[evlatests] Results from Switch Power Calibration Test

Thu Jun 21 21:13:06 EDT 2012

you'll get the wrong opacity, because values from the old weather station are still being stored in the SDM, and it thinks the humidity is 100%. when i get home i can calculate the magnitude of the difference...

       -bryan

Sent from my iPhone.

On Jun 21, 2012, at 17:30, Rick Perley <rperley at nrao.edu> wrote:

>    Finally, the actual results from the calibration test run two days 
> ago.   (Previous reports all dealt with problems found in the system).   
> This is a long report!  Impatient readers should skip to the bottom, and 
> read the 'Summary' and 'Recommended Action' sections. 
> 
>    The goal of the experiment was to see if simple application of the 
> switched power (and the associated Tcal and antenna efficiencies) 
> provided correct gain calibration for a source of know flux density. 
> 
>    To do this, I observed 3C286, at an elevation of 55 degrees, using 
> the old narrowband (two subbands) OSRO mode, with a single pair of 
> 128-MHz-wide subbands within each of the eight cassegrain bands.  For 
> each band I chose two frequencies, reasonably well separated, which are 
> known to be free of RFI. 
>    The target source (3C286) was observed twice at each band.  The band 
> sequence followed was:  L, S, C, X, Ku, K, Ka, Q.  Prior to the X-band 
> observation, referenced pointing was done to try get the main beam 
> squarely on the target source, and the results applies to the subsequent 
> high frequency bands. 
>    The weather was clear, but rather breezy (winds 15 to 20 mph).  
> Unfortunately, the time chosen was about 6PM, so the referenced pointing 
> offsets were remarkably large -- over an arcminute for most antennas!  
> They also changed significantly for some antennas between the two 
> referenced pointing determinations, perhaps a result of the wind.  These 
> problems certainly affected the two highest frequency bands, as the 
> diligent reader will note in the results given below. 
> 
>    The data will filled using the latest gain curves (updated this 
> morning), and using the default atmospheric opacity model. 
>    The data (both visibilities and switched power values) were edited 
> for the problems noted in previous reports.   The AIPS program 'TYAPL' 
> then produced a corrected visibility file.  Basic calibration (delays 
> and bandpasses) was done.  The antenna gains were then generated, using 
> modern (and we think correct) values for the flux density of 3C286.  The 
> results of this are the gain corrections needed to convert the 
> visibilities to correct flux densities.  A perfectly calibrated system 
> will then show gain values of 1.0.  Values greater than 1.0 mean the 
> visibilities are too low, and values less than one mean the visibilities 
> are too high. 
> 
>    The process of converting raw visibilities into corrected 
> visibilities involves the values of the switched power noise diode 
> (Tcal), and the antenna efficiency, epsilon.  The sense of the 
> correction is such that:
> 
>    - If the values of Tcal that are utilized are lower than the true 
> value, then the resulting visibilities will be too low, causing the 
> derived gain values will be greater than 1.0
>    - If the values of the antenna efficiency that are used are lower 
> than the true value, then the resulting  visibilities will be too high, 
> causing the derived gain values to be less than 1.0. 
> 
>    From the derived gains, I plotted histograms for each frequency at 
> each band.  For each of these, I give below the median offset, and the 
> fraction of the antennas which are with 5% and 10% off this median. 
>    For each band, there are usually a small number of antennas which 
> are truly discrepant.  These are identified in the listings below.  Some 
> effort into understanding why these antennas are discrepant would be 
> useful. 
> 
>    1)  L-band
> 
>    Median gain = 0.95 for all IFs.  50% of the antennas are with .05 of 
> this, 85% are within 0.10. 
>    Four antennas are very discrepant:
>        ea04 on all IFs.  Median gain = 1.35
>        ea17 on IF 'A' only, gain = 1.28.  (PDif values are clearly 
> weird for this IF).
>        ea19 on all IFs.  Median gain = 1.35
>        ea28 on all IFs.  LCP was flagged out (PDif = 0), RCP median 
> gain = 1.35. 
> 
>    2) S-band
> 
>     Five antennas are greatly discrepant.  The statistics below ignore 
