|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
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
|
|||
|
|||
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. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
....AN OPEN LETTER TO THE IGNORANT MASSES !!!!! | jonathan | Astronomy Misc | 11 | November 8th 06 10:40 AM |
Open Letter to Sky and Telescope | Larry Curcio | Amateur Astronomy | 22 | December 31st 05 01:12 AM |
Open letter to Mr Joseph Min | jacob navia | Astronomy Misc | 0 | December 15th 05 07:03 PM |
Open letter to Rod Mollise | Joe S. | Amateur Astronomy | 4 | December 13th 04 06:23 PM |
an open letter to Pete | Christopher M. Jones | Policy | 2 | February 22nd 04 09:56 AM |