Taking the long view

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Profile David Anderson
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Message 2066219 - Posted: 21 Jan 2021, 5:47:45 UTC
Last modified: 31 Jan 2021, 3:47:19 UTC

Wow! A lot has happened since we last spoke. Democracy prevailed in the U.S., for the time being. And COVID vaccination has started - Yay!

Lots of progress on Nebula too. We solved a nagging issue, and in the course of doing so we improved our understanding of what SETI@home is able to detect.

The nagging issue has to do with birdies - specifically, where we position them in the sky. We were creating 4000 birdies, and putting each one in a pixel in which spikes had been detected. The choice of pixel, and the position within the pixel, were random.

A Nebula program, birdie_gen.cpp, generate spikes for the birdies by scanning the trajectories of the 7 beams over the ~15 years of observation, seeing when a beam gets close to the birdie, and computing a set of spikes that the SETI@home client might have found.

A bit surprisingly, we ended up making any spikes at all for only 2700 out of the 4000 birdies. Then Eric made a bunch of changes to make the simulation more realistic:

  • Taking into account the difference between signal bandwidth and width of the FFT bin.
  • Taking into account the movement of the beam during the duration of the FFT bin.
  • Modeling transmitters that are the surface of a planet and are therefore detectable only ~half the time.

After these changes only 1300 of the birdies had any spikes! This was no good - our plans to estimate sensitivity require finding birdies, and we can't find a birdie if it doesn't have any spikes.

So we started thinking about where we were putting the birdies. Pixel observation varies widely. Some pixels have been observed thousands of times and/or for long periods. But many have been observed only once or twice, for a few seconds, If we put a birdie in such a pixel, we might not generate any spikes, especially if the birdie and the beam path are in opposite corners of the pixel.

So I restricted birdies to pixels with at least 60 seconds of observing time. This helped, but only slightly.

Then Eric made a key realization: we can "detect" a birdie best at the FFT length whose frequency resolution is closest to the birdie's intrinsic bandwidth. But this will happen only if we have an observation that includes the time interval of the FFT bin. So if the birdie bandwidth is small (say, .05 Hz) we'll detect it best at our longest FFT length: 128K samples, or 13 seconds, or .07 Hz. But we'll detect it with maximum power only if our observation includes an entire FFT bin. A 26-second observation is guaranteed to include one 13-second FFT bin; a 39-second observation is guaranteed to contain two.

Now, a pixel could have 60 seconds of observing time, but if that consists of a lot of 1 or 2 second observations, we probably won't generate any spikes for that narrow-bandwidth birdie.

This was like a light bulb going on. We've always thought of observing time as the key factor in sensitivity - in other words, that we could say "for pixels that we've observed for at least 10 minutes, our sensitivity is X". But what matters - at least for narrow-bandwidth signals, which are inherently easier to find because they're less like noise - is not total time but rather just the intervals that are long enough to hold the optimal FFT bin.

Nebula has a program "pointing.cpp" that analyzes the beam pointing trajectories and generates per-pixel observing information. I extended this so that for each pixel it computes the length of the longest and send-longest observations.

Recall that a multiplet must have at least two detections. So we can't "find" a birdie unless it has at least two spikes. To make this likely, we need to put it in a pixel with sufficiently long observations for the birdie's intrinsic bandwidth. Specifically: if the duration of the closest FFT length is T, then we want a pixel for which

longest observation >= 3T
or
second longest observation >= 2T

I made this change to birdie_gen. We also had a discussion about planetary transmitters. If the transmitter is hidden half the time, it might not be on during the critical observation, in which case our careful selection of pixel is for naught. Dan argued that if ET is serious about being detected, they'd put two transmitters on opposite sides of the planet, so that one of them is always visible. We decided to go with this.

We re-ran birdie_gen with high hopes! But alas there was no improvement.

Then, today, Eric found two critical bugs in the function (which I'd written) to get information about a pixel's observations: the arguments to fseek() were reversed, and one of them was a file offset that was overflowing 32 bits. After fixing these problems, we're now generating spikes for 3945 of the 4000 birdies, which is plenty. Phew!


This means we have to recalibrate our view of what SETI@home is good at. Previously, we thought: SETI@home has covered a lot of the sky - about 23% - and we did 128K FFTs everywhere, so we can find narrow-band signals in 23% of the sky. But that's not the case. We can actually hope to find narrow-band signals only in the parts of the sky where we had long enough observations. And for 128K FFTs, that's only 0.5% of the sky.

That's a little disappointing, I guess. Of course, we're sensitive to broad-band signals and pulsed signals over a wide area of sky.

But there's an important lesson here for future radio SETI sky surveys: if possible, make each observation long enough for your longest FFT. I'm sure Dan and Eric already knew this, but since we never controlled the pointing of Arecibo, we didn't think about it (until recently).

BTW: covering the Arecibo sky with 13-second observations - given the ALFA beam size - would take about a year, according to my calculations.

