Three Years of SETI@home: A Status Report

The SETI@home project has recently completed its third year of active data analysis. Over 4 million volunteers have joined the search, providing a combined total of over 1 million CPU-years of processing power. SETI@home performs a sensitive search for extraterrestrial signals in a 2.5 MHz band centered on 1420 MHz. SETI@home searches a wide parameter space including 14 octaves of signal bandwidth and 15 octaves of pulse period with Doppler drift corrections from -50 Ha/s to +50 Ha/s, We will briefly describe the SETI@home project and the algorithms used in the SETI@home client. We will describe the postprocessing methods we use to reject RFI and select candidate signals from the nearly 4 billion "hits" returned by SETI@home clients. 1. Observing Methodology The UeB SETI searches use the 1420 MHz line feed on Carriage House 1 at the National Astronomy and Ionospheric Center's 305 meter radio telescope in Arecibo, Puerto Rico. This unique arrangement allows observations to be conducted without interference with other uses of the telescope. This results in two main modes of observation. If the primary observers feed is stationary or stowed the beam scans across the sky at the sidereal rate. If the primary observer's feed is tracking a position on the sky, the beam scans the sky at twice the sidereal rate. At twice the sidereal rate, the beam width corresponds to a 12 second beam transit time (Korpela et al. 2001). Figure 1 shows the path of the telescope beam over the course of 15 hours. Since the start of the project, the telescope has covered about 90% of the sky visible from Arecibo. The time domain data for the sky survey is recorded as follows: first, a 30 MHz band from the receiver is converted to baseband using a pair of mixers and low pass filters. The resulting complex signal is digitized and then filtered to 2.5 MHz using a pair of 192 tap FIR filters in the SERENDIP IV instrument. (Werthimer et al. 1997) One bit samples are recorded on 35 GByte DLT tapes (one bit real and one bit imaginary per complex sample). These tapes are shipped to Berkeley for use in the SETI@home program. In Berkeley, the data from the tapes are split into work units of duration 107 seconds and bandwidth 9766 Hz. These work units are shipped over the Internet