Publications DepartmentCopyright © 1996 by Dr. H. Paul Shuch, N6TXAbstract
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.orgPresented to the AMSAT Annual Meeting and Space Symposium, Tucson AZ, 9 November 1996
The Ohio State University SETI program is the longest continuously running electromagnetic Search for Extra-Terrestrial Intelligence to date. In 1977 that sky survey detected a signal which seemed to fit all the characteristics anticipated for communications of intelligent extra-terrestrial origin. The so-called Wow! signal is interesting in its own right, and we analyze it here, applying time-honored reverse engineering principles in trying to ascertain its nature. But it is also important as a possible benchmark, in defining the signal characteristics for which future SETI projects might be searching. Microwave antenna, receiver and digital signal processing technologies have all advanced significantly in the two decades since the detection of the Wow! signal. If we take it as being typical of the types of signals which we are seeking, we can use it to calibrate the effectiveness of future generations of SETI receiving stations. We see that the amateur state-of-the-art is today easily capable of detecting any future Wow! signals which happen our way.
Introduction -- Is Amateur SETI Practical?
The ambitious NASA SETI (Search for Extra-Terrestrial Intelligence) program, modestly funded at five cents per American per year, was terminated by Congress in October of 1993, reducing the Federal deficit by 0.0006%. Several organizations arose to privatize the research, including the membership-supported, non-profit SETI League, Inc. The SETI League differs from other space advocacy organizations in that it is a grass-roots movement, composed mainly of radio amateurs, which encourages its individual members to build and operate their own modest SETI receivers. A tax-exempt educational and scientific corporation, we are modeled in large part after the amateur communications satellite organizations, AMSAT and Project OSCAR.
The professional radioastronomy community has voiced an understandable skepticism as to the contributions to science which might be made by a handful of amateurs, funded at a small fraction of the former NASA SETI budget. The late SETI pioneer Dr. Bernard M. Oliver articulated this skepticism well. Barney Oliver's credentials are impressive. Longtime vice-president of engineering for the Hewlett-Packard Company, he served as president of the IEEE, and was principle author of NASA's ambitious 1971 Project Cyclops design study for detecting intelligent extra-terrestrial life. [1] He said of amateur SETI, "If your system wouldn't detect the strongest signal the ETI might radiate, then years of listening, or thousands doing it, won't improve the chance of success. To cross the Golden Gate, we need a bridge about 10,000 feet long. Ten thousand bridges ... one foot long won't hack it." [2]
Barney made a good point, even if he was something of a dinosaur. The burden of proof falls to us in the amateur SETI community to demonstrate that our systems are indeed capable of detecting, at the very least, that strongest signal which an extra-terrestrial civilization might generate. We do so through the following analysis of the Ohio State "Wow!" signal. As for the Golden Gate analogy, it would be valid only if SETI proved a serial process. I suggest that it is more of a parallel enterprise, and hope to show in this paper that 10,000 volunteers can, if properly coordinated, accomplish something which Barney Oliver had never contemplated. For we seek to cross not just the Golden Gate, but the gulfs of space, in all directions at once, in real time.
Review of the "Wow!" Signal
Modern SETI was born in 1959, with the publication in Nature of a short paper by Cocconi and Morrison [3] proposing a search of nearby Sun-like stars, near the 1420 MHz neutral hydrogen frequency, for artificially generated signals. Unbeknownst to the authors, even as they wrote their paper Frank Drake was preparing to perform the very experiment which they were proposing. Project Ozma searched only two stars, at that single frequency, for two months during the summer of 1960. During the succeeding years, several dozen other SETI experiments have been performed, many still concentrating on the hydrogen line as a likely frequency for interstellar communications.
The longest running of these is the Ohio State sky survey, which has been continuously operational since 1973. It was the Ohio State Radio Observatory which on August 15, 1977 detected the most tantalizing and promising candidate signal to date, the so-called "Wow!" signal. The computer printout of this historic signal is shown in Figure 1.
The "Wow! received its name from the marginal note on the computer printout, penned by SETI volunteer Dr. Jerry Ehman. "I came across the strangest signal I had ever seen, and immediately scribbled 'Wow!' next to it," Ehman explained. "At first, I thought it was an earth signal reflected from space debris, but after I studied it further, I found that couldn't be the case." [4]
The letters and numbers in the printout are today widely misinterpreted as a message. "What does the progression 6EQUJ5 actually stand for?" asked one SETI enthusiast. "A sequence in need of completion? A matrix in need of expanding? A computer malfunction? The ASCII equivalent to a binary code?" [5]
Let me emphasize that the "Wow!" sequence itself is not a message. What was received appeared to be a CW (unmodulated) signal. The numbers and letters in the much-reproduced computer printout are merely a time-series representation of the signal amplitude, as received at the Big Ear radiotelescope. Specifically, the symbols represent the number of standard deviations by which the received signal exceeded average background noise, on a scale of 0 to 35. So a 0 means no stronger than background noise, 1 is one sigma above noise, 9 means nine sigma above noise, an A would be ten units, and U (the strongest peak of the actual signal) is 30 standard deviations above the mean background noise in the receiver. If you graph the sequence as amplitude values over time you get roughly a Gaussian distribution, consistent with the antenna pattern of the Big Ear in drift-scan mode. The data set depicts signal amplitude over both frequency and time.
