From: Jim Choate <ravage@EINSTEIN.ssz.com>
Date: Mon, 9 Nov 1998 05:10:18 +0800
To: cypherpunks@EINSTEIN.ssz.com (Cypherpunks Distributed Remailer)
Subject: Re: IP: Discover Alien Life With Your PC And SETI (fwd)
Message-ID: <199811081829.MAA05664@einstein.ssz.com>
MIME-Version: 1.0
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Forwarded message:

> Date: Sat, 7 Nov 1998 20:39:58 -0800
> From: Steve Schear <schear@lvcm.com>
> Subject: Re: IP: Discover Alien Life With Your PC And SETI

> Perhaps I'm missing something, but it seems to me that if I were an alien
> civilization and wanted to send out a beacon, in as wide an angle as
> possible, across the vast reaches of space and overcome as much of the path
> losses as possible using the least energy I certainly wouldn't use a narrow
> band signal. Quite the contrary, I'd want to spread a low bandwidth
> information signal across the widest practical spectrum. Its much easier to
> increase process gain (the ratio of the baseband information signal to the
> final carrier bandwidth) than transmit power.

The problem becomes the 1/r^2 losses. The receivers on this end have fixed
lower end to their sensitivity that sets a minimal effective energy output
at the transmitter. If you do the math you find that the entire Earth
doesn't generate enough energy to send a spread-spectrum wide-band signal
more than a few light-years out, not worth the effort or the social costs
(nobody'd know about it because radio and television and such wouldn't be
available since we're using all *that* energy for sending the signal).

This might work for a hive mentality society but not for people.

> While a narrow band signal from Arecibo's powerful transmitter/antenna
> combination can be detected at a distance of about 300 light years.

Since it's only been transmitting intermittenly about 30 years that's more
than a tad moot.

> Switching to a spread spectrum approach could allow broadening the antenna
> pattern, and thereby its chances of detection, significantly without
> reducing its effective range.

Spread spectrum won't effect the beam angle of the dish, that's set when the
dish is designed - spread spectrum or not. Also you'll need to build a new
set of LMB's since the old ones won't be able to handle the frequency range
or the gross power input required to keep the new signal at a given
frequency at a respectible power level with the other frequencies we're now
operating on in parallel. That baby's gonna get hot.

> Notice how 63 dB (or over 2,000,000 fold
> effective increase in transmit power) of process gain enables handheld GPS
> receivers to pull in signals from satellites, sent using only a few watts
> of transmit power, without much of an antenna.

It's amazing how much you can pull in when you increase the sensitivity of
the front-end and use mechanisms to reduce noise.

> If all this seems to make sense, then why are the SETI people apparently
> seaching the skies with lots of narrow band receivers? They don't seem to
> be employing any broadband correlator techniques, so spread signals will
> probably be missed.

Then you haven't been keeping up with the Million Channel Receiver system
that has been operating at Aricebo for several years. You also haven't been
keeping up with the folks out at White Sands either who also impliment
receivers of the same spread-spectrum type. Just about all the current big
money (there ain't any since the feds cut the budget several years ago)
projects use this system and the small-money and individual projects also
impliment it but at a lower range of bandwidth sampled.

It isn't that these folks don't want to use the latest technology it's that
they can't.....$$$.


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