The influence of atmospheric loading on VLBI-experiments
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Air pressure lows and highs with periods of some days and seasonal variations of mean air pressure can be regarded as loading functions on the Earth's surface. They result in quasi-periodic surface deformations. The influence of such displacements on the results and the accuracy of VLBI experiments (Very Long Baseline Interferometry) is investigated by simulation calculations for the transatlantic Effelsberg-Haystack baseline. Different models for the time dependence of air-pressure-induced station displacements are considered. It is shown that today's standard VLBI data analysis, including model parameters for clock- and atmospheric-refraction effects, is not able to correct the measurements for atmospheric loading effects. It leads to erroneous baseline vectors. Hence, for every geodetic VLBI experiment, the amount by which (1) the local air pressure at the station and (2) the mean air pressure in a surrounding area of 2,000 km radius has changed during the experiment should be tested. These two values give an estimate of the resulting vertical displacments by the use of a regression formula. The corresponding delay-time corrections have to be applied to the VLBI data. Most of the radiotelescopes participating in geodetic VLBI experiments are situated in regions with small seasonal variations of the station position. However, an increasing VLBI accuracy and an expanded and denser VLBI network will also require the consideration of seasonal displacements.
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