The bandpass-filtered signal from each
photodetector provides two separately-filtered 1st I.F and 2nd L.O.
signals to an electronic mixer. The 2nd I.F. produced after the low-
pass filter (LPF), has all the laser local-oscillator and atmospheric-
induced phase noise dphi eliminated.
The frequencies given in brackets in Figure 11 are arbitrary, and used
to help clarify the technique. Each pixel of the 128 X 128 array has
one of these circuits, whose in-phase outputs are simply added (in a
summer) and taken to a single MCSA.
If we heterodyne a local-oscillator laser operating at frequency wo
with both these signals, we obtain the difference frequency signals or
1st I.F. from the photodetector proportional to:
Ep(t).Eo.sin[(wp-wo)t+dphi] (26)
Es(t).Eo.sin[(ws-wo)t+phi(t)+dphi] (27)
where dphi now also includes the effects of local-oscillator jitter.
EJASA, Vol. 3, No. 6, January 1992
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The pilot-tone signal as stated by Equ. (26), may be passed through a
narrow-band filter and amplifier, to produce what is effectively a
strong electrical second local oscillator (2nd L.O.) signal for an
electrical mixer. It may also be used to lock a narrow-band Phase
Locked Loop (PLL) whose Voltage Controlled Oscillator (VCO) is used as
the strong, amplitude-stable and clean 2nd local oscillator. The
information signal as stated by Equ. (27), may be passed through a
wideband filter and applied to the other port of this electrical mixer.
The 2nd I.F. output of the electrical mixer is proportional to:
Ep(t).Es(t).Eo(t)^2.cos[(ws-wp)t+phi(t)] (28)
The phase disturbances dphi introduced by the atmospheric turbulence
and laser jitters have been eliminated by the process of electrical
mixing. Thus, if the image of the transmitter is instantaneously or
sequentially smeared out over many pixels, all the second I.F. contri-
butions are in phase, and may be simply summed to provide predetection
diversity combining and a substantial reduction in amplitude
instability (scintillation).
It also provides the best type of predetection summation in the form of
Maximal-Ratio Combining. Although the system appears to implement
Equal-Gain Combining, the effect of the electronic mixer is to cause
the weakest signals to be automatically weighted downwards, and hence
cause Maximal Ratio Combining of the photodetector signals. Those
pixels producing the weakest signal also produce the lowest quantum,
Planckian or background noise contributions to the input of the
electrical mixer, so that the summed electrical signal power is not
degraded by noise from pixels with little or no optical signal. This
occurs because when no optical signal is present, the noise output of
each electronic mixer is essentially that due to a noise^2 term, and
hence is very small. Only a single MCSA would be required, which would
be effectively continuously "looking" at the combined outputs of all
16,384 pixels. We would have only one MCSA, but 16,384 electronic
front-end systems for predetection combining of the photodetector
outputs, based on the mixing technique illustrated in Figure 11.
A predetection combining system with a single MCSA would not detect
directly any Planckian starlight noise from a star in the array field-
of-view alone, only that which overlapped and mixed (downconverted)
with an ETI signal on one or more pixels. However, for nearby stars
where the transmitter and star are separately resolved, we would lose
any Planckian suppression effect of a (single pixel) diffraction
limited telescope. Also, if there are significant interstellar or
atmospheric group-delay dispersion effects between the signal and
pilot-tone, the technique would not work. This consideration may
affect the choice for the value of (fs-fp) and may itself limit
modulation bandwidth to be less than a few GHz, notwithstanding SNR
considerations. Of course, to use this technique will require the
cooperation of the ETI.
Would they be so obliging? It would be difficult to justify building
such a receiving signal processing system without foreknowledge that
EJASA, Vol. 3, No. 6, January 1992
Page 86
ETIs employ this technique - this could be said to be putting the cart
before the horse! Anyway, before implementing such a system, assuming
ETIs would use such a modulation format, we would have had to
previously detect this modulation format to know what electrical
filters to use!
