C<sub>20</sub> and C<sub>30</sub> Variations From SLR for GRACE/GRACE‐FO Science Applications
Minkang Cheng, John Ries
Abstract
Abstract 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 × 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, https://doi.org/10.1029/2019GL082929 ), 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.