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Peer-to-Peer by Andy Oram

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How SETI@home works

We decided that SETI@home would use SERENDIP’s antenna. Like all previous radio SETI projects, SERENDIP analyzes its signal using a dedicated supercomputer at the telescope; it doesn’t record the signal. For SETI@home, we needed to digitally record the signal and transport it to our computers at Berkeley. The network connection from Arecibo to the mainland is too slow. Instead, we record the data on digital tapes and mail them to Berkeley. The largest-capacity digital tape available in 1998 was the 35-GB digital linear tape (DLT).

We had to decide what frequency range to record. Covering a wide range is good from a scientific point of view, but it means more tapes and more network bandwidth. We decided to record a 2.5 MHz frequency band. Using 1-bit samples, this gives a data rate of 5 Mbps, meaning that a tape fills up in about 16 hours. Like most radio SETI projects, we centered our band at the hydrogen line, 1.42 GHz. This is the resonant frequency of the hydrogen molecules that fill interstellar space. Since hydrogen is the most abundant element in the universe, we hope that if aliens are sending an intentional signal, they will use this frequency. Our 2.5 MHz band is wide enough to contain Doppler shifts (frequency shifts due to relative motion) corresponding to any likely velocity of a transmitter in our galaxy.

SETI@home and SERENDIP are complementary: SETI@home looks at a narrower frequency range than SERENDIP (2.5 MHz versus 140 MHz) but does better ...

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