A few days ago I looked in on a SETI run to see what it was doing (using the little graphics thingie), and I noticed a definite artificial structure in the sample. Instead of being a signal sitting on top of the noise, it was a groove-like structure, a trough in the noise. You could see little echoes of the trough in the noise field surrounding it. I noticed that the base frequency was right at 1.40000000 GHz, but it was doing a frequency shift. It was late at night and I was tired, so I didn't think to note the date of the sample, I figured that the SETI post-processors would catch it if it was important.

A few days later I was thinking about it and realized that most signal processing looks for signal sticking out of the noise field, but it probably isn't looking for a trough in it. Has anyone else seen anything like this? Do you think it might be something?

Pardon my ignorance if this is a newbie question, this is my first post here...

This is normally a known type of terestrial interference. However it might be a good idea for another SETI project to look for the lack of signals where they would be expected.

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BOINC WIKI

BOINCing since 2002/12/8

This could be an effect of the "radar blanking" that is being applied to some of the data recorded at Arecibo.

See Technical News archives for more information. A nearby military radar site somtimes causes interferance, which can cause many of the -9 overflow "found too many pulses" problems.

SETI staff are now compensating for some of this local Terrestrial noise by applying a blanking compensation.

This may be what you have seen.

Keith.

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Sir Arthur C Clarke 1917-2008

I wasn't aware of the blanking to compensate for known terrestrial signals. The thought had occurred to me that if some extraterrestrial civilization was able to create an anti-noise signal and then project it through astronomic distances, _that_ would be a neat trick, indeed! But alas, a processing artifact is the more likely explanation...

At Arecibo there's a radar installation nearby. The Seti crew has implemented a way of blanking out these bogus signals in the calculations. I've heard that a mechanical radar shield was going to be installed but I haven't heard anything more about that.

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At Arecibo there's a radar installation nearby. The Seti crew has implemented a way of blanking out these bogus signals in the calculations. I've heard that a mechanical radar shield was going to be installed but I haven't heard anything more about that.

Actually more than one, but not really nearby. There's a technical discussion with interesting plots from a test in May 2006 which indicates a large part of the problem is the radar signals bouncing back from airplanes, so arriving from any direction. Any shield would have to go all the way around the dish, be quite high, and absorbent rather than reflective.

Anyhow, the blanking is done by replacing the data recorded during time periods affected by radar with pseudo-random data. That should not cause an observable trough in either the frequency or time direction.

The original post specified 1.40000000 GHz base frequency, but we're only looking at data from 1.41875 through 1.42125 GHz. I suspect the trough was actually at 1.42000000 GHz., at that center frequency the math of FFTs produces a DC component so the splitter compensates by notching out that frequency. That trough has been seen and reported several times.

                                                                Joe

The original post specified 1.40000000 GHz base frequency, but we're only looking at data from 1.41875 through 1.42125 GHz. I suspect the trough was actually at 1.42000000 GHz., at that center frequency the math of FFTs produces a DC component so the splitter compensates by notching out that frequency. That trough has been seen and reported several times.

You're correct. As I noted in the OP, I was quite tired at the time...