[evlatests] "pulsing" explanation

[Ken Sowinski]

On Sat, 23 Jun 2007, Rick Perley wrote:

>    A)  Pulsing Data.
>
>    At all bands and all IFs and all sources, at all times, data from
> most VLA antennas is 'pulsing'.  There are three very clear behaviors:

Here are the gory details as we understand it this afternoon.
Corrections welcome.

Rick reported this based on data taken Friday night, June 22.
It has been happening on and off since then.  After much
confusion this afternoon we think we know that this was a
result of two recent changes.

1.  A change was made to the vxWorks half of the CMP which
made it more prone to lose track of time.

2.  A change was made to the executor which as a side effect
caused the CMP synchronizing command to be sent only once
per executor execution rather than once at the beginning of
each script.

The combined effect of these is that once the CMP lost sync
it never recovered unless a new executor was started.  The
insidious part was that we thought the problem was deep
becuase we all assumed that the CMP was being resynchronized
at every new script.


The change to the CMP has been backed out and we will revisit
it on Hichem's return.  This should make the CMP less likely
to lose sync.  Barry has fixed the executor bug; we will test
it tomorrow or Thursday.


I think that the phase errors which lead to large apparant antenna
position changes are independent of this.  Data is being taken
now to verify or disprove that statement.