ITSG-Grace2018

The ITSG-Grace2018 gravity field model is the latest GRACE only gravity field model computed in Graz, providing unconstrained monthly and Kalman smoothed daily solutions. It is a reprocessing of the whole GRACE time series starting from 2002-04.

When using ITSG-Grace2018 data , please cite:

Kvas, A., Behzadpour, S., Ellmer, M., Klinger, B., Strasser, S., Zehentner, N., & Mayer‐Gürr, T. ( 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

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

Operational Processing of GRACE Follow-On Data

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.

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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. For most applications a spectral resolution of degree 60 is sufficient. In some cases a higher resolution is preferable but in some rare months the orbit configuration doesn't allow to solve for high degrees (e.g. 2004-09).

The spherical harmonic coefficients of the solutions and background models are provided in ICGEM format and can be accessed through our FTP server.

The 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.

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Daily solutions using Kalman smoothing

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.

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

Total water storage anomaly in the Danube basin from daily GRACE solutions during the 2006 flood event.
Total water storage anomaly in the Danube basin from daily GRACE solutions during the 2006 flood event.

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High-resolution Static Field

The high-resolution static field ITSG-Grace2018s is computed from GRACE L1B data in the time span from 2002-04 to 2016-08. Next to the unconstrained mean gravity field, which is modelled up to degree and order 200, secular and annual gravity field variations up to degree and order 120 are estimated. To account for the regionally varying signal characteristics of these temporal variations, an adaptive regularization strategy was developed. The basis of this novel regularization strategy are isotropic noise models for individual regions with different temporal behavior  (e.g. Greenland, Caspian Sea, ...). The optimal regularization factor for each region was determined via variance component estimation. Large instantaneous mass redistributions are taken into account by estimating corrections for the three major earthquakes, Sumatra 2004, Chile 2010 and Tohoku 2011, which occurred during the GRACE period. This prevents the coseismic gravity change beeing mapped into the estimated linear trend.

ITSG-Grace2018s is the GRACE contribution to the combined satellite-only gravity field model GOCO06s.

The spherical harmonic coefficients of static field, temporal variations, and coseismic gravity changes are provided in ICGEM format and can be accessed through our FTP server.

Gridded Mass Variation Products

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.

Processing Details

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.

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References

Mayer-Gürr, T., Behzadpour, S., Ellmer, M., Klinger, B., Kvas, A., Strasser, S., & Zehentner, N. (2018). ITSG-Grace2018: The new GRACE time series from TU Graz. Abstract from GRACE / GRACE-FO Science Team Meeting 2018, Potsdam, Germany.

Ellmer, M., & Mayer-Gürr, T. (2017). High precision dynamic orbit integration for spaceborne gravimetry in view of GRACE Follow-on. Advances in space research. DOI: 10.1016/j.asr.2017.04.015

Klinger, B., & Mayer-Gürr, T. (2016). The role of accelerometer data calibration within GRACE gravity field recovery: Results from ITSG-Grace2016. Advances in space research, 58, 1597. DOI: 10.1016/j.asr.2016.08.007

Behzadpour, S., Mayer-Gürr, T., Kvas, A., Ellmer, M., Klinger, B., Strasser, S., & Zehentner, N. (2018). Reducing systematic errors in the GRACE observations using parametric models. Abstract from GRACE / GRACE-FO Science Team Meeting 2018, Potsdam, Germany.

Kvas, A., Behzadpour, S., Ellmer, M., Klinger, B., Strasser, S., Zehentner, N., & Mayer-Gürr, T. (2018). Incorporation of background model uncertainties into the gravity field recovery process. Abstract from GRACE / GRACE-FO Science Team Meeting 2018, Potsdam, Germany.

Mayer-Gürr, T.:
Gravitationsfeldbestimmung aus der Analyse kurzer Bahnbögen am Beispiel der Satellitenmissionen CHAMP und GRACE, Dissertation, University of Bonn. URN: urn:nbn:de:hbz:5N-09047

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. DOI: doi.org/10.1016/j.jog.2012.02.006

Klinger, B.; Mayer-Gürr, T.:
Combination of GRACE star camera and angular acceleration data. EGU General Assembly 2014 (EGU2014-5340), Vienna, 28.04.2014. 

Zehentner, N.; Mayer-Gürr, T.
Kinematic orbits for GRACE and GOCE based on raw GPS observations. - in: IAG Scientific Assembly 2013. Potsdam, Germany am: 01.09.2013

Zehentner, N.; Mayer-Gürr, T.:
Gravity variations from precise LEO orbits of GRACE and GOCE. - in: EGU General Assembly 2014 (EGU2014-6018). Wien am: 01.05.2014

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Contact
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Torsten Mayer-Gürr
Steyrergasse 30/III
8010 Graz
Austria
Tel: +43 316 873-6359
Fax: +43 316 873-6845
mayer-guerrnoSpam@tugraz.at

Andreas Kvas
Steyrergasse 30/III
8010 Graz
Austria
Tel: +43/316/873-6347
Fax: +43/316/873-6845
kvasnoSpam@tugraz.at

Felix Öhlinger
Steyrergasse 30/III
8010 Graz
Austria
Tel: +43/316/873- 6333
Fax: +43/316/873-6845
felix.oehlingernoSpam@tugraz.at

Related Projects
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Various research projects have contributed to the GRACE/GRACE-FO processing chain used to procuce solutions presented here:

  • G3P: Global Gravity-based Groundwater Product
    This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No  870353. 

  • MAGIC
    This project is funded within the Austrian Space Applications Program (ASAP) Phase XIII by the Austrian Research Promotion Agency (FFG). 

  • EGSIEM: European Gravity Service for Improved Emergency Management
    This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No  637010.

  • GeoQ
    This Project is funded by the German Research Foundation. 

  • SPICE
    This project is funded by the Austrian Research Promotion Agency (FFG) within the Austrian Space Applications Programme (ASAP, Phase 11).