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The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes the standardisation of height systems worldwide. The GGOS Focus Area Unified Height System (GGOS-FA-UHS) was established to lead and coordinate the efforts needed towards the establishment of a global standard for the precise determination of physical heights. During the 2011–2015 term, various discussions focused on the best possible definition of a global unified vertical reference system, resulting in the IAG Resolution for the Definition and Realisation of an International Height Reference System (IHRS), which was adopted at the 2015 General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Prague, Czech Republic. During the period 2015–2019, activities were undertaken to investigate the best strategy for the implementation of the IHRS; i.e., the establishment of the International Height Reference Frame (IHRF). A preliminary selection of stations for the IHRF reference network was made and different calculation methods for the determination of potential values as IHRF coordinates were evaluated. For the period 2019–2023, the objectives of the GGOS-FA-UHS focused on (i) compiling detailed standards, conventions and guidelines to support a consistent determination of the IHRF at global, regional and national levels; (ii) coordinating with regional/national experts in gravity field modelling the computation of a first IHRF solution; and (iii) designing an operational infrastructure that will ensure the long-term sustainability and reliability of the IHRS/IHRF. This infrastructure was approved by the IAG Executive Committee in December 2023 and will operate under the responsibility of the International Gravity Field Service (IGFS). With these objectives achieved, the GGOS-FA-UHS completed its goals and was closed during the IUGG 2023 General Assembly in Berlin, Germany. This paper presents a comprehensive report on the activities and achievements of the GGOS-FA-UHS.
The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes the standardisation of height systems worldwide. The GGOS Focus Area Unified Height System (GGOS-FA-UHS) was established to lead and coordinate the efforts needed towards the establishment of a global standard for the precise determination of physical heights. During the 2011–2015 term, various discussions focused on the best possible definition of a global unified vertical reference system, resulting in the IAG Resolution for the Definition and Realisation of an International Height Reference System (IHRS), which was adopted at the 2015 General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Prague, Czech Republic. During the period 2015–2019, activities were undertaken to investigate the best strategy for the implementation of the IHRS; i.e., the establishment of the International Height Reference Frame (IHRF). A preliminary selection of stations for the IHRF reference network was made and different calculation methods for the determination of potential values as IHRF coordinates were evaluated. For the period 2019–2023, the objectives of the GGOS-FA-UHS focused on (i) compiling detailed standards, conventions and guidelines to support a consistent determination of the IHRF at global, regional and national levels; (ii) coordinating with regional/national experts in gravity field modelling the computation of a first IHRF solution; and (iii) designing an operational infrastructure that will ensure the long-term sustainability and reliability of the IHRS/IHRF. This infrastructure was approved by the IAG Executive Committee in December 2023 and will operate under the responsibility of the International Gravity Field Service (IGFS). With these objectives achieved, the GGOS-FA-UHS completed its goals and was closed during the IUGG 2023 General Assembly in Berlin, Germany. This paper presents a comprehensive report on the activities and achievements of the GGOS-FA-UHS.
The International Association of Geodesy (IAG) introduced the International Height Reference System (IHRS) in 2015 as an international standard for the accurate determination of physical heights worldwide. Primary vertical coordinates are geopotential numbers referenced to a conventional W0 value. The realisation of the IHRS is the International Height Reference Frame (IHRF), which corresponds to a global network of reference stations with precise reference coordinates specified in the IHRS. The spatial position of the stations, at which the geopotential numbers are calculated, is defined by their respective coordinates (X, Y, Z) in the International Terrestrial Reference Frame (ITRF). The realisation of the IHRS is thus based on the combination of a geometric component, given by the positions of the stations in the ITRF, and a physical component, given by the determination of the potential values W at these positions. Through a strong international collaboration, framed by the IAG, it has been possible in recent years to pave the scientific foundations of the IHRS, to compute a first solution of the IHRF, and to identify the key requirements for a long-term sustainability of the IHRF. Much progress has been made and continuity is needed to ensure the maintenance and availability of the IHRF in the future. Following IAG practice, the development of theory and methods for the continuous improvement of the IHRS/IHRF should be promoted by the IAG Commissions and the Inter-Commission Committee on Theory (ICCT), while the operational performance should be ensured by the IAG Services. In this paper, we highlight the organisational challenges in maintaining the IHRS/IHRF, discuss how the existing gravity field related IAG Services could contribute to the IHRS/IHRF, and identify the elements needed to establish an operational infrastructure for the IHRS/IHRF that addresses the organisational challenges. Our proposal is to establish a central coordinating body under the responsibility of the International Gravity Field Service (IGFS), composed of individual modules taking care of the main components of the IHRS/IHRF. The central management body is the IHRF Coordination Centre and its modules are the IHRF Reference Network Coordination, the IHRF Conventions’ Coordination, the IHRF Associate Analysis Centres, and the IHRF Combination Coordination. The IGFS presented this proposal to the IAG Executive Committee at its meeting on 10 December 2023 and it was unanimously approved. Thus, a new component of the IGFS dedicated to the IHRF has been created and will ensure the long-term availability and reliability of the IHRF.
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