C ₂₀ and C ₃₀ Variations From SLR for GRACE/GRACE‐FO Science Applications

It is essential that the Satellite Laser Ranging (SLR) estimates of C20 and C30 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 C20 and slightly overestimates the annual variation. The data are insufficient for separating C30 and C50 within a month from the 5-satellites used in GRACE Technical Note 11 (TN11) (Cheng and Ries, 2016). LARES’s eccentricity is extremely sensitive to the gravitational perturbations of C30 and plays an important role in the reliable determination of C30. 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 C30, C50 and C60 when a GRACE-based empirical orthogonal function (EOF) analysis is also applied. A better time series of C20 and C30 (represented by TN11E as the updated TN11) was determined from 7 satellites (including -1 and 2, Starlette, Ajisai, Stella, BEC and LARES) and adjusting a degree and order 5 (5×5) field + C60, C61 and S61 based on the TVG model and an EOF analysis of GRACE. The estimated rate of the C20 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 C30 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.