In-Depth Characterization of the First Inter-Satellite Laser Ranging Interferometer on GRACE Follow-On

authored by: Malte Matthias Misfeldt
supervised by: Gerhard Heinzel
Abstract: The GRACE Follow-On (GRACE-FO) satellite mission was launched in May 2018. It continues the time series of monthly estimates of the Earth’s gravity field, started by its predecessor, the Gravity Recovery And Climate Experiment (GRACE). Scientists all around the globe use these monthly snapshots to study hydrological processes and the climate crisis. The GRACE-FO twin satellites orbit the Earth approximately every 90 minutes in a polar orbit, with an along-track distance of about 220 km. The Earth’s gravity information is encoded in subtle distance variations between the two, which are measured by the conventional Microwave Instrument (MWI), and in GRACE-FO also by the novel Laser Ranging Interferometer (LRI). The LRI is an optical interferometer split into two units, one on each spacecraft. Its sensitivity of 200 pm/sqrt(Hz) at 5 Hz surpasses the MWI by several orders of magnitude. Further, it has proven very reliable, with very few instrument-related outages. This dissertation is concerned with data analysis of the various telemetry channels of the LRI to deepen the understanding of the instrument. It begins with explaining the LRI instrument and its subunits. New formulas for converting the measured optical phase to the inter-spacecraft range in meter are presented, considering the effect of a time-varying laser frequency. Furthermore, the dominant error sources in the data processing, namely the determination of the absolute value of the laser frequency and a time-tag error of the measurements, are modeled. Lasers form the heart of the LRI, and their reliability and stability are closely monitored. The two LRI lasers show no sign of performance degradation after five years in orbit, but a small bi-modal behavior of some laser telemetry channels was observed. Further, short periods with increased laser frequency noise are investigated. The absolute optical frequency of the LRI lasers acts as the conversion factor between the measured phase variations and the desired ranging signal. It is not measured directly on board and must be inferred in post-processing on ground. Therefore, a large part of this work is taken by developing different models for the absolute optical frequency of the LRI lasers, including the assessment of thermally induced tone errors. The triple mirror assembly is a key component in the LRI, and the perpendicular alignment of its three mirrors ensures the parallelity of the two laser beams traveling between the spacecraft. Hence, their alignment is closely monitored in orbit by analyzing particular diagnostic scans. These scans can further be used to assess various properties of the beam, like the Gaussian divergence angles, the heterodyne efficiency variations, and the effects of a particular kind of tilt-to-length coupling. In the end, single event upsets within the ranging data are investigated, which are short-lived disturbances of the ranging measurement due to charged particles that interact with the onboard electronics.
Organisation(s): Institute of Gravitation Physics
QUEST-Leibniz Research School
Type: Doctoral thesis
No. of pages: 155
Publication date: 2023
Publication status: Published
Electronic version(s): https://doi.org/10.15488/15152 (Access: Open)
: Details in the research portal "Research@Leibniz University"

BibTeX

@phdthesis{148310bdb50b44139cb9342a9b4fb195,
title = "In-Depth Characterization of the First Inter-Satellite Laser Ranging Interferometer on GRACE Follow-On",
abstract = "The GRACE Follow-On (GRACE-FO) satellite mission was launched in May 2018. It continues the time series of monthly estimates of the Earth{\textquoteright}s gravity field, started by its predecessor, the Gravity Recovery And Climate Experiment (GRACE). Scientists all around the globe use these monthly snapshots to study hydrological processes and the climate crisis. The GRACE-FO twin satellites orbit the Earth approximately every 90 minutes in a polar orbit, with an along-track distance of about 220 km. The Earth{\textquoteright}s gravity information is encoded in subtle distance variations between the two, which are measured by the conventional Microwave Instrument (MWI), and in GRACE-FO also by the novel Laser Ranging Interferometer (LRI). The LRI is an optical interferometer split into two units, one on each spacecraft. Its sensitivity of 200 pm/sqrt(Hz) at 5 Hz surpasses the MWI by several orders of magnitude. Further, it has proven very reliable, with very few instrument-related outages. This dissertation is concerned with data analysis of the various telemetry channels of the LRI to deepen the understanding of the instrument. It begins with explaining the LRI instrument and its subunits. New formulas for converting the measured optical phase to the inter-spacecraft range in meter are presented, considering the effect of a time-varying laser frequency. Furthermore, the dominant error sources in the data processing, namely the determination of the absolute value of the laser frequency and a time-tag error of the measurements, are modeled. Lasers form the heart of the LRI, and their reliability and stability are closely monitored. The two LRI lasers show no sign of performance degradation after five years in orbit, but a small bi-modal behavior of some laser telemetry channels was observed. Further, short periods with increased laser frequency noise are investigated. The absolute optical frequency of the LRI lasers acts as the conversion factor between the measured phase variations and the desired ranging signal. It is not measured directly on board and must be inferred in post-processing on ground. Therefore, a large part of this work is taken by developing different models for the absolute optical frequency of the LRI lasers, including the assessment of thermally induced tone errors. The triple mirror assembly is a key component in the LRI, and the perpendicular alignment of its three mirrors ensures the parallelity of the two laser beams traveling between the spacecraft. Hence, their alignment is closely monitored in orbit by analyzing particular diagnostic scans. These scans can further be used to assess various properties of the beam, like the Gaussian divergence angles, the heterodyne efficiency variations, and the effects of a particular kind of tilt-to-length coupling. In the end, single event upsets within the ranging data are investigated, which are short-lived disturbances of the ranging measurement due to charged particles that interact with the onboard electronics.",
author = "Misfeldt, {Malte Matthias}",
year = "2023",
doi = "10.15488/15152",
language = "English",
publisher = "Leibniz Universit{\"a}t Hannover",
school = "Leibniz University Hannover",
}