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Message 2066233 - Posted: 21 Jan 2021, 10:15:39 UTC

these 2 urls were not found on this server ...


https://setiathome.berkeley.edu/web/birdies.php
https://setiathome.berkeley.edu/web/signals.php
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Message 2066261 - Posted: 21 Jan 2021, 18:39:11 UTC

Thanks or the insight and a good read.

Would "heat maps" of pixels total observation time and/vs pixels 'long-enough-intervals' observations counts be interesting to see where we've usefully seen?


Keep searchin',
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Message 2066369 - Posted: 22 Jan 2021, 13:07:04 UTC - in response to Message 2066233.  

these 2 urls were not found on this server ...
https://setiathome.berkeley.edu/web/birdies.php
https://setiathome.berkeley.edu/web/signals.php
These may be the intended links

https://setiathome.berkeley.edu/nebula/web/birdies.php

https://setiathome.berkeley.edu/nebula/web/signals.php
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Message 2066370 - Posted: 22 Jan 2021, 13:09:31 UTC - in response to Message 2066219.  

Thank you for this update and education on the progress on Nebula. It's very welcome by many more than just myself, I'm sure.
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Message 2066476 - Posted: 23 Jan 2021, 10:43:30 UTC

It's shitty that the most computational effort devoted to 128K FFT end up not useful - perhaps over 50% of all FLOPs are wasted! If then, SETI@home would only be marginally more sensitive than SENDRIP - while costing hundreds of times more!
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Message 2066551 - Posted: 23 Jan 2021, 23:40:09 UTC - in response to Message 2066476.  
Last modified: 23 Jan 2021, 23:43:35 UTC

... the most computational effort devoted to 128K FFT end up not useful - perhaps over 50% of all FLOPs are wasted! If then, SETI@home would only be marginally more sensitive than SENDRIP - while costing hundreds of times more!

We don't know anything like those sort of ('99%' of statistics) guesses yet... Hence the research to find out!

Note that just one of the ideas of the s@h processing was to gain greater sensitivity by processing over longer sample intervals...

Note that there is a physical compromise of longer integration time for greater sensitivity but then also, that also reduces the proportion of the observation time. Hence a balance of guesses were made. We now get to understand what was found and to what sensitivity.


Also specially note: this part of the discussion is about "birdie" tests. There are more steps yet for the development and tests before we get to see the final results and learn further from what those results mean. The most important bit is what we learn.

Such is the good and real world way of Science!

Enjoy the ride and the great search!


Keep searchin',
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Message 2066618 - Posted: 24 Jan 2021, 10:29:31 UTC

So the lessons learned is that SETI should be using dedicated observation time, rather than piggy-back the telescope while somebody else is doing a basket weave sky survey. Ok, BLC data from GBT will be much more sensitive for SETI, since they are targeted observations. I really hope there will be an all-sky SETI@home, not just 0.5%! And even then the chance that ET could be detected is still vanishing.

So ET might be there, but we are unable to detect them, because nobody on earth has conducted a comprehensive and sensitive SETI project. Just like we will never be talking about astronomy if there is no telescope.
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Message 2067299 - Posted: 31 Jan 2021, 3:50:30 UTC - in response to Message 2066369.  

Thanks - I fixed the links.
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Message 2068487 - Posted: 15 Feb 2021, 18:46:45 UTC

So, what I understand as a totally layman:
SETI@home was "consuming" whatever data were made available, regardless of obeservation time at a single spot or where this spot in the sky might have been. This was due to the fact, that S@h never hat control over Arecibo or other sensors. This resulted in Terabytes of data, generated from the observation of about 23 % of the sky, but only data of 0.5 % of the sky are worth processing it through Nebula.
Am I right so far?
So, IMHO for the next start of S@h, it must be a prerequisite to have control over the observation sensors to meet your defined minimum observation time of 13 seconds per spot.
What are your plans to achieve this?

Best regards
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Message 2068727 - Posted: 19 Feb 2021, 2:41:57 UTC

Keep going!Best wishes to you.
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Message 2069375 - Posted: 26 Feb 2021, 16:51:51 UTC

We can actually hope to find narrow-band signals only in the parts of the sky where we had long enough observations. And for 128K FFTs, that's only 0.5% of the sky. But there's an important lesson here for future radio SETI sky surveys: if possible, make each observation long enough for your longest FFT. I'm sure Dan and Eric already knew this, but since we never controlled the pointing of Arecibo, we didn't think about it (until recently).


Is it true also for the GBT data? Or does the observation time is longer there?
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Message 2070123 - Posted: 7 Mar 2021, 10:30:22 UTC - in response to Message 2066219.  

Politics and the work we did on Seti@home is less useful than we thought. I was once proud of my efforts toward Seti@home, meager though they were. We gave up too soon.

- Jim[/quote]
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Message boards : Nebula : Taking the long view


 
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