Potential and scientific requirements of optical clock networks for validating satellite-derived time-variable gravity data

The GRACE and GRACE-FO missions have provided an unprecedented quantification of large-scale changes in the water cycle. However, it is still open problem how these missions' data sets can be referenced to a ground truth. Meanwhile, stationary optical clocks show fractional instabilities below $10^{-18}$ when averaged over hour, continue improved terms precision accuracy, uptime, transportability. frequency clock affected by gravitational redshift, thus depends on local geopotential; relative change corresponds geoid height about $1$ cm. Here we suggest that this effect could further exploited for sensing temporal geopotential via network distributed at Earth's surface. In fact, several projects already proposed create ensemble connected across Europe fibre links. Our hypothesis with collocated GNSS receivers spread - which physical infrastructure partly place would enable us determine variations gravity field time scales days beyond, provide new means validating satellite such as or future mission. Here, through simulations ice, hydrology atmosphere observed comparisons follows current design concepts metrology community. We assume different scenarios uncertainties find even under conservative assumptions error vertical control $1.4$ mm daily measurements hydrological signals annual scale atmospheric down weekly observed.