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

modest size.A largedatabaseontapeordiskgivingthe
coordinates of hundreds of thousands of target stars is
needed to automate the search, but the data rates are
slow enough so that only a relatively small central
processor is sufficient. A great deal more fast storage
capacity is needed for data processing than for controlling
and monitoring the entire array.
The system power and communication (telephone)
needs have also been assessed. The power requirements
are substantial and, in view of the remote location, may
require a dedicated power-generating plant. The costs of
this have been included. The telephone costs have also
been included but are a negligible part of the total
system
cost.
COMBING THE SPECTRUM FOR SIGNS OF LIFE
As described so far the Cyclops system is simply a
huge phased microwave antenna array that could be used
for many purposes. And indeed, if built, it would be! It
is the signal processing system of Cyclops that both
distinguishes it and qualifies it for its unique primary
mission. The signals we are searching for are earmarked
by their coherence. They will cause very narrow,
needlelike peaks in the power spectrum that may drift
slowly with time but can be seen if we can resolve the
power spectrum sufficiently and follow the drift. As
delivered, in the 100 MHz band, the needles are literally
buried in the haystack of receiver and sky noise. Yet the
proposed signal processing system will find a signal even
it" its coherent power is 90 dB below the total noise
power in the IF band.
The first step in the signal processing is to transform
the received signal (amplitude versus time) so as to
obtain successive samples of its power spectrum (energy
versus frequency). This converts the nearly sinusoidai
waveform of any coherent signal to a "needle" in the
frequency domain. The second step is to add the power
spectra under a variety of offsets between adjacent
samples to allow for any reasonable drift rate the signal
may have had during the observation time. In one of
these additions, the one that matches the drift rate, the
signal "needles" (which in any given sample may still be
inconspicuous compared with the noise peaks) will all
add to form a spike that is clearly above the noise level.
Thus, the final step is to determine whether any of the
added spectra contain spikes above a certain threshold.
In the proposed system the IF signals are first
subdivided by filters and heterodyne mixers into bands
from 1 to 10 MHz wide, depending on the bandwidth
capabilities of the subsequent equipment. The time
signal in each of these subbands is then recorded as a
continuous raster on photographic film. A constant bias
is added to prevent negative values of amplitude. After
processing, the film passes through the gate of an optical
Fourier transformer where it is illuminated by coherent
light. The amplitude distribution in the aperture plane
(f'llm gate) is transformed by a lens into the signal
spectrum in the image plane. The intensity of the light in
this image plane is the power spectrum of the signal
sample in the gate at any instant of time. The power
spectrum is automatically displayed in raster form also.
Thus, the full two-dimensional Fourier transforming
power of the lens is used for a one-dimensional signal.
Each line of the power spectrum represents a frequency
band equal to the scanning frequency used in the
recording process. The frequency resolution is the
reciprocal of the time represented by the total segment
of the signal in the gate. Thus, if a l MHz band has been
recorded, using a I-KHz sweep frequency, and if 1000
lines of the raster are in the gate at any time, this
represents one second's worth of signal. The power
spectrum will also consist of 1000 raster lines, each of
which represents 1 kHz of the spectrum, displayed with
a resolution of 1 Hz.
Optical spectrum analyzers with a time-bandwidth
product of 10 6 are currently available. Two hundred
such units would be needed to comb the 200-MHz total
IF band (both polarizations) into l-Hz channels. Analyzers
with a time-bandwidth product of l0 7 are
believed to be within the state of the art. No other
known method of spectrum analysis even approaches the
capability of the optical analyzer. (It is interesting to
note that, in principle, the Fourier transformation can
take place in about 10-_ 1 sec. The time need only be
long enough to allow about 5000 cycles of the coherent
light used to pass through the film. Shorter times would
broaden the spectrum of the coherent light too much.
Thus, in principle, the optical spectrum analyzer can
handle about 1018 data samples per second. No practical
way of utilizing this speed is known.)
In the proposed system the power spectrum is imaged
on a high resolution vidicon tube, where it is scanned
and converted into a video signal, which is then recorded
on magnetic disks. As many as a hundred or more
complete power spectra, representing successive frames
of film in the gate, are recorded for each observation.
The power spectra are then played back simultaneously
and added with various amounts of relative delay
between successive spectra. This is accomplished by
sending all the signals down video delay lines and adding
the signals from taps on these lines that are disposed in
slanting rows across the array of lines.
In each observation of a star we will thus record 200
MHz of IF signal for something on the order of 1000
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