Radio Frequency Signal-To-Noise Ratio:
The Carrier-To-Noise Ratio (CNR) in the Microwave Heterodyne [5,8,85]
100-meter diameter, 1 kW dish system operating at 1.5 GHz over a range
of 10 light years:
Pr
CNR = ---- (29)
kTBe
where Pr = received power (1.72 X 10^-22 W),
T = effective system temperature (10 K),
Be = electrical intermediate frequency bandwidth (1 Hz).
CNR = 1 dB
A symmetrical Cyclops array system [5] with 900 such dishes at both the
transmitter and receiver would have a CNR = 60 dB.
Optical Signal-To-Noise Ratio:
The dimensions of all signal and noise components the following optical
expressions are in units of amperes^2, and by multiplying by the
photodetector load impedance, may be turned into units of power. The
numerators are representative of the electrical signal power in the
photodetector load, while the denominators represents the electrical
noise power in the photodetector load. [71-78]
For coherent receivers, dual-balanced photodetection is assumed so that
all the received signal power is utilized, and the noise floor is not
raised by excess intensity noise on the local-oscillator laser. It is
further assumed that the linewidths of the received signal and local-
oscillator laser are sufficiently small compared to the modulation
bandwidths, as to not raise the noise floor.
The effective system noise temperature of an optical receiver may be
expressed in the form:
h.f
Teff = ----- K (30)
eta.k
where h = Planck's constant (6.63 X 10^-34 J.s),
f = frequency (4.57 X 10^14 Hz).
Teff = 43,900 K
EJASA, Vol. 3, No. 6, January 1992
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Incoherent Signal-To-Noise Ratio:
Direct Detection and Photon-Counting
Pr^2(MRi)^2
SNR = -------------------------------------------------------------- (31)
[2e{Ri(Pr+NbBo)+Ib}M^(2+x)+2eIs+2Nb{Pr+NbBo}(MRi)^2+4kTF/RL]Be
where Pr = received optical power (W),
Po = local oscillator power (W),
M = avalanche gain,
eta = photodetector quantum efficiency (0.5),
Ri = unity gain responsivity (W/A),
e = electronic charge (1.6 X 10^-19 C),
Nb = background radiation spectral density (W/Hz),
Ib = bulk dark current at unity gain (A),
Is = surface dark current (A),
x = excess noise factor,
k = Boltzmann's constant (1.38 X 10^-23 J/K),
T = front-end amplifier temperature (K),
F = front-end amplifier noise figure,
RL = front-end load (Ohms),
Bo = optical pre-detection bandwidth (Hz),
Be = noise equivalent electrical bandwidth, which for a single-
pole filter = PI/2 x maximum modulation frequency (Hz).
The electrical signal power is proportional to Pr^2, and the noise
components proportional:
1. To the quantum noise produced by the signal photons.
2. To the fluctuation noise produced by the background radiation Pb
(NbBo). Notice that this noise is proportional to the optical
bandwidth, and the ratio of this noise to the quantum noise
component is inversely proportional to the received optical power.
3. To the shot noise produced by the bulk dark current in the photo-
detector.
4. To the shot noise produced by the surface leakage dark current.
5. To the background radiation beating with the signal, which is
independent of optical bandwidth. The noise spectral density is
the important factor here.
6. To the noise beating with noise, which is proportional to both the
noise spectral density squared and the optical bandwidth. The
latter two noise components are insignificant and may be safely
omitted for this application where the background is very small.
7. To the thermal kT noise in the photodetector load and front-end
amplifier, and may be neglected for shot noise limited direct
detection receivers, and ideal photon-counting receivers.
EJASA, Vol. 3, No. 6, January 1992
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The total noise produced is proportional to the electrical post-
detection bandwidth Be. To an approximation at high avalanche gain,
the surface dark current component Is, which is not subject to gain,
is sometimes ignored, and Ib is called Id.
