Determination of physical heights via time transfer

We present a project of the research unit (RU) 'TIME' (Clock Metrology: A Novel Approach to TIME in Geodesy) which aims to determine gravity potential or height differences between remote sites by comparing optical clocks. A strontium optical lattice clock at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig will be connected with the German Research Centre for Geosciences (GFZ) in Potsdam through a delay-compensated optical fiber. Then, optical time transfer is carried out between the geodetic observatories in Potsdam and Wettzell (where a second optical clock will be operated) via the Atomic Clock Ensemble in Space (ACES) using the Satellite Laser Ranging (SLR) telescopes. The key innovation is using time transfer, not frequency and optical free-space links over an extended period to determine physical height differences. Challenges include clock/link variations, atmospheric effects, visibility constraints and data gaps, etc. We investigate the major error sources and apply corrections like tidal effects. This approach showcases accurately transferring physical heights via time transfer and demonstrates the RU’s time concept for integrating geometric and physical heights in future height systems, especially for Global Geodetic Observing System (GGOS) core stations like the Geodetic Observatory Wettzell (GOW).

In this presentation, we introduce the principles, special properties and challenges of this specific measurement scenario. We also provide preliminary numbers for the expected accuracies of the various components and the resulting height difference based on simulations.

We acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 490990195 – FOR 5456.