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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.....$$$. ____________________________________________________________________ To know what is right and not to do it is the worst cowardice. Confucius The Armadillo Group ,::////;::-. James Choate Austin, Tx /:'///// ``::>/|/ ravage@ssz.com www.ssz.com .', |||| `/( e\ 512-451-7087 -====~~mm-'`-```-mm --'- --------------------------------------------------------------------