Seite - 41 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

ITSG-Grace2016 6 This chapter will give an outline of the state of the art of GRACE data processing in the frame of the ITSG-Grace2016 gravity field solution. This information will give important background and context to the later chapters in this thesis, which detail some of the author’s specific contributions to ITSG-Grace2016 and the subsequent ITSG-Grace2018 solution. The processing, as outlined in fig. 6.1, begins with the download of GRACE level 1B data from the SDS. Following SCA/ACC sensor fusion and data screening, kinematic and dynamic orbits are determined for both GRACE-A and GRACE-B. This is followed by the estimation of stochastic models for both hl-SST and ll-SST observations, which are then used in the determination of the sought monthly Stokes coefficients. All computation steps are performed in GROOPS. GROOPS is is the Gravity Recovery Object Oriented Programming System, an in- house multi-purpose geodetic software suite developed at IfG. GROOPS is used for all GRACE data processing at IfG, and is capable of performing all necessary processing steps to generate level 2 and level 3 products from GRACE level 1B data. To enable full GRACE-FO processing, level 1A to level 1B processing is also being integrated into GROOPS. GROOPS is written in C++, and makes extensive use of the low-level linear algebra routines defined by LAPACK and BLAS (Anderson, 1999; Blackford et al., 2002). 6.1 Data Preprocessing TheITSG-Grace2016gravityfieldsolutioniscomputedfromGRACElevel1Brelease2.0 data published by GFZ and JPL (GRACE, 2001). The data provided is of high quality, however some additional screening and preprocessing steps have been found to improve the gravity field solutions derived from them. These additional steps have been described in much detail by Klinger (2018). Much of the preprocessing of GRACE data will remain unchanged for the upcoming ITSG-Grace2018 release, with notable exceptionof thesensor fusionsteptocombinethestarcamera level1Bproduct (SCA1B) with the accelerometer level 1B product (ACC1B) to yield an improved orientation product. The upcoming release 3.0 of GRACE level 1B data by JPL will already include an orientation product derived from SCA/ACC sensor fusion (GRACE, 2018), which was previously computed in-house at IfG. For the purpose of this document, a SCA/ACC sensor fusion approach refined from the ITSG-Grace2016 approach 41