1. SETI@Home Sunny Gleason COM S 717 November 29, 2001 (Based on the article, “SETI@Home: Massively Distributed Computing for SETI.”)

2. In This Presentation What is SETI? Partitioning the Job The SETI@Home Client Server Post-processing Project Status

3. SETI@Home SETI: Search for Extra-Terrestrial Intelligence –Private / Academic efforts –NASA –SETI Institute –SETI@Home SETI@Home : Project led by researchers at University of California - Berkeley (1997) “Piggyback SETI” receiver at Arecibo radio telescope

4. SETI: The Task What is the complexity of detecting signals sent by an extra-terrestrial civilization? Category: massively difficult –Signal parameters unknown –Sensitivity of analysis depends on available computing power

5. SETI: Task Assumptions Aliens would broadcast a signal that is easily detectable, distinguishable from natural radio emission Narrowband signals stand out from natural broadband sources of noise Thus, SETI efforts concentrate on narrowband signals The hydrogen line: 1420 MHz

6. Narrowband Signals Use a narrow search window (channel) around a given frequency Earlier systems: –Analog narrow bandpass filters Newer systems: –Dedicated banks of Fast-Fourier Transform (FFT) processors –Separate signal into up to 1 billion 1-Hz channels

7. Signal Problems Signals are unlikely to be stable in frequency –Example: A listener on Earth’s surface for 1.4GHz signals undergoes acceleration of up to 3.4cm/s 2 due to Earth’s rotation Corresponding Doppler drift rate: 0.16 Hz/s Alien transmission would drift out of channel in about 6 seconds

8. Signal Problems We can compensate for Earth’s rotation, but what about remote planet? Solution: –Correct for Doppler drift at the receiving end –Search for signals at multiple Doppler drift rates Computation-intensive! Allowed remote drift rates are between -10Hz/s and +10Hz/s (+50/-50)

9. Other Parameters Signal frequency / bandwidth? Is it pulsed? If so, what period? Solving over the full range of parameters is beyond even the world’s most powerful supercomputers Fortunately, the task is easily partitioned

10. Distributing the Load Break the data up into separate frequency bands Observations of different portions of the sky are essentially independent Partition the huge dataset into smaller chunks that ordinary PC’s can handle

11. Data Collection Observations come from 305-meter radio telescope in Arecibo, Puerto Rico Dedicated instrumentation within telescope Passively monitors the telescope’s field of view (0.1 degrees) Stationary telescope: objects pass through in 24 seconds When telescope is tracking: 12 s

12. Data Collection Over the course of the project, SETI@Home will see visible portions of the sky 3 or more times Covers stars with declinations from -2 to 38 degrees Approximately 25% of the sky

13. Data Collection System records a 2.5MHz band, centered at the 1,420MHz hydrogen line Records 2-bit samples onto 35GB DLT tapes (Recall: Nyquist Rate) Each tape: 15.5h of data 39TB of data total

14. Data Collection Data tapes shipped to Berkeley Split into work units using 4 splitter workstations –Divide 2.5MHz data into 256 subbands using 2048-point FFT followed by 256 8-point inverse transforms –Subbands are 9,766Hz wide –2 20 samples, thus each work unit is ~10KHz wide and 107 s long –Work units overlap to detect overlapping signals Work units are stored on separate server for distribution

15. Data Collection Main SETI@Home Server –3 Sun Enterprise 450 Series Computers User Database –Contains account information for each of the 2.4 million users –Also aggregates statistics by platform Science Database –Contains information about each work unit »Time, sky coords, frequency range »How many times each work unit has been downloaded –Stores parameters of candidate signals »Signal power, frequency, arrival time sky coords »1.1 billion candidates (Oct. 2000) Work unit storage

16. Data Collection Work unit storage server –Distribution of work units, storage of results Client communications via HTTP –Important to get through firewalls –Request to download new work unit Work units that have not been downloaded yet have priority Then, work units for which no results have been returned –Request to post results Data contains signal characteristics Updates user statistics

