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;

2. Start with the smallest system for which a significant a priori chance of success exists 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).

7. Carry out design studies for ground and space-based experimental, dedicated, small 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 beamwidth, 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.

6. THE SCIENCE OF SETI

SETI is a manifestation of man's drive to explore. This drive is one of the oldest and most fundamental aspects of our nature; the very origin of the hominidae as a distinct biological entity is owed, at least in part, to the boldness of our venturesome simian ancestors who abandoned their familiar forest environment to probe the savannah, there to seek fleet-footed prey. Our forebears pushed into almost every corner of the globe. They explored by climbing hills, by walking through forests, and even by crossing large bodies of water. Sometimes they may have had in mind some material purpose, but certainly they sometimes went where they did for no other purpose than to see what was there. Modern man still explores, but the arena for his exploration is now the planets and the stars beyond. But we are not limited to physical exploration. We can use the fruits of our intelligence to conduct exploration from a distance. Though some day we may wish to build space ships to travel there, to probe the stars now we need only telescopes. Yet the excitement and exhilaration that comes with this kind of exploration, as larger telescopes and more sensitive and sophisticated data acquisition techniques lead to discovery after discovery, is akin to that the Viking seamen must have felt.

Exploration has always required knowledge and understanding of the physical world. Viking boats could not have been built without knowledge of what wood to use; Viking navigation could not have been accomplished without an understanding of the winds and tides. We call this understanding of nature, which we gain from observation and experiment, scientific knowledge. To explore the stars in search of other intelligent life also requires scientific knowledge; indeed, because it can only be done using highly sophisticated technologies and methods, it requires more scientific knowledge than have man's classical explorations.

A SETI program should embrace not only a search for evidence of other civilizations, such as radio signals, but also a wide range of related scientific studies. We need knowledge of nature primarily for two purposes. One of these is to enable us to narrow the scope of the search by distinguishing promising volumes of search space; for example, we might be better able to identify promising target stars or frequency bands. The other purpose is to enable us to be able to interpret any evidence of other civilizations we obtain, and to decide what course we should follow once we are sure that other intelligent life has been discovered.

The scientific knowledge needed by a SETI program can perhaps best be illustrated in the context of the Drake equation, which relates the expected number N of intelligent, technologically advanced communicative species in the Galaxy to the product of several factors. It should be understood that the Drake equation is not a fundamental expression of the way nature behaves, as is, for example, the deceptively simple law, f= ma. Rather, the Drake equation is simply a device to enumerate the factors that influence N and hence must be considered in any attempt to estimate this number. One form of the Drake equation (there are several) is

N = R✩fgfpnefififcL