[evlatests] Interesting behavior of PDif at S-band

[Rick Perley]

    The 23 or so S-band databases that I've been processing over the 
past two weeks have taught much about how to best handle S-band 
calibration. 

    One of the key lessons is that for observations within ~30 degrees 
of the 'Clark Belt' (say, declinations from -35 to +25), application of 
the switched power is important -- and essential if the observations are 
with ~20 degrees of that geostationary belt.   A later circular will 
give the details. 
   
    This discovery has led me to look closely at the S-band switched 
power quality.  The good news is that, with the rather minor exceptions 
noted below, it is of excellent quality.  There are only two antenna-IFs 
with any kind of problem with the PDif.  I've attached plots for these, 
so interested parties can see directly what the issues are.   Both of 
the attachments show, from top to bottom:

    PDif (the demodulated synchronous power)
    PSum (the total power as measured at the correlator)
    Tsys (the ratio of the above two, multiplied by the tabulated value 
of Tcal. 

    Comments  on the three wayward antenna-IFs:

    1) ea06L.   PDif is oscillating over a fixed range of about 10%.  
The oscillations are not sinsuoisal, but are certainly smooth.  The 
period is typically 30 seconds.  The PSum values are not affected -- 
hence this is not due to a gain change.   The antenna is at N56.  This 
behavior has been seen since the receiver was first installed.  It has 
never behaved normally. 
   
    2) ea16R.  Exactly the same behavior as ea06L, but at a lower level 
-- 3%.  What is *truly amazing* is that these oscillations in PDif are 
an exact mirror reflection of those in ea06L!!!  (Check it out!).  How 
can the synchronous systems for these two antennas 'know' about each 
other?  This antenna is at N08.  As for ea06, this behavior (including 
the mirror behavior) has been in this antenna since the beginning. 

    Neither of these problems is critical -- application of these PDif 
values does 'fuzz up' the antenna gains a bit, but  not by a sufficient 
factor to hurt sensitivity on weak objects.  If the target is strong, 
then these gain variations do matter, but self-cal will then repair the 
damage.   



   
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ea06S-LCP.png
Type: image/png
Size: 60943 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20140129/4d53b420/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ea16S-RCP.png
Type: image/png
Size: 64067 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20140129/4d53b420/attachment-0001.png>