> these:
>     Median gain = 0.92 for A&C, 0.90 for B&D.  75% of antennas are with 
> .05, 100% within .10
>    The five discrepant antennas are:
>           ea01 on all IFs. Median gain = 0.30.  (The Tcal values must 
> be way to low --  I bet it's still at 1.0)
>           ea07 on all IFs.  Median gain = 0.6
>           ea04 on all IFs.  Median gain = 1.5
>           ea10 on all IFs.  Median gain = 1.3
>          ea12 on all IFs.   Median gain = 1.3
>    In addition to these, ea16 is bad on the AC side only:  median gain 
> = 1.15 
> 
>    (It gets a lot easier from here).
> 
>    3)  C-Band
> 
>    Median gain = 0.95 for all IFs.  75% are within .05, 100% are within .10
>    There are no seriously discrepant antennas. 
> 
>    4) X-Band (Wideband systems only)
> 
>    Median gain = 0.97 at 8.2 GHz, and 0.92 at 11.3 GHz.  50% are within 
> .05, 90% within 0.10. 
>    There are no seriously discrepant antennas. 
> 
>    5) Ku-Band
> 
>    Median gain = 0.95 at 16.8 GHz.  50% within .05, 90% within 0.10.  
> Because of the phase wrapping problem noted yesterday, no useable data 
> from the other IF were gained.
>    There wee no discrepant antennas.  However, the gain distributions 
> show that antennas low (or high) in one polarization are also low (or 
> high) in the other -- suggesting an efficiency origin.  For example, 
> ea22 and ea23 are at the low end of the distribution in both 
> polarizations, (tabulated efficiency probably too low) while ea11 and 
> ea20 are both on the high end (tabulated efficiency probably too high). 
> 
>    6) K-Band
> 
>    At 19 GHz:  Median = 0.92; 70% within .05, 85% within .10
>    At 25 GHz:  Median = 0.90;  50% within .05, 80% within .10
>    ea10 is quite discrepant on all IFs:  median gain = 1.15.   
>    ea23 gain = 1.2 in IF 'D',
>    ea14 gain = 1.15 in IF 'D'
> 
>    7) Ka-Band
> 
>    At this band, the troubles with pointing stability begin to show up, 
> causing the much wider distribution, with the tail stretching to the 
> high gain end (the visibilities are too low due to the pointing, making 
> the gain corrections too high).  For both this band and Q-band, I've 
> used the smaller of the two gain solutions, since this one is much more 
> likely to be closer to the truth. 
> 
>    At 29 GHz:  Median = 0.90.  65% within .05, 90% within .10
>    At 36 GHz:  Median = 0.87.  60% are within .05, 75% within .10
>    Serious Discrepant Antennas:
>        ea17 on all IFs:  gain = 1.25
>        ea08 on all IFs:  gain = 0.72 at 29 GHz, and 0.62 at 36 GHz.
>        ea14 on all IFs:  gain = 0.72 at 29 GHz, and 0.62 at 36 GHz. 
> 
>    8 Q-Band
> 
>    At 41 GHz:  Median = 0.80.  40% within .05, 60% within .10
>    At  48 GHz:  Median = 0.85.  30% within .05, 50% within .10.
>    But in fact, the median is probably lower than 0.80 at 48 GHz, as 
> the distribution shows a long tail extending to higher values, doubtless 
> due to the pointing offset. 
>    The only antenna identifiably bad (separate from pointing, unless 
> its pointing is truly abysmal) is ea03, whose gain is 1.10 at 41 GHz, 
> and 1.25 at 48 GHz. 
> 
>    Summary
> 
>    At all bands -- ignoring the few discrepant antennas, and presuming 
> the spread at the high frequencies is pointing-induced -- the agreement 
> in the gains is very good.  The only significant issue other is the 
> median offset.  Correcting for this should enable quite accurate 
> amplitude gain calibration to be made -- good to 5% for sure -- without 
> use of a standard source. 
> 
>    Recommended Actions
> 
>    I think the most likely cause of the median offset is an error in 
> the tabulated antenna efficiency.  I suggest that a special version of 
> TYAPL be generated in which the antennas gain value is modified by the 
> appropriate values, to produce gains with no median offset.  Another 
> test observation of 3C286 should then be made, but this time late at 
> night and under clear calm conditions to confirm the sense and magnitude 
> of the changes. 
>    If this proves good, then a more extended observation, utilizing a 
> wider range of elevation, should next be made, to check the robustness 
> of the opacity and elevation correction procedures.   (This can be part 
> of a real science observation, simply by adding in 3C286 once an hour, 
> for example). 
> 
> 
> 
> 
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