Gravity field modelling with regard to optical clock comparisons
21/09/2016 | 17:15 | Session 5: Height systems and vertical datum unification
Author(s): Heiner Denker, Ludger Timmen and Christian Voigt
Heiner Denker, Ludger Timmen and Christian Voigt
A coordinated programme of clock comparisons was carried out within the EMRP-funded project “International Timescales with Optical Clocks” (ITOC), aiming at a validation of the uncertainty budgets of the new optical clocks in view of an optical redefinition of the SI second. Based on Einstein’s general relativity theory, clocks are affected by the gravitational field and the velocity of the clocks. For an Earth-bound clock at rest, the corresponding relativistic redshift effect is directly related to the (geodetic) gravity potential. As optical clocks are now targeting a relativeaccuracy of 10-18, corresponding to a sensitivity of about 0.1 m2/s2 in terms of the geopotential or 0.01 m in height, precise knowledge of the gravity potential is required at the respective clock sites. Alternatively, optical clocks may also be employed for deriving the gravity potential
(denoted as “chronometric levelling” or “relativistic geodesy”) and hence offer completely new options for geodetic height determination.
The ITOC project involved clock sites at the national metrological institutes in France, Germany, Italy and the United Kingdom. In order to determine the gravity potential with best possible accuracy at these sites, two approaches were considered, namely geometric levelling and GNSS ellipsoidal heights in combination with a gravimetric (quasi)geoid model. Additional absolute and relative gravity observations were carried out around the clock sites and then used to compute an updated quasigeoid model. The general strategy, the work performed, as well as the obtained potential values and their accuracies are presented.
The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.