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View Full Version : A new "WOW signal"?



jumpjack
2011-Feb-07, 09:24 AM
http://apod.nasa.gov/apod/ap110206.html

At the beginning I thought this was one of the two "alien" signals just detected as coming from one of the 54 "earth-like" planets just discovered by Kepler:
http://shrewdraven.org/webfm_send/126

Then I figured out this image is from 2006...
Are instead these 2 new "WOW signals" available to the public?
And why does SETI need a signal is repeated more and more to be able to analyze it?!? Don't they record the signals for further processing?!? :?

Elukka
2011-Feb-07, 01:54 PM
If they didn't record them they couldn't make pretty pictures to post on APOD. :p
I believe the reason they need it to be repeated is to make sure it's really an alien signal. From the description of your first link, it says a likely possibility is that the signal was "an unusual modulation between a GPS satellite and an unidentified Earth-based source".

JustAFriend
2011-Feb-07, 09:44 PM
Ever watch or read "Contact"?

You have to get another hit when the Earth is in another position so that you can confirm the signal is truly coming from outside the Solar System and not something local.

It would help if we had radio telescopes scattered throughout the system, but we're stuck with the stuff here right now.

jumpjack
2011-Feb-08, 08:28 AM
Ever watch or read "Contact"?

Both. :)


You have to get another hit when the Earth is in another position so that you can confirm the signal is truly coming from outside the Solar System and not something local.

So we wasted our time by sending Arecibo signal just once? ;-)

djellison
2011-Feb-08, 08:56 AM
So we wasted our time by sending Arecibo signal just once? ;-)

Basically, yes. The chances, even if our galaxy is teeming with life, of someone listening, at the right frequency, at the right time, in the right direction is so mathematically remote as to be barely worth considering.

baric
2011-Feb-08, 01:27 PM
Basically, yes. The chances, even if our galaxy is teeming with life, of someone listening, at the right frequency, at the right time, in the right direction is so mathematically remote as to be barely worth considering.

Sounds like a Hollywood script to me.

flynjack1
2011-Feb-08, 05:54 PM
Hence multiband recievers on a huge array...AKA Allen Array.

kamaz
2011-Feb-09, 11:31 AM
And why does SETI need a signal is repeated more and more to be able to analyze it?!? Don't they record the signals for further processing?!? :?

There's recording and recording... Alright, a crash course in signal processing:

How do we transmit signals on the radio? We take an audio signal we want to transmit (20kHz band) and superimpose it on a carrier signal (say 100MHz) in a process known as modulation. (There are of course several ways to do this). For a simple FM modulation (here's an image: http://upload.wikimedia.org/wikipedia/commons/d/df/Frequency-modulation.png), this process results in a signal occupying a (100MHz +/- 20kHz) frequency band. When you "tune in" to a radio station with your receiver, you just tune the oscillator inside to the carrier frequency (100MHz). This carrier, together with the radio signal received from the antenna, is then fed to the demodulator, which outputs an audible 20kHz signal which is then send to the speaker. Here's the diagram: http://frrl.files.wordpress.com/2009/01/amfm_radioblock.jpg

Now, suppose you want to detect a radio station, but don't know its (carrier) frequency. What do you do? Well, you take a machine called a spectrum analyzer. You know that the station is somewhere between 80 and 120MHz. The machine goes from 80 to 120MHz in small steps (say 1Hz) and measures signal level at each frequency. Recall, that the radio station occupies the range between 100MHz-20kHz and 100MHz+20kHz. So once you reach 100MHz-20kHz, you'll see an increase in signal level (above the noise level). The signal will peak at 100MHz (carrier), and slowly decrease back to the noise level when reaching 100MHz+20kHz. It will look something like this: http://beaconreviews.com/transceivers/images/Frequency_M2_minus63.jpg Once you have found the carrier frequency, you just tune in with your radio.

And this is precisely what SETI is doing. They take a 100MHz wide band and run it through a spectrum analyzer working in 0.01Hz steps. Next, they average the output over ~1000 seconds. The averaging step improves carrier detection, but also destroys any possibility of actually recording the signal. One reason for doing it this way is that, since SETI antennas are not aimed at particular targets, but observe a wide field, the recorded signal is going to be very weak anyway, so there's not that could be done with it except saying that it's there.

So the basic problem is that SETI is hunting for carriers, not signals. Apparently the idea is that once they locate a carrier, they can target the source with big dishes, "tune in" with different equipment, record the signal at high resolution and try to decode it.

The issue with "Wow" signals? They just show up and disappear in a few seconds, before the SETI people get any chance to actually investigate them.

KaiYeves
2011-Feb-10, 02:03 AM
When was this signal recorded? The APOD link doesn't say.

jumpjack
2011-Feb-10, 07:53 AM
Thanks Kamaz, wonderful explanation.

Ross 54
2011-Feb-13, 01:22 AM
Actually, the very same image of a 'waterfall' signal display has been a NASA 'picture of the day' at least three times over the years, and with exactly the came caption attached. The earliest I'm aware of was in July, 2002. This signal was detected by a member of the SETI League, date not given, using amateur SETI equipment ( dishes on the order of 3 to 5 meters diameter). The SETI League has a substantial list of such one shot detections on their website, including images of their displays. Without repeat observations it is not possible to ascertain what made the signals, ETs, our own satellites, or whatever. The 1977 wow signal didn't repeat either, but fit the time a fixed sky source would take to cross through that telescope's beam so well that its believed the source would have to have been at least as far away as the Moon. Know satellites were ruled out, and this not a very likely distance for a secret (military surveillance) satellite.

jumpjack
2011-Feb-13, 09:28 AM
I think a good solution to prevent "false positives" would be using "stereo" antennas: two antennas pointing at same part of the sky, some kilometers apart, would receive same signal only if coming from outer space rather than Earth orbit.
But this would also double the costs...

Ara Pacis
2011-Feb-16, 11:46 PM
Another way to reduce false positives is to use antennae/dishes in space.

Blackhole
2011-Feb-20, 06:54 AM
Hubble was launched to take images of far off galaxies, Kepler was launched to observe the light emissions of far off stars to look for orbiting planets. I think NASA needs to launch a satellite into space that specifically focuses on detecting radio wave signals from space.

baric
2011-Feb-21, 01:21 AM
Hubble was launched to take images of far off galaxies, Kepler was launched to observe the light emissions of far off stars to look for orbiting planets. I think NASA needs to launch a satellite into space that specifically focuses on detecting radio wave signals from space.

Our atmosphere does not block radio waves so we can observe them much more easily (and cheaply!) from Earth. The huge radio dish arrays that we can build across a countryside could never be duplicated in orbit.

kamaz
2011-Feb-24, 08:45 PM
Our atmosphere does not block radio waves so we can observe them much more easily (and cheaply!) from Earth. The huge radio dish arrays that we can build across a countryside could never be duplicated in orbit.

However, NRO does (allegedly...) put 100m radio telescopes in space, so in principle, a couple of these could be used to build a large interferometer. There was a thread on it before: http://www.bautforum.com/showthread.php/109858-Putting-radio-telescopes-in-space