Data Processing and Investigations for the GRACE Follow-On Laser Ranging Interferometer

Abstract

This thesis presents first in-depth results of the Laser Ranging Interferometer (LRI) onboard the Gravity Recovery And Climate Experiment - Follow On (GRACE-Follow On) mission. The LRI is a novel instrument, which was developed in a U.S.-German collaboration including the Albert-Einstein Institute (AEI) in Hanover. It successfully demonstrated the feasibility of ranging measurements by means of laser interferometry between two distant spacecraft and will push space-borne gravimetry missions to the next sensitivity level. The author of this thesis contributed to this project by programming a comprehensive framework for ground-processing of LRI telemetry and analyzing various kinds of instrument data streams. Therefore, the title of this thesis covers both topics, data processing and investigations within the data. Within this thesis, an introduction to laser interferometry is given and the various payloads of the GRACE-Follow On satellites are presented. Furthermore, the design of the LRI itself is discussed, in order to understand the profound causal relations when getting into the details of investigations. The various kinds of telemetry data and their processing levels are presented, giving an insight about the variety of data sets, that are downlinked from the satellites. The investigations cover various major topics. These reach from different models to assess the absolute laser frequency, which sets the scale to convert the raw phase measurements into corresponding inter-satellite displacements, and comprise a detailed investigation of the carrier to noise ratio, which provides information about the signal quality. Furthermore, the laser’s beam properties in the far-field are investigated by means of the intensity and the phasefront. These investigations even lead to a proposal for a new scan pattern, which has actually been performed. Last but not least, a comprehensive assessment of the LRI spectrum was performed, which reveals correlation between the satellite’s attitude and orbit control system (AOCS), i.e. the star cameras for attitude determination and thruster activations for attitude control, and the ranging signal, measured by the LRI. In summary, this thesis is concerned with several aspects of the LRI characterization and data analysis. Since the overall data quality and sensitivity of the LRI exceeds the needs and expectations for the current gravimetric mission, many of the discussed effects are rather of academic interest, e.g. to deepen the instrument understanding of the LRI team and for the development of future missions in the field of geodesy or the space-based gravitational wave detection (LISA mission).