Publications Department
Copyright © 1996 by H. Paul Shuch, Ph.D.Abstract
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.orgPresented at the Fifth International BioAstronomy Conference, Capri Italy, 5 July 1996
Project Argus, a global effort of the non-profit SETI League, Inc., seeks to achieve continuous microwave monitoring of all four pi steradians of space, in real time. This most ambitious SETI project ever undertaken without benefit of Government support will ultimately involve 5,000 small radiotelescopes worldwide, built, maintained and operated by private individuals (primarily radio amateurs and microwave experimenters), coordinated so as to miss no likely candidate signals, and providing independent verification of any interesting signals detected. Prototype stations went into operation in 1996; full sky coverage is planned for 2001. Sensitivity and range are assessed by comparison of current capabilities to those in place at the Ohio State Radio Observatory nineteen years ago, when the so-called "Wow!" signal was detected. The "Wow!" signal serves as a convenient benchmark, even though its exact nature remains unknown. Should a similar candidate signal appear during the fully deployed phase of Project Argus, it will not evade detection. Though utilizing just a small satellite TV dish as its antenna, each station achieves range and sensitivity on a par with the Ohio State Big Ear radio telescope, circa 1977. This paper explores the technological breakthroughs which have made this level of performance possible.
Key Words: Project Argus, Radio Astronomy, Microwave, SETI, Amateur Radio.
Probabilistic Considerations
Thirty six years after implementation of the first modern SETI study, it is beginning to appear that the most interesting candidate signals are highly intermittent in nature, of just a few seconds to a few minutes in duration, and never repeating. If such signals are the norm, then even the most advanced temporally displaced signal verification scheme (such as the FUDD approach used by Project Phoenix) will be prone to a high incidence of false negatives. Further, as large radio telescopes scan only a tiny fraction of the cosmic sphere at any given time, we can expect that the overwhelming majority of interesting candidate signals will evade initial detection.
The probability of detection of any given extra-terrestrial radio emission is a multivariate parameter space with at least six degrees of freedom, three spatial and three temporal. Spatial parameters include sky coverage (expressed in ranges of azimuth and elevation, or alternatively right ascension and declination), and capture area. Temporal factors include frequency coverage, resolution bandwidth and observing time. The latter two factors are highly correlated through integration time constant. There are also thermodynamic factors, specifically involving sky noise and receiver noise, but these can be negated. Judicious choice of operating frequency (such as within the transparent portions of the microwave window) can minimize noise sources associated with the interstellar medium, and the state of the art in receiver design makes equipment noise contributions almost negligible. [1]
Much attention has been given in past SETI efforts to maximizing frequency coverage, through the use of elaborate multi-channel spectrum analyzers (MCSA's), and capture area, through the use of very large parabolic reflector antennas. This emphasis has traditionally been at the expense of sky coverage and observing time. As MCSA's are inordinately expensive and in short supply, they have thus far been utilized at only our largest radio telescopes. Such large antennas are appropriate for targeted searches of specific stars, but perhaps not for all-sky surveys. When a drift-scan sky survey is performed, capture area and observing time (or its complement, sky coverage) are mutually exclusive.
It is hypothesized that the most likely microwave signals of intelligent extra- terrestrial origin will be highly intermittent in nature. The best known candidate signal to date, the Ohio State "Wow!" signal, is a case in point. The duration of this signal was insufficient to be captured by both feedhorns employed for terrestrial interference elimination. Over one hundred follow-up observations of the same region of sky, at the same frequency, failed to turn up a repetition of this tantalizing candidate. If the "Wow!" represented incidental radiation leakage rather than an interstellar beacon, we should not reasonably expect it to repeat over time scales consistent with the human life span.
Consider that the Big Ear radio telescope at the Ohio State Radio Observatory is narrow in beamwidth, viewing about one part in ten to the sixth of the sky at a given time. Let us imagine that the "Wow!" emanated from a similar antenna some tens to hundreds of light years distant. As both antennas can be assumed to be situated on rotating planets, the likelihood that each will be pointing at the other is one part in ten to the twelfth.
Granted, there could be other, equally interesting candidate signals emanating from other directions, the reception of which would be just as significant as a reprise of the "Wow!" That, after all, is the justification for the sky survey approach to SETI. But wherever a signal might originate, the narrow beamwidth of Big Ear suggests that if we happen to be listening on exactly the correct frequency, at exactly the instant an interesting signal arrives at Earth, there's still a 99.9999% chance our antenna will be pointed the wrong way!