Coherent Signal-To-Noise Ratio:
Heterodyne Detection (Reception)
HPrPo(MRi)^2
CNR = ---------------------------------------------------------------- (32)
[e{Ri(Pr+Po+NbBo)+Ib}M^(2+x)+eIs+2Nb{HPo+NbBo}(MRi)^2+2kTF/RL]Be
Homodyne Detection
2HPrPo(MRi)^2
CNR = ---------------------------------------------------------------- (33)
[e{Ri(Pr+Po+NbBo)+Ib}M^(2+x)+eIs+2Nb{HPo+NbBo}(MRi)^2+2kTF/RL]Be
The electrical signal power is proportional to Pr and the optical
mixing efficiency H, and the noise components proportional:
1. To the quantum noise produced by the signal photons.
2. To the shot noise produced by the local oscillator.
3. To the fluctuation noise produced by the background radiation Pb
(NbBo). This noise is also proportional to the optical bandwidth
and its ratio to the quantum shot noise is effectively inversely
proportional to the local oscillator power Po.
4. To the shot noise produced by the bulk dark current in the photo-
detector.
5. To the shot noise produced by the surface leakage dark current.
6. To the background radiation beating with the local oscillator,
which is very small, the noise being proportional to the noise
spectral density and independent of optical bandwidth.
7. To the background noise spectral density squared, which is again
very small, the noise being proportional to the optical bandwidth.
8. To the thermal kT noise of the optical front-end, which like the
case for all other noise components except that due to the local-
oscillator quantum shot-noise, is negligible for sufficient local-
oscillator power.
The local-oscillator (L.O.) is assumed to have negligible excess
intensity noise or it is balanced out, so that the Relative Intensity
Noise (RIN) is at the theoretical quantum noise level.
EJASA, Vol. 3, No. 6, January 1992
Page 89
Note, the excess noise due to a non-Poisson distribution of arriving
photons in a power-starved situation, is not included in this expres-
sion. Poisson statistics imply that sufficient photons arrive during
the observation time to take the probability of the arrival of a photon
as being given by a binomial distribution. [83] In situations where
the optical receiver is power-starved, i.e., when there are relatively
few photons arriving during the signal integration time so that Bose-
Einstein [73] statistics apply, the non-white noise associated with
statistics of the photon arrival times will lower the effective CNR.
The total noise produced is again proportional to the electrical post-
optical detection bandwidth Be. Usually Po >> Pr and Pb, and thus
other multiplicative noise components relating to Pr and Pb are not
included in these expressions, since they are negligible. For this
application the nearest star is several light years away, Po is much
larger the background Pb, and the latter component is also negligible
for all optical bandwidths, unlike the case for incoherent detection.
This is also generally true for large diffraction limited telescopes
operating in daylight. For SETI to be practical, the EIRP needs to be
extremely high, but since the star is distant, the background Nb is
very small. However, for communications within the solar system, these
background noise components (from the Sun or reflected light from Earth
or another planet) can be significant. [94-95]
For the Amateur Optical SETI analysis, a more conservative approach for
assessing the performance of various receiving systems has been
employed. Account has been made for the efficiencies of atmospheric
transmission, telescope aperture, monochromator (incoherent systems
only) and in the case of coherent receivers, an allowance for the
optical (heterodyne or homodyne) mixing efficiency.
Expression (31) relates to incoherent detection, while (32) and (33)
relate to coherent detection. The ideal shot-noise limited direct
detection receiver approaches the performance of the photon-counting
receiver at higher received powers. For substantially cooled photon-
counting receivers, the dark currents Is and Ib may be taken as zero,
and thermal noise is insignificant. In the quantum noise limit, the
CNR of the homodyne system is 3 dB more than the heterodyne, which is
itself 3 dB more than the direct detection or photon-counting receiver.
Quantum-Noise Limited Signal-To-Noise Ratio:
The Carrier-To-Noise Ratio in a perfect quantum noise limited (656 nm)
optical heterodyne system where the L.O. has negligible intensity and
phase noise, and where the shot noise from the L.O. swamps all other
sources of noise, is given by:
eta.Pr
CNR = ------ (34)
hfBif
where Pr = received optical power (1.6 nW),
Bif = Intermediate Frequency bandwidth (30 MHz).
