Determination of physical heights via time transfer

We present a project from the research unit (RU) "TIME" (Clock Metrology: A Novel Approach to TIME in Geodesy), which seeks to determine gravity potential or height differences between distant locations by comparing optical clocks. A strontium optical lattice clock at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig will be connected to the German Research Centre for Geosciences (GFZ) in Potsdam via a delay-compensated optical fiber link. Optical time transfer will then be applied between the geodetic observatories in Potsdam and Wettzell (hosting a second optical clock) through the Atomic Clock Ensemble in Space (ACES) using Satellite Laser Ranging (SLR) telescopes.

Additionally, a third optical clock located in Grasse, France, will be included for comparison. This clock, assumed to have similar characteristics to the one in Braunschweig, will connect to PTB via microwave terminals and to Wettzell using both laser and microwave links, forming a triangular measurement configuration.

The innovative aspect of this approach lies in utilizing time transfer, rather than frequency transfer, and employing free-space links over an extended period to measure physical height differences. Key challenges include managing clock and link variations, atmospheric disturbances, visibility limitations, and data gaps. The clock and link errors are modeled specifically for this constellation, involving the ACES system.

This approach demonstrates the accurate determination of physical height differences via time transfer, particularly for Global Geodetic Observing System (GGOS) core stations such as the Geodetic Observatory Wettzell (GOW).

We discuss the underlying principles, unique properties, and specific challenges of this measurement scenario. Additionally, we provide preliminary estimates of the expected accuracies for the various components and the resulting height differences, based on simulations with varying error budgets for comparison.

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