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Surveying and navigation became much easier, more accurate and operational thanks to the Global Positioning System. The use of this technology in height determination, bathymetry and 3-D navigation is not only limited by the reduced accuracy compared to the horizontal component. It supposes additional information about the geodetic height reference surface in order to leverage the full potential of this technology. The corresponding models, measurements and activities which are necessary to improve this part of the geodetic infrastructure are usually behind the curtain. This article emphasizes the need of a common cross-border geodetic infrastructure and the relevance of precise models of the height reference surface for GNSS-aided height determinations. The need of gravimetric surveys for the determination and improvement of these models is explained. Finally, it gives an overview about the gravimetric surveys which were carried out in the German Exclusive Zone of the North and Baltic Sea over more than one decade and provides some insight into practical aspects and challenges of this kind of surveys.

Gravity determination at sea is essential for the definition of physical height systems at sea and adjacent coastal areas. Since 2017, four shipborne gravimetry campaigns were conducted using a Chekan-AM gyro stabilized platform gravimeter containing quartz spring sensors and an iMAR Inertial Measurement Unit serving as strapdown gravimeter. The comparison of both gravimeter types exposed complementary properties of both sensors. With proper maintenance, the gyro stabilized gravimeter shows a very stable linear drift, but its recordings are impaired by steering manoeuvres and harsh sea conditions. In contrast, the strapdown gravimeter recordings are typically reliable during the whole cruise. But with up to 80 hours, the durations of some of the analysed cruises were exceptionally challenging for strapdown measurements. Under these conditions, the accuracy of the strapdown recordings is slightly degraded due to non-linear sensor drift, although the major part of the drift can be eliminated by thermal calibration or stabilisation. Under smooth sea conditions at the Baltic Sea with cruise durations of less than 35 hours, a precision of 0.4 to 0.5 mGal was obtained for both individual gravimeter results. A combination of both gravimeter types using complementary high- and low-pass filtering provided drift reduction and the possibility of gravity determination during the entire cruise. In the latest campaign, two strapdown gravimeters were installed in addition to the gyro stabilized platform gravimeter allowing for a detailed comparison of the drift behaviour of all three gravimeters., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The almost 20-year-old ICGEM is one of the five services coordinated by the International Gravity Field Service (IGFS) of the International Association of Geodesy (IAG). Hosted by the GFZ Potsdam, it provides the scientific community with a comprehensive archive of static, temporal, and topographic global gravity field models (GGMs) of the Earth. Furthermore, ICGEM offers an interactive calculation and visualization service of gravity field functionals and tutorials on the theory. The current service cannot address the advancements in the field of gravity modelling and the wide range of applications of such models and their deliverables in Earth sciences. There is an immediate need to expand, modernise, and upgrade the existing database, research data and metadata included in all facets of the service. In this presentation, we will introduce an upcoming project that will improve, actualize and expand ICGEM with providing additional data and a sustainable infrastructure that follows the FAIR Principles (Findable, Accessible, Interoperable and Reusable). New gravity field related data of transdisciplinary importance (e.g., altimetry grids, DEMs) as well as new calculated gravity field functionals (e.g., isostatic gravity anomalies) and readily computed gravity functional grids will be added. The evaluation tools will be improved and data and metadata standards will be established. The new portal will be interlinked to other national and international research data initiatives (e.g., GGOS, NFDI4Earth). Finally, an improved public outreach strategy shall be developed. The realization of this project shall be done in close contact and communication with the international Earth Gravity Field community., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The global gravitational potential generated by the attraction of the Earth’s topographic masses has been computed in spectral domain. The mass-source information is provided by the 1 arcmin resolution Earth2014 relief model and four averaged density values for rock, ocean, lake, and ice areas. The topography and bathymetry are split into confocal ellipsoidal shells of a defined thickness. Based on the provided mass-source information, the gravitational potential is expanded for each shell and then summed up to represent the complete gravitational potential of the topography (and bathymetry). In this contribution, we present the impact of different shell thicknesses to the model accuracy and computation time. Moreover, we expanded our topographic gravity field model up to spherical harmonic degree and order 5,494. Such short scale mass information represented by the topography can be used to complement high-resolution combined static gravity field models for the very high-frequency components of the gravity field. As an example, we enhanced (augmented) EIGEN-6C4 model with the high frequency components retrieved from the topographic model. The deflections of vertical values computed from the augmented model are compared w.r.t. ground truth observations in Germany, Southern Colorado and Iowa (USA) which suggest as expected a considerable improvement over rugged mountainous regions and comparable residuals in areas of moderate topography.

