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SETI System Performance Comparison Table

Radobs 12

 
The following is a comparison table for three different interstellar
communication systems.  It is assumed that the links are symmetrical, i.e,
the receiving dish or telescope is identical to the respective transmitter. 
The system performances assume diffraction-limited telescopes at all
wavelengths, and are normalized to a 1 kW transmitter power, a 1 Hz
bandwidth and a range of 10 light years.


     SETI SYSTEM PERFORMANCE FOR RECEIVERS AT A RANGE OF 10 LIGHT YEARS.

 ========================================================================== 
| Parameter                     Microwave      Infrared       Visible      |
|--------------------------------------------------------------------------|
| Wavelength                    0.20 m         10,600 nm      656 nm       |
| Frequency, Hz                 1.50 X 10^9    2.83 X 10^13   4.57 X 10^14 |
|==========================================================================|
|                               TRANSMITTERS                               |
|--------------------------------------------------------------------------|
| Antenna Diameter, m           300            10             10           |
| Gain, dB                      73.5           129.4          153.6        |
| Power, W                      1,000          1,000          1,000        |
| EIRP, W                       2.22 X 10^10   8.78 X 10^15   2.29 X 10^18 |
|==========================================================================|
|                                RECEIVERS                                 |
|--------------------------------------------------------------------------|
| Antenna Diameter, m           300            10             10           |
| Gain, dB                      73.5           129.4          153.6        |
| Beam Diameter, A.U.           514            0.818          0.051        |
| Intensity, W/m^2              2.0 X 10^-25   7.8 X 10^-20   2.1 X 10^-17 |
| Signal, W                     1.4 X 10^-20   6.1 X 10^-18   1.6 X 10^-15 |
| Photon Count Rate, s^-1       -----          164            2,640        |
| Equivalent Magnitude (Mag)    -----          -----         +23           |
| Quantum Efficiency            -----          0.5            0.5          |
| Noise Temperature, K          10             2,700          44,000       |
| Planck Noise, W/m^2.Hz*       8.8 X 10^-33   1.1 X 10^-25   2.4 X 10^-24 |
| Star Stellar Magnitude (Mag)  -----          -----         +2            |
| Alien Planet Magnitude (Mag)  -----          -----         +24           |
| Normalized SNR, dB/Hz         20.0           22.1           34.2         |
| Signal-To-Planck, dB/Hz*      71.0           55.7           65.8         |
| Signal-To-Daylight, dB/Hz*    -----          51             106          |
| Doppler Shift, Hz          +/-1.5 X 10^5  +/-2.8 X 10^9  +/-4.6 X 10^10  |
| Orbital Chirp, Hz/s        +/-1.1 X 10^0  +/-2.1 X 10^4  +/-3.4 X 10^5   |
 ========================================================================== 

* Polarized



1 Astronomical Unit (A.U.) = 1.496 X 10^11 m
1 Light Year (L.Y.) = 9.461 X 10^15 m = 63,242 A.U.
1 Parsec (psc) = 3.26 L.Y.


1.   The effective or equivalent magnitude of the visible transmitter has
     not been corrected for wavelength.  Because the wavelength chosen is in
     the red region of the visible spectrum, it will appear somewhat dimmer
     than the stated magnitude of +23.

2.   Alien Planet Magnitude is the apparent stellar magnitude of reflected
     Planckian light from a Jupiter-type extra-solar planet.

3.   Signal-To-Planck Ratio (SPR) per pixel at the heterodyned I.F.
     frequency is based on a noise spectral density of 2Npl, no separation
     (resolution) of alien star and planet, and no Fraunhofer dark line
     suppression.  The SPR rises to about 86 dB at 656 nm if 20 dB of
     Fraunhofer H-alpha absorption is assumed.  Under these conditions,
     Planckian noise does not exceed quantum noise, and hence degrade the
     SNR, until the effective optical bandwidth is greater than about
     150 kHz.

4.   Signal-To-Daylight Background Ratio is based on the per pixel
     (resolution element) signal-to-daylight noise ratio at the heterodyned
     I.F. frequency.  At 656 nm it is 72 dB below quantum noise.  It does
     not degrade the SNR until the effective optical bandwidth is greater
     than about 15 MHz.

5.   Doppler Shift is the maximum frequency shift due to the local motion of
     the transmitter or receiver along the line-of-sight, for a transmitter
     orbiting a Sun-type star at about 1 A.U..

6.   Orbital Chirp is the maximum Doppler shift drift due to local
     accelerations along the line-of-sight, for a transmitter or receiver in
     geosynchronous orbit around an Earth-type planet.


 -------------------------------------------------------------------------- 
| The bottom line as far as system performance is concerned is summarized  |
| below:                                                                   |
|                                                                          |
| Transmitter Power = 1 kW                                                 |
| Bandwidth = 1 Hz                                                         |
| Range = 10 L.Y.                                                          |
|                                                                          |
| Microwave: SNR = 20 dB                                                   |
| Infrared:  SNR = 22 dB                                                   |
| Visible:   SNR = 34 dB                                                   |
 -------------------------------------------------------------------------- 


Note that while the star appears to be of 2nd magnitude, the 1 kW ETI
transmitter is only a very dim 23rd magnitude object, so it is vastly
outshone by its star.  It is doubtful whether the largest (conventional)
telescopes could detect this signal, even after considerable signal
filtration and integration.  As far as is known, there are presently no
high-resolution (incoherent) large ground-based telescope spectrographs in
existence that can detect this signal.  The transmitter would appear
slightly brighter than a Jupiter-size planet in orbit about the alien star,
which is approximately a 24th magnitude object.  Even if the transmitter
power is increased by six orders of magnitude to 1 GW, the transmitter as an
8th magnitude object, is only 0.6% of the apparent intensity of its star.

