The gravity field model ITSG-Grace2018 (operational)
====================================================
This gravity field model is computed with the software GROOPS (https://github.com/groops-devs/groops) 
by the GRACE/GRACE-FO processing team at Graz University of Technology:

Felix Öhlinger, Andreas Kvas and Torsten Mayer-Gürr 

When using the data please cite:

Mayer-Gürr, Torsten; Behzadpur, Saniya; Ellmer, Matthias; Kvas, Andreas;
Klinger, Beate; Strasser, Sebastian; Zehentner, Norbert (2018):
ITSG-Grace2018 - Monthly, Daily and Static Gravity Field Solutions from
GRACE. GFZ Data Services. http://doi.org/10.5880/ICGEM.2018.003

Kvas, Andreas, Behzadpour, Saniya, Ellmer, Matthias, Klinger, Beate, Strasser, Sebastian, 
Zehentner, Norbert, & Mayer‐Gürr, Torsten (2019).
ITSG‐Grace2018: Overview and evaluation of a new GRACE‐only gravity field time series. 
Journal of Geophysical Research: Solid Earth, 124. https://doi.org/10.1029/2019JB017415

Unconstrained monthly solutions
-------------------------------
For each month of the observation period sets of spherical harmonic coefficients for different
maximum degrees (60, 96, 120) were estimated without applying any regularization. Daily variations
are co-estimated and eliminated from the normal equations. 

Kalman smoothed daily solutions
-------------------------------
In addition to the standard monthly products, a set of daily gravity solutions is computed in order to recover
sub-monthly gravity variations. As the GRACE data coverage within one day is not sufficient to resolve the
global gravity field, the solutions are stabilized using an autoregressive (AR) model of order three. This AR model
is derived by fitting coefficients to the dealiasing  error estimates contained in the ESA ESM and detrended LSDM residuals.

For each day of the observation period a set of spherical harmonic coefficients for degrees n=2...40 was estimated.
The adjustment delivers daily solutions, even if there are no GRACE data available for a specific day. These days should be
handled with care, as they tend towards the trend and annual signal of the static background field.

Processing Details
------------------
The ITSG-Grace_operational 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 (Kvas and Mayer-Gürr, 2019).

K-band range rates with a sampling of 5 seconds and kinematic orbits with a sampling of 60 seconds were used as observations.
The kinematic orbits of the GRACE-FO satellites (Zehentner and Mayer-Gürr, 2016) were processed using 
in-house GPS orbits and clock solutions (Strasser et al., 2018). 
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
    Ocean pole tide: Desai 2004 (IERS 2010)
    Atmospheric tides: AOD1B RL06
    Atmosphere and Ocean Dealiasing: AOD1B RL06
    Sub-monthly continental hydrology: LSDM (not for daily solutions)
    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.
The models are provided as mean value over the specific time spans (daily, monthly)
in the background directory.

References
----------
Kvas, A. & Mayer-Gürr, T. J Geod (2019). https://doi.org/10.1007/s00190-019-01314-1

Zehentner, N. & Mayer-Gürr, T. J Geod (2016) 90: 275. https://doi.org/10.1007/s00190-015-0872-7

Strasser, S., Mayer-Gürr, T. & Zehentner, N. J Geod (2019) 93: 1045. https://doi.org/10.1007/s00190-018-1223-2

Beate Klinger, Torsten Mayer-Gürr, The role of accelerometer data calibration within GRACE gravity field recovery: Results from ITSG-Grace2016,
Advances in Space Research, Volume 58, Issue 9, 2016, Pages 1597-1609, ISSN 0273-1177, https://doi.org/10.1016/j.asr.2016.08.007.