Geodetic measurements and quantitative evaluation for reduced gravitational redshift uncertainty of optical frequency standards

The National Institute of Information and Communications Technology (NICT) has developed a Sr optical lattice clock and optical ion clocks employing In+and Ca+. The centimeter-level uncertainty of site elevation is the cause of 10^(-18)-level frequency uncertainties of optical frequency standards. It is significantly important to understand frequency changes caused by solid-earth tides that often range from 10 to 20 cm in amplitude, by oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes for the stable operation of optical atomic clocks.NICT and partners including the Geospatial Information Authority of Japan (GSI) have been jointly conducting leveling surveys and relative gravimeter observations at NICT’s headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the optical lattice clock to the 10^(-19)level. With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in August 2019 and May 2022 to evaluate the effects of the 2011 off the Pacific coast of Tohoku Earthquake on postseismic crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal, which matches the trend of GNSS vertical movement obtained by GSI. We have introduced a Micro-g LaCoste’s gPhoneX gravimeter for continuous gravity measurements nearby the optical clocks in the end of 2021 and have started to investigate the temporal variation of the ground water level in Koganei. We will present preliminary results of these geodetic measurements.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)