The objective of GGOS-FA-UHS is the unification of the existing vertical reference systems around the world. This should be achieved through the definition and realization of a global vertical reference system that
- supports geometrical (ellipsoidal) and physical (normal, orthometric, geoidal) heights world-wide with centimetre precision in a global frame;
- enables the unification of all existing physical height systems (i.e., all geopotential differences shall be referred to one and the same reference equipotential surface with potential W0); and
- provides high-accuracy and long-term stability of the vertical coordinates.
A first step towards the establishment of a worldwide unified (standardized) height system was the release of an IAG resolution for the definition and realization of an International Height Reference System (IHRS) that was issued during the 2015 IUGG General Assembly. This resolution outlines the conventions for the definition of the IHRS in terms of potential parameters: the vertical coordinates are geopotential numbers referring to the equipotential surface of the Earth's gravity field realized by the conventional value W0 = 62 636 853.4 m2/s2.
At present, the main challenge is the realization of the IHRS, i.e., the establishment of the International Height Reference Frame (IHRF). It is expected that the IHRF follows the same structure as the ITRF: a global network with regional and national densifications, with known geopotential numbers referring to the global IHRS. To guarantee a precise combination of physical and geometric parameters and to support the vertical datum unification worldwide, this reference network should be collocated with fundamental geodetic observatories, geometrical reference stations, reference tide gauges, local levelling networks, and gravity reference stations. For this purpose, it will use contributions from all IAG Commissions, and the available databases, standards, and infrastructure of the IAG/GGOS Services.