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Abstract:
This study contributes to the effort of space geodesy to reach the 1mm accuracy level on a global scale. This stringent requirement is vital to realize a global reference system upon which phenomena such as sea-level rise can be reliably monitored. The study deals with two interrelated challenges: modeling geophysical loads imposed on the crust of the Earth and quantifying the atmospheric propagation delay of signals employed by space geodetic techniques, namely VLBI, SLR, GNSS, and DORIS. If not adequately modeled, both geophysical loading and propagation delay corrupt space geodetic data analysis results, thus distorting the implied reference frame and compromising the physical interpretation of other parameters. The explicit purpose of this work is to understand how these effects propagate into the parameters estimated within the geodetic adjustment, and to develop models that alleviate geodetic results from these effects. To achieve this goal, the scientific framework was divided into two contributions to be understood and enhanced: the theory governing the effects of geophysical loading and atmospheric propagation, and the space geodetic technique data analysis pipeline, largely using VLBI as a test-bed. In essence, the research conducted here includes: (i) the development of software capable of realistically simulating VLBI, SLR, GNSS, and DORIS observations within a Monte Carlo framework, (ii) the homogenization of in situ meteorological data recorded at VLBI and SLR stations, (iii) the development of ray-traced delays, mapping functions and higher-order gradients for all four space geodetic techniques, (iv) the comprehensive investigation of inter-frequency and inter-system atmospheric ties, (v) the development of models to describe the displacement induced by mass redistribution within Earth’s fluid envelope including the atmosphere, the ocean, and the continental hydrology, (vi) the development of empirical models to describe the signal propagation delay (GFZ-PT) and the non-tidal geophysical loading displacement (EGLM), and (vii) the study of the impact of the atmospheric refraction and non-tidal geophysical loading models in space geodetic data analysis on station coordinates, the terrestrial reference frame, the Earth orientation, and the integrated water vapour trends. A number of developments were carried out herein for the first time, for example, the simulation of space geodetic measurements based on ray-traced delays, the study of systematic errors on the reference frame induced by not properly accounting for the orbital altitude of the satellites in the calculation of atmospheric refraction corrections, and the assessment of the probability of successful laser ranges based on integrated cloud fraction along the ray path. It was found that microwave and optical atmospheric gradients are starkly different both spatially and temporally, and cannot be scaled to fit each other. Failing to account for non-tidal geophysical loading and atmospheric asymmetries induces a scale bias in the SLR reference frame as well as a spurious geocenter motion predominantly along the Z-axis. Employing a VLBI-tailored atmospheric refraction model to reduce DORIS observations displaces stations in the radial component thus inducing a large scale bias in the implied frame. Employing homogeneous in lieu of raw meteorological data in VLBI data analysis reduces the scatter of station coordinates and improves the baseline length repeatability. Employing the mapping functions developed herein in lieu of VMF1 yields an overall improvement in VLBI data analysis. Applying the geophysical loading models developed herein reduces the response of almost all station coordinate and baseline length series at seasonal and synoptic timescales. Based on the investigations carried out herein, differences in Earth orientation induced by the quality of the atmospheric refraction and geophysical loading models — or their very application for the latter — are not statistically significant in the framework of the modern VLBI system. Nevertheless, to fulfill the 1mm requirement, proper treatment of geophysical loading and atmospheric refraction is a necessity.
