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SETI System Performance Comparison TableRadobs 12The 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" * * __________ * * FIBERDYNE OPTOELECTRONICS / \ * * 545 Northview Drive --- hf >> kT --- * * Columbus, Ohio 43209 \__________/ * * United States .. .. .. .. .. * * Tel. (614) 258-7402 . . . . . . . . . . . * * skingsle@magnus.ircc.ohio-state.edu .. .. .. .. .. * * CompuServe: 72376,3545 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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