The IAG Resolution No. 1 released during the IUGG 2015 General Assembly outlines five conventions for the definition of the International Height Reference System (IHRS). The definition is given in terms of potential parameters: the vertical coordinates are geopotential numbers referring to an equipotential surface of the Earth's gravity field realized by the conventional value W0 = 62 636 853.4 m2s‐2. The spatial reference of the position P for the potential WP = W(X) is given by coordinates X of the International Terrestrial Reference Frame (ITRF). This Resolution also states that parameters, observations, and data shall be related to the mean tidal system/mean crust. 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, whose geopotential numbers referring to the global IHRS are known. According to the GGOS objectives, the target accuracy of these global geopotential numbers is 1 x 10-2 m2s-2. In practice, the precise realization of the IHRS is at present limited by different aspects; for instance, no unified standards or methods for the determination of the potential values WP; application of different conventions for the gravity field modelling and the estimation of the position vectors X; inhomogeneous distribution of the geodetic infrastructure; restricted accessibility to terrestrial gravity data to increase the GGM resolution; insufficient modelling of geodynamic phenomena, etc. This may restrict the expected accuracy of 1 x 10-2 m2s-2 to some orders lower (from 10 x 10-2 m2s-2 to 100 x 10-2 m2s-2). This contribution discusses the required steps to outline a sustainable realization of the IHRS.

Studying, understanding and modelling global change require geodetic reference frames with an order of accuracy higher than the magnitude of the effects to be actually studied and with high consistency and reliability worldwide. The International Association of Geodesy, taking care of providing a precise geodetic infrastructure for monitoring the Earth system, promotes the implementation of an integrated global geodetic reference frame that provides a reliable frame for consistent analysis and modelling of global phenomena and processes affecting the Earth’s gravity field, the Earth’s surface geometry and the Earth’s rotation. The definition, realization, maintenance and wide utilization of the International Terrestrial Reference System guarantee a globally unified geometric reference frame with an accuracy at the millimetre level. An equivalent high-precision global physical reference frame that supports the reliable description of changes in the Earth’s gravity field (such as sea level variations, mass displacements, processes associated with geophysical fluids) is missing. This paper addresses the theoretical foundations supporting the implementation of such a physical reference surface in terms of an International Height Reference System and provides guidance for the coming activities required for the practical and sustainable realization of this system. Based on conceptual approaches of physical geodesy, the requirements for a unified global height reference system are derived. In accordance with the practice, its realization as the International Height Reference Frame is designed. Further steps for the implementation are also proposed.

Friedrich Robert Helmert died in Potsdam in 1917 at the age of 74 after serving for over 30 years as director of the Royal Prussian Geodetic Institute and of the Central Bureau of the Internationale Erdmessung, the forerunner of today's International Association of Geodesy. He dedicated his life and his scientific career to the field of geodesy. His teachings on theoretical and physical geodesy were incorporated into university curricula around the world and hence into international endeavours to measure planet Earth. The purpose of this article is to illustrate the impact he has had on the development of modern geodesy and on the related sciences.

Three different approaches for the unification of height reference frames have been used to compute datum offsets between national vertical reference frames in Europe: the oceanographic approach, the spirit leveling approach and the gravity field approach. All three methods are discussed and advantages and drawbacks are evaluated. A set of tide gauge locations is used to compute height datum offsets between national height reference frames in Europe based on all three approaches. The results agree on a level of 5–10 cm. In summary, the gravity field approach is the most flexible approach. The ESA satellite gravity mission GOCE ensures a uniform global level in the range of 1–2 cm. Hence, the gravity field approach is a suitable approach for future realizations of the European Vertical Reference Frame as well as for the establishment of a World Height System.

Very Long Baseline Interferometry (VLBI) is a primary space-geodetic technique for determining precise coordinates on the Earth, for monitoring the variable Earth rotation and orientation with highest precision, and for deriving many other parameters of the Earth system. The International VLBI Service for Geodesy and Astrometry (IVS, http://ivscc.gsfc.nasa.gov/) is a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU). The datasets published here are the results of individual Very Long Baseline Interferometry (VLBI) sessions in the form of normal equations in SINEX 2.0 format (http://www.iers.org/IERS/EN/Organization/AnalysisCoordinator/SinexFormat/sinex.html, the SINEX 2.0 description is attached as pdf) provided by IVS as the input for the next release of the International Terrestrial Reference System (ITRF): ITRF2014. This is a new version of the ITRF2008 release (Bockmann et al., 2009). For each session/ file, the normal equation systems contain elements for the coordinate components of all stations having participated in the respective session as well as for the Earth orientation parameters (x-pole, y-pole, UT1 and its time derivatives plus offset to the IAU2006 precession-nutation components dX, dY (https://www.iau.org/static/resolutions/IAU2006_Resol1.pdf). The terrestrial part is free of datum. The data sets are the result of a weighted combination of the input of several IVS Analysis Centers. The IVS contribution for ITRF2014 is described in Bachmann et al (2015), Schuh and Behrend (2012) provide a general overview on the VLBI method, details on the internal data handling can be found at Behrend (2013).

