Recent Analyses with Alternative LRI1B Datasets of AEI

Although the Laser Ranging Interferometer (LRI) on GRACE Follow-On has a higher precision than the K/Ka-Band Ranging (KBR) system, special circumstances lead to the decision to operate the satellites in nadir pointing mode and, starting in July 2023, in wide deadband mode to improve the quality of the accelerometer transplants and reduce propellant consumption. During these periods, the LRI is unable to record ranging data due to its narrow pointing requirements. However, the LRI will be the primary and only ranging instrument on future missions, and it is important to understand its data and optimize the data processing.

Therefore, we have continued to (re-)analyze the raw telemetry from the LRI and have advanced the data processing, resulting in alternative LRI1B datasets that we are making publicly available. These data products include the Tilt-to-Length Coupling Correction (aka Antenna Offset Correction in KBR), an alternative calculation of the Light Time Correction, different models for a time-dependent absolute laser frequency, consideration of relativistic effects acting on the spacecraft proper time when converting the phase to a range, and also a quality flag to mark special events such as sun blinding periods.

We have recently produced a new LRI1B v54 release that includes all available LRI data (June 2018 to June 2023) and a dataset with a sampling rate of 0.2 Hz, the same as the KBR sampling, to facilitate comparisons. We will present an analysis of the differences between LRI1B v54 and KBR1B v04 in the spectral, geographic, and time domains to support ongoing investigations into why the ocean RMS values of the monthly gravity fields from LRI are often slightly higher than those from KBR.

In addition, Duwe et al. [2024] recently showed that the post-fit residuals of LRI1B v04 exhibit some artifacts of the order of +/- 2.5 nm/s², visible when the residuals are high-pass filtered. We were able to confirm most of these features with the LRI1B v04 data, but could not observe them in our internally derived alternative product, LRI1B v54. We further show that the spurious signatures appear even without gravity field recovery by comparing LRI1B v04 and v54 at the level of the raw biased range (or range rate), i.e. without applying correction terms for light-time and antenna offset. We also show that the effects are most likely not present in the LRI1A v04 phase data, leading to the conclusion that these artifacts arise in the v04 Level1a to Level1b processing of SDS.

All of our data products are publicly available at https://www.aei.mpg.de/grace-fo-ranging-datasets.

_{Duwe et al [2024]: Residual Patterns in GRACE Follow-On Laser Ranging Interferometry Post-Fit Range Rate Residuals, Advances in Space Research, https://doi.org/10.1016/j.asr.2024.03.035.}