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open letter to the seti institute



 
 
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Old December 18th 06, 12:12 AM posted to sci.astro.seti
Chris
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Posts: 24
Default open letter to the seti institute


SETI
Is the present form of the search for extraterrestrial signals likely to
succeed?
I attach, below a letter sent to the SETI Institute.

Dear Sir,
I see that you are attempting to find carriers in your search for
extraterrestrial signals. I have a little knowledge of radio and TV
transmission theory. Our most modern radio and television signals do not
emit a carrier. The SSB signals (Single Side Band) have no detectable
carrier and will not be detected by your search. The Mpeg signals sent by
digital television also have no carrier. FM when used properly also does
not have a carrier and is equivalent to SSB.
These signals do have something in common, they contain repeated sequences
of data. If you want to detect this sort of signal below noise level (as it
will be) then a broad band (20KHz for sound and 4MHz for pictures) detector
is needed. The noise is then recorded and compared with itself in sections
of various periods with previously collected noise within the same band.
The waveform of the noise over periods of 1 sec, 1 minute, 1 hour, is
compared with similar periods over the total length of time of the recoded
data.
If you compare each section (effectively by sliding the signal recording
along a copy of itself and looking for autocorrelation
(=Integral(S(t)XS(t-p)Xdt)) (over t now to t start) where p is the time
shift. Alter p and plot the autocorrelation against p. For a significant
signal there will be peaks in the graph, these may be at regular intervals.
This interval could mean something and will be the first clue of an
organised signal. Over a long integrating time, signals below noise level
would be detectable.
I hope this reaches the right people and is acted on. I see no hope for
the project if you only look for carriers. You do have plenty of past data
to analyse, so I suggest that this is the place to start.
Chris,


  #2  
Old December 18th 06, 01:16 AM posted to sci.astro.seti
David Woolley
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Posts: 32
Default open letter to the seti institute

In article ,
"Chris " (forged, really ) wrote:

Is the present form of the search for extraterrestrial signals likely to
succeed?


Short answer. It will probably only succeed in setting reduced upper
limits for what signals are available to detect, but it is the best
that can be done given the current technology, and, possibly more
importantly, the current amount of real estate dedicated to radio
astronomy.

I see that you are attempting to find carriers in your search for
extraterrestrial signals. I have a little knowledge of radio and TV
transmission theory. Our most modern radio and television signals do not
digital television also have no carrier.


They are well aware of this.

FM when used properly also does
not have a carrier and is equivalent to SSB.


Unmodulated FM produces a narrow band signal at the nominal carrier
frequency, whereas unmodulated single side band suppressed carrier has
no signal when unmodulated.

These signals do have something in common, they contain repeated sequences
of data. If you want to detect this sort of signal below noise level (as it


Not really true of (suppressed carrier) SSB, except in as much as human
vowels sounds have strong frequency components which are shifted by
mixing with the carrier).

will be) then a broad band (20KHz for sound and 4MHz for pictures) detector
is needed. The noise is then recorded and compared with itself in sections
of various periods with previously collected noise within the same band.


Errrrrr! Auto-correlation is a standard technique for detecting
carriers! The repetition in the frequency domain is equivalent to
a weak leakage of carrier. Unfortunately, the amount leaked is very
much smaller than a full carrier, so is undetectable for reasonable
broadcast transmission powers. Actually even AM and vestigial sideband
(i.e. analogue TV) entertainment broadcast carriers are too weak to
detect with currently available antenna sizes, at insterstellar distances.

(I am also pretty sure that about the most efficient way of doing an
auto-correlation is to run a fast Fourier transform!)

What the SETI Institute's project Phoenix, and UCB's Serendip and
SETI@Home projects (note that the SETI Institute is a, SETI searching
organisation - there is no *the* SETI organisation) are currently
searching for is transmissions that are intended to be easily detectable.
We have a very few signals that would also be detectable, but the
characteristic that makes them detectable is that they are unmodulated
(although in some cases chirped) carriers. Unfortunately, any detection
would not be repeated so would not be confirmable.

 




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