The Experimental Gravity Field Model XGM2016
20/09/2016 | 08:45 | Session 2: Model Development
Author(s): Roland Pail, Thomas Fecher, Daniel Barnes, John K. Factor, Simon A. Holmes, Thomas Gruber and Philipp Zingerle
Roland Pail, Thomas Fecher, Daniel Barnes, John K. Factor, Simon A. Holmes, Thomas Gruber and Philipp Zingerle
In December 2015, the United States National Geospatial-Intelligence Agency [NGA] publicly announced that it has begun work on a replacement for its Earth Gravitational Model 2008 [EGM2008]. This new model, Earth Gravitational Model 2020 [EGM2020], is scheduled for release in late 2019. Like EGM2008, EGM2020 will take the form of a spherical harmonic expansion of the geopotential, complete to degree (n) 2190 and order (m) 2159. NGA is collaborating with its national and international partners to ensure that EGM2020 makes best use of its new data sources and methodology.
Towards this end, NGA has agreed to provide the Technical University of Munich [TUM] with a new, global 15’x15’ grid of ‘terrestrial’ gravity anomaly area means. This grid incorporates the majority of NGA’s new altimetric and terrestrial survey data, as well benefiting from new procedures for processing this data. At this early stage, TUM has agreed to provide NGA with an independent assessment of this new data grid, in terms of its suitability for supporting an improved EGM. One outcome of this effort is the Experimental Gravity Field Model 2016 [XGM2016]. XGM2016 will extend to spherical harmonic degree of 719, which is maximum resolution supported by its 15’x15’ terrestrial grid.
For XGM2016, a significant focus will be the optimal combination of the new terrestrial data with the latest satellite gravity information. This includes 11 years of GRACE (2002-2013), and the entire GOCE mission (2009-2013). The combination is based on a full normal equation system up to the maximum degree (n=719) of the expansion. A regionally dependent weighting strategy is applied, thus allowing consideration of the individual accuracy of each altimetric and terrestrial data cell so that the transition from satellite to terrestrial and altimetric data can be modeled individually for each cell. Stochastic models for the terrestrial/altimetric data base are informed by comparisons with satellite-only solutions in the long to medium wavelength range.
Validation of XGM2016 will include internal metrics from the final adjustment, demonstrating the benefit from the GOCE data. External validation will include (for example) GPS/leveling comparisons over land, and improvements in the derived global mean dynamic topography (MDT) model, as compared against independent models and data for ocean current velocities. Other assessments, and comparisons against existing models, such as EGM2008, will also be included.
XGM2016 will be publicly available in late 2016.