Three gravity field models, parameterized in terms of spherical harmonic coefficients, have been computed from 71 days of GOCE (Gravity field and steady-state Ocean Circulation Explorer) orbit and gradiometer data by applying independent gravity field processing methods. These gravity models are one major output of the European Space Agency (ESA) project GOCE High-level Processing Facility (HPF). The processing philosophies and architectures of these three complementary methods are presented and discussed, emphasizing the specific features of the three approaches. The resulting GOCE gravity field models, representing the first models containing the novel measurement type of gravity gradiometry ever computed, are analysed and assessed in detail. Together with the coefficient estimates, full variance-covariance matrices provide error information about the coefficient solutions. A comparison with state-of-the-art GRACE and combined gravity field models reveals the additional contribution of GOCE based on only 71 days of data. Compared with combined gravity field models, large deviations appear in regions where the terrestrial gravity data are known to be of low accuracy. The GOCE performance, assessed against the GRACE-only model ITG-Grace2010s, becomes superior at degree 150, and beyond. GOCE provides significant additional information of the global Earth gravity field, with an accuracy of the 2-month GOCE gravity field models of 10 cm in terms of geoid heights, and 3 mGal in terms of gravity anomalies, globally at a resolution of 100 km (degree/order 200).

/pdf/first-goce-gravity-field-models-derived-by-three-different-55q4c3j26r.pdf

First GOCE gravity field models derived by three different approaches

Ocean-driven basal melting of Antarctica's floating ice shelves accounts for about half of their mass loss in steady-state, where gains in ice shelf mass are balanced by losses. Ice shelf thickness changes driven by varying basal melt rates modulate mass loss from the grounded ice sheet and its contribution to sea level, and the changing meltwater fluxes influence climate processes in the Southern Ocean. Existing continent-wide melt rate datasets have no temporal variability, introducing uncertainties in sea level and climate projections. Here, we combine surface height data from satellite radar altimeters with satellite-derived ice velocities and a new model of firn-layer evolution to generate a high-resolution map of time-averaged (2010-2018) basal melt rates, and time series (1994-2018) of meltwater fluxes for most ice shelves. Total basal meltwater flux in 1994 (1090±150 Gt/yr) was not significantly different from the steady-state value (1100±60 Gt/yr), but increased to 1570±140 Gt/yr in 2009, followed by a decline to 1160±150 Gt/yr in 2018. For the four largest "cold-water" ice shelves we partition meltwater fluxes into deep and shallow sources to reveal distinct signatures of temporal variability, providing insights into climate forcing of basal melting and the impact of this melting on the Southern Ocean.

/pdf/interannual-variations-in-meltwater-input-to-the-southern-4slwdtv474.pdf

Interannual variations in meltwater input to the Southern Ocean from Antarctic ice shelves

. The International Centre for Global Earth Models (ICGEM,
 http://icgem.gfz-potsdam.de/ , last access: 6 May 2019) hosted at the
GFZ German Research Centre for Geosciences (GFZ) is one of the five services
coordinated by the International Gravity Field Service (IGFS) of the
International Association of Geodesy (IAG). The goal of the ICGEM service is
to provide the scientific community with a state-of-the-art archive of static
and temporal global gravity field models of the Earth, and develop and
operate interactive calculation and visualization services of gravity field
functionals on user-defined grids or at a list of particular points via its
website. ICGEM offers the largest collection of global gravity field models,
including those from the 1960s to the 1990s, as well as the most recent ones,
which have been developed using data from dedicated satellite gravity
missions, CHAMP, GRACE, GOCE, advanced processing methodologies, and
additional data sources such as satellite altimetry and terrestrial gravity.
The global gravity field models have been collected from different
institutions at international level and after a validation process made
publicly available in a standardized format with DOI numbers assigned through
GFZ Data Services. The development and maintenance of such a unique platform
is crucial for the scientific community in geodesy, geophysics, oceanography,
and climate research. In this article, we present the development history and
future plans of ICGEM and its current products and essential services. We
present the ICGEM's data by means of Earth's static, temporal, and
topographic gravity field models as well as the gravity field models of other
celestial bodies together with examples produced by the ICGEM's calculation
and 3-D visualization services and give an insight into how the ICGEM service
can additionally contribute to the needs of research and society.

/pdf/icgem-15-years-of-successful-collection-and-distribution-of-30zzdralnf.pdf

ICGEM – 15 years of successful collection and distribution of global gravitational models, associated services, and future plans

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. 
Plain Language Summary: In a context of global change and increased human vulnerability to terrestrial hazard, monitoring the Earth system is one of the key challenges of geoscience. In particular, terrestrial gravimetry, with its precision at the level of one part of a billion, allows the monitoring of many phenomena, from water resource availability to volcanic activity. This paper reviews the technique, its advantages and limitations, how it has been used in the Earth monitoring, and the next challenges to be met by ground gravimetry.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017RG000566

Geophysics From Terrestrial Time-Variable Gravity Measurements

XGM2019e is a combined global gravity field model represented by spheroidal harmonics up to degree and order (d/o) 5399, corresponding to a spatial resolution of 2′ (~ 4 km). As data sources, it includes the satellite model GOCO06s in the longer wavelength range up to d/o 300 combined with a ground gravity grid which also covers the shorter wavelengths. The ground data consist over land and ocean of gravity anomalies provided by courtesy of NGA (15′ resolution, identical to XGM2016) augmented with topographically derived gravity information over land (EARTH2014). Over the oceans, gravity anomalies derived from satellite altimetry are used (DTU13 with a resolution of 1′). The combination of the satellite data with the ground gravity observations is performed by using full normal equations up to d/o 719 (15′). Beyond d/o 719, a block-diagonal least squares solution is calculated for the high-resolution ground gravity data (from topography and altimetry). All calculations are performed in the spheroidal harmonic domain. In the spectral band up to d/o 719, the new model shows a slightly improved behaviour in the magnitude of a few mm RMS over land as compared to preceding models such as XGM2016, EIGEN6c4 or EGM2008 when validated with independent geoid information derived from GNSS/levelling. Over land and in the spectral range above d/o 719, the accuracy of XGM2019e marginally suffers from the sole use of topographic forward modelling, and geoid differences at GNSS/levelling stations are increased in the order of several mm RMS in well-surveyed areas, such as the US and Europe, compared to models containing real gravity data over their entire spectrum, e.g. EIGEN6c4 or EGM2008. However, GNSS/levelling validation also indicates that the performance of XGM2019e can be considered as globally more consistent and independent of existing high-resolution global models. Over the oceans, the model exhibits an enhanced performance (equal or better than preceding models), which is confirmed by comparison of the MDT’s computed from CNES/CLS 2015 mean sea surface and the high-resolution geoid models. The MDT based on XGM2019e shows fewer artefacts, particularly in the coastal regions, and fits globally better to DTU17MDT which is considered as an independent reference MDT.

/pdf/the-combined-global-gravity-field-model-xgm2019e-3u8slfejpd.pdf

C. Foerste

Papers

EIGEN-6C3 - The latest Combined Global Gravity Field Model including GOCE data up to degree and order 1949 of GFZ Potsdam and GRGS Toulouse

A preliminary update of the Direct Approach GOCE Processing and a new release of EIGEN-6C

Earth Gravity Field Solutions from Several Months of CHAMP Satellite Data

A New Glocal Mean Gravity Field Model From The Combination Of Satellite Mission And Altimetry/Gravimetry Surface Gravity Data

EIGEN-5C - the new GeoForschungsZentrum Potsdam / Groupe de Recherche de Geodesie Spatiale combined gravity field model