Warning: This paper on SETI@home is written for scientists and engineers. In order to confuse and obfuscate the public, we utilize substantial technical jargon:
SETI@home is a commensal SETI sky survey at the National Astronomy and Ionospheric Center's 305 meter radio telescope in Arecibo, Puerto Rico. The sky survey covers a 2.5 MHz bandwidth centered at 1420 MHz, as many researchers have suggested the 21 cm Hydrogen line as the most likely band for intentional interstellar transmissions to be lurking. The survey covers 28% of the sky (declinations ranging from +1 to +35 degrees) with a sensitivity of 3E-25 W/m2. SETI@home observations will span a total of two years, during which most of the sky will be observed two or three times. Observations began in October 1998.
Much of the SETI@home data analysis is accomplished using distributed computing with the help of hundreds of thousands of participants and their internet connected computers (Sullivan et al 1997).
SETI@home, together with the SERENDIP IV seti program (Werthimer et al 1997), utilize a dedicated L band receiver at the Arecibo telescope. Using an independent feed and receiver permits SETI observations to be carried out simultaneously with ongoing ionospheric and astrophysics research programs at the telescope; in this way, SETI@home can observe year round without interfering with other scientific endeavors. This commensal technique, also called "piggyback SETI", was developed by the SERENDIP seti program at UC Berkeley (Bowyer et al 1983).
SETI@home's search space is roughly orthogonal to the Serendip IV sky survey; although SETI@home has 40 times less frequency coverage than SERENDIP IV, it's sensitivity is ten times better. The SETI@home search also covers a much richer variety of signal bandwidths, drift rates, and time scales than SERENDIP IV or any other SETI program to date.
SETI@home Receiver and Data Recording
SETI@home and SERENDIP IV utilize a dedicated flat feed and cryogenic receiver mounted on the carriage house of the Arecibo telescope. The feed provides a single linear polarization with a gain of 3K/Jy and a 0.1 degree beam width. System temperature is 45K.
The receiver output is down converted with quadrature analog mixers and filters, then digitized and and converted to baseband by a digital quadrature mixer and a pair of 256 tap finite impulse response low pass filters (Backer et al, 1997). The resulting 2.5 MHz band is recorded continuously on 35 Gbyte DLT IV tapes with two bit complex sampling, along with data on telescope coordinates, time and engineering monitors. Tapes are mailed to UC Berkeley for analysis; the complete sky survey requires 1100 tapes to record a total of 39 terabytes of data.
We expect to record high quality data 65% of the time, observing each of the one million beams two or three times during the two year program. It's important to observe each beam several times because sources may scintillate (Cordes, 1991) or have short duty cycles, and most of our robust detection algorithms require multiple detections. SETI@home is able to collect useful data whenever the telescope is stationary or the Gregorian feed is tracking a source. When the Gregorian system tracks a source, the SETI@home feed is moving at 1 to 2 times sidereal rate on the sky and a source remains in the beam for 12 to 24 seconds. When the telescope is stationary, a source is in the beam for 24 seconds. We are not able to collect useful data when a transmitter is on or when the telescope is slewing rapidly across the sky (since we can't get an accurate position) or when the carriage house is tracking (we can't reject short term RFI when the feed tracks a source).
SETI@home data tapes from the Arecibo telescope are divided into small "work units" as follows: the 2.5 MHz bandwidth data is first broken down into 256 sub-bands by means of a 2048 point fast Fourier transform (FFT) and 256 eight point inverse transforms. Each work unit consists of 107 seconds of data from a given 9,765 Hz sub-band. Work units are then sent over the internet to hundreds of thousands of client "screen saver" programs around the world for the bulk of the data analysis.
Because an extraterrestrial civilization's signal has unknown bandwidth and time scale (eg: the signal may be pulsed, continuous, wide or narrow band), the client software searches for signals at 15 octave spaced bandwidths ranging from 0.075 Hz to 1220 Hz, and time scales from 0.8 mS to 13.4 seconds. The rest frame of another civilization's transmitter is also unknown, (eg: their transmitter may be on a planet that is rotating and revolving) so extraterrestrial signals are likely to be drifting in frequency with respect to the observatory's topocentric reference frame. Because the reference frame is unknown, the client software examines 6761 different doppler acceleration frames of rest (dubbed "chirp rates"), ranging from -10 Hz/sec to +10 Hz/sec.
De-chirping the data is accomplished by multiplying the time domain data by the complex vector V:
|where||t = time, and|
|c = chirp rate, which ranges from -10 Hz/sec to +10 Hz/sec.|
At each chirp rate, peak searching is implemented by computing non-overlapping FFT's and their resulting power spectra. FFT lengths range from 8 to 131,072 in 15 octave steps. Peaks greater than 22 times the mean power are recorded and sent back to the SETI@home team for further analysis.
Besides searching for peaks in the multi-spectral- resolution data, SETI@home also searches for signals that match the telescope's gaussian beam pattern. Gaussian beam fitting is computed at every frequency and every chirp rate at spectral resolutions ranging from 0.6 to 1220 Hz (temporal resolutions from 0.8 mS to 1.7 seconds). The beam fitting algorithm attempts to fit a gaussian curve at each time and frequency in the multi-resolution spectral data, of the form:
|where||P = predicted power|
|B = baseline power|
|A = peak power|
|t = time|
|t0 = time of gaussian peak|
|b = half power beamwidth|
B, A, and t0 are free parameters in the fit, but the beamwidth is known, calculated from the slew rate of the telescope beam for each work unit. Gaussian fits whose A/B exceeds 3.2 and whose chi-squared < 10 are reported by the client software to UCB for further analysis. A typical gaussian fit is shown in Figure 1.
Most of the signals found by the client programs turn out to be terrestrial based radio frequency interference (RFI). We employ a substantial number of algorithms to reject the several types of RFI (see Cobb et al, 1997). After the RFI is rejected, we search the remaining data set for multiple detections in any reference frame, giving higher weights to drifting or pulsed signals, those that repeat in the barycentric frame, that match the antenna beam pattern, or detections coincident with newly detected planets, nearby stars (from the Gliese catalog) or globular clusters (again, details in Cobb, et al). We compare candidates signals with SERENDIP IV data, and will follow up interesting candidates with dedicated observations.
Figure 1: a gaussian fit found by a client in a typical work unit. This work unit contains only noise (no signal is present).
Backer, Dexter, Zepka, Ng and Werthimer (1997): A programmable 36 MHz digital filter bank for radio science, Publications of the Astronomical Society of the Pacific, 109, January 1997
Bowyer, Zeitland, Tarter, Lampton and Welch (1983): The Berkeley Parasitic SETI program, Icarus 53, 147-155
Cobb, Donnelly, Bowyer, Werthimer and Lampton (1997): The SERENDIP IV Interference Rejection and Signal Detection System; in the book "Astronomical and Biochemical Origins and the Search for Life in the Universe", eds Cosmovici, Bowyer and Werthimer
Cordes, Lazio and Sagan (1997): Scintillation-induced Intermittency in SETI, the Astrophysical Journal
Werthimer, Bowyer, Ng, Donnelly, Cobb, Lampton and Airieau (1997): The Berkeley SETI Program: SERENDIP IV Instrumentation; in the book "Astronomical and Biochemical Origins and the Search for Life in the Universe", eds Cosmovici, Bowyer and Werthimer
Sullivan, Werthimer, Bowyer, Cobb, Gedye and Anderson (1997): A New Major SETI Project based on project SERENDIP data and 100,000 Personal Computers; in the book "Astronomical and Biochemical Origins and the Search for Life in the Universe", eds Cosmovici, Bowyer and Werthimer
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