What is SETI@home
Looking For?
| So, what will you be doing for us? What exactly
will you be looking for in the data? The easiest way to answer that question
is to ask what we expect extraterrestrials to send. We expect that they
would want to send us a signal in the most efficient manner for THEM that
would allow US to easily detect the message. Now, it turns out that sending
a message on many frequencies is not efficient. It takes lots of power.
If one concentrates the power of the message into a very narrow frequency
range (narrow bandwidth) the signal is easier to weed out from the background
noise. This is especially important since we assume that they are far enough
away that their signal will be very weak by the time it gets to us. So,
we're not looking for a broadband signal (spread over many frequencies),
we're looking for a tuning your radio set to various channels, and looking
at the signal strength meter. If the strength meter goes up, that gets
our attention. |
 |
Another factor that helps reject local (earth-based
and satellite-based) signals is that local sources are more or less constant.
They maintain their intensity over time. On the other hand, the Arecibo
telescope is fixed in position. When SETI@home is in operation, the telescope
does not track the stars. Because of this, the sky "drifts" past the focus
of the telescope. It takes about 12 seconds for a target to cross the focus
(or "target beam") of the dish. We therefore expect an extraterrestrial
signal to get louder and then softer over a 12 second period. Since we
are looking for this 12 second "gaussian" signal, we send you about 100
seconds of data. Also, we allow the data in the work-units to overlap a
little so we won't miss an important signal by cutting it off early in
the analysis.
| Let's look at and some examples. If you have RealAudio, you can also
listen to simulations of what it might sound like (though remember that
these signals are radio waves, not sound waves…) |
 |
Click graph for RealAudio sound
|
This graph (typical of the others below) shows time progressing along
the horizontal X-axis. The vertical Y-axis represents the frequency, or
pitch, of the signal. Here you see a broadband signal. Many frequencies
all mixed together. Note that the signal starts out weak (dim) at the left,
and gets louder (brighter), reaching a maximum in the center of the graph
6 seconds later, fading out over the next 6 seconds. This is what we would
expect from an extra-terrestrial signal as it drifts past the telescope.
Unfortunately, we are not looking for broadband sources. This is probably
what a star or other natural astronomical source would look like. Broadband
sources are rejected. |
| This is more what we're looking for. Here you can see the signal is
much narrower in frequency range. It also gets stronger and weaker over
a 12 second period. We don't know how narrow the bandwidth will be, so
we'll check for signals at several bandwidths. |
Click graph for RealAudio sound
|
Click graph for RealAudio sound
|
If our stellar friends are trying to put actual information on their
signal (very likely), the signal will almost certainly be pulsed. We'll
be looking for this too. |
| It's unlikely that our two planetary systems will be motionless with
respect to each other. Because of this, there is likely to be a "doppler
shifting" or changing frequency, of the signal because of our relative
motions. This might cause the signal to rise or fall in frequency slightly
over the 12 seconds. These are called "chirped" signals. We'll check for
this too. |
Click graph for RealAudio sound
|
Click graph for RealAudio sound
|
Of course, we'll also be checking for a doppler shifting (chirped)
signal that contains pulses too! |
Extra Credit Section: More detail on the analysis
The SETI@home software searches for signals about
10 times weaker than the SERENDIP IV search at Arecibo, because it makes
use of a computationally intensive algorithm called "coherent integration."
No one else (including the SERENDIP program) has had the computing power
to implement this method. Your computer performs fast fourier transforms
on the data, looking for strong signals at various combinations of frequency,
bandwidth, and chirp rates. The following steps are taken on each of the
work-units you get from us.
| Let's look first at the most computationally intensive portion of the
calculation. The first job is to "de-chirp" the data - that is, to remove
all the effects of the doppler acceleration. At the finest resolution,
we have to do this a total of 5000 times, from -5 Hz/sec to +5 Hz/sec in
steps of .002 Hz/sec. At each chirp-rate, the 107 seconds of data is de-chirped
and then divided into 8 blocks of 13.375 seconds each. Each 13.375 second
block is then examined with a bandwidth of .07 Hz for peaks (that's 131,072
tests (frequencies) per block per chirp rate!) This is a LOT of calculation!
In this first step, you computer does about 100 billion calculations! |
 |
We're not finished, we still have to test other bandwidths
too. The next step doubles the bandwidth to 0.15 Hz. Starting at this bandwidth,
we double the chirp range and look at chirp rates from -10 Hz/sec to +10
Hz/sec. Even though this doubles the range, we only have to examine 1/4
the number of rates due to the increase in bandwidth. So we have twice
the chirp range times 1/4 the number of chirps. You see we end up doing
about 1/2 the amount of work we did above at the highest resolution narrow
bandwidth, or about 50 billion calculations. Piece of cake...
 |
The next step doubles the bandwidth again (from 0.15 to 0.3 Hz) and
again reduces the chirps by 1/4. (We maintain the -10 Hz/sec to +10 Hz/sec
chirp range for the rest of the calculations.) This step (and all successive
steps) take 1/4 the calculation of the previous step. In this case only
12.5 billion calculations. This continues for a total of 14 doublings of
bandwidth (0.07, 0.15, 0.3, 0.6, 1.2, 2.5, 5, 10, 20, 40, 75, 150, 300,
600, and 1200 Hz) to bring you to a grand total of slightly more than 175
billion operations on the 107 seconds of data. As you can see we actually
do most of our work at the narrowest bandwidth (about 70% of the work.)
Finally, signals that show a strong power at some particular combination
of frequency, bandwidth and chirp are subjected to a test for terrestrial
interference. Only if the power rises and then falls over a 12 second period
(the time it takes the telescope to pass a spot in the sky) can the signal
be tentatively considered extra-terrestrial in nature.
How long should all these computations take? An average, current model
home home computer of reasonable power (with a CPU running around 233 MHz)
should take about 24 hours to complete one work-unit. This assumes that
the computer ONLY works on SETI@home, not playing your favorite game. Also
remember that we collect over 200,000 work-units of data every day! |
Now you know why we need your help!
|
