[evlatests] WVR outline

[Vivek Dhawan]

                                       Outline:
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Background:

  * Water vapour absorbtion (and emission) are
     enhanced near its line at 22GHz (and 183 GHz, for
     ALMA). The emission can be used to estimate the
     WV path, and correct for the delay (phase) change
     at any wavelength.

  * EA10 has a new WVR installed. 5-chan, 18-26GHz.
    It sits after the LNA, before any downconversion.

  * Raw data (Pon & Poff counts) demodulated at
     10Hz go into the monitor archive, 1sec time avg.
  * Being de-bugged. Nathan may want to say more.

Questions:

  * I believe (nay-sayers aside) the WIDAR TP/SP are
    pretty good for single-dish radiometry.  SO:

  * Are they good enough for WVR?

  * How do they compare with the 'real' WVR?

  * Can we use them (on 28 antennas) to debug the
    systematics, in preparation for the real thing?

  * VLBA next (?) leading up to NGVLA long baselines?

Answers:

  * Surprisingly good, very encouraging. See plot 1.
    Green is TP (Psum), red is phase, for several
    antennas with little gain variation, time ~45min.

  * TP has very good SNR, highly correlated with fringe
    phase.  But there are gain variations (doh!). Some
    are clearly in the T304s, some clearly in the LNA.

  * The scale factors are ~OK : 120deg phase @22GHz
    <==> 1K Tsys  <==>  5mm path length  <==>  1mm WV,
    where the numbers are variations around their means.

  * Tsys (Psys in AIPS) is gain-stabilized but lower SNR.
    Plot 3 same legend as plot 1. This is a 128MHz subband,
    improves by ~3 when averaged over a 1GHz band.

  * A new AIPS task SYVSN allows plotting SY-table Tsys
    vs. SN-table phases, with some knobs to play with.
    In progress. e.g. does smoothing of Pdif help?

  * The 'real' WVR (i.e. EA10 K band) needs Tcal corrected.
    Also in progress.

  * EA10 K-band also has a ~15sec gain variation on the LNA,
    both R and L.  No idea what makes this, its too slow for
    the cryo-pump cycle of ~2Hz(?). I went looking at other
    antennas to see if it was peculiar to the WVR, it is not,
    about 6-8 antennas show it. Will need to be squashed.
    see Plot 2.

  * etc.  - plot 4 is WVR archive data, needs some explaining.

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On 5/9/19 8:51 AM, Rick Perley wrote:> Â Â Â  10AM, Rm 317.
> 
> Â Â Â  Abbreviated agenda:
> 
> Â Â Â  1) Saving your science when 'Set and Remember Fails' (for any
> reason).  (Rick)
> 
> Â Â Â  2) Artificially induced antenna gain-elevation effects. (Rick)
> 
>     3) P-band collimation for the JVLA.  (Ken).
> 
> Â Â Â  4) Correlation of Switched Total Power with Observed Phase at
> K-band.  (Vivek and Eric).
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