SETI Re: ET's problem with Doppler.


David Woolley (david@djwhome.demon.co.uk)
Fri, 10 Sep 1999 22:35:50 +0100 (BST)


> Because of this I would take a fair guess that ET will
> sweep frequency +- some few MHz either side of the

If we went that far off the hydrogen line we would almost
certainly jam other radio users, probably including several
satellites.

> Hydrogen line, in order to be certain that anyone,
> anywhere, no matter what their relative speed to him,
> will always receive a signal on 1420 MHz. at some point
> in their listening. If you wish to contact someone then
> you would go to some slight extra trouble to greatly increase
> your chance of success, would't you?

Actually it will reduce the chance of success, because it will
generate a signal with a high chirp rate, which is difficult
to detect with simple processing. The fact is that it is quite
easy to design receivers that simultaneously search across
several MHz with a sensitivity which is only really constrained
by their stablity and the noise bandwidth; so compensating for
source unknown Doppler elements can be done at the receiver without
compromising detectablity, but sweeping all possible frequencies at
the tranmitter will make detection difficult.

> So, are we all wasting valuable PC time in listening at all
> those many 10Hz bins etc. so advocated? Would a much
> more sensitive search be done by just listening to 1420 MHz,
> and wait until ET's TX signal 'sweeps' by?

(Note this refers to Project Argus, whilst seti@sni.net is a general
SETI list.)

For most people, doing this analysis does not stretch their PCs
very much at all, so one could hardly call it wasting valuable
time.
 
> In fact, perhaps we only need to monitor somewhere near
> to 1420 MHz, and await the Doppler effect on ET's varying
> freq. signal to sweep him right into our waiting laps!

Actually one of the problems with standard Argus stations is that
they don't allow for the local Doppler chirp, but a deliberate
source chirp designed to do anything other than cancel out the
source's acceleration in the transmission direction, would exacerbate
this and require searching in chirp space for maximum sensitity
(the reason that (the reason that SETI@Home is so slow is that it is searching a
rather large range of chirp space - arguable much to much).

> As most/all of you are 'bin' searchers, I guess my argument
> is flawed somewhere.....but I'm surely tempted to stay right
> by the 'waterhole, and dream of Doppler!. :-)

There are several frames of reference in which a nominal hydrogen
line signal could be compensated, e.g. source solar system centre
of gravity, galactic centre of gravity, primary target's star's
motion, and even no compensation. The third is only applicable
if we are the primary target.

The other factor, of course, is that the neutral hydrogen line is
a guess at the psychology of the sender. So far we haven't transmitted
on anything like it.

[ Deleted from UK group ]



This archive was generated by hypermail 2.0b3 on Sun Oct 10 1999 - 15:46:34 PDT