Local Tie Analysis at Fundamental Sites in the CONT17 Campaign

In this contribution we highlight the challenges of the combination of space geodetic techniques through local ties, during the rigorous estimation of geodetic parameters, for the realisation of a global Terrestrial Reference Frame (TRF). Local ties at geodetic fundamental sites play an important role in the determination of a TRF, providing the necessary links to connect the different geodetic techniques. Moreover, the growing demands on the accuracy and stability of the ITRF, have turned the analysis of the quality of these ties into a crucial element to achieve a highly consistent frame, providing additionally the opportunity to identify technique-specific systematic biases  when comparing the space geodetic results with local measurements.

To study the impact of the local ties at fundamental sites, we use the Very Long Baseline Interferometry (VLBI) observations collected during the CONT17 campaign, together with simultaneous Global Navigation Satellite System (GNSS) observations of a subset of the International GNSS Service (IGS) global network. To this end, our approach performs the rigorous estimation of all parameter types common to the two techniques, namely station coordinates, troposphere zenith delays and gradients, and the full set of five Earth Orientation Parameters (EOPs) and their rates, and we include their full variance-covariance information during the combination process. In the central step of this processing scheme, we realise the GNSS-VLBI combination via the "official" ITRF coordinate ties, using and evaluating different weighting schemes, to obtain a unique set of consistent parameters. Moreover, we study the impact of tropospheric ties between the collocated VLBI and GNSS stations, which are essential for the height estimates. Thus, based on the analysis of the station coordinate repeatabilities and the characteristics and behaviour of the EOPs, we discuss the impact of the accuracy and weighting of the local coordinate and troposphere ties on the estimation of the different geodetic parameters.