The EUREF (Reference Frame Sub-Commission for Europe) Permanent GNSS Network (EPN) serves as the backbone for the realization of, and access to, the European Terrestrial Reference System (ETRS89). The cumulative site positions and velocities for the EPN stations are used for national ETRS89 densifications and geo-information applications. EUREF has developed specific guidelines through which European countries ask validation of their national ETRS89 densification campaigns. Today, the majority of the European countries has passed this process and a large part of European National Mapping and Cartographic Agencies have officially adopted ETRS89. In addition, ETRS89 plays a fundamental role in INSPIRE (Infrastructure for Spatial Information in the European Community).

The EUREF key infrastructures are the EUREF Permanent GNSS Network (EPN) and the Unified European Levelling Network (UELN). The EPN runs almost 250 Global Navigation Satellite System (GNSS) stations in a well organized environment and serves as the backbone of the realization of and access to the European Terrestrial Reference System (ETRS89) and as contribution to the densification of the International Terrestrial Reference Frame (ITRF2008). The upcoming European navigation system Galileo will be a big challenge for the EPN in sense of upgrading the station network. Almost half of the EPN stations are capable of streaming their data in real-time. To strengthen the robustness of the real-time data and product distribution two additional Ntrip broadcasters were set up at different premises in Europe. The EVRS and the ETRS89 have been adopted by the European Community as coordinate reference systems in INSPIRE for environmental policies, or policies and activities that have an impact on the environment.

A suitable representation of the regional gravity eld is used to estimate relative offsets between national height system realizations in Europe. The method used is based on a gravimetric approach and bene ts from the signi cant improvements in the determination of the global gravity eld by the recent satellite gravity missions the Gravity Recovery and Climate Experiment (GRACE) and the Gravity eld and steady-state Ocean Circulation Explorerr (GOCE). The potential of thesemissions for the uni cation of height reference frames is analyzed in terms of accuracy and spatial resolution. The results of the gravimetric approach are compared to the independent results of the geodetic leveling approach. Advantages and drawbacks of both methods are discussed.

Time variable gravity field models derived from the satellite mission GRACE have been demonstrated to be consistent with water mass variations in the global hydrological cycle. Independent observations are provided by terrestrial measurements. In order to achieve a maximum of reliability and information gain, ground-based gravity observations may be deployed for comparison with the gravity field variations derived from the GRACE satellite mission. In this context, the data of the network of superconducting gravimeters (SG) of the 'Global Geodynamics Project' (GGP) are of particular interest. This study is focused on the dense SG network in Central Europe with its long-term gravity observations. It is shown that after the separation and reduction of local hydrological effects in the SG observations especially for subsurface stations, the time-variable gravity signals from GRACE agree well with the terrestrial observations from the SG station cluster.Station stability of the SG sites with respect to vertical deformations was checked by GNSS based observations. Most of the variability can be explained by loading effects due to changes in continental water storage, and, in general, the stability of all stations has been confirmed.From comparisons based on correlation and coherence analyses in combination with the root mean square (RMS) variability of the time series emerges, that the maximum correspondence between the SG and GRACE time series is achieved when filtering the GRACE data with Gaussian filters of about 1000 km filter length, which is in accordance with previous publications.Empirical Orthogonal Functions (EOF) analysis was applied to the gravity time series in order to identify common characteristic spatial and temporal patterns. The high correspondence of the first modes for GRACE and SG data implies that the first EOF mode represents a large-scale (Central European) time-variable gravity signal seen by both the GRACE satellites and the SG cluster. (C) 2011 Elsevier Ltd. All rights reserved.

The Federal Agency for Cartography and Geodesy (BKG) in Germany is in charge of the German quasigeoid as a reference surface for the German height system. A new quasigeoid GCG2011 for Germany was developed in collaboration with the colleagues from Institut für Erdmessung (IfE), Hannover university. GCG2011 is based on an extensive database of terrestrial gravity data. The case-study presented here refers to the BKG gravity solution of GCG2011. This approach is based on a point mass representation in combination with remove-compute-restoretechnique for the determination of the geoid.

In order to improve the gravity field and geoid modelling in the border area between Germany and Denmark including the Baltic Sea and North Sea, three airborne gravity surveys have been undertaken since 2006: BalGRACE-06, NorthGRACE-07, NorthGRACE-08 (Baltic resp. North Sea GRavity Airborne Campaign and Examine). These endeavours were aiming to give a substantial contribution towards comparison, verification and improvement of the vertical reference in this area. During these campaigns more than 25,000 km of track data were gathered with three different models of LaCoste&Romberg gravimeters mounted in two different aircrafts.Owing to the recent airborne gravity efforts we were able to cover most of the mentioned area yielding gravity anomalies with an accuracy better than 2 mgal according (i) to cross-over estimates and (ii) to comparison with non-airborne gravity data. Due to considerable turbulences on some flights not all of the airborne measurements could be successfully used. Data from turbulent flights were partly too noisy and did not meet the quality criteria. Attaining high-quality airborne gravity anomalies is still a demanding business.In this study we present the data handling of the airborne gravity data from three campaigns carried out under quite different conditions. It is described how the airborne data have been checked, homogenized and used to fill-up various existing data gaps.A first result of improved regional gravity field and geoid modelling is presented, using the newly obtained airborne gravity data in connection with existing terrestrial data and satellite observations.