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

The National Institute of Information and Communications Technology (NICT) has developed optical ion clocks employing In$^{+}$ and Ca$^{+}$, as well as a Sr optical lattice clock that provides TAI calibration data to BIPM. We have started geodetic measurements near these NICT optical atomic clocks to evaluate vertical ground movements, where centimeter-level uncertainties correspond to $10 -^{18}$ order frequency shifts.