Geodetic measurements of the gravity potential difference to validate a 1000-km scale optical lattice clock comparison in Japan - preliminary result

Advances in the developments of optical clocks have enabled 10^-18-level frequency comparisons between fibre-linked clocks. This leads to chronometric leveling on the Earth’s surface with an uncertainty on the order of 1 cm, based on the observation of the gravitational red shift. Since measurement uncertainty does not deteriorate with increasing fibre length, applications of chronometric leveling in geodesy, particularly unification of height reference systems, have been actively studied. When the uncertainty of chronometric leveling reaches a few cm in height, uncertainties in the verification by geodetic survey must be carefully evaluated. In a previous study, we determined the height difference between two clock sites approximately 100 km apart in the Tokyo area, based on geometric leveling and estimated its uncertainty to be roughly 2 cm. Comparison with chronometric leveling using optical lattice clocks is currently underway. In addition to this, frequency comparison between the clock sites in Tokyo and the Mizusawa VLBI observatory 400 km apart is under preparation. In this study, we will report preliminary results on the geodetic determination of the height difference between the latter two sites. Due to the long distance, we combine so called GNSS-geoid method and local geometric leveling. In this approach, the largest uncertainty comes from the error of the geoid model, precluding a few cm-level verification. Confirmation by geometric leveling over 400 km distance should be considered. Uncertainties in the determination of not only static but also dynamic height difference due to tides etc. are discussed, for future repeated frequency comparison experiments to detect tectonic gravitational potential change due to the postseismic relaxation caused by the M9 Tohoku earthquake in 2011.