17. The SETI@Home Client Available for 47 different combinations of CPU and OS Dominant platforms: Windows, Mac –Feature graphical “screensaver” display –UNIX works as daemon (display program available for X)

18. The SETI@Home Client Downloads work unit from server Performs “baseline smoothing” to eliminate wideband features, help reduce false signals Performs main data analysis loop (shown on next page)

19. Main Data Analysis Loop for Doppler Drift rates from -50 to 50Hz { for bandwidths from 0.075 to 1220Hz in 2x steps { Generate time-ordered power spectra Search for short-duration signals above a constant threshold for each frequency { Search for faint signals matching beam parameters (Gaussians) Search for groups of 3 evenly spaced signals Search for faint repeating pulses (pulses) } } }

20. The SETI@Home Client Client examines signal at various drift rates –10 to -10 Hz (fine-grained) –50 to -50 Hz (~twice as course) Although drift rates are most likely negative, examine both sides –For statistical comparison –To detect deliberately chirped signals

21. The SETI@Home Client For each drift rate, examines the signal at different bandwidths between 0.075 and 1,221 Hz –Using a variety of FFT –Not all bandwidths are examined at every drift rate (only when drift rate becomes significant compared to the frequency)

22. The SETI@Home Client Transformed signals are examined for spiked exceeding 22 times the mean noise power Threshold: 7.2 x 10 25 W/m 2 (at the finest frequency resolutions) “Detecting a cell phone on one of the moons of Saturn” These spikes are what the client reports

23. The SETI@Home Client Other transformations to detect Gaussians and pulse patterns Specialized algorithms (fast-folding algorithms) for detecting pulses efficiently Work by “folding” portions of the signal together in time, to detect gain over the pulse period

24. The SETI@Home Client Typical workload: –2.4 to 3.8 trillion floating-point operations (teraflops) –Typical 500MHz PC takes 10 to 12 hours to complete a work unit –Within the average work unit: 4 spikes, 1 Gaussian, 1 pulsed signal, 1 triplet signal

25. Postprocessing Client uploads candidate signal data to server (exact data formats are kept quiet) Server examines results for errors Keeps track of user statistics

26. Error detection SETI@Home uses thousands of CPU years every day With heat, floating-point units are the first to give incorrect results High error rates are offset by easy error detection Replication of work units is the primary error detection mechanism 60% of work unit results must agree in order to be considered for further analysis

27. Candidate Signals Vast majority of detected signals correspond to terrestrial RFI –Extra-terrestrial signals can not last more than 12 s –Also, signals should repeat when viewing the same portion of the sky at a later time

28. Project Status October 2000 –2.4 million users –520,000 active clients donating 437,000 years of CPU time (4.3 x 10 20 flop) –Average processing rate: 15.7 Tflops “Largest supercomputer in existence” “Largest computation ever performed”

29. Project Status 1.1 Billion signals in SETI@Home database Candidate signals being submitted faster than the server can confirm them So far, no extra-terrestrial signals

30. Future Work Expand coverage by adding new telescope in southern hemisphere Expand frequency bandwidth (up to double the data rate) Expand number of volunteers, increase SETI education efforts

31. Summary Seemingly impossible problem Easily partitioned Good publicity, marketing Achieves incredible performance –But, high latency –High redundancy/replication of computation

32. Related Work Distributed.net –Cracking of encryption keys (DES,rc5) –Search for optimal Golomb rulers Folding@Home –Stanford project - distributed protein folding PiHex –Distributed effort to calculate Pi GIMPS –Great Internet Mersenne Prime Search

33. Discussion Potential comments on: –System architecture –Fault-tolerance –Security

34. References Seti@Home Web Site –http://setiathome.ssl.berkeley.edu/ NASA Science Newsletter –http://science.nasa.gov/newhome/headlines/ast23may99_1.htm Papers –Korpela, et al. “SETI@Home: Massively Distributed Computing for SETI.” –Sullivan, et al. “A new major SETI project based on Project Serendip data and 100,000 personal computers.” –“The SETI@Home Sky Survey.” Available from the SETI@Home web site.