Methods for an Enhanced Co-Location of Space Geodetic Techniques

Abstract

The modern definition of Geodesy goes beyond the classical task of determining the shape and figure of the Earth and its (external) gravitational field in a three-dimensional time-varying space. Nowadays, Geodesists also deal with the analysis of geodynamical phenomena (Torge, W., 2001). The different space geodetic techniques, Global Navigation Satellite Systems (GNSS), Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) contribute to quantify these “Geodetic Earth Observation” tasks. These techniques provide both, in time and space, global and regional observations to measure geodynamical processes. Undoubtedly, the requirements for the appropriate observation of these phenomena are growing. Consequently, the geodetic infrastructure to monitor them requires high accuracy and stability so that the “real” signal in the observations can be separated from the noise and the not-so-important information contained in the measurements. Currently, the International Terrestrial Reference System (ITRS) and its realisation, the International Terrestrial Reference Frame (ITRF), constitute the infrastructure to support this geodetic Earth observing system. The quality of the ITRF relies, to a large extent, on the sufficiency of the combination of the different space geodetic techniques at co-location sites. The predominant threats to establishing a reference frame are the deficiencies in the local tie measurements and technique-specific systematic biases in the individual space geodetic techniques. An alternative to get an insight into these biases is the analysis of co-located instruments of the different techniques. Therefore, the study of co-location strategies constitutes the central element of this work. To improve the understanding of the error sources which affect each technique, we performed a series of intra-technique studies on short baselines, to analyse technique-specific biases, to monitor local ties, and to propose alternative ways to link the different techniques, such as clock and tropospheric ties. Ties among the different techniques are realised by parameters common to more than one technique. Therefore, we assess the performance of various parameters in experiments on GNSS-to-GNSS, SLR-to-SLR, and VLBI-to-VLBI short baselines, where multiple local and environmental effects, such as snow, meteorological data, antenna phase centre variations, and multipath, are investigated. We propose new methodologies and processing strategies to quantify and mitigate these error sources and to achieve a more accurate reference frame. Furthermore, we performed inter-technique experiments including GNSS and VLBI observations, where the task lies in the analysis of biases among the space geodetic techniques and the study of the benefits from a rigorous GNSS-VLBI combination of all common parameter types, including all types of ties available. These experiments contribute to the realisation of a consistent reference frame and constitute a necessary step to improve the realisation of the ITRF.