As the geocenter is one of the focuses of the GNSS satellite orbits, it is also implicitly at the origin of the AC solutions. By “comparing” the weekly AC solutions to IGS08, it is therefore possible to estimate the geocenter motion in the IGS08 Reference Frame. The term “comparison” can cover different approaches discussed by e.g. Lavallée et al. (2006).
However, the geocenter determination by GNSS suffers from mismodeling issues and correlations with other parameters such as empirical solar radiation pressure parameters (Hugentobler et al., 2006). This results in discrepancies at the centimeter level between the individual AC geocenter estimates. During the weekly SINEX combinations, it is therefore necessary to “reconcile” the AC geocenters. This is concretely done by estimating translation parameters between the AC solutions, the origin of the combined solution being defined as the mean of the AC origins. This non-rigorous approach raises practical issues addressed by Rebischung and Garayt (2011).
Finally, seven Helmert parameters are estimated between the combined solution and IGS08. The translation parameters are the so-called combined coordinates of the apparent geocenter. Note that this approach based on a Helmert transformation (network shift approach) was “found to perform particularly poorly with GPS” by Lavallée et al. (2006).
For the reasons given above, we recommend particular caution using this product.