SP-419 SETI: The Search for Extraterrestrial Intelligence

 

[75] COLLOQUY 5

SEARCH STRATEGIES

 

Prepared by:
Charles L. Seeger
SETI Program Office
Ames Research Center

 

[76] blank page

[77] INTRODUCTION

 

The objective of SETI strategies is the detection of the existence of other intelligent species in the Universe by examining the spectrum of electromagnetic radiations in the vicinity of the Earth. Communication, one-way or two-way, does not directly concern us now. only detection .

Another characteristic of this objective is that out of all the intelligent species that may exist, we are seeking another member of the subset to which we belong. The essential property of our subset is that its members radiate electromagnetic energy into interstellar space in such large amounts and in such a distinctive fashion that at interstellar distances we can recognize it as an artifact against the background of "natural" radiations.

In the past several decades we have developed a trenchant technology appropriate to searching in the spectral range of the free-space microwave window. No comparable technology has yet been generated for shorter wavelengths, so the remarks below are directed in the main toward strategies suited to an initial search in the microwave window and give special attention to the water hole (see Section II-4). However, our suggested strategies do encompass the spectrum above the microwave region, for relevant physical knowledge and suitable technologies are burgeoning (e.g., in the infrared region of the spectrum).

A final distinction - search strategies as discussed here, though closely related, are not equivalent to search programs. The latter are a topic unto themselves since many factors not touched on here enter their design.

 

GENERAL CONSIDERATIONS

 

We cannot afford to search for all kinds of radiation at all frequencies from all directions at the lowest detectable flux levels. It is necessary to put some bounds on the volume of our multidimensional search space. At the same time it is important not to narrow the space too much: to put all our eggs into one basket. We submit that a rational approach is to assess all strategies and to attempt to assign relative a priori probabilities of success per unit cost. If only one strategy can be pursued at a time, one chooses the most likely and continues until success is achieved or until the accumulated negative results have depressed the probability/cost ratio below that of some other strategy, in which case the other strategy would then be pursued. If several strategies appear to have comparable probability/cost ratios they may all be pursued in proportion to these ratios.

[78] The case for preferring electromagnetic to any other form of radiation seems compelling (see Sections II-4 and III-1). The case for preferring the microwave window (approximately 1-102 GHz) seems very strong but not necessarily compelling. The case for preferring the low frequency end of the window to the high seems strong but not so strong that no attention should be paid to higher frequencies. The case for the water hole is very appealing (as a starting place) if one has already decided on the low end of the microwave window.

The case for searching nearby main sequence F, G, and K stars at ever-increasing range seems very natural; the only life we know lives on a planet around a G2 dwarf star. To adopt this strategy takes us only as far into space as necessary to find perhaps our closest neighbor. Communication with a close neighbor would permit more two-way exchanges than with a civilization in the Andromeda nebula, for example.

On the other hand, cultures only slightly older than ours may be able to exploit enormously greater communicative capabilities. As is true for stars, the nearest transmitters may not be the strongest. The strongest signals may come from more advanced cultures at great distances. For these reasons it would be a mistake to pursue only one search strategy, such as that suggested in the Cyclops report. One should in addition examine other options. To cover other possibilities, it seems prudent to conduct a complete search of the sky over as wide a frequency band as practicable (see Section III-3). To be significant, such a search should extend to frequencies and down to flux levels not reached by existing radio astronomy surveys.

Our great uncertainty as to the likely flux levels of extraterrestrial signals argues that all search strategies should assume at the outset the strongest signals not likely to have been detected yet, and that the sensitivity should be increased with time until success is achieved or until the strategy is no longer thought to be sufficiently promising.

 

PARTICULARS

 

The fund of relevant physical knowledge and available technologies has a profound effect on estimates of a priori probability of success per unit cost of a proposed strategy. This is particularly true with respect to individual subelements. Furthermore, certain concepts may be unattractive simply because they promise significant returns too far in the future even though the total costs might be relatively low. Human time scales and human patience are important factors in the present context.

