In article <40958A0B.F39AD023@nospam.com>,
"Andrew Nowicki <andrew@nospam.com>" <> wrote:

Mark wrote:

At least we'll know that aliens with big non-directional
radio transmitters are rare. 

We have already learned that.

Even if we did detect a signal, we couldn't tell if it was directional
at source.  (We are almost at the point of being able to detect our
analogue TV carriers at interstellar distances (see the 1 sq km array
below), and those, whilst directed to try and avoid wasting power, 
are not strongly directional.

What we need is a non-directional microwave receiver
on the far side of the Moon. It would be a sort of
phase-array radar in reverse -- lots of small receivers

I would consider this to be multiple directional beams,
rather than non-directional reception.  As far as I know, the
1 square km array, which will be an earth based equivalent, won't
normally attempt to form as many beams as there are elements, but
rather a small number.

However, such an array is beyond the wildest dreams of even
conventional radio astronmers, let alone cash starved SETI 
searchers.  As well as the problems of surveying and placing
each element, the large amount of electronics involved will
result in a system that will start to lose elements quickly and
have no maintenance man to replace them.

but no directional antennas. To reduce the noise, it
would be used only during lunar night, when the surface
of the Moon is cold.

Modern receiver noise temperatures are generally a long way below
ambient.  The cryogenically cooled SERENDIP (S@H) receiver was actually
replace by a non-cooled one with a lower noise.  I believe the total
system noise for this receiver is about 45K, of which about 12K is 
galactic noise and big bang backround, which can't be eliminated.

Whilst I argue in another thread, that a pure phased array transmitter
is future, rather than current technology, it is not that far off that
it wouldn't be reasonable to assume that an ETI would form multiple
beams to allocate the total power more effectively to likely targets.

The advantage of the non-directional receiver is that
it can detect signals coming from a broad solid angle.

But only at the cost of doing a two dimensional fourier transform,
and then doing the followup signal processing a number of times 
comparable to the number of elements.  Remember that processing capability
for space qualified equipment is rather moderate compared with 
what is on your desk top.

Incidentally, with respect to the subject, there was concern amongst
the professionals that the general public might expect a contact from
S@H when the professionals, whilst hoping for one, thought that setting
new upper bounds was the more likely outcome.  This was before it went
fully live.  The concern was that there might be a backlash from the
modern instant gratification culture, that might prevent funding of
more sensitive future work.