Since its start of operations in July 2019, IAGâ?Ts Combination Service for Time-variable Gravity fields (COST-G) is providing a complete time-series of combined monthly GRACE gravity fields and a regularly updated time-series of monthly gravity fields derived from kinematic Swarm orbits. Starting from October 2020, the COST-G product line is complemented by a time-series of operationally combined and monthly updated GRACE-FO gravity fields. All these combinations are performed by variance component estimation on the solution level. We report on new developments, i.e., a planned extension of COST-G to include Chinese analysis centers of GRACE and GRACE-FO data, a re-consideration of the combination strategy to better focus on the range of spherical harmonic coefficients most relevant for the users, and the potential application of COST-G products for orbit determination of altimeter satellites.

The Combination Service for Time-variable Gravity Fields (COST-G) of the International Association of Geodesy (IAG) provides combined monthly gravity fields of its associated and partner Analysis Centers (ACs). In November 2020, the combination of monthly GRACE-FO gravity fields started its operational mode, providing consolidated L2 (spherical harmonics) and L3 (gridded and post- processed) products with a latency of currently 3 months. We present an overview and quality assessment of the available products.COST-G aims at the extension of its service to include further GRACE and GRACE-FO analysis centers. In January 2020 a collaboration with representatives of five Chinese ACs was initiated, who provided GRACE time-series according to the COST-G requirements. We present the results of a test combination with the Chinese AC models, including comparison and quality assessment of all contributing time-series and validation of the combined gravity fields.

FAMOS (Finalizing Surveys for the Baltic Motorways of the Sea) is a recently run project which was supported by EU to improve the safety of ship navigation on the Baltic Sea. One task of FAMOS was to work on a common height reference for the Baltic Sea, the Baltic Sea Chart Datum 2000 (BSCD2000), which will be based on a new gravimetric geoid for this sea. The German institutions German Research Centre for Geosciences (GFZ), Federal Agency for Cartography and Geodesy (BKG), Federal Maritime and Hydrographic Agency (BSH) and Technical University of Darmstadt performed several marine gravimetry surveys to contribute to this project. The primary measurement equipment was GFZ’s Chekan-AM gravimeter. On two of these campaigns an iMAR strapdown inertial measurement unit (IMU) was used together with the Chekan. The data records from both instruments are of very good consistency. Since the FAMOS project ended ahead of the planned schedule already in 2019, finalization of the new chart Datum is now coordinated by the Chart Datum Working Group within the Baltic Sea Hydrographic Commission. It is planned to implement the new BSCD2000 by 2023.

The International Geodynamics and Earth Tide Service (IGETS) was established in 2015 by the International Association of Geodesy IAG. IGETS continues the activities of the Global Geodynamics Project (GGP) between 1997 and 2015 to provide support to geodetic and geophysical research activities using superconducting gravimeter (SG) data within the context of an international network. As part of this network, the Helgoland Gravimetric Observatory Germany (HELGOG) was established by the GFZ German Research Centre for Geosciences in March 2020. Continuous time-varying gravity and atmospheric pressure data from the iGrav SG at HELGOG are integrated in the IGETS data base hosted by GFZ. HELGOG is located in the basement of the Biological Institute Helgoland (BAH) of the Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Research. The operation and maintenance of the HELGOG instrumentation is jointly done by staff of AWI and GFZ. The iGrav SG is directly installed on the concrete floor of the basement. The distance from the iGrav SG to the North Sea is only 10 m. Being located at sea level (H=2.059 m), the iGrav SG is predominantly sensitive to vertical displacements. Helgoland is Germany’s only island in the open sea 50 km away from the mainland. The gravimetric observations enable the analysis of ocean tides and non-tidal mass redistributions, especially in connection with storm surges in the North Sea (see also Weise et al. 2020). Next to the iGrav SG, there is space for absolute gravimeters (first measurements in July 2020 by Ludger Timmen from Leibniz Universität Hannover) and other instruments. The nearby continuous Global Navigation Satellite Systems (GNSS) stations HELG and HEL2 are operated by BKG (Federal Agency for Cartography and Geodesy). HELGOG is also part of the Observation Platform Helgoland including seismometers, infrasound, spring gravimeters and tiltmeters. HELGOG is equipped with the iGrav SG 047 manufactured by GWR Instruments. The time series of gravity and barometric pressure from the iGrav SG 047 starts in March 2020. The SG is active and the time series is kept up to date regularly with a time delay of a few months. The time sampling of the raw gravity and barometric pressure data of IGETS Level 1 is 1 second and 1 minute. For a detailed description of the IGETS data base and the provided files see Voigt et al. (2016, https://doi.org/10.2312/GFZ.b103-16087).

