In this study, however, the monthly solutions generated by the SDS processing centers (CSR Release 05,13 GFZ Release 05,15 and JPL Release 05.114) and GRGS Release 03-v112 were used. The processing center solutions of CSR, GFZ, GRGS, and JPL are computed based on the integration of variational equations (dynamic method), where monthly solutions (GFZ also provides weekly solutions) have been estimated throughout the whole GRACE mission period. Currently, an alternative approach using radial basis functions is under investigation at GFZ with promising results.50 Despite the approach being the same, possible differences can be related to, for example, the background models. As such, it is important to determine the noise deviations of each solution relative to an unknown true value (see Sec. 2.1) to assess their strengths and weaknesses. The Stokes coefficients from the four processing centers were considered up to d/o 60 in Eq. (11) (i.e., , which can be associated with the shortest resolution as at the equator). The degree 1 coefficients (, , and ), which represent the changes in the geocenter, and the degree 2 coefficient (), which is associated with the oblate shape of the Earth, were not considered here. Since they would be a common-mode signal for the four centers, through Eq. (2), the contribution of these low-degree coefficients is canceled out.