[105237] in Cypherpunks
Re: IP: Discover Alien Life With Your PC And SETI (fwd)
daemon@ATHENA.MIT.EDU (Steve Schear)
Mon Nov 9 00:40:02 1998
In-Reply-To: <199811081829.MAA05664@einstein.ssz.com>
Date: Sun, 8 Nov 1998 21:22:45 -0800
To: Jim Choate <ravage@einstein.ssz.com>
From: Steve Schear <schear@lvcm.com>
Cc: cypherpunks@cyberpass.net
Reply-To: Steve Schear <schear@lvcm.com>
>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).
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.
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.
>
>> 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.
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.
>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.
The LNB must already have considerable bandwidth, otherwise SETI's million
channel parallel receivers would be useless.
>
>> 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.
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.
>
>> 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.
Can you provide a citing to SETI use of SS correlation techniques?
--Steve
