SLR-determined Low-Degree Geopotential Harmonics and their use With GRACE Data Products

GRACE provides estimates of the temporal variations in the Earth's gravity field with extraordinary precision, but as with any non-synoptic measurement system, the problem of aliasing of high- frequency signals errors is an important concern. In the case of GRACE, errors in the models for the diurnal and semi-diurnal (solid earth and ocean) tides will alias into long-period variations in the low-degree geopotential harmonics. This issue is particularly apparent for the degree-2 zonal harmonic, C20. In addition, after removing the tidal aliases, the GRACE C20 series is still noisier than SLR by about a factor of 3. As a consequence, GRACE data product users typically replace the C20 estimates from GRACE with estimates derived independently (but using consistent modeling) from SLR tracking of several geodetic satellites. For the other low degree harmonics, the GRACE estimates do not appear to suffer from significant tidal aliasing or excessive noise. In this paper, we will discuss the characteristics of the GRACE and SLR time series for the low-degree harmonics. In particular, we discuss the SLR and GRACE estimates for the harmonic terms describing the orientation of the Earth's principal figure axis, C21 and S21, and compare them to the conventional model for the long-term evolution of these terms.