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Abstract

The principle of chronometric leveling allows determining differences in gravity potential and physical height based on the relativistic gravitational frequency redshift. The technique paves the way to introduce an atomic standard to measurements of potential and height, instead of using classical references such as tide gauges or gravimetric geoid models. Developments on the field of optical frequency standards at the 10^-18 relative frequency accuracy level are progressing at National Metrology Institutes and other institutions. Significant efforts to prepare for a redefinition of the SI second based on optical frequency standards are ongoing, led by the Consultative Committee on Time and Frequency (CCTF) of BIPM. Activities include the further extension of phase-stabilized optical frequency transfer through fiber links at continental distances, a technique that has been pioneered in Europe. Recently, in the remarkable Tokyo SkyTree experiment two transportable optical lattice clocks have been used to demonstrate the capability of centimeter-level chronometric leveling (Takamoto et al 2020, doi:10.1038/s41566-020-0619-8). With the key elements of the technique successfully demonstrated, the realization of frequency redshift measurements in a continental network seems feasible. Measurements could be performed with stationary and transportable frequency standards. The involved stations could serve as a prototype backbone height network for geodesy. Long measurement runs would not be necessary at least in the early phases of network. Nevertheless, very significant coordination efforts, support, and resources will be required. We will discuss the perspective of remote optical frequency comparisons and give an overview of activities of IAG QuGe Working Group Q3.