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

11. SIGNAL PROCESSING
The Cyclops system as it has been specified in the last
three chapters amounts to a very large radio telescope
with an effective clear aperture of a few kilometers,
capable of simultaneous reception of both orthogonal
polarizations of a received signal over a I O0-MHz band.
The received band can be quickly tuned anywhere in the
low end of the microwave window and, if desired, could
be extended to higher frequencies in that window. The
system up to this point is a very high resolution, high
sensitivity instrument that would find many applications
in radio astronomy, radar astronomy, and space probe
communications. Each of these applications would
require further processing of the signals delivered by the
phased outputs of the array. Much of this processing
would use standard equipment and techniques peculiar
to the application involved. These will not be discussed
here.
This chapter is primarily concerned with the signal
processing techniques and equipment needed to allow
the Cyclops system to carry out efficiently its primary
mission of detecting signals originated by other intelligent
life-spectrally narrow band information-bearing
signals. A second concern of this chapter is the techniques
and equipment needed to form wide band images
of the radio sky, and thereby greatly speed the construction
of detailed maps of natural sources. The chapter
concludes with a discussion of interfering signals the
Cyclops system must contend with and what might be
done about these.
THE DETECTION OF NARROW BAND SIGNALS
We concluded in Chapter 6 that signals of intelligent
origin, and particularly signals from intentional beacons,
were most likely to contain strong, highly monochromatic
components. We saw that these coherent
components would probably be best detected using a
receiver having a predetection bandwidth on the order of
0.1 to 1 Hz, but that to search sequentially across the
spectrum with such a narrow band receiver would result
in prohibitively long search times per star. What we are
seeking, therefore, is a receiver with some 108 to 109
channels each 1 or 0.1 Hz wide so that we can monitor
the entire 100-MHz IF band simultaneously. In this
section, we describe such a receiver, but before doing so
we will dispose of some alternative methods that have
been proposed.
Total Power Detection
In principle we do not need to divide the spectrum
into narrow channels to detect the increase in total
power in the IF band produced by a coherent signal in
one (or more) channels. All we need to do is to integrate
for a long enough time to be able to measure the
increase over the noise power alone that is produced by
the signal. A little reflection shows that, while possible
in principle, such an approach is totally impractical.
Suppose there is a coherent signal that doubles the noise
power in a 1 Hz band. This produces an increase of 1
part in l0 s in the total noise in a 100 MHz band. To
detect such a tiny increase would require integrating
101_ samples of the wide band noise. (See e.g.,
equations (8) (9) and (10) Chap. 6). This is an
integration time of 10 s seconds or roughly 3 years. Even
accepting this time, the method fails, for we cannot
assume we know the noise bandwidth to this accuracy
nor that it is this constant, nor that the system noise
temperature is this constant, nor even that the radio sky
is this constant.
Cross Power
Detection
Simpson and Omura I have proposed that, instead of
a simple noise power measurement, a measurement of
=NASA unpublished report, 1970.
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