SCARIe: Realtime software correlation Nico Kruithof, Damien Marchal

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SCARIeRealtime software correlationNico
Kruithof, Damien Marchal
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Introduction

• VLBI
• Correlation
• Software correlator
• SCARIe
• EXPReS / Fabric
• Conclusion

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VLBI

• VLBI (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

• Resolution ??? / b
• Sensitivity ??b2
• ? wavelength
• b diameter telescope

Arecibo, Puerto Rico b 305 m
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Westerbork/Very Large Array
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VLBI
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e-VLBI
Past
Today
Results
Today
Research
Today
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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

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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
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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 information
• Nico Kruithof
• JIVE
• Kruithof_at_jive.nl