Differences
This shows you the differences between two versions of the page.
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projects:meerkat:compest [2016/05/23 14:23] wucknitz |
projects:meerkat:compest [2016/12/13 12:17] (current) 134.104.31.74 |
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| Summing up the stations now "only" takes 4*N_a*N_b*bw floating-point operations per second (factor 4 is for two polarisations and complex beams). This is typically 90 TFLOPS, but these can be combined with the phase corrections using fused multiply/accumulate operations. | Summing up the stations now "only" takes 4*N_a*N_b*bw floating-point operations per second (factor 4 is for two polarisations and complex beams). This is typically 90 TFLOPS, but these can be combined with the phase corrections using fused multiply/accumulate operations. | ||
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| ==== Incoherent sums ==== | ==== Incoherent sums ==== | ||
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| We can form additional beams from visibilities. For this we assume that we have only one polarisation product (typically Stokes I) and a decimation factor in time/frequency of N_d. Per (decimated) sample, each beam requires the addition of N_a^2/2 visibilities after applying phase factors (4 MUL and 2 ADD). This means 3*N_a^2*N_b*bw/N_d operations. With a decimation of N_d=10 and N_b=400 beams, this means 430 TFLOPS, not so much more than the direct beam-forming. But now we can trade resolution in time/frequency for number of beams. Going from 50 microsec time resolution to 500 microsec boosts the number of beams to 4000. At higher frequencies (e.g. for GC searches) we can average more in frequency without losing any science. | We can form additional beams from visibilities. For this we assume that we have only one polarisation product (typically Stokes I) and a decimation factor in time/frequency of N_d. Per (decimated) sample, each beam requires the addition of N_a^2/2 visibilities after applying phase factors (4 MUL and 2 ADD). This means 3*N_a^2*N_b*bw/N_d operations. With a decimation of N_d=10 and N_b=400 beams, this means 430 TFLOPS, not so much more than the direct beam-forming. But now we can trade resolution in time/frequency for number of beams. Going from 50 microsec time resolution to 500 microsec boosts the number of beams to 4000. At higher frequencies (e.g. for GC searches) we can average more in frequency without losing any science. | ||
| This sounds realistic, provided we can search all these beams. But they do not have to be searched in real-time. Still, they may have to go into the switch again, which may limit us. | This sounds realistic, provided we can search all these beams. But they do not have to be searched in real-time. Still, they may have to go into the switch again, which may limit us. | ||
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| + | Also we must not forget that decimation in time and/or frequency reduces ouf field of view because of bandwidth and time-averaging smearing! | ||
| An FFT can be used for this "imaging" step, but this makes everything much more complicated, because visibilities would have to be gridded, which is probably not very efficient on GPUs. | An FFT can be used for this "imaging" step, but this makes everything much more complicated, because visibilities would have to be gridded, which is probably not very efficient on GPUs. | ||