 

The Satellite Problem

 

It is difficult to survey the entire sky over the entire 1-22 GHz spectral range from the surface of the Earth. The glare of satellite transmissions over more than half this band makes it difficult to reach attractive sensitivities in the satellite bands (see fig. l).

 


[
79]

Figure 1. Frequency bands in the free-space microwave window, allocated to certain transmitting services and the

Figure 1. Frequency bands in the free-space microwave window, allocated to certain transmitting services and the "water hole" allocation recommended for SETI.

 

Radio Astronomy Surveys

 

Radio astronomers have carried out few surveys of the sky to what are believed to be flux levels of interest here. It is even possible that one or more of the "point" sources appearing in a survey in one frequency band but not in another survey in a different band, could be an unexpectedly powerful artificial signal.

Radio astronomical surveys of large areas of the sky can be divided into two classes: broadband continuum surveys (1 MHz to 5 GHz bandwidth), and spectral line surveys (102 to 106 Hz bandwidth). Large bandwidths, absence of adequate spectral resolution, customary on-line instrumental data compaction, and absence of specific interest in the existence of extraterrestrial artifacts, all have reduced enormously the chances of recognizing an artificial extraterrestrial signal. In fact, a signal so strong as to be noticed, generally would have been ascribed to man-made radio frequency interference (RFI).

To illustrate these points, we describe briefly two surveys reaching low flux levels by radio astronomical standards. The Parkes 2700-MHz source survey of the sky visible from Australia employed a bandwidth of 200 MHz and a sensitivity such that a signal flux level of about 10-20 W/m2 would have produced unity signal-to-noise ratio (SNR = 1). No procedures were in effect to recognize a narrow band signal at this or much higher levels, unless the absence of this "radio source" on other surveys at adjacent frequencies caught an astronomer's interest and the source was then reexamined with suitable care.

[80] Westerhout used the NRAO meridian transit 300-ft antenna to survey a region along the Galactic equator 225° in longitude by 4 in latitude. Frequency resolution was 9.5 kHz in a band about 1 MHz wide. Unity SNR corresponded to about 3 x 10-23 W/m2, but automatic data handling algorithms and RFI subtraction procedures would generally have prevented output registration of a coherent signal orders of magnitude stronger.

The radio astronomy situation (and parallel situations in other areas) can be summarized this way. Procedures in use in high data volume observations tend to discriminate against the discovery of unexpected phenomena. Scientists generally do not explore a range of phenomena merely because it is not forbidden by any known laws. They usually have more immediate objectives in mind. The polarization of starlight was observed half a century after the work of Maxwell and Hertz; and the discovery of pulsars was a fortuitous accident due to the presence and curiosity of an astute graduate student in a favorable observational situation. Analog pen and paper recording of data was in use. For a third and final example, proof of the polarization of the diffuse background radio radiations was delayed at least 5 years by an unwillingness to test for polarization just because it was an unexplored degree of freedom in nature.

 

Signal Classes

 

Postulating the characteristics of signals we might be able to detect from another species has enlivened many a casual moment. We have only our experience over the past 30 years or so, our electromagnetic technology in which we take some pride, and our projections of how in the near future we might exploit this technology to suit our manifold desires.

Above some tens of megahertz most of the power from all our surface transmissions is dissipated in the endless reaches of outer space. For over a quarter century and with increasing intensity, we have been generating an expanding aura of fairly powerful signals about the Earth, one we could detect if we had a sufficiently sensitive radio telescope situated 20 light years from Earth. As has been suggested many times, this may be a transient phase in our development, one that may be over in a time period microscopic on the cosmic scale. Nevertheless, a reversal of the historical trend toward higher transmitter powers is not yet in evidence. It does seem likely that some of our present transmissions will vanish from our scene in favor of more efficient, more directive procedures; cable and low power satellite transmissions may replace high power TV broadcasting, for instance. The future of high power radar is perhaps less clear. Table I is a fairly complete tabulation of published current high power radar signals in the 1 GHz to 2.5 GHz frequency range. They generally sweep out the sky continuously in regular patterns both in time and in direction.

