Towards Clock Ties for a Global Geodetic Observing System

Over the last two decades, the precision of individual measurements of Space Geodesy improved to a millimeter level. However, the overall achieved accuracy remains at a centimeter level due to systematic errors. The fundamental stations operating more than one space geodetic measurement technique present a keystone in systematic error investigation and mitigation. Due to regular surveys, the distances and mutual movements of the reference points are established with millimeter accuracy. The problem arises in the combination at the observation level, where the residuals of the measurements do not match with the established geometrical ties sufficiently well. Internal instrumental signal delays within each technique are causing this detrimental effect.

We have identified time coherence between the individual techniques and fundamental stations as the proper tool to overcome this problem. Within IAG Project QuGe we examine referencing the instrumentations to the optical clocks. In this scenario, the clock parameter in geodesy does not need to be adjusted any more, and all systematic effects would promote. To transfer clock stability within an entire station campus, we use a mode-locked fs-pulse laser, which is distributed using actively delay compensated fiber links, provides the necessary means to identify and remove these systematic errors. This talk illustrates some results and introduces the novel time distribution system of the Geodetic Observatory Wettzell, which realizes an ideal test bench for these clock ties.