C₂₀ and C₃₀ Variations From SLR for GRACE/GRACE-FO Science Applications

It is essential that the Satellite Laser Ranging (SLR) estimates of C-20 and C-30 are as reliable as possible for GRACE and GRACE Follow-on science applications. Experiments in combining SLR and GRACE data indicated that not accounting for the influence of the higher-degree even zonal terms results in a lower trend for C-20 and slightly overestimates the annual variation. The data are insufficient for separating C-30 and C-50 within a month from the five satellites used in GRACE Technical Note 11 (TN11) (Cheng & Ries, 2016, https://podaac-tools.jpl.nasa.gov/drive/files/allData/grace/docs/TN-11_C20_SLR.txt). LARES's eccentricity is extremely sensitive to the gravitational perturbations of C-30 and plays an important role in the reliable determination of C-30. Applying a time-variable gravity (TVG) forward model (based on GRACE) for the higher-degree coefficients improves the recovery of the lower-degree coefficients, such as C-30, C-50, and C-60 when a GRACE-based empirical orthogonal function (EOF) analysis is also applied. A better time series of C-20 and C-30 (represented by TN11E as the updated TN11) was determined from seven satellites (including -1 and 2, Starlette, Ajisai, Stella, BEC, and LARES) and adjusting a degree and order 5 (5 x 5) field + C-60, C-61, and S-61 based on the TVG model and an EOF analysis of GRACE. The estimated rate of the C-20 is comparable with that from TN14 (Loomis et al., 2019, ), but the seasonal variations appear to be better recovered from this study. The time series of C-30 from this study is comparable with the GRACE RL06 solution for the period prior to March 2012 and appropriate for application to the GRACE-FO mission.