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ALMA Integrated Computing Team Coordination & Planning Meeting #3 Socorro, 17-19 June 2014. Observation with ACA correlator for Cycle3 Manabu Watanabe NAOJ. Background & purpose. ACA correlator has a bug to get bad channels .

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alma integrated computing team coordination planning meeting 3 socorro 17 19 june 2014

ALMA Integrated Computing TeamCoordination & Planning Meeting #3 Socorro, 17-19 June 2014

Observation with ACA correlator for Cycle3

Manabu Watanabe

NAOJ

background purpose
Background & purpose
  • ACA correlator has a bug to get bad channels.
  • In MBR telecon on 16 May 2014, we have made decisions for Cycle 2 as:
    • Full array mode will be used with ACA correlator.
    • FPS will be used even when bad channels will be output.
    • The fact is that 5 B grade (1.4%) among 353 A+B projects for Cycle 2 require ACA where bad data will be emerged at up to tens channels near the band edges.
  • The bug will not be likely fixed yet for Cycle 3.
  • How could we live with the ACA correlator for Cycle 3 given the persisting bug?
problem overview
Problem overview
  • ACA correlator frequently outputs bad data at multiple channels during observations.
  • We believe the problem is caused by a few bugs in logic circuit of FPGAs in the ACA correlator.
  • The location of failure in the logic circuit of FPGA is not determined yet.
  • Experience has shown that the occurrence or absence of bad channels depends only on the spectral spec, but not on the input data.
investigation and fix
Investigation and fix
  • It will take a long time to understand thoroughly & resolve the underlying cause.
  • Plan for 2014 fiscal year
    • May- and Jun-2014: a preparation phase. the posterior analysis of the problems.
    • Jul-, Aug- and Sep-2014:the logic design analysis, mapping between failures and circuits. small-scale simulations.
    • Oct-, Nov- and Dec-2014:large-scale simulations to reproduce the problem and to give a theoretical grounding to the occurrence condition.
    • Jan-, Feb- and Mar-2015: the proposal of the fix and/or countermeasures
study of occurrence condition
Study of occurrence condition
  • No theory for the error prediction
    • Brute force attack
  • 56,344 spectral specs have been confirmed at present
    • Up to 4-spw for the full-array
    • 1-spw for 2-subarrays
  • Bad channels only with FPS (No errors w/o FPS, so far)
    • Full-array: 3% cases affected (360: errors/11,478: no errors)
      • 7.6kHz- or 15.3kHz-channel spacing for 1-polarization.
      • 15.3kHz-channel spacing for 2- or 4-polarizations.
      • 1GHz BW, 1ms dump duration, 1-polarization & 64ch for the last-spw.
    • 2-subarrays (808: errors/888: no errors)
      • Error for about a half of confirmed spectral specs.
workaround proposal for cycle 3
Workaround proposal for Cycle 3
  • Full-array mode with ACA correlator.
    • But at the same time, we will continue the brute force attack for the remaining spectral specs.
    • Up to 4-spw for 2 subarrays which share the same spectral specs between them by March 2015.
  • Add the usage information of FPS to ASDM
    • This information is needed intrinsically for ARCHIVE users.
  • Align spw in ascending order of bandwidth by ALMA-OT
    • This will dramatically reduce the number of spectral specs to be tested in the light of errors with the 64ch last spw.
  • 4 options for the FPS (next slide)
    • We recommend the “FPS online with CDP computers” option
4 options for fps for cycle3 1
4 options for FPS for Cycle3 (1)
  • No FPS
    • Difference will be 2%-3% typically, 10% at most compared to the frequency profile of the BL correlator.
  • FPS online with ACA correlator
    • Live with the bad channels problem.
  • FPS online with CDP computers
    • Difference will be 1%-2% typically, 5% at most compared to that for FPS with ACA correlator.
  • FPS offline with CASA
    • Equal to FPS with CDP for the spectral averaging factor = 1
    • No simulation yet for the spectral averaging factors > 2