EJASA, Vol. 3, No. 6, January 1992
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One of the major advantages of using the normalized CNR approach is
that we can express the CNR for the perfect diffraction-limited
ten meter diameter symmetrical heterodyne system, for any transmitter
power, range and electrical bandwidth, in the form:
------------------------------------------------------
| |
| CNR = 54 + 10.log(Pt) - 20.log(R) - 10.log(Be) dB | (35)
| |
------------------------------------------------------
where Pt = transmitter power (kW),
R = range (L.Y.),
Be = I.F. bandwidth (Hz).
For Pt = 1 GW, R = 10 L.Y., and Be = Bif = 30 MHz:
CNR = 19 dB
Again, it should be remembered that this relationship (Equ. 35) only
holds out to distances where interstellar attenuation is insignificant,
and will over-estimate the CNR at very low received optical powers (Pr)
and/or higher bandwidths (Be). For a huge transmitting array, the
Rayleigh near-field range may be so large (Equ. 7), that the 20.log(R)
term disappears from the above expression, and the 54 dB constant has
a higher value.
We see that one advantage of coherent detection for this application is
that the effective bandwidth determining the relative level of detected
background noise is the electrical bandwidth Be, not the optical
bandwidth Bo. Since Be can be much less than Bo, coherent receivers
have a considerable sensitivity advantage over incoherent receivers in
the presence of weak signals and/or significant background radiation,
besides being able to allow for the demodulation of phase or frequency-
modulated signals. In the case of the heterodyne receiver, Be
corresponds to the I.F. bandwidth, and the signal has still to be
demodulated. A further stage of "detection", either square-law or
synchronous, must be applied to demodulate the intelligence on the
signal. For this reason, the signal-to-noise ratio for the radio
frequency heterodyne and optical heterodyne systems is denoted as CNR
and not SNR.
Signal Integration:
In practically all SETI systems, what is being looked for is an ETI
beacon. In such systems, the sensitivity of the receiver is enhanced
by post-detection signal integration, perhaps over many seconds. This
increases the detected signal level, and reduces the noise level; both
at the expense of increasing the search time. This can only be done
for detecting the presence of a signal beacon, not for the demodulation
of a continuously and rapidly changing non-repetitive signal.
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In the case of a microwave or optical receiver with square law
detection and an input SNR less than unity, the Signal-To-Noise Ratio
can be increased by (post-detection) integration of a number of
detected pulses over a period of time. In such a situation, the SNR is
proportional to the square-root of (Nc), where Nc is the total pulse
count during the observation integration time. [83,88] The same
relationship applies to the post-detection counting of individual
photons, but not to pre-detection. That is why the quantum limited
CNRs (SNRs) for both incoherent and coherent optical detection systems
are proportional to the photon count rate. See Equ. 36 below.
Photon-Count Rate:
The equivalent photon-count rate for the heterodyne receiver is given
by:
eta.Pr
Nph = ------ s^-1 (36)
hf
Alternatively, this can be expressed as CNR.(Bif). For the 1 GW
transmitter that results in a CNR = 19 dB re 30 MHz:
Nph = 2.64 X 10^9 s^-1
This count rate is more than adequate for the photon arrival (and
detection) statistics to be taken as Gaussian (Poisson), and hence the
CNR expressions should give an accurate figure for the Carrier-To-Noise
Ratio. This is reasonably true even for the 1 kW transmitter, where
on average, only 5,280 photons arrive per second, of which on average,
2,640 photons are detected every second. However, the method of
expressing CNRs in this analysis, even in the power-starved case,
allows for a simple linear extrapolation for CNR at any received
optical power (Equ. 35).
Bit Error Rate (BER):
This analysis has concentrated on optical signal detectability in terms
of SNR not Bit Error Rate (BER), as would be applicable for a digital
system. For the sake of completeness, the following expression may be
used to predict the photon-count rate for a required BER: [78]
-ln(2.BER)
m = ---------- (37)
log N
2
where m = average number of photons per bit required by an ideal N-PPM
(pulse position modulation) system to achieve a given BER.
