The effect of the dynamic wet troposphere on radio interferometric measurements

Abstract

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 ø elevations in midlatitudes, the wet troposphere induces about 2 cm of delay fluctuation for two-station, 3-hour observations. At 20 ø elevations for 200-s time intervals, water vapor induces approximately 1.5 x 10 -xa s/s of delay rate fluctuation for two-station observations, which corresponds to an Allan standard deviation of 1.2 x 10-t3 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. the differential phase of an electromagnetic wave be-phase delay rate observables will in turn suffer errors tween two antennas on the earth's surface. Typically due to the fluctuating water vapor, or wet tropothe waves originate from compact extragalactic radio sphere. Typical estimated parameters include basesources, such as quasars. The inferred group delay, line length, baseline orientation, radio source posiwhich is the primary VLBI observable, reflects the tions, and clock offsets [Shapiro, 1976; Sovers et al., distance between the two antennas as well as the 1984]. In most intercontinental VLBI data reduction, angle between the vector connecting the antennas zenith tropospheric delay parameters are also estiand the vector pointing to the radio source [Gold, mated. This technique essentially accounts for a spa-1967; Rogers, 1970; Thomas, 1972]. In astrometric tially and temporally average troposphere, for each and geodetic applications, delays other than the geo-station, for the time over which the zenith parameter metric delay are regarded as errors in the VLBI is estimated. The wet troposphere fluctuations measurement. The presence of atmospheric water around these averages are then the dominant tropovapor along the lines of sight from each antenna to spheric errors, which map to errors in VLBI astrothe source will affect the index of refraction of the metric and geodetic parameters. traversed medium, and will therefore corrupt the geo-When centimeter-level VLBI observable ...

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