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Nebula :
Finding persistent non-barycentric signals (work in progress)
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David Anderson Send message Joined: 13 Feb 99 Posts: 173 Credit: 502,653 RAC: 0 |
SETI@home is supposed to find persistent non-barycentric signals. We currently do a bad job of this:
Our current algorithm for finding non-bary multiplets goes like this:
This is what I'd call a "wishful thinking" algorithm. I developed it based on a mental model of what the data looked like, rather than on actual data. A while back I added web features for plotting (in time/freq space) the detections making up non-bary multiplets. When I got around to actually looking at these plots, I realized that the algorithm wasn't working at all. The multiplets jumped all over the place in frequency, way too fast to be from any plausible transmitter, and with none of the sinusoidal structure we'd expect. My brilliant algorithm was outputting garbage. Aside: in developing Nebula I wrote algorithms first, then created data visualizations. In retrospect, I should have done it the other way around. Anyway, let's think about what a non-bary multiplet should look like. The orbital shift would be sinusoidal (if the orbit is circular; a bit different if it's eccentric, but we'll ignore that for now). The rotational shift would also be sinusoidal, most likely with a smaller amplitude and higher frequency. The shift depends on the nature of the star/planet system; the overall width can pretty big, like hundreds of KHz. We'd expect to see detections (e.g. spikes) scattered along this frequency/time curve. So when I saw the garbage multiplets my algorithm as finding, my first instinct was to think about curve-fitting: take the set of detections and fit (in the least-squares sense) a sine wave (or a sum of 2 sine waves, if you want to include rotation) to them. I spent some time looking at libraries that did this: GSL (which seemed impossibly complicated), NLopt, and ALGLIB. I wrote programs to test these with actual sinusoidal data, with and without noise. And with sporadic (but regular) data: This is a nice idea but there are problems:
At this point I did what I should have done in the first place, and added a web feature for showing all the detections in the pixel disc in the 200 KHz band - i.e. the input to the non-bary multiplet finding algorithm. This showed that, in each observation interval, there were tons of detections randomly distributed across the entire frequency band. Here's the data for the multiplet shown above: There were no well-defined clumps. The algorithm was finding essentially random clumps, tossing out a few to enforce the long-term consistency constraint, and calling that a multiplet. And it was clear that fitting sine waves to this data would be meaningless. It was clear that we had to somehow "thin out" the data. Eric and I decided to do this by restricting our search to signals from F and G type stars. Transmitters in this case would be accelerated relatively slowly, so the chirp rate of the resulting detections would be small (like in the range 0 to 1 Hz/sec; SETI@home searches all the way out to +/- 100 Hz/sec). Here's the above pixel disc with chirp-rate thinning: This F/G star restriction also means that the "long-term consistency constraint" can be quite a bit tighter: the signal can't change frequency that fast over time. So we're in the middle of changing the way we generate non-bary birdies, and the way we find non-bary multiplets, to reflect these restrictions. We'll see how things look after that. My view is: if we reach the point of being able to easily see birdie signals, and their sinusoidal structure, in the pixel-disc plots, then we can return to the task of fitting curves to them. Notes on curve fitting:
It remains to be seen whether SETI@home's multiple observations gives us greater sensitivity to non-bary signals. It seems like it should: signals presumably persist, but noise doesn't. The problem is that, for non-bary signals, the detected frequency can change widely over time, But we have a bunch of ideas to explore, and maybe some of them will pay off. |
rob smith Send message Joined: 7 Mar 03 Posts: 22504 Credit: 416,307,556 RAC: 380 |
Well, you never said it was going to be a simple, smooth ride to plough through all the first part filtered data. It now looks as if you are getting into the tiddly step forward each time instead of the one forward, two back of your initial attempts. Bob Smith Member of Seti PIPPS (Pluto is a Planet Protest Society) Somewhere in the (un)known Universe? |
Bill F Send message Joined: 18 May 99 Posts: 42 Credit: 5,653,653 RAC: 2 |
Thank you for the update... your volunteer's are out here waiting to hear from you and these updates are great. Like the Arecibo Observatory ... we are "all ears" Bill F |
Rich Send message Joined: 25 Apr 08 Posts: 15 Credit: 3,727,057 RAC: 65 |
Thanks very much for the update – that’s really interesting. And... I have nothing useful to offer, so I won’t interrupt further. Please keep on sharing, when you can. |
Steven Send message Joined: 5 Aug 18 Posts: 1 Credit: 1,600,364 RAC: 1 |
Could you create some kind of dataset for an AI to chew on and see what it could spit out (I am not fully caught up on that stuff)? |
Helium Head Send message Joined: 27 Feb 19 Posts: 3 Credit: 3,026,745 RAC: 7 |
If/when you devise a suitable algorithm, will we be running 'old' datasets again? |
Peter Hucker Send message Joined: 3 Apr 99 Posts: 39 Credit: 7,555,481 RAC: 0 |
7 PCs at the ready when you are. |
rob smith Send message Joined: 7 Mar 03 Posts: 22504 Credit: 416,307,556 RAC: 380 |
Nebula will never be a "BOINC Style" project - which you would have learned if you read the introduction written by D.Anderson when he started this phase of SETI. Bob Smith Member of Seti PIPPS (Pluto is a Planet Protest Society) Somewhere in the (un)known Universe? |
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