The ITSG-Grace2014 gravity field model consists of three parts:
(Reference: Mayer-Gürr et. al. 2014)
The gravity field solution ITSG-Grace2014s is a satellite-only gravity field model derived from GRACE data in the time span of 2003-02 to 2013-12. In addition to the static gravity field, daily, secular and annual variations have been modelled in the estimation process, taking into account the correlations between each component. The result is a high-resolution static solution up to degree and order 200 and complementary secular and annual variations up to degree and order 100. These temporal variations were regularized using a Kaula-like function starting from degree 21.
Each solution component contains the complete gravity field signal including atmosphere and ocean masses (i.e. the mean, secular and annual parts of AOD1B have been restored to the model).
The reference epoch of this model is 1 January 2008.
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 elimniated 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 (e.g. 2004-09).
For each monthly solution and for each resolution the corresponding full variance-covariance matrix is provided. Further information can be found in READE_monthly_covariance.txt.
In order to recover fast gravity field variations as detailed as possible, it is reasonable to increase the temporal resolution. The goal is the calculation of daily GRACE solutions. This increase in temporal resolution results in less observations per time span and therefore a reduced redundancy in the parameter estimation process. This leads to a decreasing accuracy of the estimated parameters with decreasing time span. It can be assumed, however, that the gravity field does not change arbitrarily from one time step to the next. The information about the temporal correlation patterns can be derived from geophysical models. Utilizing this knowledge, the temporal resolution can be enhanced without losing spatial information within the framework of a Kalman smoother estimation procedure (Kurtenbach et al. 2012). The following geophysical models were used to derive the temporal correlations: the WaterGAP global hydrology model (WGHM), the atmospheric model ECMWF, and the ocean circulation model OMCT. In order to guarantee that the GRACE solutions are not biased towards the model values themselves but that only the stochastic behavior is exploited, the model output of the years 1976 - 2000 (i.e. outside the GRACE time span) was applied.
For each day of the observation period a set of spherical harmonic coefficients for degrees n=2...40 was estimated. Of course, these sets are not independently estimated, but the gravity model is updated daily by the GRACE observations. The Kalman smoother delivers daily solutions, even if there are no GRACE data available for a specific day. These days should be handled with care, as they are predictions only and tends towards the mean trend, and annual signal of ITSG-Grace2014s.
The ITSG-Grace2014 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 2013, 2014) 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 to estimate K-band antenna center variations (One set per month). Additionally, accelerometer scale factor were estimated per axis and day. The accelerometer bias were modelled as polynomial of degree 3 per axis and day.
The following background models were used during the data processing:
The above models were reduced during the analysis process. They are not present in the solutions. The AOD1B product is provided (separately for ocean and atmosphere) as mean value over the specific time spans (daily, monthly) in the download section below.
Kurtenbach, E.; Eicker, A.; Mayer-Gürr, T.; Hohlschneider, M.; Hayn, M.; Fuhrmann, M.; Kusche, J.:
Improved daily GRACE gravity field solutions using a Kalman smoother. - in: Journal of geodynamics 59-60 (2012) , S. 39 - 48
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