Generation of Level 1 Data Products and Validating the Correctness of Currently Available Release 04 Data for the GRACE Follow-On Laser Ranging Interferometer

Abstract

The satellite pair of the Gravity Recovery and Climate Experiment (GRACE) Follow-On orbits the Earth, while their inter-satellite distance changes are measured with an accuracy never reached before. This is achieved with the first Laser Ranging Interferometer (LRI) that oper- ates between two distant spacecraft. The mission is based on a US-German collaboration for investigating Earth’s gravitational field and its temporal variations. The LRI was developed with the involvement of the Albert Einstein Institute (AEI) and the instrument has been running reliably for about 3 years now. The AEI has an interest in verifying and validating the LRI Level 1 data products, to ensure that the officially provided LRI data (Release 04 or v04) is correct and useful for gravity field determination. Level 1 data results from the raw telemetry of the spacecraft and serves as an intermediate step before the actual gravity field solutions can be created. Furthermore, the Level 1 data is divided into Level 1A and Level 1B products, where Level 1B is the result of further processing of Level 1A. The author of this thesis has implemented a processing chain in the existing framework of data processing and data analysis at AEI. The new processing chain generates alternative LRI Level 1A data products and especially the LRI1B product. They are referred to as v50 data. The data sets of v04 and v50 were compared in order to identify discrepancies between both versions. It turns out that the LRI Level 1A v04 products show some minor imperfections like a few missing packets of the data, incorrect units or time frame identifiers which do not match with the product description. However, the LRI phase measurements within the LRI1A product are provided correctly, which is the most important data for deriving a correct LRI1B product and the subsequent gravity field solutions. In the case of LRI1B, the range measurement in v50 shows a lower noise level on some individual days than v04. This might be related to instrument reboots, incorrect clock data, and to jumps in the phase measurement, which result for example from thruster activation, but were probably not completely removed from v04 data. In summary, this thesis will introduce some theoretical basics on laser interferometry and occurring effects of relativity in space. Afterwards, the GRACE Follow-On mission and the functionality of the LRI are presented in detail. Furthermore, the different levels of data processing are discussed and the LRI Level 1A and LRI1B processing steps are explained. Finally, the differences of v04 and v50, and their origins will be clarified.