archive: Re: SETI modulation of ETI transmission

Re: SETI modulation of ETI transmission

David Woolley ( david@djwhome.demon.co.uk )
Sun, 30 Aug 1998 13:17:35 +0100 (BST)

> That is exactly what I meant. I read in the book "Contact" that Hadden
> said that pm doesn't mess up the spectrum a lot and requires a phase
> correlator. If it is leakage, it might appear to be a fluke.

In the book, the problem was that the carrier WAS detectable, so it would
be a hit from the point of view of the Argus search, but there were
two levels of modulation on top of it. The first was polarization modulation,
which is probably easy for a radio astronomer to handle, but is not used
in normal earthly communication systems. The second was phase modulation,
which is used on earth, but isn't something that a radio astronomer would
need to deal with. On the other hand, I think that it would, in reality,
have been detectable in the analysis phase, as I think, in reality,
coherent detectors would have been used, and the phase modulation would
have been converted to some level of amplitude modulation. What you
might find is that, for a low modulation rate, the receiver would shift
to the new phase, and you would only get blips at the bit edges.

>
> I also had a strange idea. Is it possible for another civilization to
> move just perfectly so there is no apparent doppler shift. This might
> make a candidate signal look terrestrial. I am not to familiar about the

It's possible, but unlikely. In addition, the source would move relative
to a fixed antenna, so would pass a beamwidth test, and would be blocked
by the earth twelve hours a day.

What's more likely is that the signal would be pre-corrected to cancel
out the source's doppler variations (or for an established link, possibly
to cancel those of the target - planet positions in the solar system are
known with extreme accuracy over the likely flight time of such a signal,
so such correction is certainly possible if the recipient sends the
necessary ephemerides).