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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
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
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
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
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/s2 due to Earth’s rotation
Corresponding Doppler drift rate: 0.16 Hz/s
Alien transmission would drift out of channel in about 6 seconds
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)
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
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
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
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
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
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
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
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
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)
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)
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) } } }
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
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)
The SETI@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
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
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
<Insert Demonstration Here>
Postprocessing
Client uploads candidate signal data to server(exact data formats are kept quiet)
Server examines results for errors
Keeps track of user statistics
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
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
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”
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
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
Summary
Seemingly impossible problem
Easily partitioned
Good publicity, marketing
Achieves incredible performance
But, high latency
High redundancy/replication of computation