Within the European Union FAMOS project, GFZ performed two gravimetry campaigns on commercial ferries in the Baltic Sea in 2017 and 2018. The nature of such “non-dedicated” campaigns is different from “dedicated” campaigns which are performed on research vessels with tracks planned according to gravity measurement needs. The non-dedicated campaigns use non-survey vessels (e.g. ferries) or survey vessels running for other purposes (e.g. hydrographic measurements). Measurements conducted on such conditions may require additional corrections besides the typical marine gravimetry corrections. We investigated two additional corrections, namely the vertical accelerations due to the motion of the ferry in the vertical direction and the dynamical effect due to the cross-coupling between horizontal and vertical acceleration components. To assess the usefulness of non-dedicated campaigns, we analysed gravity measurements collected on two commercial ferries and demonstrated that the standard processing without the above mentioned two corrections, as used in dedicated campaigns, already delivers good quality end products that fulfil the requirements of a typical marine gravimetry survey with an uncertainty of about 1 mGal for a much lower cost. Therefore, the data published in this contribution is a product of the same algorithm we use for dedicated campaign measurements. Based on our findings, we suggest that gravimetry campaigns on commercial ferries can be used to complement dedicated marine gravimetry campaigns and contribute to geodetic purposes.

The Zugspitze Geodynamic Observatory Germany (ZUGOG) has been setup on the summit of mountain Zugspitzeat an altitude of almost 3000 m during 2018 with the main scientific objective to obtain a better understanding ofseasonal and long-term mass redistributions in the European Alps. This knowledge is known to be very important,e.g. with regard to water storage and its high sensitivity to climate change, but not very advanced due to sparsedata availability. ZUGOG is connected to the Environmental Research Station Schneefernerhaus as the home baseof a large research consortium operating a dense sensor network for almost 20 years as well as providing technicalsupport and infrastructure. It is accessible all around the year with cable cars.In September 2018, the Observatory Superconducting Gravimeter (SG) 052 has been installed at ZUGOGafter removal from Sutherland, South Africa, as one of two SGs operating in parallel and refurbishment at themanufacturer GWR. This former laboratory of the Max Planck Society on the summit of Zugspitze has no massvariations above the sensor, no snow accumulation on the roof and a largely increased gravimetric footprint dueto the high altitude. Moreover, this location fits very well into the central European network of SGs. In addition,a GNSS station has been setup as well as meteorological and hydrological sensors in the local vicinity. Thecompleted installations are described, and the first months of data are analysed with regard to noise characteristics,drift and signal separation.This location offers a large variety of connections to other scientific groups and disciplines, which are dis-cussed. These include episodic absolute and monthly relative gravimetric observations with the continuous SGobservations filling an important gap in the concept of hybrid gravimetry. From a hydrological perspective, theconditions of the Research Catchment Zugspitze are very suitable being regarded as a natural lysimeter. In additionto the point measurements of individual storage compartments of the hydrological sensor network, the SG providesthe total water storage variations from a bird’s eye perspective on the summit in the sense of hydrogravimetry.These will also be applied for the validation of GRACE Follow-On. For the monitoring of mountain permafrost inZugspitze, geoelectric and seismic observations but also permanent temperature observations are used. The SG onthe summit directly above the permafrost supports the analysis of seasonal and long-term evolution of permafrostand allows for joint interpretation with complementary geophysical observations.

The 15 year old ICGEM is one of the five services coordinated by the International Gravity Field Service (IGFS)of the International Association of Geodesy (IAG). The aim of the ICGEM service is to provide the scientificcommunity with a state of the art archive of static and temporal global gravity field models of the Earth in astandardized format with a possibility to assign DOI number and develop interactive calculation and visualisationservices of gravity field functionals. Development and maintenance of such a unique platform is crucial for thescientific community in geodesy, geophysics, oceanography and climatology and has a positive impact in thegovernmental organisations and industrial practice. In this presentation, we will cover the maintenance, and the newfeatures and future plans of the ICGEM Service. New features include the calculation of gravity field functionalsat a list of distributed points and new topographic gravity field models whereas the future plans aim to meet theneeds of the scientific community.

The determination of the global gravity field has gained momentum due to high accuracy satellite-derived observations and development of forward gravity modelling. Forward modelling computes the global gravitational field from mass distribution sources instead of actual gravity measurements and helps improving and complementing the medium to high frequency components of the global gravity field models. In this study, we approximate the global gravity potential of the Earth’s upper crust based on ellipsoidal approximation and a mass layer concept. Lateral density variations within a sequence of thin volumetric shells bounded by confocal lower and upper shell ellipsoids are used in the computation of the ellipsoidal harmonic coefficients which are then transformed into spherical harmonic coefficients on the Earth’s surface in the final step. The main outcome of this research is a spectral representation of the Earth’s upper crust’s gravitational potential, computed up to degree and order 3660 in terms spherical harmonic coefficients (ROLI_EllApprox_SphN_3660).

