SETIHome

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SETI_at_Home

• Sunny Gleason
• COM S 717
• November 29, 2001
• (Based on the article, SETI_at_Home Massively
  DistributedComputing for SETI.)

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In This Presentation

• What is SETI?
• Partitioning the Job
• The SETI_at_Home Client
• Server Post-processing
• Project Status

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SETI_at_Home

• SETI Search for Extra-Terrestrial Intelligence
• Private / Academic efforts
• NASA
• SETI Institute
• SETI_at_Home
• SETI_at_Home Project led by researchers at
  University of California - Berkeley (1997)
• Piggyback SETI receiver at Arecibo radio
  telescope

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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

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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

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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

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Signal Problems

• Signals are unlikely to be stable in frequency
• Example
• A listener on Earths surface for 1.4GHz signals
  undergoes acceleration of up to 3.4cm/s2 due to
  Earths rotation
• Corresponding Doppler drift rate 0.16 Hz/s
• Alien transmission would drift out of channel in
  about 6 seconds

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Signal Problems

• We can compensate for Earths 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)

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Other Parameters

• Signal frequency / bandwidth?
• Is it pulsed? If so, what period?
• Solving over the full range of parameters is
  beyond even the worlds most powerful
  supercomputers
• Fortunately, the task is easily partitioned

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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 PCs can handle

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Data Collection

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

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Data Collection

• Over the course of the project, SETI_at_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

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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

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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
• 220 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

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Data Collection

• Main SETI_at_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

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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

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The SETI_at_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)

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The SETI_at_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)

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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)

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The SETI_at_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

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The SETI_at_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)

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The SETI_at_Home Client

• Transformed signals are examined for spiked
  exceeding 22 times the mean noise power
• Threshold 7.2 x 1025 W/m2 (at the finest
  frequency resolutions)
• Detecting a cell phone on one of the moons of
  Saturn
• These spikes are what the client reports

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The SETI_at_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

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The SETI_at_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
• ltInsert Demonstration Heregt

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Postprocessing

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

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Error detection

• SETI_at_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

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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

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Project Status

• October 2000
• 2.4 million users
• 520,000 active clients donating 437,000 years of
  CPU time (4.3 x 1020 flop)
• Average processing rate 15.7 Tflops
• Largest supercomputer in existence
• Largest computation ever performed

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Project Status

• 1.1 Billion signals in SETI_at_Home database
• Candidate signals being submitted faster than the
  server can confirm them
• So far, no extra-terrestrial signals

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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

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Summary

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

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Related Work

• Distributed.net
• Cracking of encryption keys (DES,rc5)
• Search for optimal Golomb rulers
• Folding_at_Home
• Stanford project - distributed protein folding
• PiHex
• Distributed effort to calculate Pi
• GIMPS
• Great Internet Mersenne Prime Search

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Discussion

• Potential comments on
• System architecture
• Fault-tolerance
• Security

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References

• Seti_at_Home Web Site
• http//setiathome.ssl.berkeley.edu/
• NASA Science Newsletter
• http//science.nasa.gov/newhome/headlines/ast23may
  99_1.htm
• Papers
• Korpela, et al. SETI_at_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_at_Home Sky Survey. Available from the
  SETI_at_Home web site.