ALMA Data Rate Concerns

Comparison of ALMA and SMA correlator capacities for maximum uniform resolution observations

Telescope Polarization mode Max channels Bandwidth Resolution Band 3 (90GHz) Band 6 (230GHz) Band 7 (345GHz)
ALMA single 8192*4 4*(2 GHz) = 8 244 kHz 0.82 km/s 0.32 km/s 0.21 km/s
dual 4096*4   488 kHz 1.64 km/s 0.64 km/s 0.43 km/s
cross 2048*4   976 kHz 3.28 km/s 1.28 km/s 0.86 km/s
SMA single 6144*2 2*(2 GHz) = 4 320 kHz N/A 0.43 km/s 0.28 km/s
dual 3072*2 2*(2 GHz) = 4 640 kHz N/A 0.56 km/s
2 bands 3072*2 2*2*(2 GHz) = 8 0.86 km/s

  • The peak data rate quoted in the project book = 60 MB/sec
  • This implies a peak data rate per baseline with 50 antennas = (60 MB/sec) / 1225 baselines = 49.0 kB/sec
    • For 64 antennas, it would be (60MB/sec)/2016 kB = 30 kB/sec
  • With 50 antennas, if I want 8192 channels * 4 quadrants * 4 bytes (i.e. 4 bytes per complex number visibility), then I need 131kB per baseline per integration. So I am limited to T=(1225*0.131MB)/(60MB/sec) = 2.67 second integrations.
    • With 64 antennas, this number becomes 5.0 seconds.
  • But the minimum dump time is listed as 0.256 seconds, so this implies that the maximum supported data rate needs to be 626 MB/sec when the data rate is upgraded in the future.
    • For 64 antennas, this number becomes 1.03 GB/sec
  • With the current data rate limit, if I want to use 0.512 sec dumptimes, today I would be limited to 6283 total channels.
    • If I spread these evenly across 8 GHz, then I would have 1.3 MHz channels.
    • If I require 0.3 km/s resolution at 230 GHz (0.23MHz), then I could only cover 1.4 GHz at once.
    • If I require 0.3 km/s resolution at 90 GHz (0.09MHz), then I could only cover 0.55 GHz at once.

Data rate calculations for OTF interferometry

Topic attachments
I Attachment Action Size DateSorted ascending Who Comment
datarate_new.xlsxls datarate_new.xls manage 98 K 2009-10-15 - 11:06 ToddHunter Data rate calculations for OTF inteferometry of 2 lines; and highest uniform correlator resolution
Topic revision: r5 - 2017-12-31, ToddHunter
This site is powered by FoswikiCopyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding NRAO Public Wiki? Send feedback