# ALMA Memo # 515

## Calculation of Integration Times for WVR

## Alison Stirling, Mark Holdaway, Richard Hills, John
Richer

2005-03-02

**Abstract**:In this memo we address the issue of how to apply water
vapour radiometer estimates of atmospheric phase to visibility data. We
consider the impact of smoothing the radiometer data over a period of time
to reduce the noise in the w.v.r. estimate, and applying a multiplicative
factor to decrease the impact of the w.v.r. estimate when the phase
fluctuation amplitude is small compared with the radiometer noise.

We find that when fast-switching is taken into account, for fully
three-dimensional turbulence, and r.m.s. path length fluctuations of order
75 micros', the optimal smoothing timescale is 11 seconds. This timescale
decreases to around 3 seconds as the thickness of the turbulence layer
becomes small compared with the baseline length. These values are found to
decrease as the r.m.s. fluctuations increase. A multiplicative factor is
required to modify the w.v.r. correction term for fluctuations less than ~
50 microns', where the noise in the radiometer becomes comparable to
fluctuations at the site.

View a pdf version of ALMA Memo #515.

*Last modified: 2005-03-03*

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alma-memos@nrao.edu