Application of ultra‐precise GRACE Follow-On LRI measurements for validation of the state-of-the-art static gravity field models and examination of sub-monthly time-variable gravity signals

The test Laser ranging interferometer (LRI) on the GRACE Follow-On satellites provides complementary inter-satellite ranging measurements to the baseline K-band microwave ranging (KBR) system that can be used to examine standard, and create novel, GRACE-FO data products.  We first calculated the KBR and LRI inter-satellite ranging residuals using dynamic orbits computed from non-gravitational accelerations, a static gravity field model and other background geophysical models like ocean tides. To accurately quantify the improvement by LRI, we directly examined the inter-satellite ranging residuals in the time and frequency domains. The frequency-domain analysis reveals that LRI enhances the accuracy of gravity measurements by ~1 order of magnitude over 60-200 CPR (10-37 mHz) frequencies with the signal dominated by static gravity field of the Earth. The time-domain analysis shows that LRI is capable of detecting static gravity signals as small as a few 0.1 nm/s² in 100-200 CPR frequency band. We made use of such LRI data acquired in 2019 to validate the state-of-the-art gravity field models GGM05S, GGM05C, GOCE-TIM-R6e, EIGEN-6C4, ITSG-Grace2018s and GOCO06s. We found that LRI data can identify subtle un-/mis-modeled static gravity signals in these models in the spectral as well as spatial domains, and thus, suggest how the next generation of gravity field models could be improved. We also examined the high‐frequency (sub-monthly) variations of the Argentine Gyre using LRI measurements along with satellite altimetry data. Through comparison of measured gravity change by LRI with synthetic gravity change from altimetry sea surface data (evaluated at GRACE Follow-On altitude), we clearly demonstrate how the high-frequency Argentine Gyre signal is fully captured by instantaneous LRI measurements by individual data arcs, but not in the monthly mean Level-2 data. Such along-orbit analyses of LRI data could be employed for, among others, validation of high-frequency non-tidal ocean models used in GRACE and GRACE Follow-On de-aliasing products.