11. Studies of biological evolution: The basic mechanism for biological evolution, namely mutations that alter the structure of the DNA molecule in living systems, is understood in a general sense. However, the full panorama of events that might affect or control mutation rates, the tolerance of living systems to mutation rate and to environmental factors is not well understood. Studies in these areas could be of significant value in the field of molecular biology.

12. Studies of cultural evolution (see Section II-2): We are just beginning to appreciate the complexity and diversity involved in the interactions between cultures and individuals within a culture. Studies of social science and the behavior of organisms of various levels of biological complexity are clearly a significant aspect of a SETI program, perhaps most significant following a successful search for extraterrestrial intelligence. Attempts to understand the evolution of, and factors controlling, behavior will not only provide useful input to a SETI program, but would benefit the disciplines of psychology and psychiatry.

13. Radar astronomy and deep space communications: Data processing techniques including the strong capabilities in spectrum analysis will be very useful for deep space communications, permitting multiple spacecraft monitoring with small antennas, thus releasing larger aperture facilities from routine tasks. In addition, the application of this technology to monitoring radio frequency interference will greatly assist deep space operations in our crowded spectral environment.

Although no transmitters are presently being considered for any interstellar search system, their inclusion might have enormous advantages with regard to planetary radar astronomy and large distance deep space communications. The great potential of such a system warrants its consideration and further studies should be undertaken.

This listing is not intended to be, nor could it be complete. The intent is to provide some indication of specific areas of scientific research that would provide useful input to a SETI program while also making a major contribution to man's general storehouse of knowledge.

247-831 O 78 13

A PRELIMINARY PARAMETRIC ANALYSIS OF SEARCH SYSTEMS

This study was carried out during 1975 and 1976 by the Stanford Research Institute (SRI), Palo Alto, California. The study was concerned with evaluating the comparative cost effectiveness of several different microwave receiving systems that might be used to search for signals from extraterrestrial intelligence. Specific design concepts for such interstellar search systems were analyzed in a parametric fashion to determine whether the optimum location is on Earth, in space, or on the Moon.

System evaluations were performed in terms of a great number of parameters, including the hypothesized number of transmitting civilizations in the Galaxy (N), the number of stars that must be searched to give any desired probability P of receiving one of these signals, the required antenna collecting area, the necessary search time, the maximum search range, and the overall cost. The underlying principle of the method of parametric analysis was that systems are compared only after performance is normalized by requiring that all systems have the same probability of detecting a given signal for any postulated value of N. In practice, this performance normalization was accomplished by requiring that each system examine the same number of stars for any given case (as defined by the values assumed for P and M). Systems with limited sky coverage thus need to search to greater ranges than do systems with more complete sky coverage to examine any given number of stars.

For systems on Earth, primary consideration was given to a Cyclops-type array of conventional parabolic dishes and to an Arecibo-type array of fixed spherical dishes. Preliminary consideration was also given to a Cyclops-type array with a solar power capability. This system has the advantage that costs can be offset by the sale of electricity, but its design must be worked out in detail in a future study. Arecibo-type arrays were found to be slightly more cost effective than Cyclops-type arrays under the assumption that sinkholes are freely available for many dishes to be constructed in the same manner (and for a comparable cost) as the existing dish. This assumption clearly breaks down for large numbers of dishes, and in a future study the possible additional cost of excavating should be evaluated.

For a system in space, a concept was developed that consists of a spherical primary reflector made of lightweight mesh, with three Gregorian subreflectors and feed assemblies that permit three different stars to be searched simultaneously. A disk-shaped shield placed between the antenna and the Earth protects against radio frequency interference (RFI); a series of relay satellites permits the IF signal to be beamed to Earth for processing and to transmit instructions from Earth for station-keeping and attitude control. A number of orbital locations are possible, but for the SRI analysis it was assumed that the system would be at the L3 libration point of the Earth-Moon system, a point at lunar distance but on the opposite side of the Earth from the Moon. Among several desirable features of an orbiting antenna system are the complete sky coverage, the ability to track a given star continuously if a signal is detected, the reduced system temperature, and the potential structural simplicity and longevity made possible by the weightless and benign environment.