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

practical limits of narrow beamwidths in the microwave
region as well, the photon energy disadvantage shows
through. But there are additional specific disadvantages
that are revealed by each of the laser systems studied.
Optical system A suffers from too little energy per bit. If
its pulse energy were equal to that of the other systems,
its range would increase over 10 times. Optical system B
does not adequately override star noise, so about 30
times as many signal photons must be received per pulse
as for optical system A and, because of the filter loss,
the received energy per pulse is only 25 times as great at
the same range. Infrared system A suffers from the
inefficient array utilization because of incoherent addition
and the spontaneous emission noise associated with
each receiver. Finally, infrared system B suffers from the
small size of receiving antenna possible in a coherent
system
at 10.6/a.
Except possibly for infrared system B, all the laser
systems would appear to be far more expensive than
microwave A. Certainly to provide two acres worth of
optical collecting area in the form of precisely steerable
2- to 5-m telescopes would cost much more than a single
l O0-m dish. All the systems require cooled detectors.
Optical B requires costly precision narrow band filters at
each antenna. Infrared A requires precision local lasers
and mixers at each of 1975 antennas, while the precision
needed for the local oscilla{or of infrared B may be
beyond the capabilities of even the most advanced
technology.
The mechanical stability and precision problems of
the laser systems have their counterpart in the precision
of electrical control needed to compensate for Doppler
drifts. The drift due to Earth diurnal Doppler could be
as great as 3 kHz/sec at 10.6/,( for stars near tile meridian
and a station near the equator. Locking infrared system
B onto such a rapidly drifting signal would be a
nightmare. The total interstellar Doppler shift is about
_+10-3 so the filters in the optical systems would require
tunability over a +-300 GHz range while the infrared
systems would need a -+30 GHz tuning range.
Finally, laser systems suffer from atmospheric absorption
even in clear weather and are unusable in cloudy
weather. Sky light does not materially affect the
performance of any of the systems, except for optical
system B. In the daytime, the background count from
the sky would reduce the range of this system from 24
to about 20 light-years. Thus all systems would be usable
in clear daytime hours as well as at night, but the
microwave systems are the only all-weather systems.
In summary
1. The sole advantage of lasers is that they permit
narrow beamwidths to be realized with small
transmitting mirrors, but this advantage turns into
a disadvantage at the receiver where we need a
large collecting
area.
2. The collecting surface is much cheaper and more
durable in the microwave region, because the
tolerances are much greater and polished surfaces
are not needed.
3. Microwave systems offer substantially more range
for the same power, even with wider beamwidths.
4. Because of the wider beams, automatic positioning
and tracking are feasible at microwaves on a dead
reckoning basis.
5. Doppler shifts and drift rates are orders of
magnitude less at microwave frequencies and can
easily
be compensated.
6. Microwave systems are all-weather systems.
We have gone to some length to compare microwaves
and lasers because the ease of obtaining sharp beams has
led many people to propose laser systems (ref. 3). We
have made the comparison for point-to-point communication
links where the transmitting directivity of the
laser is used. When we come to consider the search
problem in the next chapter we will see that transmitter
directivity is not an asset, and lasers lose out completely.
In fact, we believe that if lasers had been known for the
last hundred years and microwaves had only recently
been discovered, microwaves would be hailed as the long
sought for answer to interstellar communication.
COMMUNICATION RATE OF A MICROWAVE LINK
At the range limits given, the links compared in the
last section are all 1 bit/sec systems with a bit error rate
of 0.07865. As long as the received energy per bit is held
constant, the error rate will not change. Thus the
connnunication rate varies inversely as the square of the
_,
,ookw7s,_o-_ / / / gi/ .-ry
F 2ookw2._,,o -2 / / /._'/.-"/7
Lmlk
I I01 10 2 10 3 10 4 I0 5
CLEAR APERTURE DIAMETER, m
FigureS-8. Information rates of microwave links;
X = 3 cm.
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