Project Cyclops, A Design... - Department of Earth and Planetary ...

The price of incomplete sky coverage is hard to
assess; the one intelligent life form accessible to us
might lie in the regions of the sky not covered. About all
we can say, a priori, is that the probability of contact is
roughly proportional to the fraction of the sky covered,
with a slight premium attached to the portions of the
sky toward the galactic center, because of the increasing
stellar density in that direction. Under this premise, an
element mount that reduces the sky coverage by a factor
of two should result in a cost saving of at least a factor
of two to be worth considering.
SITE
SELECTION
If Cyclops were to be located in the territorial
United States, sites along the southern border deserve
primary consideration. In addition to having a low
latitude, the site should have the following characteristics:
1. Geologic stability. The area should not be subject
to earthquakes nor lie across faults. The land
should not be subsiding nor warping. Otherwise,
the geometry of the array will be disturbed and
require resurvey and reprogramming of coordinate
data for the phasing and delay.
2. Low relative humMity. Atmospheric turbulence in
the microwave region is determined primarily by
inhomogeneities in the water vapor content of the
air. For good "seeing" the air masses should exist
in horizontal layers each of uniform composition.
High, dry plateaus are preferable to moist low
lying regions prone to cumulous clouds and thunderstorms.
3. Mild, calm climate. High winds deform antenna
surfaces and cause loss of gain. Heavy snow loads
require stronger more expensive structures to
avoid permanent deformation.
4. A large plane area. The array need not be level, in
fact, in northern latitudes a slight tilt toward the
south is an asset. However, to avoid distortions in
the imaging process, and complications in the
delay and phase shift computing programs, the
array should be plane or nearly so.
5. Remoteness from habitation and air routes, preferably
ringed by mountains. The Cyclops system is
of necessity an extremely sensitive detector of
weak radiation. Although the antenna gain would,
in general, be small for radiation generated within
several thousand kilometers of the array, the low
noise receivers can pick up weak interference even
without the antenna gain. The problem is worse
for Cyclops than for the usual radio telescope
because of the high spectral resolution and because
interfering signals may have strong coherent
components of just the type we are searching for.
All in all, a remote site in the southwestern United
States seems indicated. The Very Large Array (VLA)
siting study is pertinent for Cyclops, and any of the sites
recommended in this study should be considered.
RECEIVER
SYSTEM
Since we concluded that the low end of the microwave
window is best suited for interstellar acquisition
and communication, the Cyclops receiver system was
designed to cover the frequency range from 0.5 to 3
GHz. The antenna elements and system design would
very likely permit higher frequency receivers to be used
for radio astronomy, deep space probe communication,
and radar astronomy, but these higher bands are not
believed essential to the primary mission.
To avoid having to transmit the entire RF spectrum
back to the central station, heterodyne receivers are used
at each antenna. These convert the received RF band to
a fixed (IF) band for transmission. The use of heterodyne
receivers requires that local oscillator signals of
precisely known phase be available at each antenna. In
the proposed Cyclops system these are synthesized, at
each antenna, from two standard frequencies, which, in
turn, are distributed from the central station. The
technique used for distribution is an extension and
refinement of the two-way transmission technique described
in the VLA report. The Cyclops technique
virtually eliminates the residual errors present in the
VLA system. We believe that local oscillator signals with
phase errors of only a few degrees at 10 GHz can be
generated throughout the Cyclops array using the
proposed
method.
Two primary design requirements of the Cyclops receivers
are low noise and remote tunability. Halving the
receiver noise is equivalent to doubling the antenna area.
The remote tunability is a practical requirement; with a
thousand or more antennas, local retuning of each
receiver would be virtually impossible. Band changing
must occur at all antennas in response to a single
command over the control system. The proposed design
achieves both these requirements by the use of cooled
up-converters followed by a fLxed frequency maser. It is
believed that with the appropriate cryogenic cooling this
combination can yield noise temperatures as low as
20 ° K. To change receiver bands the local synthesizers at
the antennas are simply commanded to synthesize a new
pump frequency, and, if necessary, the up-converter and
feed horn are switched. This approach to radio telescope
front end design is believed to be novel and should find
application in other telescopes and arrays where flexible
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