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Forming up to 450 Tied-Array Beams

Submitter: Jan David Mol, Jason Hessels, John Romein
Description: Tied-array beams are created by coherently adding the signals of multiple LOFAR stations. This results in a beam with a sensitivity equivalent to the sum of the station sensitivities, but with a primary field-of-view (main lobe) that decreases with the distance between stations. To regain field-of-view, multiple tied-array beams are synthesized within the station field-of-view.

The number of beams that can be formed depends on both the amount of data that will be generated and the amount of computing power required to form the beams; this is shown in the top-left figure. At full spectral and temporal resolution, each beam generates 6 Gbit/s of data and we can create 11-13 such beams before reaching our maximum output capacity of 70-80 Gbit/s (depending on the amount of station input). If just the signal's total power is recorded (Stokes I), a full beam is only 1.5 Gbit/s of data, which can be further integrated in time to allowing for an additional reduction in data volume. If the data rate is reduced enough (16x downsampling in the figure), our supercomputer reaches its limits in the number of beams it can form. More stations to add means fewer beams are possible.

Three cases are highlighted for which the figures at the bottom show the load across the supercomputer. The left figure represents the 64 I/O nodes, which handle station input and beam output. On the right, the system load is shown for the 4096 compute cores, which perform the actual data exchanges and beam forming. Maximum load is avoided for reliability reasons. Case A represents the highest number of possible beams, 450, and is limited by the high number of data streams flowing through the supercomputer's internal network (> 100,000). The computational limit is reached in case B, and the output bandwidth limit is reached in case C.
Copyright: ASTRON
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