Since GRACE's launch 17 March 2002, the official GRACE Science Data System continuously releases monthly gravity solutions from three different processing centers:
- GFZ (GeoforschungsZentrum Potsdam)
- CSR (Center for Space Research at University of Texas, Austin)
- JPL (Jet Propulsion Laboratory)
There's just one Earth gravity field, and all centers start off with identical GRACE Level-1 observations - so why are there three different gravity solutions? And - perhaps of most interest to Level-3 users - which solution should be used for an analysis?
Why three different solutions?
Deriving month-to-month gravity field variations from GRACE observations requires a complex inversion of relative ranging observations between the two formation-flying GRACE spacecraft, in combination with precise orbit determination via GPS and various corrections for spacecraft accelerations not related to gravity changes. Many parameter choices and solution strategies are possible, and have been explored by GFZ, CSR, and JPL. The differences in the resulting Level-2 gravity fields have helped to better understand the characteristics of the various approaches, and differences between the centers have generally decreased over the Releases (currently, we're at RL05).
Which solution to use?
The differences between solutions from JPL, CSR, and GFZ can be used to infer the uncertainty in Level-2 and Level-3 GRACE fields that arises from the choice of solution strategy. Recent papers (e.g., Sakumura et al., 2014) found that the ensemble mean (simple arithmetic mean of JPL, CSR, GFZ fields) was most effective in reducing the noise in the gravity field solutions within the available scatter of the solutions.
What are the differences?
The following figures show differences between the three Release-05 gravity field solutions, mapped into typical units of water-equivalent-height. These maps should guide users when deciding if an analysis of GRCTellus Land or Ocean data may depend on a particular solution. The differences between JPL, GFZ, and CSR are mostly very small, and lie within the error bounds of the GRACE solution itself. For further details, please consult Sakumura et al. (2014).
We recommend that users obtain all three data center's solutions (JPL, CSR, GFZ) and simply average them. The JPL Tellus GRACE team is in the process of making this average available to users as a stand-alone Level-3 product by 09/2015.
Sakumura, C., S. Bettadpur, and S. Bruinsma (2014), Ensemble prediction and intercomparison analysis of GRACE time-variable gravity field models, Geophys. Res. Lett., 41, 1389–1397, doi:10.1002/2013GL058632.
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