LDCmgm90: an improved multiple-data based monthly geopotential model set

This is an updated version of LDCmgm90, which has assimilated CSR RL06 mascon and GSFC SLR degree-2 Stokes coefficients besides all the data sets used in the previous version (see https://www.nature.com/articles/s41597-019-0239-7 for more details).In addition, The ICE-6G_C used for RL05 mascons, and ICE-6G_D used for RL06 mascons are added back because there are still somewhat large uncertainties in GIA modeling. Instead, we provide a corrected GIA model ICE6Gavg(GPS) as a separate data set (see the attached pdf for more details about this GIA model).