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

12. CYCLOPS AS A BEACON
Although the Cyclops array was conceived as a large
aperture receiving system, nothing inherent in the design
prevents its use in the transmitting mode as well. This
would allow radar astronomy to be performed to greater
precision and over greatly extended ranges as discussed
in Chapter 14. It also opens the possibility of using
Cyclops as an interstellar beacon. Of course, transmission
and reception could not go on simultaneously.
Nevertheless, if our first search of the nearest 1000
target stars produced negative results we might wish to
transmit beacons to these stars for a year or more before
carrying the search deeper into space. We could then
re-examine these stars at the appropriate later times
looking for possible responses.
There are two natural ways to use Cyclops as a
beacon: to focus the entire array on a single star, and to
train each element on a different target star. The second
method would require a different coding method for the
antenna positioning information than the one described
in Chapter 10, which permits only a few subarrays.
However, the added cost of providing completely independent
positioning for each element would be very
small. It is therefore of interest to determine Cyclops's
beacon capabilities in both modes.
Let us assume that the nominal Cyclops array of a
thousand 100-m dishes is equipped with a 100-kW
transmitter at each antenna. The total transmitted power
would then be 100 MW. (To achieve this total would
probably require on the order of 300 MW of power to be
fed to the array over a somewhat heavier distribution
system than that specified in Chapter 10.) At an
operating frequency of 1.5 GHz (X = 20 cm), the
effective radiated power with the entire array aimed at
the same star would then be 2.5×1017 watts. With the
elements used individually as beacons the effective
radiated power for each would be 2.5× 10 _1 W.
Of course, we do not know all the characteristics of
the receiver that might pick up our beacon signals.
However, if it is intended for radio astronomy or deep
space communication, and the other race is technologically
advanced, it is reasonable to assume a noise
temperature
of 20 ° K or less. Using a valHe of 20° K we
can then plot the reference range limit as a function of
the receiver antenna diameter and bandwith.
Figure 12-1 shows the ranges for various antenna
diameters when the entire array is focused on a single
target star. We see that a small receiver with a 10-m
diameter antenna and a 10-kHz bandwidth could detect
the Cyclops beacon at a range of about 80 light-years. A
100-m antenna would extend the range to 800 lightyears
or permit detection at 80 light-years with a 1 MHz
bandwidth. We also see that with a 1 Hz receiver bandwidth,
the Cyclops array beacon could be picked up
at 50 light-years with no reflector at all, merely on an
isotropic antenna or dipole. The significance of this is
that the signal might be detected by a search system at
this range even if the latter were not pointed toward the
Earth.
I
,°° i
ISOTROPIC _ _ :
RECEIVER
"_..
I01 I0 2 10 3 10 4 )0 5
RECEIVER BANDWIDTH, HZ
Figure 12-1. Range limits oJ 100-roW, 3-kin Cych_ps
array used as a beacon.
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