Momentum Transfer Events and Other Disturbances in LRI Data

The Laser Ranging Interferometer (LRI) is a technology demonstrator instrument on board the GRACE-FO twin-satellite mission that has been in orbit since 2018. It uses laser interferometry to measure the range between the two satellites (GF1 and GF2) with a nanometer level of accuracy. The gravity field measurements using the ranging data quantify the spatial and temporal variation in the Earth's mass distribution.

The change in the range measured by the LRI has gravitational and non-gravitational origins. Many non-gravitational effects, like atmospheric drag and thruster firings, are known and can be delineated from the LRI data. However, other sporadic instantaneous changes in the range are described as possible momentum transfer events (MTEs) candidates. Some potential sources are meteoroids and space debris's physical impacts on the spacecraft. 

To analyze MTE candidates, we first present the process of detecting them by correlating the LRI ∆v with that of the accelerometer (ACC) for both GRACE-FO spacecraft. Through this, 129 events were found on GF1 and 138 in GF2 between mid-2018 and the end of 2022. Some events show a better correlation between LRI and ACC data than others. Hence, further analysis is conducted by categorizing them accordingly and studying their spatial distribution and periodicity. The occurrence of many events coincides with the period of the beta angle of the sun. Structural changes in the spacecraft could cause these events upon exposure to solar radiation. However, many events have ∆v < 5 × 10 −8 m/s, which is too low to be considered an MTE. They are instead categorized as disturbances to the spacecraft. The origin of these remains unknown.

Furthermore, simulations are performed using the ESA MASTER v8.0.3 to render the number of events per year that cause MTEs via physical impact during the same observation period. The observed events are consistent with the simulation for ∆v ≥ 5 × 10 −8 m/s. 

This analysis is beneficial in creating a repository of known effects in the first-ever space laser interferometer. In future missions, the knowledge of these occurrences in the LRI and ACC data will be helpful during post-processing.