The successor to the GRACE mission, GRACE Follow-On (GRACE-FO), was launched on May 22, 2018 and has provided science data since June of that year. To continue the ITSG-Grace2018 gravity field time series, we will extend the GRACE data record with GRACE-FO solutions in an operational manner. As with GRACE, we provide unconstrained monthly solutions up to degree and order 60, 96, and 120 and Kalman smoothed daily solutions up to degree 40. The processing standards are consistent with ITSG-Grace2018 - the GRACE-FO solutions can therefore be treated in the same fashion.  

The computed potential coefficients and normal equation matrices can be found on our FTP server and on ICGEM (solutions only). This time series will be continually updated on a best effort basis.

In order to alleviate post-processing of the spherical harmonic solutions, we provide a Python package which allows detrending, filtering and signal separation of the solutions.

The source code of the package can be found on GitHub.

The ITSG-Grace2018 gravity field solutions are computed using variational equations with an arc length of 24 hours. In addition to satellite state and instrument calibration parameters, daily gravity field variations up to degree and order 40 were modeled in the adjustment process.

K-band range rates with a sampling of 5 seconds and kinematic orbits with a sampling of 5 minutes were used as observations. The kinematic orbits of the GRACE satellites (Zehentner and Mayer-Gürr 2013, 2014) were processed using the GPS orbits and clock solutions provided by CODE. Additionally, a full accelerometer scale factor matrix was estimated per day (Klinger and Mayer-Gürr, 2016). The accelerometer bias was modelled through cubic splines with a node interval of six hours and estimated for each axis and day. 

The following background models were used during the data processing:

• Earth rotation: IERS 2010
• Moon, sun and planets ephem.: JPL DE421
• Earth tide: IERS 2010
• Ocean tide: FES2014b, co-estimated
• Pole tide: IERS 2010 (linear mean pole)
• Ocean pole tide: Desai 2004 (IERS 2010, linear mean pole)
• Atmospheric tides: AOD1B RL06
• Atmosphere and Ocean Dealiasing: AOD1B RL06
• Sub-monthly continental hydrology: LSDM (only for monthly solutions and static field)
• Relativistic corrections: IERS 2010
• Permanent tidal deformation: included (zero tide)

The above models were reduced during the analysis process. They are not present in the solutions (sub-monthly LSDM variations were not reduced in the computation of the daily solutions).  The background models are provided as mean value over the specific time spans (daily, monthly) on the FTP server.

All daily and monthly solutions have the same signal definition as the official GRACE products. They contain the full hydrological, cryospheric and GIA signal. Atmosphere and ocean signals are relative to AOD1B RL06.