Clock networks for geodetic applications

authored by: Asha Vincent
supervised by: Jürgen Müller
Abstract: Relativistic geodesy enables a novel approach that utilises atomic clocks for deriving geodetic parameters. A clock lifted by 1 cm or affected by a gravity potential variation of 0.1 m^2/s^2 observes a fractional frequency change of 10^-18. To study the detection of time-variable gravity signals with clocks, case studies have been conducted in five regions characterised by different mass change processes: the Himalayas, Amazon, Greenland, Fennoscandia, and Japan. Clock observations, affected by both mass changes and vertical land deformations, have been simulated as gravity potential variations. In the Himalayas and Amazon, seasonal potential variations reveal regional precipitation and hydrological cycles, while in Greenland, long-term ice mass loss is captured. In Fennoscandia, glacial isostatic adjustment mainly leads to vertical deformations, and in Japan, clock measurements provide insights into co-seismic and post-seismic potential variations, illustrated for the 2011 Tōhoku earthquake. The reference clock for the comparisons was assumed to be realised by a combination of ground-based and space-based clocks in known satellite orbits, with overall uncertainties better than 10^-18. These case studies demonstrate the great potential of clock networks to detect subtle geophysical signals, enhancing our understanding of the Earth's dynamic processes. To realize an international height reference system, a comprehensive study using closed-loop simulations has been carried out with the goal of unifying regional/local height systems in Europe and Brazil, targeting an accuracy of 1 cm. Various error sources were investigated such as datum offsets, slopes (both in latitude and longitude direction), accumulated tilts based on the distance from reference tide gauges, and elevation-dependent errors. Clocks with fractional uncertainties of 10^-18 and 10^-17 were assumed, accounting for intrinsic clock uncertainties, temporal correlations, external factors like tidal effects, propagation delays, and the presence of outliers. Different clock distribution strategies were tested to determine some optimised network setup for a good estimation of these errors considering clocks at distant levelling points, reference tide gauges, and elevated locations. A network design involving master and local clocks with reduced linkages appears quite good. A unification accuracy of 1 cm can be achieved. Moreover, the unified height systems of Europe and Brazil related to the global geoid can be realised with a height accuracy of 3 cm. The third application of clock networks focuses on monitoring the global sea level, which has been increasing over several decades due to climate change. Up to now, Absolute Sea Level (ASL) changes can only be accurately determined from Relative Sea Level (RSL) measurements by properly accounting for vertical land movements at tide gauge benchmarks. Atomic clocks at these tide gauges can provide real-time, absolute physical height changes. Since RSL is affected by regional tidal datums, local variations must be considered to ensure a consistent, global ASL measurement. By incorporating land motion derived from clock observations, it is possible to establish a uniform reference datum, enabling accurate, geoid-based assessments of global sea level changes. These applications show that relativistic geodesy with clocks can revolutionize geodesy by near real-time, pointwise and direct measurements of the time-variable gravity potential.
Organisation(s): Institute of Geodesy
Type: Doctoral thesis
No. of pages: 127
Publication date: 04.07.2025
Publication status: Published
Sustainable Development Goals: SDG 13 - Climate Action
Electronic version(s): https://doi.org/10.15488/19225 (Access: Open)
https://publikationen.badw.de/de/050352196 (Access: Open)