One class of signal for which we might search is the intraspecies type of transmission. We may be fortunate in having a relatively close neighbor on whom we can eavesdrop. From our own experience, this would require not only proximity or exceptionally great sensitivity because of the modest power levels we may perhaps expect, but also a complex pattern recognition capability in the time, frequency, and modulation domains. Most of our transmissions have a strong carrier component. If this holds with our nearest neighbor (and it may because it is a simple way to....


[81] TABLE 1. HIGH POWER TERRESTRIAL RADIATIONS IN THE 1 - 2.5 GHz RANGE(a)

 

EIRP, W(b,c)

No. of transmitters(d)

.

>1012

1

1011 -1012

2

1010 - 1011

0

109 - 1010

15

108 - 109

23

107 - 108

78

106 - 107

462

105-106

2680

 

a This table is a summary of published radar installations.
b Equivalent Isotropic Radiated Power in watts.
c UHF color TV (460-890 MHz provides many carriers in the 105-106 W range. They are radiated with high stability and the toroidal radiation patterns rotate with the Earth. Television signals may constitute the most readily detectable terrestrial radiations at moderate distances in spite of their lower frequency.
d See table 3 in Section III-2 for further details.


 

.....maintain coherence), it eases the pattern recognition problem since stable, monochromatic signals are relatively easy to detect. Finally, we are in total ignorance of their spectrum utilization and have little idea even how they might utilize transmitters in "space activities." We can only assume that bandwidth and propagation requirements dictate some frequency allocation scheme there as here. We have, so far, no reason to search any frequency band outside the low noise, free-space microwave window; and since signal stability so directly affects our achievable carrier search sensitivity, the water hole seems a likely place to start an eavesdropping search.

Eavesdropping can be classified as a non-cooperative search situation. Alternatively, "they" may be transmitting in a cooperative, beacon mode, carefully arranged to assist discovery by others, particularly newcomers like ourselves. Any concern on their part for range would surely highlight the advantages of the low end of the free-space microwave window. Since we have conceived it and it is feasible for us today, it is quite possible (but who knows how likely) that other species are curious as are we, and indulge in their allotted share of an obvious cooperative search strategy.

[82] Beacons could exist in many modes and for many differing applications. All who study extraterrestrial phenomena should be alert to the possibilities, regardless of the portion of spectrum of immediate interest to them.

It is worth noting that beacons can be of any power, isotropic or beamed, and continuous or repetitive. If I were asked how to construct a beacon (to announce our presence) to be built on a crash basis by the year 2000, I would suggest these chief properties: 1 GW continuous radiated power, an isotropic radiation pattern, a frequency in the water hole some megahertz above 1421 MHz, frequency stability to 10-14 or better for the circularly polarized carrier, and modulation by polarization reversal in three modes: (1) carrier alone, (2) a bit every few seconds of binary, "acryptic" information to assist first decoding, (3) large information transfer at a bandwidth up to perhaps 104 Hz, and the transmitter and all that it requires in Earth's solar orbit on the other side of the Sun in order to provide a low Doppler drift rate and to minimize pollution of the local terrestrial electromagnetic environment. Modulation modes (1) and (2) would be present over 90 percent of the time in order to assist first detection. Such a beacon installation would be at about the limit of our own technology, and it would be detectable by a system equivalent to a modest Cyclops at a distance of 1000 light years or more.

