archive: Re: SETI Fwd: Scientist: Reason for Optimism in ET Search

Re: SETI Fwd: Scientist: Reason for Optimism in ET Search

David Woolley ( david@djwhome.demon.co.uk )
Sat, 22 Aug 1998 11:56:24 +0100 (BST)

John Marcus wrote:
>
> In a message dated 8/21/98 6:37:04 PM Eastern Daylight Time, AOL News writes:

There were two copies of this, one with HTML constructs but a text/plain type.

> > Traditional thinking dictates doing this by using a powerful and expensive
> > transmitter or through the magnification or 'gain' from a very large antenna
> > dish. This produces an inefficient and expensive technological tradeoff.
> >
> > Cohen says the two are likely to be linked as an optimized, aperture
> > engine. The transmitting antenna also is a stellar power collector and

"aperture engine" reads like a marketing label, rather than a technical
description, although I think I understand it to mean a phased array,
in space, with distributed power amplifiers, and with each amplifier getting
its power from solar cells in its neighbourhood.

It is still very large, and has lots of 'gain'. The news clip doesn't
justify why it is a better proposition than a large dish, although
detailed construction considerations may mean that it is cheaper to
build for the same ERP.

(The main uses of pure phased arrays that I'm aware of are military radar,
where the beam can be steered quickly, and domestic environments where
a dish is cosmetically unacceptable (e.g. the squarial for the proposed
second DBS satellite system in the UK - because a second dish would have
caused planning problems - this is not the whole story).

Steerability without physical rotation might be an advantage here - I
believe that one of the problems with the largest earthbound telescope,
Arecibo, is you can only steer it a small amount, by moving the feedpoint.
But if that was a key advantage, the journalist completely missed it.)

> > thousands of stars at a time", explains Cohen, " also you have to be smart
> > enough to know that the changing, thin plasma of space requires a special
> > spread spectrum method called polychromatic SETI." At the moment, says
> Cohen,

Again "polychromatic SETI" reads like a marketing label, not a technical
description. Does anyone have a translation into the core technical
features? The two guesses I can think of are:

- signals on an established insterstellar channel might use a modulation
system like that used in the HF amateur and commercial system, Clover,
where there are a lot of low baud rate sub-carriers, rather than one
high baud rate one (some early high speed modems used this tactic).
In that case, we would be talking of strategies to detect such
transmissions (the advantage of this system in HF radio is that it
tolerant of variations in propagation delay with frequency) - Argus
probably is not in the running for such chance reception;

- dithering of the received frequency (presumably at the 10s of mHz level,
not the 10s of Hz level) would produce a noise like result in an FFT
frequency analyser, with channels narrower than the dithering range.
There may be an algorithm that can detect that can discriminate
between a signal wandering in frequency and one that occupies all the
frequencies at once.

> > ongoing Earth SETI's are receive-only and fall substantially short of
> > optimized efforts. The most comprehensive survey has looked at less than 700

I think that this indicates that this is really a political stance,
rather than a criticism of privately funded efforts. I.e. he's saying
that SETI needs the level of funding that can only come from public funds
and that governments should encourage active SETI.