ATMO-DEL: Determination of the atmospheric delay via line-of-sight modelling and measurements of the tropospheric composition and the ionospheric electron distribution.

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In the sub-project of the research unit "Clockmetrology" the atmospheric delay from a transmitter on a GNSS (Global Navigation Satellite System) satellite to the receiver on the ground will be specified with higher accuracy than before to meet the requirements specified in the proposal to the research unit. This goal will be reached via a combination of a dedicated delay model of the line of sight with measurements of atmospheric conditions. The delay model consists of a tropospheric part, which uses flexible profiles of refraction indices and an ionospheric part. The tropospheric measurements will be taken at the Geodätic Observatorium Wettzell, GOW, in Southern Germany (49.14N, 12.86E), a GNSS reference station operated by the "Bundesamt für Karthographie und Geodäsie", BKG. Measurements of atmospheric water vapor, temperature, clouds and aerosols along the line-of-sight will be done using microwave and infrared spectrometers. The electron content of the ionosphere is determined from satellite measurements and interpolated to the place of the GNSS ground station. The measurements together with the model will be used to investigate the effect of the variability of the atmospheric composition and temperature on the accuracy of the delay determination. This includes fast spatial and temporal fluctuations in the water vapor content of the atmosphere, clouds and aerosols. The research unit provides an unique environment to reach this goal and separate the error caused by a miscalculation of the atmospheric delay from other error sources, e.g. uncertainties in the electronics of the receiver. The measurements will be used as an input to the delay model, but also supplied to other projects within the research unit.