 

Signal Propagation Paths

 

The interstellar medium, the interplanetary medium, and the Earth's atmosphere and ionosphere can all affect the coherence of signals passing through them. The physical processes, dispersion, scatter, and multipath transmission, are well understood theoretically, but our observational knowledge leaves much yet to be learned. In all these media there are systematic and turbulent motions of matter and free-electron density and magnetic field intensity which all show variations in direction, distance, and time. In the lower atmosphere, corresponding variations in the water vapor density add their contributions to the total possible coherence loss. Present information suggests that in the water hole and over distances as great as 1000 light years, the coherence of interstellar intelligent signals may suffer an appreciable loss unless the initial bandwidth (B) is limited to about the range

 

10-3 Hz < B < 106 Hz

 

These limits are uncertain by perhaps a factor of 10 and, since bandwidth is an important search dimension, point source scintillation and pulsar pulse-shape studies should be encouraged.

 

Radio Frequency Interference (RFI)

 

Solving the RFI problem is crucial to SETI. Section III-8 discusses RFI extensively and Section III-9 presents the unanimous Science Workshop resolution on the matter. Here, we mention briefly only certain salient points.

[83] A SETI system, regardless of antenna area, requires protection from man-made radiations down to a level which is 50 to 100 dB more stringent than the usual communication system requirements (see Section III-8). The protective measures required depend on the location of the SETI system and upon the frequency band or bands being searched.

Figure 1 shows that most of the terrestrial microwave window below 22 GHz has been allocated to services such as radar and satellite communications, and therefore is generally difficult for SETI. It is fortuitous that, so far, the 1400 to 1727 MHz "water hole" band is used chiefly by a multiplicity of low and very low power services with which, in the main, SETI is compatible. SETI does not require an exclusive frequency band allocation. Only a moderate degree of worldwide and local cooperation is needed in order to preserve the terrestrial water hole for SETI. We should:

1. Choose a site for SETI over the horizon from centers of high population density, and out from under heavily used commercial aviation lanes.

2. Avoid the use of water hole frequencies by even low power services within some 100 to 200 km of the SETI site.

3. Obtain worldwide agreement to keep the water hole band significantly free of interference at the one or more SETI sites agreed upon.

4. Allow the present, mild use of the band by satellite services to come to a natural, perhaps hastened, termination over the next 5 or 10 years.

 

Item 3 involves more than just setting limits on EIRP. All transmitters radiate some power into adjacent bands and into harmonic bands. The official national and international standards on spurious emissions are old and well behind the state of the art; and the actual situation is often worse because of the effort required to challenge effectively operations believed to be below even the standard requirements. Some civil and federal communications groups are already trying to improve both the standards and the practice to levels much closer (at least several powers of ten) to the knee in the performance/cost curve of current first-rate technology.

To summarize the Earth-based search site situation, only relatively mild allocation problems are foreseeable with respect to sharing frequency allocations with many kinds of Earth-based transmitters. Searches cannot compete with line of sight satellite down transmissions. Any overall search strategy should contain an element that proposes actively to support other groups trying to bring transmission practice closer to that permitted by the state of the art. If adequate RFI protection for Earth-based search cannot be provided, it will be necessary to develop space search systems shielded from Earth.

Space-based search systems are without the natural and effective shielding properties of the Earth. In descending order of estimated relative cost, we list below four desirable and feasible (before the year 2000) off-Earth sites for long-term search systems - long-term because much useful initial search and research could be carried out on Earth and in low Earth orbit while [84] developing and testing space-based hardware for more sensitive and extensive systems, if they are needed. The four sites are:

 

1. On the far side of the Moon

2. In Earth orbit around the Sun and 60° from the Earth as viewed from the Sun

3. At lunar distance at L-3, on the opposite side of the Earth from the Moon

4. In synchronous orbit around the Earth and at the longitude of the data processor

 

Site 1 seems hardly worth consideration until well after the year 2000, for reasons of cost. International efforts are already under way to keep the electromagnetic environment essentially pure as seen from the far-side lunar surface. The Moon itself is an excellent shield against terrestrial radiations.

Site 2 employs distance rather than an RFI shield, to protect itself from Earth's radiations. It is likely to be more costly in the long run chiefly because of servicing costs, reliability requirements, and multiple, long distance relay link demands. Also, some minor but necessary limits on maximum permissible EIRP on Earth would be required in the search frequency bands. The greater distance provides only about 50 dB improvement over Site 3.

