Memo Review

Memo: 415 - Phase Correction using Submillimeter Atmospheric Continuum Emission
      Matsushita, Matsuo, Wiedner, and Pardo

Reviewer: Mark Holdaway

Date Received: 2002Aug23


I think it is likely that the basic points of 415 are correct, ie,
that if you are doing WVR with continuum measurements, they will be
more sensitive in the submillimeter, less sensitive to liquid water,
and also that multi-frequency measurements will permit rejection of
liquid water.

However, because of a host of questions about this method's utility,
I don't think we should change the design of the ALMA to meet the
requirements of this calibration method.

Some issues I have, small and large:

* 11.5 um is the specification for the instrumental part of the
phase error.  Larger total phase errors (ie, in part due to the
atmosphere) will occur.

* It is stated that 183 GHz WVR is the most convincing phase
correction method.  It would be more accurate to say that 183 GHz is
the most hoped for phase correction method.  So far, no radiometric
phase correction scheme has produced the required accuracy for ALMA,
and worse, we still don't have a quantitative understanding of the
errors involved.  Fast switching should at least be mentioned.

* It is never stated how much better (or worse) this technique
might be than 183 GHz WVR in the presence or absence of liquid 
water.  In part that is because we don't know how to quantify
the WVR residual errors.  

* A critical piece of missing information that is required to
judge how useful the submillimeter continuum monitoring might
be is: "what fraction of the time will there be appreciable liquid
water over chajnantor"?  I am guessing 10-20%

Now, of that time when liquid water is a problem over the site, 
we have other questions to ask:
      - can spectral line 183 GHz WVR succesfully reject
	the liquid water?  (183 GHz WVR is in the project 
	to stay)
      -	what fraction of that time has poor opacity, precluding
	observations at any but the lowest frequencies?
      -	what fraction of that time would result in system
	temperature fluctuations so large as to make amplitude
	calibration unreliable?
      -	the phase recovered by fast switching is not affected by
	liquid water, so if the opacity and Tsys fluctuations
	were not too severe, fast switching could recover some
	time affected by liquid water.

I contend that these additional factors would make the utility of
this method a small incremental improvement over what we are
currently planning for the ALMA.

Now, another set of issues:  how easy is it to make the submillimter

* very stable recievers or well-calibrated gains are required
* small systematic errors (variable ground pickup) will be a problem
* ALMA is not designed to perform dual frequency observations, and
  only two bands will be ready to go, one at a time, at any
  given time. 
* we don't yet know how well we can remove the liquid water
  with the dual frequency technique

Some submillimeter observations could use this technique "for free".
Only software and some algorithmic work will be required (assuming the
recievers are stable enough, which they may not be if we are not
forcefully backing this proposal).  If we were observing at the
millimeter, we would need to perform "fast frequency switching" to
utilize this method.  In that case, the errors we experience will be
similar to the residual errors in fast switching:  we will
perform the calibration measuring the sky at submillimeter
frequencies, switch to the target frequency (but not moving the
antenna), and integrate on source while the atmosphere changes
above us.  (Yes, we DON'T need to move the antenna, but the residual
errors in fast switching are always dominated by the time terms,
ie, the atmosphere changing above us with time.  The time
overhead required to change frequencies is about the same as the
antenna move time and settle time.)

If we desired the multi-frequency approach to explicitly remove the 
liquid water, we have the same sort of problems, but we will ALWAYS
have to do the frequency switching thing.

For the SMA, this is probably a good research topic.  Multi-frequency
observing is possible, and the SMA may provide more of a research and
development environment.  For the ALMA, I think we need more solid
evidence of improvement for a major change in design to be implemented.