[asac][almanews] ALMA Memo 496 Released

[Stacy Oliver]

ALMA MEMO #496 

183 GHz water vapour radiometers for ALMA: Estimation of phase errors
under varying atmospheric conditions. 

Alison Stirling, Richard Hills, John Richer, Juan Pardo 

2004-05-18
 
We investigate the use of water vapour radiometers as a tool for
estimating phase due to atmospheric water, focusing on the impact of
differing atmospheric conditions on the relationship between path length
and brightness temperature. We provide a formula for converting between
the two for a variety of atmospheric conditions, and outline how the
radiometer channel temperatures may be combined to give an optimal
estimate for the path difference. This estimate gives an error of about
2% per mm of precipitable water vapour due to atmospheric variations.
The presence of hydrometeors such as ice or water droplets is also
considered, and we show that radiometers possessing sideband separation
could be used to detect the presence of 0.02 mm of column integrated ice
for crystals of size 75µm,
and about 10¯³mm of water droplets. 

View a pdf version of ALMA Memo #496. 
http://www.alma.nrao.edu/memos/html-memos/alma496/memo496.pdf



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