[evlatests] Subreflector rotation statistics

[Rick Perley]

     Paul, et al.:

     Are you sure it is only *rotation*, as opposed to focus?  In the 
various test I do, both are involved.  I'll soon have better 
information, as I'm nearly ready to seriously reduce the 'flux density' 
test data.

     Rick


On 04/03/2019 02:15 PM, Paul Demorest wrote:
> hi all,
>
> This is a report on an analysis of VLA subreflector rotation times I 
> recently did (some of you will have seen a version of this already; 
> there is a little new info in here but no change in basic 
> conclusions).  This was motivated by recent anecdotal reports from 
> operators and analysts about specific antennas often being flagged due 
> to subreflector rotation for much longer than expected, sometimes 
> resulting in their missing calibrators, etc.  I thought it would be 
> useful to take a more systematic look at recent data for problems like 
> this.
>
> First, the main conclusions are:
>
>  - There are several "bad" antennas that frequently spend >~10x the 
> time flagged due to subreflector rotation as the rest, often for 
> minutes at a time.  These are ea05, ea11, ea22, ea23, and ea25.  These 
> should be prioritized for FRM maintenance if possible.
>
>  - There are a few more "marginal" ones that show similar behavior but 
> not quite as severe (ea09, ea10, ea12, ea13, ea15).
>
>  - All the "bad" and "marginal" antennas have old ACUs.
>
>  - Not all old-ACU antennas act badly, for example ea03 and ea04 look 
> generally pretty well-behaved.  But even these "good" examples spend 
> typically ~50% more time flagged than new-ACU antennas.  So the new 
> ACUs and associated mechanical overhaul are clearly an improvement 
> (this is probably not news to many of you!).
>
> More details about this analysis:
>
> I gathered data on this from the SDMs currently available in the MCAF 
> workspace.  Right now this goes back to the beginning of the year.  To 
> avoid confusion from test/maint time, I only counted real science 
> observations, identified as those datasets that start with '1' or 'V'.
>
> For each day (MJD) I add up all the time each antenna is listed as 
> being in the SUBREFLECTOR_ERROR state in Flag.xml.  This only counts 
> rotation errors (I haven't looked at focus but could in the future).  
> Since there will be different numbers of band changes each day, I then 
> divide all the times by the median of the 10 best (least flagged) 
> antennas for that day.
>
> For a second statistic, I also looked at the duration of each flag 
> event.  For reference, a typical subreflector rotation for a band 
> change should take somewhere between 5 and 25 seconds depending on 
> which bands are in use; Rick took a close look at this recently, see 
> his emails to this list in Nov 2018 titled "Band Change Times."  The 
> assumption that band changes take ~20s is baked into our software in 
> several places (OPT, observing scripts).  I counted up all the 
> instances where an antenna was flagged for >30s or >120s, these will 
> be potentially bad for observations.
>
> Both of these metrics are plotted versus antenna number for a week's 
> worth of data at a time (starting on Wednesday evenings). The rotation 
> time plot has one point per antenna per day for a week.  The flag 
> duration counts are cumulative for the whole week.  See attached png 
> showing the most recent week, and pdf showing all available data.
>
> This analysis has an implicit assumption that all antennas are getting 
> commanded to do the same thing.  This will occasionally not be true, 
> for example if an antenna is removed from observing for part of a day 
> for some reason.  So isolated data points away from 1.0, or small 
> non-zero numbers of long-duration flags can probably be ignored.  But 
> long-term patterns where certain antennas have consistently 
> high/scattered points or many long-duration flags are meaningful, for 
> example the "bad" ones I mentioned above.
>
> The other situation that may confuse this analysis somewhat is 
> subarray observations.  To help avoid this, I've excluded all datasets 
> that used less than 24 antennas.  There may be some residual effect on 
> the first full-array observation following a subarray project since 
> the antennas will have different starting subreflector positions.  
> These have not been removed since they are more difficult to 
> automatically identify.  But I think this happens infrequently enough 
> that it's not a big problem.
>
> Please let me know if you have comments, suggestions, or questions 
> about any of this.
>
> Cheers,
> Paul
>
>
>
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