03jul03 bsm

Questions and Issues in Connection with 
-the GB site
-mm observing at the GB site
-PTCS boundary conditions

=======================================================
The Basic Questions

 A. To what extent is anomalous refraction likely to be a
	limitation for the GBT at > 40 GHz (say)? Does this
	influence the PTCS pointing strategy?

	Is there anything we can do with existing or planned
	instruments to mitigate the impact? Do we need to
	consider building something else and is this even
	remotely feasible?

 B. How much observing time, REALLY, can we expect at 3mm in
	Green Bank?

 C. Do we have the right suite of instruments to give a zeroth-
	order realtime assessment of the suitability of
	weather conditions for high frequency observing?

=======================================================

A way to break them down:

I. Atmospheric Opacity Models (ie the opacity of the flat slab)
 A. Critical Comparison of the usefulness of existing 
	atmosphere codes: AT (Grossman-- old i think),
	MPM (Pardo-- better?); ATM (Pardo -- best?).
	Kim C. points out there are some military models
	which would be worth looking into. At the level
	we care about, and at 3mm wavelengths, I'm not
	sure there are important differences, but I'm not 
	sure there aren't.

 B. From the Models: is there a reasonable opacity to
	pwv translation (eg, at 86 GHz)? how much does
	this depend on other variables? What is the
	pwv=0 (dry) opacity? How robust is an 86 GHz to
	45 or 20 GHz tau translation, and what is it?

II. Models of Atmospheric Fluctations
 A. For Large Single Dishes: there is a substantial body
	of work where people have attempted to model the
	atmosphere to understand interferometer or
	small single dish experiments (see papers by
	Lay; Lay & Halverson; and S. Church [1995]).  
	The fluctuations for large single dishes 
	are not as well studied. Are near field effects
	important? The contribution through sidelobes?

 	A key question: what levels of atmospheric 
	spatial variation across an array detector 
	(30'' FOV) can be for the GBT expected, and 
	how much variation in this with time?  

 B. Anomalous Refraction: there has been some empirical
	characterization of AR with single dishes (Olmi 2001;
	Altenhoff 1987), but little attempt to relate
	these models to the fluctuation models. The relation
	has been considered for interferometers by Lay (1997)
	and others. It would be interesting and useful to
	look at this for the GBT.  Also, to relate this
	to the 12 GHz interferometer measurements we have.
	Also to relate this to the total power gradient
	in emission (which could potentially be seen by 
	an array detector)

--> In all of this the atmosphere models are probably very uncertain,
but even order of magitude, or best/reasonable/worst, estimates, would
be helpful.

III. GB Site Data

 A. Tipper Data: is the current way of analyzing the
	tips reasonable & robust? is there a better way?
	Is the tipper useless and we should consider something
	else (tipping the GBT-- but why would this be better?;
	or GPS line-of sight measurements)?  Is the difference
	in the tipper far-field, and the GBT far field important?
	
 B. Cumulant fcns of tau(86) for a number of years. How
	much does this change from year to year? -- at
	the Atacama site, a lot; and at GB, this also seems
	likely.

 C. Similar cumulants for the interferometer would be 
	interesting.

 D. Combine wind, interferometer, and opacity constraints
	to estimate how much observing time per year we
	will get for several interesting opacity thresholds
	at 3mm (tau>0.1; tau>0.15; tau>0.2 say).  

 	Note that constraints for 3mm continuum and spectral
	line are potentially different. 3mm (large array)
	continuum wants low tau most; 3mm spectral line wants
	reasonably low tau, but loses more sensitivity to
	uncorrected pointing errors.

	Are there other environmental envelope effects which,
	from the PTCS side, should be considered in this
	analysis? Converseley, do the results of this analysis
	indicate that the PTCS environmental envelope should
	be more (or less) stringent? (whatever the envelope
	is)

IV. Observational Tests of Anomalous Refraction

 A. As per II.B above: can a reasonable analytical 
	or numerical relation of the actual (or RMS?) 
	interferometer phase on the fixed 100m baseline, 
	to the expected 1-axis anomalous refractive 
	pointing correction at a given az/el 
	(or its RMS...?) be made?

	if so and this is significant:

 B. Attempt to assess the contribution of anomalous refraction
	to halfpower tracks under a variety of conditions.	

    Perhaps this could be done even in the absence of an understood
	correlation, by transforming the data into the appropriate
	coordinate system and looking for correlations.