Abstract
In this study, we assessed recent GOCE-based Global Geopotential Models (GGMs) and EGM2008 in Niger. The combined GGMs EIGEN_6C4, GECO and EGM2008 were evaluated up to their maximum degree and order (d/o) 2,190 to select the one for gravity database densification. The following pure satellite GGMs were assessed for the modelling of the long and medium wavelengths in geoid computation: GGM05G, ITU_GGC16, EIGEN_6S4v2 and the fifth releases from direct (DIR5), space-wise (SPW5) and time-wise (TIM5) approaches. The GGMs are compared to terrestrial gravity data and geoid heights from GNSS/Levelling points before and after applying spectral enhancement method (SEM) by residual terrain model (RTM) for combined models and by RTM and the coefficients of selected combined GGM for pure satellite models. The agreements of combined GGMs with terrestrial gravity data and GNSS/Levelling points, in terms of root mean square (RMS) are about 4.88 to 5.02 mGal and 0.14 to 0.16 m, respectively. EIGEN_6C4 was selected as it showed the best performance in terms of geoid height differences and the probability of 3-sigma rule for gravity anomaly differences. At d/o 200, DIR5 showed a good agreement with terrestrial gravity data (5.04 mGal) and GNSS/Levelling points (0.15 m) after applying SEM, it was then retained. All GOCE-based models exhibited a good performance in long and medium wavelengths confirming the good recovery of the gravity field by the spatial gravity mission in these spectral bands.
Highlights
The Global Geopotential Model (GGM) is a set of coefficients of a spherical harmonic expansion to a degree and order (d/o) of the Earth gravity field for global analysis
The 3-σ rule confirms this fact for all GGMs, the probabilities are less than 99% (Table 2)
The quantities derived from combined GGMs EIGEN_6C4, GECO and EGM2008 evaluated up to their maximum d/o 2,190 were compared to free air gravity anomalies from terrestrial data and geoid undulations from GNSS/ Levelling points before and after applying spectral enhancement method (SEM) by residual terrain model (RTM)
Summary
The Global Geopotential Model (GGM) is a set of coefficients of a spherical harmonic expansion to a degree and order (d/o) of the Earth gravity field for global analysis. There are two categories of GGMs: pure satellite model and combined model which is obtained by merging spatial gravity measurements and terrestrial data eg. The last three satellite gravity missions are CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery and Climate Experiment (GRACE) and Gravity Field and Steady State Ocean Circulation Explorer (GOCE). Five releases of GOCE-based GGMs have been developed, based on observation data in different time spans, by ESA’s (European Space Agency) High Level Processing Facility (HPF). Besides the ESA’s solutions, some models are based on the combination with other data sources, such as terrestrial gravity, satellite altimetry, or with CHAMP and GRACE eg. GECO is an update of EGM2008 with GOCE data (Gilardoni, Reguzzoni, & Sampietro, 2015)
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