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

needto process this spectral information in some
automated system in a manner that will assure detection
of any faint coherent signals that may be present and yet
not give too many false alarms because of noise alone. In
this section we will describe some possible ways of doing
thi_ processing.
The power spectra will show a Boltzmann distributed
background intensity due to noise. This background will
differ in detail from sample to sample but the statistics
are stationary and the long-term average at any point
will approach a constant value. If there is a strong
coherent signal present, there will be a bright spo t in the
spectrum at the frequency of the signal. If the coherent
signal is weak-comparable in power to the noise power
in a resolvable interval-the intensity in successive
samples will fluctuate, being sometimes stronger, sometimes
weaker than the average noise brightness; but the
average intensity over many samples will be greater than
that produced by noise alone. Thus, if the coherent
signals showed no frequency drift, a simple detection
scheme would be to integrate the power spectrum for a
sufficient time and then scan it for points having a
greater intensity than that likely to be produced by
noise alone.
Unfortunately, because of Doppler rates and inherent
source instability, we must expect any coherent signal
spot in the power spectrum to drift in position with
time. This means that if we are to detect weak drifting
signals we must accumulate the power spectra with
successive samples shifted by a constant amount frame to
frame corresponding to an assumed drift rate. Since we
do not know the drift rate a priori we must do this
accumulation for a range of assumed drift rates, positive
and negative, up to the maximum rate expected (or up
to the maximum rate that will allow full response in the
spectrum analyzer). Because Doppler drift rates change
slowly with time and because interstellar sources,
particularly beacons, may be assumed to be inherently
rather stable, we probably need only allow for constant
drift rates during any observation period.
Our problem is illustrated nicely by Figure 11-7,
which is actually a photograph of a pulsar pulse. Each
scanning line in the picture is the power output of a
receiver as a function of time. The receivers in adjacent
scanning lines are tuned to adjacent fiequency bands.
Because of dispersion in the interstellar medium, the
lower the frequency to which the receiver is tuned, the
tater the pulsar pu_:_ arrives, lfwe covet all but _me line at
:_ time, i_ b._co.n.e_,evident that the pulsaJ v oi_Id not be
'.'isib!e h_,;he ,_!p
,t of any single receiver. Y_.:-_i!s signature
stamts o'_' cie,:_ly in faster of traces prod_;ced by all
_'.,e receivers.
°
Figure 11-7. Signature of a pulsar produced by simultaneous
observation on ad/acent frequency channels.
(Photograph courtesy of Martin Ewing, Calif. Inst. of
Technology)
For our purposes we can take the photo to represent
the intensity versus distance along a particular raster
line, in the output of an optical spectrum analyzer, for
successive samples of the power spectrum. The noise
background has the same significance as before, but the
"pulsar" is now a coherent signal drifting at a constant
rate. Again it is evident that this signal could not be
detected in any single sample of the power spectrumthat
is, any single line of the photo-but the drifting
trace stands out clearly in the raster of lines.
If the photo were to be scanned with a vertical slit,
which moved horizontally across the picture, and the
average brightness measured through this slit were
plotted versus displacement, the resulting curve would
show a small fluctuation about a mean value. So long as
part of the signal trace were in the slit, the fluctuations
might appear to be about a slightly higher mean value,
but it is doubtful if this difference would be visible. If
the slit were now tilted to be parallel to the signal trace
and again scanned horizontally, the same sort of
fluctuations about a mean value would occur until the
trace lay entirely within the slit. At this point, there
would be a large pulse in brightness clearly in excess of
the normal noisefluctuations. By scanning the picture in
this fashion we have converted the pattern, clearly
visible to the eye, into a waveform clearly "visible" to
threshold decision circuits. This is the principle we
propose for use for the atttomatic detection of coherent
signals.
Oplical
Processing
!i _mc '.,.'ere simply to, fl.,!_rgraph successive samples
,)f the p_:xv,v spcctru:n 2p,d ._r,"a'lge to p:ojcct #z such
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