10:15 AM - 10:30 AM
[SGD02-06] Impact of Earth Tide Models on Reducing Uncertainties in Optical Atomic Clocks During Continuous Operation
Keywords:Gravity, Optical Atomic Clock, Earth Tide, redefinition of the second
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. By applying this potential for detecting frequency changes, it is possible to measure height differences in real-time and with high temporal resolution, with an accuracy of about 1 cm, similar to conventional leveling surveys, regardless of the distance. 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. Though the earth tide models are expected to reduce the uncertainties of clocks in practical and continuous operations, unmodeled periodic signals may still propagate into frequency monitoring by clocks. Based on this background, various geodetic observations near these optical atomic clocks were started in 2021. As part of this effort, a Micro-g LaCoste gPhoneX relative gravimeter was introduced at the end of 2021 for continuous gravity observation.
The continuous gravity datasets include various influences that cause changes in gravity, one of which is Earth tide variations. In this study, we estimated in-situ earth tide variations at the NICT headquarters, Koganei, by analyzing gPhoneX datasets using BAYTAP-G to evaluate Earth tide variations predicted by theoretical models. Comparing the earth tide variations predicted by the theoretical model and those estimated by BAYTAP-G over the seven-month period from February to August 2024, we found that the predictions by the theoretical model tended to be overestimated, with differences reaching up to more than 10 μGal. This corresponds to about 3.2 cm in height variation and a frequency shift of 3.2 x 10-18, which is significant in the uncertainty evaluation of optical atomic clocks.
As a geodetic application of optical atomic clocks, real-time, high-precision, and high-resolution vertical displacement monitoring is proposed. In this observation, it is assumed that the frequency differences between optical atomic clocks installed at multiple locations will be measured by connecting them with optical fibers. Therefore, we conducted a similar analysis of the gPhoneX continuous observation data for about two and a half months in the summer of 2022 at the NICT headquarters and the National Astronomical Observatory of Japan's Mizusawa VLBI Observatory. We obtained the BAYTAP-G Earth tide estimates for Koganei and Mizusawa and compared them with the theoretical model's Earth tide predictions at each location.
Assuming actual frequency variation observations, the average of the differences between the two estimates was 0.6 μGal, with a standard deviation of 1.6 μGal. This corresponds to about 5 mm in height variation and a frequency shift of 5 x 10-19, which is below the detection limit compared to the current frequency uncertainty of optical atomic clocks. However, this discrepancy is expected to vary regionally, so we will continue similar analyses for other observation sites and present these results.
The continuous gravity datasets include various influences that cause changes in gravity, one of which is Earth tide variations. In this study, we estimated in-situ earth tide variations at the NICT headquarters, Koganei, by analyzing gPhoneX datasets using BAYTAP-G to evaluate Earth tide variations predicted by theoretical models. Comparing the earth tide variations predicted by the theoretical model and those estimated by BAYTAP-G over the seven-month period from February to August 2024, we found that the predictions by the theoretical model tended to be overestimated, with differences reaching up to more than 10 μGal. This corresponds to about 3.2 cm in height variation and a frequency shift of 3.2 x 10-18, which is significant in the uncertainty evaluation of optical atomic clocks.
As a geodetic application of optical atomic clocks, real-time, high-precision, and high-resolution vertical displacement monitoring is proposed. In this observation, it is assumed that the frequency differences between optical atomic clocks installed at multiple locations will be measured by connecting them with optical fibers. Therefore, we conducted a similar analysis of the gPhoneX continuous observation data for about two and a half months in the summer of 2022 at the NICT headquarters and the National Astronomical Observatory of Japan's Mizusawa VLBI Observatory. We obtained the BAYTAP-G Earth tide estimates for Koganei and Mizusawa and compared them with the theoretical model's Earth tide predictions at each location.
Assuming actual frequency variation observations, the average of the differences between the two estimates was 0.6 μGal, with a standard deviation of 1.6 μGal. This corresponds to about 5 mm in height variation and a frequency shift of 5 x 10-19, which is below the detection limit compared to the current frequency uncertainty of optical atomic clocks. However, this discrepancy is expected to vary regionally, so we will continue similar analyses for other observation sites and present these results.