Our alien neighbors may not be sending out a nice, even tone for us to
detect. They may be sending a series of spaced pulsesa more economical use of power.
For all frequency resolutions greater than or equal to .59 Hz, the SETI@home screensaver searches for repeating
pulses and triplets.
To find a series of repeated pulses, the SETI@home screensaver applies a special test called a "fast folding algorithm." If the routine finds a set of repeating pulses, it will display them with statistics describing what it found:
The "power" tells us how strong a pulse is relative to noise. The "period" tells us how far apart the pulses are (in seconds). Because both RFI and random noise can simulate a pulsed event, we've set a minimum threshold for acceptance. This threshold is calculated dynamically and depends upon the period and the number of times the data has been folded. (For you math nerds, it involves inverting a function known as the "incomplete gamma function.") The "score" for a pulse is the ratio of the pulse amplitude to this threshold value. A pulse with a score of greater than 1 will be reported when your screensaver client returns a result to Berkeley.
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SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.