| NEWSLETTER #9 - August 29,
Eric Person, Steve Fulton
As of August 29, 2001, SETI@home users have detected billions of spikes and hundreds of
millions of gaussians,
pulses, and triplets. (For a
current tally of detected signals, visit our page on finding
Clearly, identifying these signals is only one step in the
search for extraterrestrial communication. First of all, these results need to be
verified, as discussed in Newsletter #8.
Second, signals due to radio frequency
interference (RFI) need to be removed from consideration. Also, we try to identify
signals that are "persistent"consistent in location and frequency across
time. The goal of this newsletter is to explain SETI@home's current procedure in
identifying persistent signals and provide specific examples to illustrate this process.
Why is persistency important?
We expect most (if not all) of our detected radio signals to originate from
transmitters on Earth, satellites, galactic noise, and other natural astronomical
phenomena. If our radio telescope happens to point toward Alpha Centauri while detecting
radio signals from a television broadcast that passes overhead, all we know from our data
is that we detected a signal while looking at Alpha Centauri. However, if we continue to
detect a signal on multiple occasions while pointing toward Alpha Centauri, it becomes
less likely that a signal from Earth is the culprit and more likely that we're truly
getting a signal from that region in space. Thus, a key task is to identify signals that
detected at multiple times from the same location in the sky.
To find persistencies, we match signals together that were detected at different times
within 2.5 arcminutes of the same right ascension (RA) and declination (DEC) coordinates
in the sky, and that share the same frequency band (within 50Hz). (For an explanation of
RA and DEC, read our page on features of skymaps.)
|The more matches we find for a
particular location in space, the more confident we are about a persistent signal coming
from that area of the sky. The bar graph on the right provides a snapshot (from August 29,
2001) of an ongoing persistency matching process for gaussian signals. It reports the
number of persistent gaussians found, with separate bars for 1, 2, 3, and 4 (or more)
matches at specific locations in the sky. More matches means better persistency. A similar
graph on our current progress
summary page is updated regularly.
|# of Persistency Matches
out of 80,704 Gaussians
||Let's take a closer look at a
gaussian with persistent matches. Clicking the image to the left will take you to a set of
four persistency graphs. (To learn more about these graphs please see "Key
Features of Persistency Graphs".) The main purpose of these graphs is to check
the context around a persistent signal to find clues to the source of the signal. Some
graphs are clickable, allowing you to view plots broken down by time, power, or score, as
well as view plots showing nearby gaussians, spikes, pulses, or triplets. As you can see,
there's quite a bit to explore for just one persistent signal.
Some graphs in this set break down signal distributions for gaussians, spikes,
triplets, and pulses occurring closely in time and frequency to matched persistent
signals. For example, this
graph shows a pattern of signals occuring in the same vertical band as the target
signal. Such a pattern suggests a source of rfi that is continuously detected (at a
specific frequency and power) even as the telescope moves across the sky.
By finding persistent signals and removing RFI-related signals from consideration, we
can significantly narrow the number of viable candidates requiring further detailed
examination. We will continue to keep you updated as our process continues.