One answer is to scatter a million Big Ears across the surface of the Earth. We would surely then be able to look in all directions at once, but at tens of millions of dollars per antenna, the costs could quickly exceed the Gross Planetary Product. So consideration must be given to smaller, less costly radio telescopes, if true all-sky coverage is to be achieved.
Quantifying the Wow! Signal
Any radiotelescope which we might propose for any SETI survey must, of course, be capable of detecting signals of likely power levels. Let us assume for this analysis that the Wow! signal is a valid SETI candidate, of just such a likely level. We know the Signal-to-Noise Ratio (SNR) of this candidate signal as received in 1977, and can easily compute the sensitivity of the Ohio State Radio Observatory at that point in time. Thus we can readily determine the flux density of the Wow!, which establishes for us a practical sensitivity requirement for future SETI instruments.
It is reported that when the Wow! signal was intercepted, the gain of the Big Ear radiotelescope was roughly equivalent to that of a circular parabolic reflector 52.5 meters in diameter. [2] Wow! discoverer Jerry Ehman has indicated that the equivalent capture area of the antenna was roughly 1,000 square meters. [3] At the 21 cm operating wavelength, the two figures correlate well if we assume a dish illuminated at roughly 50% efficiency, which is consistent with a feedhorn system designed to minimize sidelobes and antenna noise temperature (see Table 1). The bin bandwidth, noise temperature, and integration time used during reception of the Wow! signal are widely reported in the literature, and are also reflected in Table 1. It can be seen that the resulting sensitivity of the Ohio State Radio Observatory on 15 August 1977 was on the order of 4 x 10-23 W/m^2. The amplitude of the Wow! signal is reported as 30 sigma above receiver background noise, for a SNR of +14.9 dB. The peak of the signal was concentrated in a single channel 10 kHz wide. This suggests that the signal's flux density in a 10 kHz bandwidth was 30 times (4 x 10-23 W/m^2), or 1.2 x 10-21 W/m^2. Thus any SETI instrument with a sensitivity exceeding 1.2 x 10-21 W/m^2 will, in theory, be capable of detecting a repeat of the Wow!, or any similar signal which it should happen to intercept.
The Project Argus Concept
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. Consider that at the 21 cm neutral hydrogen line, a 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 on a par with but 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.
We can, however, buy back some of that lost range. It is axiomatic in astronomy that "there is no substitute for capture area." It turns out that, in fact, there is: integration time. Most all-sky surveys are performed with the antennas in drift-scan mode; that is, fixed in position, the rotation of the Earth bringing candidate stars within range. The narrow beamwidth of a large antenna limits the time which a given candidate signal will spend within its pattern, hence the length of time over which the signal can be integrated. The actual time of accessibility varies with declination, but in the case of the "Wow!" signal equaled 37 seconds (actual integration time used at Big Ear was then ten seconds.) The proposed five-meter dish, on the other hand, would for the same signal have enjoyed ten minutes of signal duration within its half-power beamwidth. Since sensitivity varies with the square root of integration time, wider beamwidth antennas can, through signal integration, compensate somewhat for their reduced gain. In this example, integration increases our sensitivity (hence our effective range) by a factor of eight.
Our small dish still falls short of Big Ear's range by a factor of 25. Is there anything else we can do to improve performance? It turns out there is. The state of the art in 1977 was such that the Ohio State Radio Observatory employed a 10 kHz bin width in its 50 channel receiver. Today, digital signal processing (DSP) has advanced to the point that thousands of frequency bins, each a few Hz wide, can be readily accomplished at low cost. Employing a relatively modest personal computer, 10 Hz bin width is easily achieved. This reduces background noise by a factor of 1,000. Since maximum range varies inversely with the square root of noise power, we see greater than a factor of 30 range improvement relative to Big Ear circa 1977. In other words, the proposed small SETI stations have a range and sensitivity on a par with that achieved at Big Ear when the "Wow!" was received.
