DOI: 10.5880/icgem.2019.004
title: ESA’s Release 6 GOCE gravity field model by means of the direct approach based on improved filtering of the reprocessed gradients of the entire mission (GO_CONS_GCF_2_DIR_R6)

description: "ESA’s Release 6 GOCE gravity field model by means of the direct approach based on improved filtering of the reprocessed gradients of the entire mission (GO_CONS_GCF_2_DIR_R6)" is a static gravitational model available via ICGEM (Ince et al., 2019) Model Characteristics----------------------GOCE Input Data:- Gradients: EGG_NOM_2 (re-calibrated release 2018, Siemes et al. 2019)- Orbits: SST_PRD_2 (reduced dynamic orbits)- Attitude: EGG_IAQ_2C- Data period: 20091009T000000-20131020T235959 A-priori Information used:----------------------------The a-priori gravity field for the processing of the GOCE gravity gradients was the GOCE-model 5th release from the direct approach GO_CONS_GCF_2_DIR_R5 up to its maximum degree/order 300 (Bruinsma et al. 2014). Processing Procedures:----------------------The GOCE gravity gradients were processed without applying the external calibration corrections. The observation equations were filtered with a 0 - 125.0 mHz lowpass filter. Subsequently "SGG" normal equations to degree/order 300 have been computed separately for 46 continous time segments of approximately 1270 days totally (identified after the preprocessing of the data) and for each of the gradient components Txx, Tyy, Tzz and Txz. The Txx, Tyy, Tzz and Txz SGG normal equations were accumulated with the relative weight 1.0. But within the SGG components, all observation equations have been weighted individually according to its standard deviation estimated w.r.t. the a-priori gravity field. To overcome the numerical instability of the GOCE-SGG normal equation due to the polar gaps and to compensate for the poor sensitivity of the GOCE measurements in the low orders the following stabilizations were applied: 1) The GOCE-SGG normal equation was fully combined with GRACE and SLR normal equations. Details about the latter contributions are given below.2) A spherical cap regularization in accordance to Metzler and Pail (2005) was iteratively computed to d/o 300 using the GRACE/SLR data mentioned below to degree/order 130.3) Additionally a Kaula regularization was applied to all coefficients beyond degree 180 The solution was obtained by Cholesky decomposition of the accumulated normal equations. Details of the GRACE contribution:----------------------------------The GRACE part consists of 85 monthly normal equations to degree/order 200 out of the time span January 2007 till November 2014 from GFZ's GRACE Release 06 processing based on GNSS-SST and K-Band-Range-Rate data. For details of this GRACE release see Dahle et al. 2018. The following individual months are not covered by GRACE: 2001101, 201106, 201205, 201210, 201303, 201304, 201308, 201402 and 201407 The harmonics of very-low degree, in particular degrees 2 and 3, cannot be estimated accurately with GRACE and GOCE data only. Therefore, normal equations from the following SLR missions were used in the combination in order to improve the gravity field solution:- LAGEOS-1/2, AJISAI, STARLETTE and STELLA from Jan. 2002 till Oct. 2018- LARES from Feb. 2012 till Oct. 2018 The SLR tracking data were processed according to the GRACE Release 6 standards During the combination with GOCE, the GRACE contribution was taken only up to degree/order 130 and the SLR contribution only up to degree/oder 5 As GRACE is sensitive for temporal variations in the Earth gravity field, the date 20100901 should be taken as reference epoch of this model. This date is mean of the included GRACE measurement time span by considering the mentioned missed months. This reference epoch is close to the mean of the measurement time span of the included SLR tracking data (20100701) Specific features of resulting gravity field--------------------------------------------The model is a satellite-only model based on a full combination of GOCE-SGG with GRACE and SLR tracking data, leading to both excellent orbit fits as well as GPS/leveling results Processing details are presented in Pail et al. 2011.