Daily Image
13-08-2008
First fringes with the Blue Gene/P correlator
| Submitter: | John Romein |
| Description: | On Monday August 4th, new 10 Gb/s Ethernet transceivers were installed in the network switches in Exloo and Groningen, connecting the LOFAR stations CS001, CS010, and CS016 to the correlator. The correlator itself was recently upgraded from a Blue Gene/L to a Blue Gene/P. With only a few months to prepare the upgrade, we worked hard to port the software from the BG/L to the BG/P, and a prototype of the new correlator was able to observe immediately after the wide-area network became functional. Even to our own surprise, the software did what it was supposed to do. This picture shows fringes in one of the 36 subbands (~60.5 MHz) of a 12-hour observation, taken in the night from last Tuesday to Wednesday. The picture was made with Michiel Brentjens' uvplot tool. The red and green components represent the real and imaginary parts of a correlation. The intensity represents the amplitude; pixels are colored white if the amplitude exceeds some threshold, typically due to RFI. Blue pixels are flagged, as is the case with channel 0. The picture shows the beating between a few strong celestial signals (mainly Cas A and Cyg A) on a short baseline between two dipoles in CS001. Frequency channels are on the horizontal axis, and time is on the vertical axis. Each pixel represents a 762 Hz, 30 second correlation. Delay compensation was turned off, to make more fringes visible. Most of the bandpass is taken out (a ripple of a few percent, invisible to the human eye, remains). Although the BG/P does not differ that much from a BG/L, we had to make significant changes to the software. The biggest difference comes from the fact that the BG/P moved from 1 to 10 Gb/s Ethernet technology, while the number of I/O nodes decreased from 768 to 140. As a consequence, each I/O node has to process much more data than on the BG/L (but they are not that much faster!) The I/O nodes now receive data from 4 RSP boards instead of only one, and buffer the data for about 5 seconds. Since the standard network software did not provide sufficient bandwidth, we developed a new network protocol that forwards this data at wire speed over the internal links to compute nodes. These compute nodes transpose, filter, and correlate the data. The work is not finished yet: the quality of the software is not yet exactly what we call "production level", and the processing speed on the I/O nodes must be improved to support the data rates that are required for LOFAR20. But looking at the picture that shows the first fringes feels like victory! |
| Copyright: | ASTRON |
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