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An quality assessment of the official GOCE Level 2 GRD SPW 2 products over Norway, Czechia, and Slovakia

Martin Pitoňák¹, Michal Šprlák¹, Vegard Ophaug², Ove C. D. Omang³, Pavel Novák¹

Gravity disturbances over test areas:

a) Czechia and Slovakia and b) Norway.

¹NTIS – New Technologies for the Information Society, Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czechia

²Faculty of Science and Technology (RealTek), Norwegian University of Life Sciences (NMBU), Ås, Norway,

³Norwegian Mapping Authority, 3507 Hønefoss, Norway

In this contribution, validate the official Level 2 product GRD_SPW_2 by terrestrial gravity disturbances over two test areas located in Europe, namely in Norway and former Czechoslovakia (now Czechia and Slovakia). GRD_SPW_2 product contains all six gravity gradients at satellite altitude from the space-wise approach computed only from GOCE data for the available time spans correspond to release 2, 4, 5 and 6, and are provided on a 0.2 degree grid. A mathematical model based on the least-squares spectral weighting is employed for the validation of gravitational gradients grids. This method allows as to continue downward gravitational gradients grids to an irregular topographic surface (not to a mean sphere) and transform them to gravity disturbances in one step. Prior the comparison, we removed systematic effects such as a tide system conversion or an ellipsoidal correction. Further, we estimated the high-frequency part of gravitational signal called as the omission error and subtracted it from terrestrial data because in gravitational gradients measured at GOCE satellite altitude is attenuated. To do so we employ EGM2008 (Pavlis et al., 2012) up to d/o 2160 and the omission error has been a) synthesised from dV_ELL_Earth2014_5480_plusGRS80 (Rexer et al. 2017), b) interpolated from ERTM2160 (Hirt et al., 2014) gravity model, c) calculated from a residual topographic model by forward modelling in space domain.

RTM: r₁=0.1° (polyhedron), r₂=0.5° (prism), r₃=1° (tesseroid), r₄=3° (point-mass). In zones 1-3 we employed AW3D30 (Tadano et al., 2014) with resolution of 1’’ over Czechia and Slovakia and ACE2 (Berry et al., 2010), with resolution of 1’’ over Norway. In the fourth-zone we used ACE2 with spatial resolution of 30’’. We took advantage of TGF software (Yang et al., 2020).

Conclusion:

the method for validation of the official GOCE Level 2 SPW_GRD_2_ has been presented and tested,
there are not significant differences between global geopotential models calculated by space-wise approach and their grids counterparts,
the reprocessed version of the official GOCE Level 2 SPW_GRD_2_r6 is about 10% more accurate comparing to SPW_GRD_2_r5.

Acknowledgment: This contribution was supported by project HR Award of University of West Bohemia and by the project No. 21-13713S of the Czech Science Foundation

Spectral downward continuation and combination:

The one-group estimator

The two-group estimator

The three-group estimator

Results:

Czechia and Slovakia

Norway