[evlatests] EVLA Flagging Issues

[Rick Perley]

    The (long) 'stress test' run yesterday evening has revealed clearly 
some significant issues with EVLA flagging.  I attempt a summary below. 

    One 'global' issue have already been aired thoroughly.  I simply 
summarize it here:

    1) The 'lost command' problem, which occurs when going to P-band 
from another band, from P-band to another band, and on any scan 
following U-band.  This results in bad data which are not flagged.  
About 10% of antenna-IFs are affected at any one instance.  There is no 
predictability of which ones are affected.  The fundamental cause is 
well understood, but its resolution await Barry and Hichem's return from 
travel.    For the time being, observers need to 'double up' their 
observations when the change takes place to minimize the effect (but not 
guarantee its absence). 

    2) Last evening's data showed the following at all bands:

    a) 16 B and D were oscillating in amplitude and phase -- and not 
flagged.  The cause is under investigation.
    b) 27A was not fringing at any band -- and all data were correctly 
flagged. 
    c) Antenna 11 went out near the end -- apparently a known 
subreflector issue.  The data were flagged. 

    3) We then find a number of band-dependent effects:
      
    a)  X-Band:  All flags set by the system were correct, and there 
were no data (other than 16B and D) which were bad and not flagged. 

     b)  C-Band: 
        *  Antenna 26 A and C were flagged out on one scan, but the data 
appear to be fine.  However, on this same scan, 26B and D did not 
fringe, and the data were not flagged!  Do we have some monitor point 
reversed?
        *  14B and D were flagged out on two scans, but the data appear 
fine.  OTOH, ant 23 and 25 on all IFs were flagged on one scan -- 
correctly. 
       *  1D was flagged throughout the whole observation.  The data 
were stable, but at 1/4 the proper amplitude.  So there is a real 
problem, caught by the system.  (No other band showed any problem on 
this antenna-IF). 

    c)  K-Band: 

          *  At this band, all flags set were correct, and except for 
the 'first observation past U-band problem', there were no data that 
should have been flagged. 

    d) Q-Band:
          *  As noted elsewhere, VLA antenna 8 gave no fringes, but the 
data were flagged as 'good'. 
          *  Antnenas 16, 17, 24 and 25, on IFs A and C only, gave no 
fringes on a number of scans (not all the same for these antennas), but 
there data were not flagged.  On the B and D IFs, all data were good.  I 
would presume this is a LO tuning issue -- but if so, should not the 
system detect the incorrect setup, and apply the flags? 

    e) L-Band:  We have a most curious (and perhaps alarming) situation 
at this band.   Approximately 5 to 10% of all antenna-IFs have been 
flagged bad by the system for an entire scan.  (In other words, if we 
attempted 10 observations with the 12 EVLA antennas, about 10% of the 10 
obs *12 ant *4 IFs antenna setups were flagged as bad).   The really 
interesting part about this is that in nearly all cases, these flags are 
set for a single IF only -- not all four at one time, or any pair (such 
as R,L, or R1 and R2).  The flags affect only one scan at a time.  Only 
the EVLA antennas are having this happen.  In nearly all cases, careful 
review of the data show that there is indeed a drop in correlated 
amplitude -- in most cases, the drop is quite small -- a few percent.   
There is a possible higher flagging probability of the B and D IFs -- 
but the difference from A and C is small. 
          So -- what is going on here?  Ken believes it likely we are 
seeing RFI-caused power fluctuations.   If the AGC voltage varies beyond 
a certain range, a flag is applied.  This is done independently for each 
IF.  But if this is the case, why are only the EVLA antennas being 
affected?  If due to the increased EVLA bandwidth, then the RFI is 
likely below 1.2, or above 1.9 GHz. 

    f) P-Band:  I didn't have the fortitude or patience to attempt a 
detailed comparison of flagged vs. unflagged data.  But it's clear we 
have numerous instances of antennas flagged bad which are good (antennas 
19, 23, 24, 25 and 26 stood out here), and some of antennas which are 
bad but not flagged (24D is a good example of this).