[105245] in Cypherpunks
Re: IP: Discover Alien Life With Your PC And SETI (fwd)
daemon@ATHENA.MIT.EDU (Jim Choate)
Mon Nov 9 01:15:21 1998
From: Jim Choate <ravage@einstein.ssz.com>
To: cypherpunks@einstein.ssz.com (Cypherpunks Distributed Remailer)
Date: Sun, 8 Nov 1998 23:59:56 -0600 (CST)
Reply-To: Jim Choate <ravage@einstein.ssz.com>
Forwarded message:
> Date: Sun, 8 Nov 1998 21:22:45 -0800
> From: Steve Schear <schear@lvcm.com>
> Subject: Re: IP: Discover Alien Life With Your PC And SETI (fwd)
> >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
> Not true. Arecibo claims it currently has enough transmit gain to allow its
> narrowband signals to be detected by an analogous system 300 light years
> distant.
And so what of it. Comparing a beam only a few degrees across versus a beam
that's say 180 (horizon to horizon) is apples and oranges. Do the math,
integrate the requirements of Aricebo over the entire hemisphere. Calculate
source transmitter levels such that at 100 lyr the effective field strength
is receivable with Aricebo. Don't forget that's 3-D space.
> As for spread spectrum not affecting effective reciever sensitivity...
> Without SS GPS just couldn't plain work. The satellites have a very modest
> energy budget and could not send a sufficiently powerful signal to work
> handheld receivers with small stub/patch/slot antennas. But add 63 dB of
> process gain and its a horse of a different color. As it stands GPS systems
> routinely work far below the noise floor (for narrow band signals) of the
> recivers. That's what makes them so difficult to intercept. If you don't
> know the modulation technique, spread code or time-base the signals are
> just undetectably small amounts of noise.
I used to do LORAN, I am familiar with the integration tricks that are
required to work down in the grass. The bottem line is that such technology
requires a very high gain low noise rf front end and mixer and well known
timing characteristics.
I'll have to disagree about the difficulty of detecting GPS signals.
> Pay attention. I didn't say it would broaden the pattern. I said it would
> ALLOW broadening the antenna pattern. This might be accomplished by
> slightly lowering or raising the feed outside the prime focus.
I'm assuming we're still talking about parabolic Aricebo type dishes...
Broadening the antenna pattern is a function of physical antenna design, not
modulation techniques. Spread specturm, FM, AM whatever for a given
operating frequency the beam of a given antenna is pretty fixed.
> The LNB must already have considerable bandwidth, otherwise SETI's million
> channel parallel receivers would be useless.
It isn't a question of bandwidth. It's a question of raw power. In such a
system we're talking about effectively channeling a considerable number of
MW's out through a point source. That's going to take some decidedly
non-standard design I suspect.
> Yep, but they probably won't pull in narrowband ET broadcasts unless the
> aliens have incredible transmit power and are aiming their antennas square
> at our region of the sky and we happen to be looking when they do.
True, but they're not that large a consideration in a practical situation.
The dishes we have commenly available are themselves rather narrow band. So
while it takes a while they can paint nearly the entire sky. Since it's
also reasonable to suspect that once a society begins transmitting it will
continue to do so for around a hundred years (if they're anything like us
that is) so if we don't catch it this week we have a good shot at it next
week. The reality is that if the signal is short and singular then even a
broad band wide angle search could likely miss it in the noise. So it turns
out that even a narrow band reciever setup is sufficient for reasonable ETI
searches so long as the beam width and search rate taken together keep any
given point in the sky on beam for several seconds (long enough to get a
snippet and recognize the repetition).
> >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.
>
> Can you provide a citing to SETI use of SS correlation techniques?
Not off the top of my head unfortunately.
____________________________________________________________________
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The Armadillo Group ,::////;::-. James Choate
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