Imágenes de páginas
PDF
EPUB
[blocks in formation]

The time required to search the water hole for all F, G, and K stars within

100 light years = ~1 month 1000 light years = ~50 years

CONCLUSIONS

The time required to search the entire sky for CW signals is inversely proportional to the minimum detectable flux level and, for large relative bandwidth, increases as the cube of the highest frequency. By replicating optimum systems, dollars can be traded for time. If enough systems are built to keep the search time constant the cost varies as the 3/2 power of the sensitivity.

A full-sky search of the water hole could be made to a flux level of 1024 W/m2 in only 12 days with an optimum system. To search the entire 1 to 10 GHz region to the same sensitivity would require 17 years, half of which would be spent searching between 8 and 10 GHz. Evidently if the entire microwave window is to be searched, the sensitivity should be reduced as the operating frequency increases. If the minimum detectable flux level is made proportional to v2 the search time is proportional to the frequency range covered rather than the cube. This would permit covering the 1 to 10 GHz region at from 5X1025 to 5X1023 W/m2, respectively, in about 1 year. This assumption of proportional to v2 is consistent with the assumption that the signal we will detect is being beamed at us, for then = PTAT/R2X2. Hence the full sky search covers the cases of beamed or very strong omnidirectional signals originating at great distances: the "giant" or "supergiant" transmissions of very advanced cultures.

APPENDIX A

Assume that the amplitude of the illumination of a circular antenna of radius b is

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]

The decreasing effective area and gain with increasing v are the result of illumination being confined more and more to the central region. We can compensate for this by letting

[blocks in formation]

where a is independent of v. The clear aperture case corresponds to v = 1. The scaling given by equation (A3) gives every case the same effective area and gain as a clear aperture of radius a. If we now let v → ∞, equation (A1) approaches

[blocks in formation]

The far field amplitude patterns are the Hankel transforms of (A1) and (A4) and, for finite v, are

f1(0) = ^,,(ko)

where k = 2πb/λ and A,(z) = v! 2oJ,(z). For the Gaussian case = ∞

ƒ(0) = e ̄802/8

(A5)

(A6)

where g = (2πa/)2 is the on-axis gain. The half-power beamwidth, ẞ, is twice the value of that makes A2 = 1/2 or e ̄80/4 = 1/2. We find

[blocks in formation]

If we tessellate the sky with hexagons of width B between opposite sides, each will have a solid angle = (√3/2)32 so the required number is n = 4π/. For the cases listed N

1.22 g <n<1.39 g

If we scan the sky in strips of width ẞ, the effective dwell angle per direction is

[merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
[blocks in formation]

1. Oliver, Bernard M.; and Billingham, John: Project Cyclops, A Design Study of a System for Detecting Extraterrestrial Intelligent Life. NASA CR 114445, 1972.

[graphic]

Two parabolic reflector antennas, forming the research and development site of NASA's worldwide Deep Space Network, stand out vividly against the primitive beauty of Southern California's Mojave Desert. The bowl-shaped location provides natural protection from man-made radio interference. Both antennas are equipped with Cassegrain feed systems and are steerable from horizon to horizon. The dish diameters are 26 m (85 ft) and 9 m (30 ft). The larger uses a low noise maser preamplifier to achieve a very high sensitivity for an antenna of this size. Antennas such as these can be used for sky and frequency SETI surveys. Depending on frequency, such antennas can perform all-sky surveys over plausible microwave regions with several orders of magnitude better sensitivity than has been generally achieved. The Deep Space Network is managed and technically directed for NASA by the Jet Propulsion Laboratory of the California Institute of Technology at Pasadena, California.

« AnteriorContinuar »