As replacement time series, we use here a combination of SLR and GRACE/GRACE-FO on the level of normal equations
generated at GFZ. The SLR part includes the six geodetic satellites LAGEOS-1 and -2, AJISAI, Stella, Starlette,
and LARES (starting from March 2012) and uses the same background models and standards as applied during GFZ's
GRACE/GRACE-FO processing. Relative weighting of the individual SLR normals is done by means of variance component
estimation whereas relative weighting of the SLR-combined and GRACE/GRACE-FO normals is based on empirical weights.
Gravity field coefficients up to degree/order 6x6 are estimated independently for each month. From these estimates,
the following coefficients as well as their formal standard deviations are used to replace the corresponding values
of the GFZ and COST-G solutions: C20 for the complete time series of GFZ and COST-G, C30 for
all GFZ and COST-G solutions starting from 2016/11 and later, and C21/S21 for the complete
time series of GFZ only.
It has to be mentioned that other replacement time series are available and that their choice can significantly
impact mass change results. Based on findings by Loomis et al. (2019), the operational C20 product by
the GRACE/GRACE-FO Science Data System was recently changed and is now provided as Technical Note 14 (TN-14), which
also includes a C30 time series. Another operational SLR-only product for C20, based on the
same normals as used for the above-mentioned combination with GRACE/GRACE-FO, is provided by GFZ (König et al., 2019).
Generally, the estimation and validation of low degree harmonics and their uncertainties from space-geodetic techniques
as well as their impact on mass balance studies is an ongoing research topic.
Cheng, M., Ries, J., 2017:
The unexpected signal in GRACE estimates of C20
Journal of Geodesy, 91, 8, p. 897—914,
Loomis, B. D., Rachlin, K. E., Wiese, D. N., Landerer, F. W., Luthcke, S. B., 2020:
Replacing GRACE/GRACE-FO C30 with satellite laser ranging: Impacts on Antarctic Ice Sheet mass change
Geophysical Research Letters, 47, e2019GL085488,
Dahle, C., Murböck, M., Flechtner, F., Dobslaw, H., Michalak, G., Neumayer, K.H., Abrykosov, O., Reinhold, A., König, R., Sulzbach, R., Förste, C., 2019:
The GFZ GRACE RL06 Monthly Gravity Field Time Series: Processing Details and Quality Assessment
Remote Sensing, 11, 2116,
Loomis, B. D., Rachlin, K. E., Luthcke, S. B., 2019:
Improved Earth oblateness rate reveals increased ice sheet losses and mass-driven sea level rise
Geophysical Research Letters, 46, 6910—6917,
Loomis, B. D., Rachlin, K. E., Technical Note 14:
NASA GSFC SLR C20 and C30 solutions
available at the GRACE/GRACE-FO archives ISDC and PODAAC,
König, R., Schreiner, P., Dahle, C., 2019:
Monthly estimates of C(2,0) generated by GFZ from SLR satellites based on GFZ GRACE/GRACE-FO RL06 background models. V. 1.0.
GFZ Data Services,