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Abstract

A possible new Geodetic Reference System, successor to the GRS80, has mostly been discussed in terms of updating the fundamental constants in the Somigliana-Pizzetti theory. The update would replace the geocentric gravitational constant GM and the second-degree zonal coefficient J2 of the GRS80 with the current best estimates, and adopt the W0 of the International Height Reference System as U0, the potential at the ellipsoid, the scale constant instead of the semi-major axis of the ellipsoid. Such an update is thought to advance the unification of different fields of geodesy.Nevertheless, any unifying effects could be negligible as long as a quantity of the same size as the update receives a different treatment in those different fields. This quantity is the permanent tide. The International Height Reference System IHRS is mean-tide, the International Terrestrial Reference Frame ITRFxx is (conventional) tide-free, and terrestrial gravity as exemplified by the International Terrestrial Gravity System is zero-tide. For the new GRS to serve a unifying purpose, its reference potential should include the mean tide, in addition to the Newtonian attraction and centrifugal potential. The surface of the ellipsoid would then be the equipotential surface of the sum of these three potentials, an enlarged Somigliana-Pizzetti theory (Vermeer and Poutanen 1997). Mean-tide coordinates and mean-tide gravity would be adopted. Whether such unity would be worth the major disruption caused, and whether it would be favourably received by the user community might well be questioned.