Assessment of different temporal clock-network solution strategies

Optical clocks are now one of the most precise measurement devices. They reach systematic uncertainties of a few 1018, which can be resolved after a mere few hours of measurement with optical lattice clocks based on trapped neutral atoms. This constant amelioration of their accuracy and stability permitted numerous applications in the field of metrology, fundamental physics and chronometric geodesy. Specifically, geodetic applications, like the determination of gravity potential differences over large distances, become feasible at the level of 0.1 m2 s2. In this context, optical clocks are considered as a unique tool for monitoring and understanding the daily to annual evolution of corresponding geophysical phenomena including the spatial-temporal variations of the gravity potential field. The aim of this paper is to analyze two different ground clock-network solution strategies and asses their impact on the estimation of the Earths temporal gravity field variations and their precisions. The first strategy consists of two steps. In the first step, the gravity potential differences derived from a number of ground optical clock-stations are processed independently for each session. In the second step, the gravity field variations are estimated independently for each clock-station considering the precision derived from each session solution. The second one is a one-step strategy, which estimates the gravity field variations of all clock-stations and their precisions simultaneously for all sessions. The ground clock-network solutions performed applying the least-squares adjustment procedure and different datum definition methods. In order to assess the temporal gravity field variations derived from the two strategies, we developed a simulation study that exploits the monthly gravity field solutions of the GRACE FO mission and the session solutions of the European Permanent GNSS Network (coordinate time-series along with their covariances). In addition, we simulated and applied different noise models to the measurements of the clock-network considering or not the spatial correlations existed among the stations.