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[John Lugten]

Hi All,

This is just a follow-up note to the discussion on the testing of a second
generation 183 GHz wvr.

Ideally, to best demonstrate that 183 GHz wv radiometry can correct the
atmospheric delay for the mma, one needs the wvr beam to be coincident with 
the observing beam through the lower atmosphere where most water vapor lies. 
For a test demonstration which compares the difference between two wvrs 
to the atmospheric phase meter, we would like a wvr beam to 
approximate a cylindrical column with a diameter equal to the diameter
of the phase meter beams (as Simon said, the phase meter samples a pencil
beam the diameter of the dish).

If one requires the diameter of the 183 GHz beams to have expanded by 
less than sqrt(2) at a height of 2 km above the dish, the waist size at 
the dish must be larger than,

	w_0 = sqrt(2000 m * lambda / pi) = 1.02 m 

This requires R_dish ~ 1.02 / 0.9 = 1.14 m or D_dish = 2.27 meters or
larger.

So, to really learn at what level the 183 GHz wvr system can correct the 
atmospheric phase requires a pair of fixed, good surface quality, 2 1/4 m 
diameter dishes each outfitted with one wvr and an 11.2 GHz receiver, 
preferably with coaxial beams. Because the 11.2 GHz beam pattern from a 
2 1/4 m dish is about a degree in diameter, it might be possible for the 
two feeds to share the focal plane with the wvr beam pointed directly at
the satellite and the 11 GHz feed still sufficiently on-axis to have 
only a modest loss of gain.

John