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

whereb, = (1/2 star power in bandwidth B)/beacon
power.
We see that it is not necessary, with long integration
times, for b, to be less than unity, but merely less than
x/r'ft. However, we have computed the range limits for
the high powers given above. These data are given in
Table 6-2, which includes a microwave system for
comparison. Even with the much higher powers used,
the laser systems are entirely inadequate, primarily
because of the severe limitation on antenna sizes
imposed by the requirement for coherent first detection
(in order to achieve the narrow bandwidths). We can add
the outputs of separate receivers after detection, but, as
we saw in Chapter 5, the range will then improve only as
the square root of the array diameter even assuming the
star noise were spatially uncorrelated. The use of a
100-m array would increase the range of system I to
about 1.4 light-years (and would require 197,500 antennas),
while the range of system II would increase
to about 1 light-year. If we attempt direct photon detection
at 1.061a the receiver bandwidth becomes about
30 MHz at least, b* becomes lO s while wr-h'-becomes
5.5× 104 and the range limit is imaginary.
TABLE 6-2
System I II III
Wavelength 1.06/a 10.6/a 20 crn
Beacon power 1013 W 4X10 I° W 10 9 W
Quantum efficiency 0.4 0.2 0.9
Receiving antenna
diameter 0.225 m 2.25 m 3000m
Receiver bandwidth 30 Hz 3 Hz 0.1 Hz
Integration time 1000 sec 1000 sec 1000 see
Background ratio (b,) 1 1 ---
Noise temperature
20 ° K
Range (light-years) 0.07 O. 16 1360
In going omnidirectional we have given up the major
virtue of lasers-the ability to produce tight beams with
small antennas-but still suffer the penalty of having to
use small antennas at the receiver. One possible way to
use lasers as beacons is to make a huge number-say half
a million-of them and point one at each target star (or,
as already noted at the places where these stars will
appear to be one round trip light time from now). With
l0 s W per unit for a total power of5×10 _° Wand with
receivers like those of optical system A and Infrared
System B (see Chap. V) we could achieve (because of the
longer integration time of 1000 sec.) ranges on the order
of 200 light-years. With greater antenna expense and 50
times as much power we are finally able to contact one
three hundredth as many stars as with the microwave
beacon.
Likely Beacon Frequencies
There are several reasons to prefer the low end of
the microwave window for the acquisition phase. These
include:
1. Smaller Doppler shifts as noted above
2. Less stringent frequency stability requirements
3. Greater collecting area for the narrowest usable
beam
4. Reduced cost per unit area of collecting surfaces
5. Smaller power densities in transmitter tubes,
waveguides, feeds and/or radiators, thus allowing
higher powers per unit
6. Greater freedom from O2 and H20 absorption,
which may well be more on some planets having
our life forms
Reasons for preferring the high end of the window
include:
1. Reduced spectrum clutter
2. Smaller transmitter antennas to get a given directivity
(for Doppler corrected beams)
Neither of the last two reasons seems as compelling as
the first six. The transmitting antennas are in any case
much smaller than the receiving array, and spectrum
utilization can be programmed to clear certain bands at a
time as the search involves them. Thus it appears likely
that the search should be concentrated in the region
from perhaps 1000 MHz to about 3 GHz. This is still an
enormously wide bandwidth to be combed into channels
1 Hz wide or less. So the question arises as to whether or
not there is a more sharply defined region of the
spectrum where there is a common reason to expect the
search to be located.
Several years ago Cocconi and Morrison (ref. 3)
suggested that the hydrogen line (1420 MHz) was a
cardinal frequency on which to listen. Their argument
was that this is a natural frequency known to all
communicative races, and one on which a great many
radio astronomers are busily listening. Interstellar Doppler
shifts might amount to +10 -a so we would still have
to search on the order of a 3 MHz bandwidth, but this is
still one thousandth the task of searching a 3 GHz
bandwidth.
This suggestion provided the initial impetus for
Project Ozma in which Frank Drake and his associates at
the National Radio Astronomy Observatory at Green
Bank, West Virginia, listened for about 400 hours in
April and June of 1960 for evidence of artifact signals
from two stars in the 10 to 11 light-year range: e-Eridani
b3