SCARIe: Realtime software correlation Nico Kruithof, Damien Marchal
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #2 Introduction VLBI Correlation Software correlator SCARIe EXPReS / Fabric Conclusion
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #3 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: Terabytes Output: Gbytes.
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #4 Larger dish: higher resolution Arecibo, Puerto Rico: b = 305 m Resolution: λ / b Sensitivity: b 2 λ = wavelength b = diameter telescope
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #5 Westerbork/Very Large Array
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #6 VLBI
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #7 e-VLBI Today Past Today Research Today Results
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #8 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 Direction of radio source Delay Phase Correlator
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #9 Delay correction Received signals from two stations Delay correction Doppler shift / Fringe stopping
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #10 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
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #11 Correlator Design Grid Telescopes Input nodes Correlator nodes Output node Sorting and merging Time and channel slicing
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #12 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
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #13 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
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #14 EXPReS / FABRIC Collaboration with PSNC for data processing.
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #15 Success Story: FTP Fringe Tests FTP Fringe tests: short slices of an experiment to test equipment. Cluster running NICT correlator broke down in May 2007 SFXC was successfully used for the last three FTP Fringe Tests
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #16 Benchmarks on DAS-3 40% of realtime (4 telescopes, 256mbps each) on 30 quad-cores bottleneck is CPU power GPU's / FPGA's could enhance performance
April 10th, 2008REAL-TIME SOFTWARE CORRELATIONSlide #17 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