Site 3 requires either a shield against Earth and Earth satellite radiations or a nearly sole allocation of the search frequency bands to the search effort. A shield would allow search of the entire free-space window and pose no allocation requirements. However, RFI shields could be costly.

Site 4 can tolerate almost no Earth or satellite transmissions in the search bands, unless an RFI shield is used. Even then, there would likely be restrictions on satellite transmissions in the search bands. A careful engineering assessment would be required of any concrete proposal involving this site.

The RFI shields mentioned above are worth brief description. They are needed because realizable antennas are imperfect in the sense that there is no direction of signal arrival to which they are totally unresponsive. The purpose of the shield then is to attenuate signals by reflection, by absorption, and by diffractive loss, such that the signal energy reaching the antenna does not produce a detectable signal at the antenna output terminals.

Precise definition of shield requirements in any particular instance depends on the nature of the interfering fields (direction and strength) and on the directional characteristics of the antenna. The latter are variable, of course, since the antenna is required to look in many directions relative to the direction of the source of interference. Typically, in situations studied so far, shield attenuations on the order of 50 to 200 dB (i.e., power ratios of 105 to 1020) are required. To visualize the realization of such a shield, imagine a 300-m space antenna in Earth orbit. Adjacent to it and between it and the Earth is a large disk consisting of a ring supporting a thin conducting [85] membrane. The disk diameter would be of the order of 450 to 600 m. The conducting membrane may be only several tens of microns thick; even be a fine conducting mesh. The whole is equipped with means for orbital station keeping, engines, sensors, telemetry, etc., so that it rotates about the Earth in step with but unattached to the antenna. A smaller or larger antenna would require a proportionately smaller or larger shield.

Because such shields have not yet been developed, the need for thorough study is obvious. It is also possible that the shield may somehow be incorporated into the structure of the antenna itself

Near-Earth orbit sites are omitted from the list because (as argued later on) the data processor must be on Earth, and because an antenna moving rapidly relative to the signal processor would seem to present very difficult telemetry problems, considering the SETI need for precise phase and frequency control, and the need for a very great degree of freedom from telemetry and atmospheric noise effects.

Control of miscellaneous interference from neighboring devices such as powerlines, switches, motors, etc., is well understood, but it is not a trivial matter and must not be ignored in the design stages of any SETI system.

In summary, the very real problem of RFI suggests a family of strategies for its alleviation, the nature of a particular strategy depending on a variety of wide-ranging factors such as possible sites, practical antenna characteristics, search frequency ranges desired, cost considerations, etc. There is no easily obtainable "quiet site" in view. On the other hand, the RFI protection problems for some combinations of search system parameters seem to be either minor in nature or at most only a moderate nuisance. This is true because searches are under way now on Earth in some clear bands and can continue for some time before the freedom to search is slowly closed down further by steadily increasing sources of RFI. But the time scale is not tomorrow; rather it is a decade or more in the future. This leaves adequate time for government departments, communication agencies, and so forth to make gradual adjustments toward cooperating with the search effort, thus avoiding exceptional costs and upset plans. Then too, advancing technology hurries obsolescence, so adjusting to the needs of a search system requires mainly thought and willingness, and comparatively small material cost or other inconveniences. It is most fortunate that the next and crucial World Administrative Radio Conference (WARC, see Section III-8) is scheduled for late 1979 and that so far, the water-hole band has not yet been occupied by extensive interference-producing installations such as are present in the bands on either side of the water hole.

 

Directional Search Modes

 

There are two distinct directional search modes: (1) the target search mode and (2) the area or "whole sky" search mode. Preference for one or the other, or for a mixture of the two, as major elements in an initial overall strategy for SETI, is a matter of judgment and practicality. Both modes have been and are being utilized. The point of view espoused here favors a modal mix at first, followed by increasing concentration on target search.