Figure 2 shows just such a graph of the output of the Ohio State 50-channel receiver during the transit through the antenna pattern of the "Wow!" source. Time is plotted horizontally, amplitude vertically, and frequency in the depth axis. The time increments are twelve seconds per sample. Each of the channels is 10 kHz wide; thus, a half MHz surrounding the hydrogen line is depicted. Note that the signal rises almost 15 dB above the background noise, in a single channel, then falls back into the noise, its amplitude pattern exactly coinciding with the known beamwidth pattern of the dish (including its feed-induced skew, and coma sidelobes).
From the "Wow!" signal's temporal correspondence to the antenna pattern, we know that its source was moving with the background stars. From its Doppler shift signature (the local oscillator of the receiver was being chirped at a rate which corresponds to the Earth's motion with respect to the Galactic center of rest) we can eliminate terrestrial interference, aircraft or spacecraft from consideration. The antenna coordinates indicated that the signal was coming from no known nearby sun-like star, though at any time, in any direction, the antenna pattern encompasses on average about half a dozen distant stars. Most significantly, though over a hundred follow-on studies of the same region of the sky were performed, from several different radio observatories, the signal never repeated. [6]
Of course, it should not have. Consider that the Big Ear radiotelescope at the Ohio State Radio Observatory is extremely narrow in beamwidth, viewing just one part in a million of the sky at a given time. That means if you are listening on exactly the right frequency, at exactly the instant when The Call arrives, there's still a 99.9999% chance you'll be pointed the wrong way. And if we imagine that the "Wow!" signal emanated from a similar high gain antenna, which (let us assume) illuminates only one millionth of the sky, what are the chances the two antennas will be pointed at each other at the same time? That's easy, says the statistician: (1e-6) squared equals (1e-12).
But wait, if we know the direction from which the signal emanated, and concentrate our antennas there, we've removed one factor of (1e-6), and we're back to million-to-one odds. Even still, we've only looked in that direction for a total of a few tens of hours. Not only have we not yet scratched the surface, we haven't even felt the itch.
Quantifying the "Wow!"
If the "Wow!" signal is typical of the type of evidence which SETI seeks (and we have no reason to assume otherwise), then we can expect valid SETI hits to be extremely strong, highly intermittent signals which appear once (as the transmit beam sweeps past Earth), and never repeat on human time scales. Thus we do not expect to again encounter the "Wow!" Yet there may be countless other signals, similarly strong and intermittent, falling on our heads even now. In order to determine whether our receivers are up to the task of detecting these future "Wow!" events, let us quantify the amplitude of our only known specimen. If we can show that amateur SETI is capable of detecting such signals, then Oliver's first objection is overcome.
We know a great deal about the status of the Ohio State Radio Observatory at the moment of the "Wow!" detection. It exhibited, for example, a 100 Kelvin overall noise temperature, and had channels 10 kHz wide. From the above, we use Boltzmann's Law to compute the noise threshold of the receiver:
From its reflector area and feed illumination, we determine that the antenna exhibited a gain of +55.3 dBi. Combining this figure with noise threshold, we find the incident isotropic power threshold of the radiotelescope to be:
But actual sensitivity improves with the square root of integration time, and integrating a CW signal for ten seconds, within a 10 kHz bandwidth, improves things by 25 dB. So the actual system sensitivity is:
Finally, the received Signal-to-Noise Ratio (SNR) was +14.9 dB. This suggests that, to detect a "Wow!" twin, a radiotelescope needs to provide us with an ultimate sensitivity of:
Thus, any radiotelescope with an overall sensitivity of -204 dBm would, in theory, be able to detect a "Wow!" type signal, if tuned to the right frequency, and pointed in the right direction, at the right time.
Sensitivity of an Amateur SETI Station
During the Spring of 1996, I had the pleasure of designing and assembling a demonstration amateur SETI station at SETI League headquarters in New Jersey. The design objective of this proof-of-concept station was the capability of detecting rf events of "Wow!" amplitude. Figure 3 depicts the simplified block diagram of the resulting prototype system, which employs a mix of commercial and home-brew elements.