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The photon-count rate is simply the product of m and the bit rate. For
an ideal coherent system with on-off keying (OOK) or 1-PPM,
BER = 10^-9, and very small extinction (light off/light on) ratio,
m = 10 photons/bit. However, a more realistic value is nearer to
20 photons/bit. Thus, for a 1 GHz (approx. 1 GBit/s) channel:
Minimum Photon-Count Rate = 2 X 10^10 s^-1
The modelled 1 GW system is a little deficient in being able to achieve
this goal, since this required count rate is an order of magnitude
greater than the calculated value of Nph. With digital compression
techniques, the 1 GW transmitter is capable of supporting a late
Twentieth Century digital HDTV signal, compressed into a 10 MHz
bandwidth. [87]
Range Equation:
Instead of expressing the CNR as a function of transmitter power,
range and bandwidth, we can express the quality of the optical
communications link in terms of its maximum range. As before, if we
ignore interstellar absorption, the range (in light years) required to
reduce the quantum limited CNR to 0 dB for the "perfect" 10-meter
diameter 656 nm symmetrical Professional Optical SETI system defined by
Equ. 35, can be express in the form:
Rmax = 10^[{54 + 10.log(Pt) - 10.log(Be)}/20] (38)
where Pt = transmitter power (kW),
Be = I.F. bandwidth (Hz).
For Pt = 1 GW (EIRP = 2.29 X 10^24 W) and Be = 1 MHz:
Rmax = 500 L.Y.
Doppler Shift:
The maximum Doppler Shift is given by:
v
df = -.f Hz (39)
c
where v = maximum line-of-sight velocity (29.8 km/s),
c = velocity of light (3 X 10^8 m/s),
f = frequency (4.57 X 10^14 Hz).
EJASA, Vol. 3, No. 6, January 1992
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For a ground-based receiving telescope, the maximum local Doppler Shift
at 656 nm due to the orbit of Earth around the Sun:
df = +/- 45.5 GHz
Doppler Drift:
The maximum Doppler Drift (Chirp) is given by:
w^2.r
df' = -----.f Hz/s (40)
c
where w = angular velocity (7.27 X 10^-5 rad/s),
r = radius of planet or orbit (6,378 km).
For a receiving telescope on the equator, the maximum local Doppler
Drift at 656 nm due to Earth's rotation is:
df' = +/- 51 kHz/s
Fortunately, for Amateur Optical SETI observations, the Doppler Drift
during reasonable observations times is insignificant with respect to
the bandpass of the incoherent optical filter (approximately 100 GHz).
EJASA, Vol. 3, No. 6, January 1992
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APPENDIX B
THE SETI PROTOCOLS
The following information was provided by Robert Arnold of the SETI
Institute.
November 20, 1991
Dear Colleague,
It is my pleasure to send you a copy of a document entitled
"Declaration of Principles Concerning Activities Following the
Detection of Extraterrestrial Intelligence."
The Declaration was developed over a period of several years by the
SETI Committee of the International Academy of Astronautics, with the
assistance of many experts interested in this question. In April of
1989 it was approved by the Board of Trustees of the Academy, and also
by the Board of Directors of the International Institute of Space Law.
Over the last two years it has been endorsed by the Committee on Space
Research, by Commission 51 of the International Astronomical Union, by
the members of Commission J of the Union Radio Scientifique
Internationale, and by the International Astronautical Federation.
The document is intended as a series of guidelines for individuals or
organizations, national or international, engaged in carrying out radio
searches for extraterrestrial intelligence. In the near future it will
be sent by the Academy to all such individuals and organizations with a
request that they give consideration to endorsing it.
In the meantime, the SETI Committee of the International Academy of
Astronautics will continue to review the principles and procedures of
the Declaration, and will assemble a special post-detection committee,
as indicated in Principle 9 of the document. The Committee is also
working on a second declaration, designed to expand the wording of
Principle 8 into a process for obtaining international agreement on
questions about a reply from Earth after the detection of a signal.