[86] Target search assumes the existence of a known set of most likely transmitting sites. For example, in the Cyclops study it was proposed to observe presumed planets around Sun-like F, G, and K dwarf stars, in ascending order of distance from the Sun. There are about 150 of these within 50 light years, but the number increases to about 1.7x 106 within 1000 light years. Some have argued strenuously that M0-M5 dwarf stars should be included in the target list. This would increase the size of the list by a factor of 2 to perhaps 5. Others have suggested giving first priority to a narrower selection of stars, say, only F5-K5 dwarf stars. Other discrete objects have also been proposed for special study.

The only civilization we know is on a planet orbiting a G2 dwarf star and it is appealing to expect to find other and somewhat similar civilizations on planets revolving around roughly comparable stars. There has been little change in the relevant astrophysical data since the Cyclops study in 1971. As a result, this strategy is widely supported.

Clearly, to minimize effort, target searches should be guided by the latest relevant knowledge in such areas as stellar and planetary formation and evolution, circumstance and origin of life, and cultural evolution. Because of the early stage of our knowledge in some of these fields, an overall search strategy should include a balanced substrategy for increasing relevant background knowledge; such a strategy would likely shorten the time for detection.

At present, astronomical star catalogs can identify perhaps only 0.1 percent of the stars within 1000 light years. A basic catalog listing all stars to 14th or 15th apparent magnitude just has not been constructed. Section III-4 outlines a strategy for generating an adequate Whole Sky Catalog within perhaps a decade and at moderate cost. Such a catalog would be invaluable to general astronomy as well.

In the meantime, the search can proceed by examining the truly local stars which are fairly well cataloged. Our nearest neighbors would seem to deserve rather intense study. Being near, intrinsically weaker radiations are detectable over the entire accessible spectrum. Being few in number, one can afford longer integration times. In any target search scheme, each time sensitivity is noticeably increased, one should reexamine previously observed objects.

The operation of a selected target search is clearly not a simple matter. When the committee or committees, which doubtless will control such matters, sets the following year's list of targets and areas to be searched, a fraction of the observation time should be set aside for trying inspired suggestions, as formal acknowledgment that the establishment is also working in the dark unknown. It is not beyond sensible conception that the first detection will be serendipitous.

Area search specifies nothing about the location of possible transmitting sites in the Universe. It merely characterizes the whole sky as a function of flux level, direction, and frequency.

[87] In Section III-3 an elegant theorem is developed to this effect:

The received (signal) pulse energy is independent of the antenna area and is the energy that would be received by an isotropic antenna in the full sky search time.

This holds as long as all the received data are properly used in the data reduction process. It applies whether the sky is continuously scanned in any essentially nonoverlapping fashion or whether the antenna is used in isolated target search procedures, where a fictitious search time is easily calculated.

From this theorem one can derive an expression for the flux level attainable using an ideal receiver in the real universe and compare its performance with that of a state-of-the-art receiver. This expression is:

 

mathematical equation, (Greek letter) phi (subscript O) = 4 pi(capital)psi m/lambda(squared) t(subscript s), expressed in W/m (squared).(1)

 

where the flux density, ((Greek letter) phi with subscript o) varies directly with the system noise power spectral density ((Greek letter) capital psi = kT) and with the signal-to-noise ratio, (m ), required to keep the false alarm rate due to noise peaks at an acceptable level, and inversely with the wavelength, ((Greek letter) lambda) squared and the full sky search time, (ts )

 

Substituting

 

Greek letter lambda= 0.2 m (1.5 GHz)

m = 25

k = 1.38 x 10-23 J/K

T = 2.7 K or 10 K (ideal or state of the art)

ts = 0.1 yr = 3.1 x 106 sec

 

we find

 

Two mathematical equations: 1) (Greek letter) phi ,subscript o, ideal = 3.5 x 10 to the power -25 in W/m squared; 2) (Greek letter) phi, subscript o, pract. = 1.3 x 10 to the power -24 in W/m squared.(2)

