Seite - 1 - 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

Introduction 1 The Gravity Recovery and Climate Experiment (GRACE) twin satellite mission was a joint NASA and DLR Earth observation mission, directed at detecting mass transport signals in the Earth system at an unprecedented accuracy. The mission was an over- whelming success, collecting data during its operational period spanning from April 2002 until end of science operations in June 2017, much exceeding the initial goal of 5 years of data acquisition. The successor mission to GRACE, Gravity Recovery and Climate Experiment Follow- On (GRACE-FO), was successfully launched on May 22, 2018, and is expected to continue the time series established by GRACE into the future. Further, GRACE-FO will serve as a technology demonstrator for advanced instrumentation, use of which could be made in possible future satellite gravimetry missions. This work has a three-fold purpose. First, the stability of the current state-of-the-art orbit integration technique used for GRACE processing is analysed. These results are then compared to the requirements of GRACE-FO and possible future gravity missions. Improvements to the orbit integration algorithm are proposed and imple- mented, making it suitable for GRACE-FO processing. Second, an attempt is made to introduce a stochastic model of GRACE orientation observations into the gravity field determination, the impact of which is studied in the context of the current GRACE mission. This is done in hope of improving the quality of the existing time series of GRACE gravity field solutions. Third, the newly introduced stochastic information is exploited in the co-estimation of satellite orientations and gravity field parameters in a total least squares adjustment. All of this is achieved within the context of the gravity field processing strategy employed at IfG, which is the Institute of Geodesy at Graz University of Technology, where the ITSG-Grace2016 series of gravity field solutions was computed. Chapters 2 to 6 give background on some of the relevant concepts, terminology, and algorithms. This is then followed by the three chapters 7 to 9 which detail the author’s contributions to the aforementioned fields of orbit integration, stochastic modelling, and parameter estimation in the context of GRACE. The individual chapters are as follows: Chapter 2 gives information on the employed notation, basic concepts such as the least squares adjustment, variance propagation, parameter elimination, variance component estimation, and specific interpolation techniques used in later chapters. This chapter is held general in nature, and its contents should be mostly familiar to readers with a background in satellite gravimetry. 1