Sincerely,
John Billingham
Chief, SETI Office
Enclosure:
EJASA, Vol. 3, No. 6, January 1992
Page 95
Declaration of Principles Concerning Activities Following the Detection
of Extraterrestrial Intelligence -
We, the institutions and individuals participating in the search for
extraterrestrial intelligence,
Recognizing that the search for extraterrestrial intelligence is an
integral part of space exploration and is being undertaken for peaceful
purposes and for the common interest of all mankind,
Inspired by the profound significance for mankind of detecting evidence
of extraterrestrial intelligence, even though the probability of
detection may be low,
Recalling the Treaty on Principles Governing the Activities of States
in the Exploration and Use of Outer Space, Including the Moon and Other
Celestial Bodies, which commits States Parties to that Treaty "to
inform the Secretary General of the United Nations as well as the
public and the international scientific community, to the greatest
extent feasible and practicable, of the, nature, conduct, locations and
results" of their space exploration activities (Article XI),
Recognizing that any initial detection may be incomplete or ambiguous
and thus require careful examination as well as confirmation, and that
it is essential to maintain the highest standards of scientific
responsibility and credibility,
Agree to observe the following principles for disseminating information
about the detection of extraterrestrial intelligence:
1. Any individual, public or private research institution, or
governmental agency that believes it has detected a signal from or
other evidence of extraterrestrial intelligence (the discoverer)
should seek to verify that the most plausible explanation for the
evidence is the existence of extraterrestrial intelligence rather
than some other natural phenomenon or anthropogenic phenomenon
before making any public announcement. If the evidence cannot be
confirmed as indicating the existence of extraterrestrial
intelligence, the discoverer may disseminate the information as
appropriate to the discovery of any unknown phenomenon.
2. Prior to making a public announcement that evidence of extra-
terrestrial intelligence has been detected, the discoverer should
promptly inform all other observers or research organizations that
are parties to this declaration, so that those other parties may
seek to confirm the discovery by independent observations at other
sites and so that a network can be established to enable continuous
monitoring of the signal or phenomenon. Parties to this
declaration should not make any public announcement of this
information until it is determined whether this information is or
is not credible evidence of the existence of extraterrestrial
intelligence. The discoverer should inform his/her or its relevant
national authorities.
EJASA, Vol. 3, No. 6, January 1992
Page 96
3. After concluding that the discovery appears to be credible evidence
of extraterrestrial intelligence, and after informing other parties
to this declaration, the discoverer should inform observers
throughout the world through the Central Bureau for Astronomical
Telegrams of the International Astronomical Union, and should
inform the Secretary General of the United Nations in accordance
with Article XI of the Treaty on Principles Governing the
Activities of States in the Exploration and Use of Outer Space,
Including the Moon and Other Bodies. Because of their demonstrated
interest in and expertise concerning the question of the existence
of extraterrestrial intelligence, the discoverer should
simultaneously inform the following international institutions of
the discovery and should provide them with all pertinent data and
recorded information concerning the evidence: the International
Telecommunication Union, the Committee on Space Research, of the
International Council of Scientific Unions, the International
Astronautical Federation, the International Academy of
Astronautics, the International Institute of Space Law, Commission
51 of the International Astronomical Union and Commission J of the
International Radio Science Union.
4. A confirmed detection of extraterrestrial intelligence should be
disseminated promptly, openly, and widely through scientific
channels and public media, observing the procedures in this
declaration. The discoverer should have the privilege of making
the first public announcement.
5. All data necessary for confirmation of detection should be made
available to the international scientific community through
publications, meetings, conferences, and other appropriate means.
6. The discovery should be confirmed and monitored and any data
bearing on the evidence of extraterrestrial intelligence should be
recorded and stored permanently to the greatest extent feasible and
practicable, in a form that will make it available for further
analysis and interpretation. These recordings should be made
available to the international institutions listed above and to
members of the scientific community for further objective analysis
and interpretation.