 

In perhaps a year's time, given sufficient wideband data processing equipment (see below), it should be possible to conduct a comprehensive search in the radio astronomy bands over the whole sky and with a sensitivity to narrow band signals many orders of magnitude greater than that used in existing radio astronomy observations. There would then be no need for retrospective studies of existing observations and surveys on the chance that radio astronomers have already observed an artificial signal from some fixed direction in space. Again, if we can search a 300 MHz frequency band at one time, it would take only a few years to search the whole sky over the entire microwave window. This would improve the state of our knowledge by many, many orders of magnitude.

[88] Such whole sky area searches are quick and easy to perform with modest (~25 m) antennas, given the data processing equipment. They might well detect an ETI signal. Furthermore, an attractive dividend of a search throughout the microwave window would be the characterization of the whole sky at these frequencies to a systematic, known set of flux levels, spatial and frequency resolutions. The resulting astronomical data would be valuable, and it does not seem unduly optimistic to expect new discoveries in the spectral line domain, independent of precise frequency prediction.

 

Some Technological Aspects

 

The flux received at the Earth from a transmitter r light years away, per watt of equivalent isotropic radiated power (EIRP) is

 

S = S.89 x 10-34 (EIRP) r -2 (W/m2) (3)

 

A good receiver has a power sensitivity of

 

Pr = 1.38 x 10-23 TsBr (W) (4)

 

at S/N = 1, when the system equivalent temperature lies in the range 2.7 K < TS < 10 K, and the resolution bandwidth (Br) is equal to or greater than the received signal bandwidth as observed over a unit time mathematical equation, (Greek letter) tau is congruent to (Br) to the power -1.

For purposes of this discussion, assume TS = 10 K and that we are searching for mode stable carrier signals, so Br = 0.1 Hz. Then,

 

Pr = 1.38 x 10-23 (W) (5)

 

The disparity between equations (3) and (5) can only be overcome by some combination of real transmitted power (Pt ) and transmission directivity (gt ) (EIRP-Ptgt ) at the source, and effective antenna collecting area (Ae) at the receiving end.

These relationships are illustrated in figure 2 where range in light years (ly) is plotted against effective collecting area expressed in the number of 100-m radio telescopes required. Expressed another way, the range is approximately

 

r = 20[n100(Ptgt /109)]1/2 Iy (6)

 

where n100 is the number of 100-m dishes required if they are 80 percent efficient(Greek letter) eta = 0.8The horizontal bands indicate the ranges likely needed for conditional detection probabilities in the range 0.63 < pc < 0.95 and under four assumptions about the density (N ) of transmitting civilizations in the Galaxy.

Antenna area and resolution bandwidth are interchangeable. The collecting area required dominates system costs to such a degree if high flux level signals are absent, that detection....

 


[
89]

Fig. 2.Major parameters of signal detection.

Fig. 2. Major parameters of signal detection.

 

....becomes, for all practical purposes, a simple matter of carrier recognition. This is so because (so far) we do not have an adequate adaptive matched filter technology for handling more complex coherent signals in a regime where the SNR is close to 1.

Another conclusion is that because optimum carrier detection technology has not yet been brought to bear on the problem, the first instrumental effort should be in this direction. In fact, an improvement by a factor of 103 to 105 over past and present practice can be expected merely by equipping present radio telescopes with better electronic systems.

Finally, the data processing equipment - Fourier transform spectrum processor and pattern recognition system -should be Earth-based even if the ultimate optimum strategy calls for space-based signal collectors. It is this equipment which is most subject to reorganization as optimum strategies evolve. In addition, we note that unlike maser technology, digital data processing technology is still in an early stage of development. There is, as yet, no quasiultimate horizon in view.

