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Review of instruments and techniques for tropo spheric calibration in radio science experiments

Radio science experiments aimed to the estimation of general relativity parameters request very stringent requirements in terms of observables residual noise. Among the error sources affecting observable signals, transmission media represent a significant contribution. The frequency dependant transmission media is removed with the multifrequency link, while a dedicated ground based system is necessary for the troposphere calibration. In the framework of the Mercury Orbiter Radioscience Experiment (MORE) noise level requirements have been defined. The natural choice is the use of a microwave radiometer (MWR) capable to estimate the troposphere path delay with low noise level along the line of sight of the spacecraft. The undertaken activity has focused on the evaluation of the characteristics of a MWR developed at ESA in order to satisfy the MORE requirements.