From: william edmondson (w.h.edmondson_at_cs.bham.ac.uk)
Date: Wed Sep 29 2004 - 04:36:44 PDT
Hi Pete
On 29 Sep 2004, at 04:18, Pete Heist wrote:
>
> William Edmondson wrote:
>
>> (http://www.cs.bham.ac.uk/~whe/SETIPaper.pdf)
>
> That was an interesting paper William. Thanks for
> sharing, and may your dish time leave you intrigued.
>
> I have a couple of questions that are more about the
> detection of pulsed signals in general than
> specifically about the method described, so they're
> open to anyone (omnidirectional, "weak intent"
> questions, to make a strained parallel).
>
> First I'd like to restate the proposal to verify that
> I get the gist. Pulsars may be used to determine the
> period and direction of possible pulsed signals as
> follows:
>
> P ----- E ----- ETI
>
> P = Pulsar with period A
> E = Earth
> ETI = Transmitting ETI
>
> We at Earth point our dishes opposite P and look for
> signals pulsed with period A (perhaps on the hydrogen
> line, why not)
See additional email
> . Did I get it?
On the button. Note additionally that 'Schneider mode' is when the
alignment is
E ------ ETI ------ P
and we look 'just past' ETI at the beacon pulsar. The signals at
period A from the ETI, in this alignment mode, look artefactual because
a) the signal does not have the spectral characteristic of the pulsar
(that would be wasteful of energy) and b) the directional sensitivity
of the receivers on E is good enough to separate ETI and the pulsar on
the sky. Should they overlap exactly, note, we might have found ETI
already.
>
> My questions are:
>
> - How much does knowing the period of a pulsed signal
> practically help us to detect it? Verification I
> understand.
We can tweak our signal processing algorithms to look for that sort of
periodicity (note, we cannot know the pulse rate 'exactly' but only
reasonably well, because the habstar//ETI's star system// may have real
motion relative to us - but we can cross check that in the visual
spectrum). Incidentally, one pay off is that provided the ETI doesn't
try to compensate for its own orbital motion then we can learn lots
about that - year length, day length, number of moons.....
If we are looking for fast pulse rates (recall the fast pulsars are
FAST - millisecond periods) then we may also have to use special
receivers/processing to look for such signals. Pulsar hunters do that
as a matter of course - SETI folk tend to look for CW and may not have
the response times necessary for sharp pulses (a 1ms pulse rate with
m/s of 1/10 means you need 20kHz bandwidth if my arithmetic serves me
right). No doubt I'll be put straight if that comment is out of line.
>
> - Even if we know the period, we don't know the phase.
> What kind of challenge does that present in detection,
> if any?
None so far as I know - we aren't looking for phase alignment with the
originating pulsar. Of course, in Schneider mode the ETI would not
want its transmitted pulses to be in phase with the real thing.
>
> cheers,
> Pete
>
>
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