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Abstract

The CNES/GRGS RL04 Earth gravity models are a set of gravity field solutions based on GRACE and SLR data, provided at different time samplings: (A) CNES/GRGS RL04 time series (A/1) A monthly GRACE+SLR time series of gravity field models (A/2) A 10-day GRACE+SLR time series of gravity field models (B) A mean gravity model EIGEN-GRGS.RL04.MEAN-FIELD, computed from the monthly RL04 GRACE+SLR time series and from GOCE data. (A) CNES/GRGS RL04 time series DATA: The data from the Star Camera Assembly (SCA), Accelerometer (ACC), K-Band Ranging (KBR) and GPS receiver are used. The KBR data is processed in the form of the relative velocity between the spacecrafts: K-Band Range-Rate (KBRR). In addition to the data from GRACE, the data from 5 SLR satellites are also used (Lageos, Lageos-2, Starlette, Stella and Ajisai), in order to provide an accurate and consistent description of the very low degrees of the gravity field (mainly degrees 1 and 2). The version of the GRACE data used for RL04 is L1B-v2 for the ACC and GPS data, L1B-v3 for the SCA and KBR data. INVERSION METHOD: By contrast with the GRACE solutions in spherical harmonics provided by other groups, the CNES/GRGS solutions are not obtained by a simple Cholesky inversion. The normal matrices are first diagonalized, ordered by decreasing order of the Eigen values and only the best defined sets of linear combinations of the spherical harmonics are solved. More details can be found here: https://grace.obs-mip.fr/variable-models-grace-lageos/grace-solutions-release-04/rl04-products-description/ (B) EIGEN-GRGS.RL04.MEAN-FIELD mean model EIGEN-GRGS.RL04.MEAN-FIELD is a mean model of Earth's gravity field spherical harmonics coefficients, based on the RL04 version of the CNES/GRGS time series of monthly gravity field determinations from GRACE & SLR data. EIGEN-GRGS.RL04.MEAN-FIELD is complete to degree and order 300. Between degrees 1 and 90, it contains time-variable gravity (TVG) coefficients ; above degree 90, it is a static field. EIGEN-GRGS.RL04.MEAN-FIELD is based on GOCE-DIR5 for the part between degree 91 and 300. The TVG coefficients between degrees 1 and 90 are obtained from a regression on the GRGS-RL04-v1 monthly time series of solutions (2002/09 – 2016/06). For degrees 1 and 2 this TVG part is temporally extended to 1993/01-2019/02 through the use of a GRGS SLR-only solution based on the data of 5 SLR satellites (Lageos, Lageos-2, Starlette, Stella, Ajisai). Outside of the measurements period (1993/01-2019/02 for degrees 1 and 2, 2002/09-2016/06 for degrees 3 to 90), the gravity field is extrapolated in the following way: - for degrees 1 and 2, before 1993/01 : average slope based on historical SLR data, mean annual and semi-annual periodic signals based on their average value between 1993 and 2019 - for degrees 1 and 2, after 2019/02 : average slope & mean annual and semi-annual periodic signals (based on their average value between 1993 and 2019) - for degrees 3 to 90, before 2002/09 : zero-slope extrapolation, mean annual and semi-annual periodic signals based on their average value between 2002 and 2016 - for degrees 3 to 90, after 2016/06 : average slope & mean annual and semi-annual periodic signals (based on their average value between 1993 and 2019) More details can be found here: https://grace.obs-mip.fr/variable-models-grace-lageos/mean-fields/release-04/