7. If the evidence of detection is in the form of electromagnetic
signals, the parties to this declaration should seek international
agreement to protect the appropriate frequencies by exercising
procedures available through the International Telecommunication
Union. Immediate notice should be sent to the Secretary General of
the ITU in Geneva, who may include a request to minimize trans-
missions on the relevant frequencies in the Weekly Circular. The
Secretariat, in conjunction with advice of the Union's Admini-
strative Council, should explore the feasibility and utility of
convening an Extraordinary Administrative Radio Conference to deal
with the matter, subject to the opinions of the member Admini-
strations of the ITU.
EJASA, Vol. 3, No. 6, January 1992
Page 97
8. No response to a signal or other evidence of extraterrestrial
intelligence should be sent until appropriate international
consultations have taken place. The procedures for such
consultations will be the subject of a separate agreement,
declaration or arrangement.
9. The SETI Committee of the International Academy of Astronautics, in
coordination with Commission 51 of the International Astronomical
Union, will conduct a continuing review of procedures for the
detection of extraterrestrial intelligence and the subsequent
handling of the data. Should credible evidence of extraterrestrial
intelligence be discovered, an international committee of
scientists and other experts should be established to serve as a
focal point for continuing analysis of all observational evidence
collected in the aftermath of the discovery, and also to provide
advice on the release of information to the public. This committee
should be constituted from representatives of each of the
international institutions listed above and such other members as
the committee may deem necessary. To facilitate the convocation of
such a committee at some unknown time in the future, the SETI
Committee of the International Academy of Astronautics should
initiate and maintain a current list of willing representatives
from each of the international institutions listed above, as well
as other individuals with relevant skills, and should make that
list continuously available through the Secretariat of the
International Academy of Astronautics. The International Academy
of Astronautics will act as the Depository for this declaration and
will annually provide a current list of parties to all the parties
to this declaration.
EJASA, Vol. 3, No. 6, January 1992
Page 98
INDEX
A Adaptive 17,20,24,39,43,52,58
Airy Disk 47,78
All Sky Survey 4,31,33-34,82-83
Alpha Centauri 54
Amateur Optical SETI 5-6,8,16,25,37,42,45-48,51-52,54,
60-61,90,92
Ames Research Center (ARC) 3,11,19
Arecibo 11,16,20,27-28,36
Asimov, Isaac 25
Assumption of Ineptitude 13,58
Assumption of Mediocrity 13
Avalanche Photodetector (APD) 7,42,44,48-50
B Beacon 10,28,39,52,55,90
BETA 4
Betz, Albert 5,7,9,13,20,35-37,55,61,78
Big Ear Radio Observatory 7,40
Billingham, John 5,94
Bit Error Rate (BER) 41,91
Bose-Einstein 89
Bova, Ben 3
Bulletin Board System (BBS) 6
C Carbon Dioxide Laser (CO2) 5,13,20,29,35-38,55,61,78-79
Carrier-To-Noise Ratio (CNR) 15-17,24,26-28,33-34,36,41,43,50,
82,86,88-91
Challenger 59
Charge Coupled Device (CCD) 42,46-51
