GRACE Follow-On Accelerometer Data Recovery by High-Precision Environment Modelling

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) satellites are equipped with high-precision three-axis accelerometers to measure all non-gravitational accelerations acting on the satellites. The accelerometer data are mainly used to account for the influence of these accelerations in the gravity-field-recovery process. Unfortunately, after only one month in orbit the accelerometer on one of the two satellites produced decreasingly accurate measurements. Due to this, the GRACE-D accelerometer data have to be replaced by artificial data. The procedure for the official GRACE-FO Science Data System (SDS) data products is a so called transplant of GRACE-C data. As an alternative approach, we present a modelling method, where the GRACE-D accelerometer data are based on high-precision non-gravitational force and disturbance modelling. We compare our modelled data to thruster-free accelerometer data derived from the official SDS data products. With this, we can evaluate the performance and show details of our approach. For example, the influence of an in-situ drag-coefficient estimation based on Sentman’s approach. In contrast to other GRACE-FO accelerometer-data-recovery approaches, no transplant of data is incorporated. This work is part of the Collaborative Research Center 1464 TerraQ and funded by DFG

Study of Space-Environmental Effects on Interplanetary Trajectories

Since the early 90s of the last century, when the Galileo spacecraft used the Earth for two Gravity Assist manoeuvres, scientist puzzle over the origin of an unexpected velocity difference associated with Earth-Gravity Assists. As this is not limited to one occurrence but was detected for several spacecraft, numerous scientific papers about this so called Flyby Anomaly have been published since then, trying to explain the velocity change with various physical relations. However, no scientifically sound explanation has been found up to today. The aim of this master thesis is the simulation of interplanetary trajectories of flown missions, the comparison of the simulation results to the measurement data and the evaluation of the use of the HPS in analyses of the Flyby Anomaly. The simulations are done with the High Performance Satellite Dynamics Simulator (HPS), a simulation tool developed at ZARM. For the HPS to be able to simulate Gravity Assists, the source code has to be adapted. This master thesis includes a short review of the current state of the art regarding the simulation of interplanetary trajectories and possible Flyby Anomaly explanations. The theoretical background for the the orbital dynamics of interplanetary trajectories, perturbation forces acting on a spacecraft and necessary coordinate transformations are presented as well. The results of the customized HPS are evaluated to ensure accurate simulations. Based on literature recommendations and the to examined Flyby Anomaly, the influence of gravitational effects and non-gravitational perturbations on the simulations are examined. Using the tracking data of the examined spacecraft as a comparison, the results of the simulations are discussed and evaluated. The results show which perturbations are essential for an adequate precision and which can be neglected. Further, different attitudes are compared and the mostly unknown attitude of the examined spacecraft is validated. Regarding the application of the HPS for the analysis of the Flyby Anomaly it is concluded that the type of simulation done in this master thesis provides insufficient accurate results

GRACE Follow-On Accelerometer Data Recovery by High-Precision Environment Modelling

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) satellites are equipped with high-precision three-axis accelerometers to measure all non-gravitational accelerations acting on the satellites. The accelerometer data are mainly used to account for the influence of these accelerations in the gravity-field-recovery process. Unfortunately, after only one month in orbit the accelerometer on one of the two satellites produced decreasingly accurate measurements. Due to this, the GRACE-D accelerometer data have to be replaced by artificial data. The procedure for the official GRACE-FO Science Data System (SDS) data products is a so called transplant of GRACE-C data. As an alternative approach, we present a modelling method, where the GRACE-D accelerometer data are based on high-precision non-gravitational force and disturbance modelling. We compare our modelled data to thruster-free accelerometer data derived from the official SDS data products. With this, we can evaluate the performance and show details of our approach. For example, the influence of an in-situ drag-coefficient estimation based on Sentman’s approach. In contrast to other GRACE-FO accelerometer-data-recovery approaches, no transplant of data is incorporated. This work is part of the Collaborative Research Center 1464 TerraQ and funded by DFG