The gravity field model ITSG-Grace2016
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T. Mayer-Guerr, B. Klinger, A. Kvas, N. Zehentner, M. Ellmer
http://ifg.tugraz.at/downloads/gravity-field-models/itsg-grace2016

Unconstrained monthly solutions
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For each month of the observation period sets of spherical harmonic coefficients for different
maximum degrees (60, 90, 120) were estimated without applying any regularization. Daily variations
are co-estimated and eliminated from the normal equations. For most applications a spectral
resolution of degree 60 is sufficent. In some cases a higher resolution is preferrable
but in some rare months the orbit configuration dont allow to solve for high degrees.

Processing Details
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The ITSG-Grace2016 gravity field solutions are computed using variational equations with an arc length
of 24 hours. In addition to satellite state, instrument calibration and static/long-term
spherical-harmonic coefficients, daily gravity field variations up to degree and order 40 were
modelled 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 2015)
were processed using the GPS orbits and clock solutions provided by IGS. An improved attitude product
derived from a combination of star camera data and angular accelerations (Klinger and Mayer-Gürr 2014)
was used. Additionally, accelerometer scale factor were estimated per axis and day. The accelerometer
bias were modelled as cubic splines of degree 3 per axis with nodal points every 6 hours.

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: EOT11a
    Pole tide: IERS 2010
    Ocean pole tide: Deasi 2003 (IERS 2010)
    Atmospheric tides (S1, S2): van Dam and Ray 2010
    Atmosphere and Ocean Dealiasing: AOD1B RL05
    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.