The Project Argus Prototype System
The first prototype instrument in the Project Argus all-sky survey went on the air on Earth Day, 21 April 1996, from SETI League headquarters in New Jersey. Simultaneously, four similar instruments went on the air in North America, Europe and the Pacific, launching one tenth of one percent of the total proposed Argus system. The block diagram of these first instruments is seen in Figure 1. They each consisted of a small parabolic reflector of the type used for satellite TV reception, a cylindrical waveguide feedhorn, GaAs MMIC low noise amplifier, a commercial scanning microwave receiver operated under control of a personal computer, a computer sound card as an analog-to- digital converter, and Fast Fourier Transform software for Digital Signal Processing. The cost of each of these systems was a few thousand US dollars.
Although the design standard for Project Argus participants included a five meter diameter antenna, the demonstration station built at SETI League headquarters utilized an available parabolic reflector of only 3.7 meters. The first generation LNA utilized a Gallium-Arsenide monolithic microwave integrated circuit (GaAs MMIC) with 23 dB gain and a noise temperature on the order of 150 K. We conservatively estimate the overall system noise temperature for this station at 200 K, and calculate its sensitivity accordingly. Using 10 Hz bin widths and 10 seconds of integration, the sensitivity of the prototype system is about 5 x 10-22 W/m^2. This result, reflected in Table 2, is marginally adequate for reception of the Wow! signal, though it is about an order of magnitude short of the sensitivity achieved by Big Ear, circa 1977. There really is no substitute for capture area.
Tinkering at the Margin
Or is there? By making incremental improvements to the Argus station, we can readily raise its sensitivity the dozen dB or so necessary to equal the performance of Big Ear when the Wow! was detected. [4] It is highly unlikely that these small stations will ever have the sensitivity of the world's great radiotelescopes, given equivalent technology. However, by using the Wow! as a benchmark, we hope to show that even these small stations have sensitivity adequate for SETI success.
If we increase our antenna size to 5 meters in diameter (entirely consistent with our goal of achieving real-time full-sky coverage with 5,000 instruments), we improve system sensitivity by about 2 1/2 dB. A second generation preamplifier currently under development utilizes a GaAs PHEMT in front of the existing MMIC stage, to lower preamp noise temperature to around 50 K. This makes an overall system noise temperature of 100 K or less entirely feasible, and buys us another 3 dB. But the most dramatic improvement with these small terminals comes from integration gain, facilitated by their relatively wide beamwidths.
As shown in Table 3, the beamwidth of a 5 meter dish is such that, when operated in meridian transit mode, a signal will remain within its beam for no less than ten minutes. By integrating for, say, 2 minutes, we can achieve five samples of any signal transiting our beam, which is indeed sufficient to trace out the pattern of the antenna over time. [5] Since sensitivity varies with the square root of integration time, we can see that another roughly 5 dB improvement in sensitivity can be had relatively easily.
Of course, a 120 second integration time will only yield the promised improvement if the candidate signal is a pure continuous wave (or at least confined to a 10 Hz bandwidth), and present for a sufficiently long period. This is speculative at best. However, returning to the Wow! signal as a benchmark, even though its exact nature remains unknown, we do know that its bandwidth did not exceed 10 kHz , and that it had a sustained duration of at least 37 seconds. Let us hope that either nature, or other civilizations, will provide us with other interesting signals of like amplitude, and sufficiently narrow bandwidth.
There exists a potential incompatibility between desired integration time constant and bin width, in view of the anticipated Doppler shift on signals emanating from, and received on, rotating planets. It is common practice in SETI to chirp the receiver's local oscillator so as to correct frequency to the Galactic Standard of Rest, in hopes that a transmitting civilization will do the same. Software is currently under development to allow for computer tuning of the Project Argus receivers. Of course, this places a burden on the transmitting civilization, which we can only hope they will choose to shoulder.
When we're done tinkering at the margin, we see that we have the potential, utilizing today's readily available technology, to receive with an amateur SETI station a CW signal at a power density as low as 4.1 x 10-23 W/m^2. This is on a par with the sensitivity of Big Ear, when the Wow! signal was detected. Whether this level of SETI sensitivity is adequate to bring us the existence proof we seek, only a fully implemented Project Argus network, and an indeterminate period of patient observation, can disclose.
Conclusions
During the last half-century, SETI has 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. 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. We are not discouraged. Not only have we not yet scratched the surface, we haven't even found the itch.
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. [6]
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