Clock networks and their sensibility to time-variable gravity signals

High-performance clock networks are considered as a novel tool in geodesy. Today the latest generation of optical clocks approaches a fractional frequency uncertainty of 1.0x10^-18, which corresponds to about 1.0 cm in height or 0.1 m²/s² in geopotential. The connected clocks are thus promising to enable “relativistic geodesy” in practice: Gravity potential (or height) differences can be inferred through the ultra-precise comparison of clocks’ frequencies.

In this study, we will investigate the possibility of high-performance clock networks for detecting time-variable gravity signals. In the past two decades, the satellite gravity mission GRACE, now continued by its follow-on mission, has significantly improved our knowledge on the Earth’s gravity field, especially on its changes over time. However, the results are limited in terms of spatial resolution (about a few hundreds of kilometers) and temporal resolution (standard is one month). Terrestrial clock networks can be used to observe point-wise gravity potential values at locations of interest. By continuously tracking of changes w.r.t. a reference clock, time-series of gravity potential values are obtained, which reveal the gravity variations at these locations. To elaborate this idea, we will address the following research questions:

• Are clock measurements with the accuracy of 10^-18 sensitive enough to time-variable gravity signals? Or what is the requirement on the clock’s performance for detecting time-variable gravity signals?
• Which kinds of time-variable signals can be “seen” by clocks, the long-term trends (yearly), seasonal variations or short-term changes (weekly/daily)?
• In which regions might clock networks be sensitive to time-variable gravity signals, in Amazon, Greenland or also in Europe?
• An “absolute” reference clock is required for a network that should be least affected by gravity variations. Where should it be placed?

We gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy EXC-2123 “QuantumFrontiers” (Project-ID: 390837967). This work is also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 434617780 – SFB 1464.