 

SUMMARY OF ATTRACTIVE STRATEGIES

 

A review of the requirements for search strategies seems desirable and we summarize them here. A rational strategy should:

1. Concentrate on the most likely alternatives and assign proportionally smaller effort to less likely alternatives;

[90] 2. Start with the smallest system for which a significant a priori chance of success exist and expand with time until success is achieved or until further effort is felt to be unwarranted;

3. Have substantial objectives which can be achieved within a fraction of the lifetime of the generation that begins the search;

4. Have the least cost for a given probability of success;

5. Produce valuable scientific fallout even in the absence of success (see Sections II-6, III-5, and III-6).

Three strategies are required for the guidance of three corresponding, parallel, and interrelated areas:

1. Exploration of the microwave window, both by target search and by whole sky survey.

2. Development of knowledge in relevant scientific areas.

3. Exploration of the remainder of the electromagnetic spectrum.

In each of these areas, a flexibility and multiplicity of approaches should be positively encouraged. Activity in each area should build up gradually from the present status, starting in each case with a survey of the field and emphasis on the rapid development of obviously key items, followed by an efficient buildup to some generally agreed upon steady state level.

Going one step further into details, the following substrategies are defined.

Exploration of the Microwave Window: Initial Phase

1. Place emphasis on the water hole frequency band at least in the beginning.

2. Concentrate first on carrier search technology.

3. Provide suitable low noise electronic systems for Earth-based operations and develop the equivalent for space systems.

4. Develop 106-1010 bin Fourier transform spectrum processors, simple visual, and simple automatic pattern recognition systems.

5. Using target and area search procedures, gain observational experience in characterizing the sky, using items 3 and 4 with existing antennas.

6. Develop and initiate an archival system (see Section III-13).

[91] 7. Carry out design studies for ground and space-based experimental, dedicated, antenna systems, and develop a site-choice strategy.

Relevant Scientific Studies: Initial Phase

1. Plan stellar census construction (see Section III-4).

2. Plan advanced astrometric planetary detection schemes (see Section II-3).

3. Investigate alternates to item 2, particularly direct detection possibilities (see Section II-3).

4. Increase research effort in theoretical and observational investigations of star and planet formation and evolution; in origin of life studies; in pattern recognition; in procedures for recognizing coherent signal statistics under minimum S/N ratio conditions; etc. (see Section II-6).

General EM Spectrum Exploration: Initial Phase

1. Survey the observational needs and strategies required to progressively characterize the entire external electromagnetic spectrum of objects in the solar system and beyond, because signals of intelligent origin could be found anywhere in the spectrum and could be confused with natural background.

2. Begin the development of the most obviously desirable instrumentation for item 1.

3. In view of the number of instances of failure to perceive important new discoveries in data taken for other purposes, and because many discoveries have been serendipitous, measures should be taken to encourage investigators to remain alert to the possibility of ETI artifacts in their data. "Chance favors the prepared mind." L. Pasteur.

Finally, it is important to note that search strategies should always be evolutionary and quick to respond to new experience, new knowledge, new technology, and to new inspiration. At all stages they should be in full view of humankind, and be a reflection of the spirit and intellect of the entire human species.

 


Locations within the galaxy M33, the Great Spiral in Triangulum, where the beam of the Arecibo Telescope has been pointed while searching for ETI signals from any civilizations in that galaxy. The points are superimposed on a map giving the general velocities of stars and interstellar gas throughout M33. With this distribution of points and the telescope bandwidth, every star in the galaxy falls within the coverage achieved with the Arecibo Telescope. Each location was searched for signals for a minimum of 60 sec. At any given instant, about one billion stars were within the beam of the telescope.

Locations within the galaxy M33, the Great Spiral in Triangulum, where the beam of the Arecibo Telescope has been pointed while searching for ETI signals from any civilizations in that galaxy. The points are superimposed on a map giving the general velocities of stars and interstellar gas throughout M33. With this distribution of points and the telescope bandwidth, every star in the galaxy falls within the coverage achieved with the Arecibo Telescope. Each location was searched for signals for a minimum of 60 sec. At any given instant, about one billion stars were within the beam of the telescope.