The effect of the dynamic wet troposphere on radio interferometric measurements

Calculations using a statistical model of water vapor fluctuations yield the effect of the dynamic wet troposphere on radio interferometric measurements. The statistical model arises from two primary assumptions: (1) the spatial structure of refractivity fluctuations can be closely approximated by elementary (Kolmogorov) turbulence theory, and (2) temporal fluctuations are caused by spatial patterns which are moved over a site by the wind. The consequences of these assumptions are outlined for the very long baseline interferometry (VLBI) delay and delay rate observables. For example, at 20° elevations in midlatitudes, the wet troposphere induces about 2 cm of delay fluctuation for two‐station, 3‐hour observations. At 20° elevations for 200‐s time intervals, water vapor induces approximately 1.5 × 10 −13 s/s of delay rate fluctuation for two‐station observations, which corresponds to an Allan standard deviation of 1.2 × 10 −13 s/s. The statistical model suggests that delay rate measurement error is dominated by water vapor fluctuations for most VLBI experimental situations. Water vapor induced VLBI parameter errors and correlations are calculated as a function of the delay observable errors. For example, intercontinental baseline length parameter errors due to water vapor fluctuations are of the order of 2–3 cm. The above physical assumptions also lead to a proposed method for including the water vapor fluctuations in the parameter estimation procedure, which is used to extract baseline and source information from the VLBI observables.