The recently completely recalibrated and reprocessed Gravity Field and Steady-State Ocean Circulation Explorer(GOCE) gravity gradient data were used by GFZ and CNES/GRGS to generate a new gravity field model viathe direct approach (DIR-R6). This work was done on behalf of the European Space Agency (ESA) within theconsortium of the GOCE High Level Processing Facility (GOCE-HPF).DIR-R6 is a satellite-only global gravity field model in terms of spherical harmonics to maximum degree andorder 300. It has been inferred by the combination of GOCE gravity gradient data with data from Satellite LaserRanging (SLR) tracking (LAGEOS 1/2, Stella, Starlette, Ajisai and LARES) and Gravity Recovery and ClimateExperiment (GRACE). Due to the instrumental behavior of the GOCE satellite gradiometer, the gravity gradientobservation equations must be preprocessed and filtered. Here, within the direct numerical method, the filteringhas been done using a low pass filter with a cut-off period of 8 seconds. The GOCE GPS-SST data are only usedto geolocate the gradients. The low-to-medium degree spherical harmonic coefficients of the gravity field aredetermined using GRACE GPS-SST and KBR data as well as SLR data from GFZ’s release 6 models. All dataare combined at normal equation level, which are solved using Cholesky decomposition. We applied the sphericalcap regularization to stabilize the low-order spherical harmonic coefficients for the polar gaps in the GOCE data.Furthermore, Kaula regularization is used at the high degrees. .When compared to the previous gravity field models based on GOCE data, for instance to the earlier releases ofESA’s GOCE models, DIR-R6 is more accurate, especially in its medium to high resolution. This is demonstratedamong others by GPS/leveling, orbit determination tests and an oceanographic evaluation.

System Erde; 7, A glance at the stars and, based on their location in the night sky, the calculation of your individual position on Earth was one of the three approaches used 125 years ago by the Royal Prussian Geodetic Institute on the Telegrafenberg. When “Sputnik 1”, the Earth’s first artificial escort (lat. Satellite), took up duty in 1957 the night shift for an astronomical geodesist changed dramatically. From that moment on not only objects from the catalog on stars were taken into account but satellites were now also tracked. Since then, the role of satellites in all geoscientific research has been steadily increasing. For a long time, satellites have not only provided details on position, but this modern positioning also provides exciting insights into the Earth‘s atmosphere. Furthermore, for years, satellites themselves have also provided images of the Earth‘s surface so that geo-researchers can, thus, analyze ongoing processes on the surface of our planet. This paper presents a loose outline of the development of satellite application on the Telegrafenberg and puts the reader in the mood for the following articles on geoscientific applications of satellites at the GFZ.

IGGT_R1 is a static gravity field model based on the second invariant of the GOCE gravitational gradient tensor, up to degree and order 240. Based on tensor theory, three invariants of the gravitational gradient tensor (IGGT) are independent of the gradiometer reference frame (GRF). Compared to traditional methods for calculation of gravity field models based on GOCE data, which are affected by errors in the attitude indicator, using IGGT and least squares method avoids the problem of inaccurate rotation matrices. IGGT_R1 is the first experiment to use this method to build a real gravity field model by using GOCE gravitational gradients. This new model has been developed by Wuhan University (WHU), GFZ German Research Centre for Geosciences (GFZ), Technical University of Berlin (TUB), Huazhong University of Science and Technology (HUST) and Zhengzhou Information Engineering University (IEU). More details about the gravity field model IGGT_R1 is given in our paper “The gravity field model IGGT_R1 based on the second invariant of the GOCE gravitational gradient tensor” (Lu et al., 2017, http://doi.org/10.1007/s00190-017-1089-8). This work is supported by the Chinese Scholarship Council (No. 201506270158), the Natural Science Foundation of China (Nos. 41104014, 41131067, 41374023, 41474019 and 41504013) and the Key Laboratory of Geospace Environment and Geodesy, Ministry Education, Wuhan University (No. 16-02-07).

The International Geodynamics and Earth Tide Service (IGETS) was established in 2015 by the International Association of Geodesy (IAG). IGETS continues the activities of the Global Geodynamics Project (GGP, 1997-2015) to provide support to geodetic and geophysical research activities using superconducting gravimeter data within the context of an international network. The primary objective of IGETS is to provide a service to monitor temporal variations of the Earth’s gravity field through long-term records from ground gravimeters, tiltmeters, strainmeters and other geodynamic sensors. IGETS also continues the activities of the International Center for Earth Tides, in particular, in collecting, archiving and distributing Earth tide records from long series of the various geodynamic sensors. This report is a compilation of data descriptions originating to a large part from GGP but including updates and extensions for IGETS.

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