Re: OT:SETI

So rather than just keep spouting off on this I figured it might be
interesting to put some numbers behind the discussion to better
illustrate the issues. A tool for the interstellar link budgets is
the site that came up last year when we had this discussion,
specifically

http://www.satsig.net/seticalc.htm

FWIW, he (Eric Johnston, the guy that put the site together) assumes a
carrier frequency of 10.701 GHz for whatever reason, and that's a
reasonable number for at least a starting point. As has been
discussed, moving up in frequency can be used to improve the antenna
gain, or reduce the antenna size (to make it more practical).

I've not verified his link budget computations, I'm only assuming that
the computations that the site does are correct.

The default numbers that come up on the site provide the scenario for
closing a link to Gliese 581, one of the closest, if not the closest,
known planetary system beyond our own. It does so assuming a 50m dish
on each end of the link at 10.701 GHz, 60kW, and 1bps. Using such a
low data rate (and that's the uncoded rate), allows power
concentration in frequency but also requires a demodulator good enough
to survive the phase noise at such a low rate.

Perhaps the toughest part of these assumptions is the antenna pointing
problem. Using, interestingly enough, another calculating site by the
same guy, Eric Johnston, specifically this one:

http://www.satsig.net/pointing/antenna-beamwidth-calculator.htm

a 50m dish at 10.701GHz has a 3dB beamwidth of only 0.04 degrees. This
means that BOTH antennas need to be simultaneously pointed at each
other in two dimensions with an accuracy on each end of 0.04 degrees,
tighter if one doesn't want to risk losing 3dB of link margin.

Shrinking the dish only on the earth side to 5m relaxes our pointing
requirements to 0.45 degrees to keep the target within the 3dB
beamwidth. Unfortunately, this increases the required transmit power
from 60kW to 6MW. If the aliens want to also use a 5m antenna and
relax the pointing requirements, the power goes up to 600MW. Still
the antennas would have to be simultaneously co-tracked within 0.45
degrees.

If we assume that the "waterhole" frequencies are used as was
suggested in http://www.setileague.org/general/waterhol.htm, an L-band
transmitter at 1.5GHz would make sense. At this frequency a 50m dish
has a 3dB bandwidth of 0.28 degrees, which is still tough, but
comparable to the 10.7GHz case. However, the loss of antenna gain at
this lower frequency means that even with the 50m dish on each end of
the link and simultaneous precision pointing, a 3MW transmitter would
still be required.

Using a smaller dish at 1.5GHz, say 10m, to ease construction and
pointing, provides a 3dB beamwidth of 1.39 degrees but requires a
1.8TW (yes, TeraWatt) transmitter. Again, that's for 1bps uncoded
and a 5dB C/N theshold.

And this is all to talk to the closest known candidate. If nobody's
home there and we have to increase the distance to 41 light years to
reach 55 Cancri, we're looking at 10MW for the 1.5GHz case with 50m
dishes, and 200kW for the 50m dish at 10.7GHz.

Constructing a 50m dish at 10.7GHz requires a surface accuracy of
about 3mm in order to get the expected gain. That's doable, but to
get that, maintain it, point it (a 50m dish is pretty massive), and
keep the transmit power up at the required level is no mean feat.
Assuming that the aliens are pointed exactly at (within a small
fraction of a degree) a terran receiving antenna long enough to
receive a message of any appreciable length at 1bps is also no mean
feat. Assuming that the aliens are using that type of narrow
modulation so that you coud detect it is yet another narrow
assumption. Assuming that they're transmitting right when you're
pointed at them is, well, hopeful.

Could it happen? I suppose so, but clearly (well, IMHO) the effort is
fueled by romanticism and hope rather than anything very believable.
And this analysis only covers the closest know planetary systems, and
most of the galaxy and universe are well beyond those.

Personally, I think they'd just use email. That seems a lot easier
to me.

;)
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
.

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