The antenna chosen for the prototype is a Paraclipse Classic 12 commercial satellite TV antenna of 3.7 meter diameter, on a modified horizon-to-horizon mount. The antenna feed is a monopole-fed cylindrical waveguide feedhorn from Radio Astronomy Supplies, Atlanta GA. The front end is a Hewlett-Packard GaAs MMIC preamp from Down East Microwave, Frenchtown NJ. An Icom 7000 microwave scanning receiver is employed; it drives a TI 560 CDT laptop Pentium computer for digital signal processing. Construction details of the station can be found in the SETI League Technical Manual. [7]
We analyze the sensitivity of the amateur SETI station in much the same manner as we previously quantified the Ohio State Radio Observatory. Digital Signal Processing gives us a 10 Hz bandwidth, significantly improving sensitivity over the 1977 state-of-the-art. Overall noise temperature is a modest 200 Kelvin. Boltzmann's Law thus gives us the noise threshold of the receiver:
The 12 foot reflector is poorly illuminated by the simple feedhorn, and thus achieves only about 50% efficiency. At the hydrogen line frequency, this corresponds to a gain of +31.8 dBi. Combining this figure with noise threshold, we find the incident isotropic power threshold of the amateur system to be:
which is slightly better than that achieved at Big Ear, circa 1977. But our integration gain offers us significantly less improvement, since we are starting with a channel a thousand times narrower than the bandwidth then employed at Ohio State. Still, we achieve a 10 dB improvement by integrating for ten seconds, for an ultimate sensitivity of:
Comparing our Tangential Signal Sensitivity to the Incident Isotropic Power of the "Wow!" signal, we see that this station would have achieved about a +3.4 dB SNR, had it been available to intercept the "Wow!" This is not dissimilar to the amplitude experienced by radio amateurs bouncing their signals off the surface of the moon. We can classify our sensitivity as "+3.4 dBW!" (3.4 dB more sensitive than the "Wow!" amplitude).
The above result may appear to violate the conventional wisdom. The Big Ear radiotelescope is, after all, a LARGE antenna. The amateur station we've just described uses a small one. And everyone knows there's no substitute for capture area.
Or is there? One substitute, which amateur SETI employs to good advantage, is Digital Signal Processing.
Introducing the Project Argus Network
Recall that a chief limitation of the Big Ear radiotelescope is that it can "see" only perhaps a millionth of the 4 pi steradians of space at any given time. If "Wow!" type signals are as highly intermittent as we suppose, then the odds are rather good that we'll miss the next one which comes along. It would be highly desirable to see in all directions at once. We could do so by building a global network of a million Big Ear type telescopes. But at a cost of fifty to one hundred million dollars apiece, we would very quickly exceed the gross planetary product.
There is another way, and it has been described above. Consider that at the 21 cm neutral hydrogen line, a three- to five-meter diameter parabolic antenna (such as is commonly used for satellite TV reception) will have a power gain perhaps 200 times less than that of a "real" radio telescope such as Big Ear. The reduced capture area would also imply that such an antenna would enjoy 200 times the sky coverage, so a mere 5,000 such antennas could, if properly situated, "see" the whole sky at once. And such a global array of small telescopes could be constructed at a cost far less than that of a single Big Ear.
Unfortunately, this increase in angular coverage afforded by smaller antennas was accomplished by a reduction in their capture area, hence gain. Thus, as compared to our Big Ear example, these smaller antennas will experience a reduction in their effective communications range by that same factor of 200, all else being equal. A signal which could be detected by Big Ear at a range of, say, 20,000 LY, would be detectable to our smaller antennas at a distance of only 100 LY. Since for uniform distribution of candidate stars, the number of targets varies roughly with the cube of distance, this sacrifice in sensitivity significantly reduces (perhaps by a factor of several million) the number of suitable stars which might be within range of our sky survey.
Nevertheless, for surveying the entire sky in real time, there's no better resource than the world's radio amateurs. On April 21, 1996, The SETI League launched its Project Argus all-sky survey. Over the coming years, we can envision our small network growing to perhaps 5,000 stations worldwide, operating in a coordinated manner, covering the whole sky with our modest receiving beams. Perhaps we won't cross the Golden Gate, but amateur radio has a very real opportunity to cross the gulfs of space and time.
Conclusions
Within the past half-century, SETI has finally emerged out of the realm of science fiction, and into the scientific mainstream. Every month we read about the discovery of yet another planetary system in space. Thanks to microbes detected in meteorites, we are beginning to learn about how life might have developed on other worlds. And we have completed the Copernican Revolution, finally realizing that we are not the center of all creation. Yet SETI programs continue to yield negative results. Our most promising candidate signal was detected nearly two decades ago. It may well take many more spades digging in the sands of space before we can expect to uncover another gem. But we have demonstrated here that suitable spades are readily available to any interested prospector.
The non-profit, membership-supported SETI League has launched its search on Earth Day, and flies the Flag of Earth, because SETI is an enterprise which belongs not just to one country, government or organization, but to all humankind. Like Argus, the guard-beast of Greek mythology who had a hundred eyes, we seek to see in all directions at once, that we might capture those photons from distant worlds which may well be falling on our heads even now.
Project Argus started with a mere five stations. This small step for humanity represents a humble beginning for what will ultimately be a global effort. We can foresee 500 participants within two years, and perhaps five thousand by the year 2001. When we reach that level, there will be no direction in the sky which evades our gaze. Then we can hope to find the answer to a fundamental question which has haunted humankind since first we realized that the points of light in the night sky are other suns: Are We Alone?
And when Project Argus grows
To full strength, we will show
That the suns shall never set on SETI. [8]
References
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