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SCARIe:Realtime software correlationNico Kruithof, Damien Marchal
April 10th, 2008 REAL-TIME SOFTWARE CORRELATION Slide #2
Introduction• VLBI
• Correlation
• Software correlator
• SCARIe
• EXPReS / Fabric
• Conclusion
April 10th, 2008 REAL-TIME SOFTWARE CORRELATION Slide #3
VLBIVLBI (very large baseline interferometry) is a technique, in which widely separated
radio telescopes observe the same region of sky simultaneously, in order to generate very high-resolution astronomical images of cosmic radio sources
• Telescopes are usually separated by thousands of kilometres
• Data rates of 32mbps – 1gbps per telescope
• Processing is done at JIVE
• For one experiment:• Input: 1-100 Terabytes• Output: 1-100 Gbytes.
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Larger dish: higher resolution
Arecibo, Puerto Rico: b = 305 m
Resolution: λ / b
Sensitivity: b2
λ = wavelengthb = diameter telescope
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Westerbork/Very Large Array
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VLBI
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e-VLBI
TodayPast
Today
Research
TodayResults
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Correlation• Combining signals from
different telescopes
• First aligning the signals by• Delaying signals• Stretch the signals
• Correlation function:• Measures the similarity of
two signals
• Integration to enhance the signal to noise ratio
Dire
ctio
n of
rad
io s
ourc
e
Del
ay
Delay Delay
Phase Phase
Correlator
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Delay correction
Received signals from two stations
Delay correction
Doppler shift / Fringe stopping
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Correlation function
• A measure for the similarity of two (relatively shifted) functions.
• Measures the area under the function obtained by multiplying one signal with the other shifted signal
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Correlator Design
Grid
Telescopes
Input nodes
Correlator nodes
Output node
Sorting and merging
Time and channel slicing
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Outline of the nodes• Input node
• Reads the data (at 32 mbps – 1 gbps per station)• Extracts the different frequency bands• Does the integer delay correction
(integer number of samples)• Sends two bits samples to the correlator nodes
• Correlator node• Receives samples and converts them to floating point values• Performs a fractional delay correction• Corrects for the Doppler shift• Correlates the baselines
• Output node• Sorts the data from the correlator nodes and writes it to file
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DAS-3 / StarPlane• DAS-3:
Third DAS supercomputer• 272 dual AMD Opteron
compute nodes, spread out over five clusters, located at the four universities.
• StarPlane is the Application Controlled Photonic Network connecting the DAS-3 sites
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EXPReS / FABRIC• Collaboration with PSNC for data processing.
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Success Story: FTP Fringe Tests• FTP Fringe tests: short slices of an experiment to test equipment.
• Cluster running NICTcorrelator broke downin May 2007
• SFXC was successfully used for the last three FTP Fringe Tests
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Benchmarks on DAS-3• 40% of realtime (4 telescopes, 256mbps each) on 30 quad-cores
• bottleneck is CPU power
• GPU's / FPGA'scould enhanceperformance
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Conclusion• VLBI is a technique in astronomy in which data from several
telescopes is combined to make high resolution images of the sky.
• The output of the software correlator has been validated and produces similar results as the current hardware correlator
• We are currently benchmarking and optimizing the correlator on the DAS-3 cluster.
• Contact informationNico [email protected]
