[S-GD01] Geodesy and Global Geodetic Observing System

[SGD01-P05] Performance Evaluation of the Relative Gravimeter gPhoneX for investigating uncertainties of the optical atomic clocks

*Ryuichi Ichikawa¹, Mamoru Sekido¹, Tetsuji Ono¹, Yuichi Imanishi², Ryuichi Nishiyama², Ryo Honda³, Basara Miyahara⁴, Chiaki Kato⁴, Kazuki Ichimura⁴, Yasuhiro Sugawara⁴, Kazuki Sakai⁴, Masahiro Nakashima⁴, Takuma Ogawa⁴, Koji Matsuo⁴, Takahito Kazama⁵ (1.National Institute of Information and Communications Technology, 2.Earthquake Research Institute, The University of Tokyo, 3.Mount Fuji Research Institute, 4.Geospatial Information Authority of Japan, 5.Kyoto University)

Keywords:Gravimeter, Redefinition of the second, Optical atomic clocks

Around 2015, researchers from around the world began to consider a redefinition of the second based on optical atomic clocks. Since then, the development of optical atomic clocks has progressed, and recent results have demonstrated the detection of frequency changes with 18 digits of precision. In terms of elevation differences, this provides centimeter-order precision, showing that chronometric leveling can now be realized with the same level of accuracy as conventional leveling survey. On the other hand, the redefinition of the second requires the stable operation of optical atomic clocks. Therefore, it is important to understand frequency changes caused by solid-earth tides, which often range from 10 to 20 cm in amplitude, oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes. The National Institute of Information and Communications Technology (NICT) developing optical ion clocks using In+ and Ca+, as well as Sr optical lattice clocks. Especially for calibrating International Atomic Time (TAI), data from the latter has been regularly provided to the Bureau international des poids et mesures (BIPM) since 2021. Based on these backgrounds, various geodetic observations nearby these optical atomic clocks were started from 2021. As part of this effort, a Micro-g LaCoste gPhoneX relative gravimeter was introduced at the end of 2021 for continuous gravity observation. For performance evaluation of the gPhoneX, we conducted comparative observations with absolute gravity measurement. In September 2022 at Mount Fuji Research Institute (MFRI), a comparison was made with Earthquake Research Institute (ERI) at the Tokyo University's FG5 absolute gravimeter; similarly in November at Ishioka Geodetic Observing Station of the Geospatial Information Authority (GSI), Japan with FG5 and AQG absolute gravimeters respectively were compared. We generated new data sets from the original gPhoneX time series data, which was collected every second during September 28 and 29 of 2022. This data was subsequently averaged over a moving window of 30 seconds to mitigate high-frequency noise. We then compared the FG5 drop data time series with the corresponding data points from the averaged gPhoneX time series. The root mean square (RMS) of the differences between the FG5 data sets and the averaged gPhoneX data sets was found to be approximately 22μGal. Also similar comparative observations were conducted again in September 2023 at MFRI and Tsuru University; January 2024 at Ishioka Geodetic Observing Station again respectively . In our poster, we will present the recent results of comparison measurements.