Clarke, Arthur C. 8,41,57-58
Cold Fusion 2
Columbus Telescope 40
Coherence Cell 11,18
Coherent 14-15,18,20,26,37,40,42,46,49,
60-61,78,86,88-91
Contact 1-3,27,40,56-57,63
Cosmic Catastrophes 57
Cosmic Haystack 4,32,43
Cosmic Zoo 2,62
Cullers, Kent 3,8,54
Cyclops 7,18,19-20,23,26-27,54,58,81,86
D Dark Current 38,46,49,87-89
Daylight (Optical SETI) 20,26,36,42-43,45-47,54,58,79,89
Deep Space Network (DSN) 11
Directivity 36,75
Direct Detection 18,25,36-37,40,42-46,51,60-61,78,
87,89-90
Discovery 58
Dixon, Robert 4,7,40
Doppler Drift (Chirp) 11,14,16,22,24,32-33,46,56,93
Doppler Shift 11,22,24,32,35,46,56,92-93
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Drake Equation 71
Drake, Frank 3,9,71
E Effective Noise Temperature 15,86
EIRP 17,19,21-22,28,37-38,41,43-46,60,
72,77-79
Epsilon Eradani 54
F Fabry-Perot 45
Fast Fourier Transform (FFT) 4,34,48
Fermi's Paradox 1
Field-Of-View (FOV) 31,36-37,47,56,78,80-81,85
Fraunhofer 4,17,23,25-26,30-31,34-35,43,46,
59,73
Free Electron Laser 21,29
G Gaussian Beams 73-75
H Heliographs 9
Heterodyne 4,14-18,20,22-23,25,35,37,42,45,
49-50,56,82-84,86,88-91
Homodyne 14,49,83,88-89
Horowitz, Paul 4
Hubble Space Telescope (HST) 13,20
I Image Intensifier 42,46
Incoherent Detection 18,25,36-37,40,42-46,51,60-61,78,
87,89-90
Interferometer 5,8,37,40,55
J Jet Propulsion Laboratory (JPL) 11
K Karhunen-Loeve Transform (KLT) 4,60
Kraus, John 40
L Light Pollution 26,60
Local-Oscillator (L.O.) 5,11,14-15,20,29,31,33-34,37,44,
48,56,79,82-88
M Magic Wavelength (or Frequency) 3,9,20,26,29,45,57,82
Magnitude (Intensity) 21-24,28,41,43-46,53-54,72
Microchannel Plate 46
Microwave Observing Project (MOP) 4-5,8,13,31,40,52-54,57-59,82
Monochromator 42,45,47-48,50-51,54,60,89
Morrison, Philip 3,7,9,13
MultiChannel Spect. Analy. (MCSA) 14,33-36,52,82,84-85
N NASA 3-5,7,9,11,19,27,35,38,45,58,
61-62
Neodymium YAG Laser (Nd:YAG) 18,21,27,29
Noise Equivalent Bandwidth 87
O Oliver, Bernard 3,7,9,21,31
Optoelectronics 19
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P Perkins Telescope 40,46
Photomultiplier (PM) 42,48-50
Photon-Counting 16,18,37,42,44-46,49,51,60,79,
87,89
Photon Count Rate 91
Photonics 19,62
Pilot-Tone 10-11,20,39,82-85
Planckian 17,20,22-23,25-28,39,42-44,46,49,
56,73,76,85
Planetary Report 4
Poisson Counting 15-16,89,91
Polar Response 75-76
Prime Directive 56
Professional Optical SETI 4,8,14,16,40,46,59-61
Project Ozma 9,54
Q Quantum (Shot) Noise 14-17,25-26,28,37,43,79,85,87-89
R Range Equation 92
Rather, John 7,9,21,75
Rayleigh Range 24,74-75,90
Rayleigh Resolution 80
Rayleigh Scattering 39,59
Rosetta Stone 10
S Sagan, Carl 3
Semaphores 9
Serendip 22,26,39-40,59
SETI Institute 3,5,7,12,31,54,58,62,71
SETI Protocols 63,94
Signal-To-Noise Ratio (SNR) 6,11,16,20-26,34,36-38,43-46,48,
49,59,79,85,86-91
Space Odyssey (2001 and 2010) 58
Spectrometer 36,38,44-45,47,49,51,55,78
Spielberg, Steven 4
Star Trek 62
Strategic Defense Initiative (SDI) 14
Symbiotic 22,26,39-40,59
T Targeted Search 11-12,31,33-34,46,52-54,82
Tarter, Jill 8,25,54,61
Tau Ceti 54
Thermal Noise 14,87-89
Tipler, Frank 1,56-57
Townes, Charles 5,7,9,13,20,35-37,55,61,63
Type I, II, and III Civilizations 2-3
U Unidentified Flying Objects (UFOs) 1-2,63
V Von Neumann 1-2,32,56
W Waterhole 3,32
Z Zuckerman, Ben 9,20
EJASA, Vol. 3, No. 6, January 1992