As we approach distances of 1,000 light years, the dimensions of the visible
beam becomes sufficiently large to encompass the entire biospheres of stars.
Some scientists doubt advanced technical civilization's (ATC's) prowess in
predictive targeting, i.e., the ability to hit "the bull's eye" in star
systems only ten light years away, with beams that are only 0.051 A.U. in
diameter.  For these terrene scientists, it may be more acceptable to
suggest that at least ATCs will be aware of the plane of the ecliptic of
nearby star systems.

To make it easier to hit the target, suppose the ATC's Visible beam is
expanded in one dimension to produce a fan-shaped beamwidth 1" X 0.014", and
aligned with its broadest dimension parallel to the target star system's
plane of ecliptic.  In some situations, the target's plane of ecliptic may
be close to the line-of-sight.  In this way, ATCs would not need to know
where the target planet or planets were in their orbital paths.  At a
distance of 10 light years, the beam will have dimensions approximately
3.8 A.U. X 0.051 A.U., and produce an SNR = 15 dB; only 5 dB less than for
the above Microwave beam.  For the situation where the plane of ecliptic is
more or less at right angles to the line-of-sight, a phased-array
transmitter producing a relatively thin annular ring beam might be
considered.  This would be a means of maintaining beam energy densities
several dB above that produced by a broad defocused or decollimated beam
encompassing the outer limits of the planetary biosphere.

In all previous discussions, little has been said about what modulation
techniques might be employed by ETIs.  This discussion will be left to
another occasion.  However, in passing, I would like to propose two
unconventional modulation techniques, with particular emphasis on the
problem of targeting nearby stars.  It is possible (though unlikely) that
ATCs might dither their transmission beam to scan a planetary system at a
rate determined by the modulation information - a sort of pulse position
modulation.  The sweep might be a raster or spiral scan whose sweep rate is
modulated.  As the beam passes across the receiving planet, observers would
notice a pulse.  The transmitter might be simultaneously intensity-modulated
to preserve the amplitude of the resultant pulses.

It has already been suggested that the beam might be defocused or
decollimated automatically when targeting nearby star systems.  Consider a
modulation scheme that actually modulated the collimation of the beam with
the data.  To an observer in the beam's path, this modulation scheme would
amount to intensity modulation, as the beam diameter expanded and contracted
in sympathy with the modulation.  All targets in its path would see an
intensity-modulated signal, but the strength of the signal, the modulation
depth and its phase, would depended on how well lined up the target was with
the beam axis.  This weird modulation technique might increase the
probability that some sort of signal would be picked up anywhere within the
planetary system, though it is unlikely that the signal strength or
modulation depth would be optimized.  A crazy idea - just food for thought.

It is very difficult to avoid the conclusion from the above figures that
interstellar optical communications is a very powerful technique for linking
the Milky Way Galaxy.  Whatever one might wish to argue about the beaming
skills of ATCs, Optical SETI has great merit.

Space, the final frontier, where no artificial photons have gone before - or
have they?  This document indicates that artificial photons may have indeed
been raining down upon Earth for millennia, unbeknownst to its inhabitants. 
For millennia, mankind has looked up to the heavens for inspiration and in
wonder of the majesty of the universe, completely unaware that other
intelligent creatures have been signalling in our direction.  Only now, as
we approach the end of the 20th Century, do we have the technology to see
what has always been there.

As with the Microwave SETI rationale, once the first ETI signal is detected
in the optical spectrum, the galaxy will be found to filled with such
signals.  Indeed, late 21st Century historians will be puzzled as to why it
took so long to discover the obvious.  They will uncover the fact that a
series of messages posted on a long forgotten computer bulletin board in the
late 20th Century, led eventually to a complete rethinking about the SETI
rationale and a change in the "search" emphasis.  At the time, there were
many who wondered about the wisdom of these messages, and their length! 
After all, hadn't Nobel laureate Charles Townes first suggested the optical
approach back at the dawn of SETI, but had failed to have his ideas taken up
by his colleagues.  This was not the first time, nor would it be the last,
when science has gone off on a tangent, with scientists refusing to see the
obvious, sometimes for decades or even centuries.

Thus, in a city named after the discoverer of the New World, came forth an
individual who dared to challenge the scientific orthodoxy of the day, an
individual so sure of what he had to say, that he had no qualms about
posting his thoughts on a computer bulletin board for all to read, and
copying it to the SETI Institute in California.  It occurred at a time when
most nations were united against the sort of human folly that mankind had
hoped in the late 20th Century, to have abolished for good - a moment in
Earth's history, just prior to the use of nuclear weapons for the second
time.  He even attempted to predict the future; at best a dubious pursuit as
the future is never quite what we expect.  All this occurred within six
months of the start of what we might call his professional involvement with
SETI.  These messages would eventually lead to the discovery of not one, but
many new worlds.

Science fiction or some day science fact?  Stay tuned for more exciting
developments in the world of Optical SETI!  I cannot promise these
developments to be as electrifying as the worrisome events in the Gulf, but
at least they appeal to the best attributes of the human condition.


December 26, 1990
RADOBS.12
BBOARD No. 286
Happy New Year.



* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1990        *
* AMIEE, SMIEEE                                                           *
* Consultant                            "Where No Photon Has Gone Before" *
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