HydroGeodesy & Gravity Field @ITU
Earth Observation and Modeling
 
I T U _ G R A C E 1 6
 
ITU_GRACE16 is a GRACE-only global gravity model computed up to degree and order (d/o) 180 from GRACE K-band intersatellite range-rate data, GPS tracking and GRACE accelerometer data using improved energy integral method (see Guo et al., 2015, Shang et al., 2015).
 
The purely dynamic orbits of the twin satellites were reconstructed from the reduced dynamic GNV1B orbits along with the acceloremeter calibration parameters by fitting the orbit coordinates w.r.t. a complete reference model in a least-squares sense. The reference models used for this purpose are the same used for the solution of official GFZ RL05 L2 data products. Furthermore, range-rate measurements are incorporated in order to improve the accuracy of the intersatellite relative velocity pitch which is the most important orbital component for gravity recovery.
 
Unlike dynamic approach, ITU_GRACE16 has been computed using a two step approach: First the in situ geopotential difference between the twin satellites are estimated using the improved energy integral method with the reconstructed orbit parameters. Then the geopotential coefficients are estimated by the rigourous inversion of the geopotential difference observations. 50 months (April 2009 to October 2013)of GRACE data were used in computation.
 
GIF48 up to d/o 180 served as a priori gravity field model.
 
No rate terms were modeled, and no corrections for earthquakes have been applied. For additional details on the background modeling, see the GFZ RL05 processing standards document available at:
ftp://podaac.jpl.nasa.gov/allData/grace/docs/L2-GFZ_ProcStds_0005_v1.1.pdf
 
Because ITU_GRACE16 is not regularized or constrained in any way, the errors increase with degree. We do not recommend to use ITU_GRACE16 beyond the degree 130 without smoothing.
 
Solution can be used for independent comparison and combination on normal equation level with other satellite-only models (e.g. GOCE), terrestrial gravity data, and altimetry.
 
For further info, please contact:
Dr. Orhan Akyilmaz (akyilma2@itu.edu.tr)
 
Acknowledgement:
This work is supported by TUBITAK under grant no 113Y155.

[Researchers: O. Akyilmaz, A. Ustun, C. Aydin, N. Arslan, S. Doganalp, C. Guney, H. Mercan, S.O. Uygur, M. Uz, O. Yagci]
[External Collaborators: C.K. Shum, K. Shang (OSU) ]
 
References:
Guo, J. Y., Shang, K., Jekeli, C., Shum, C. K., (2015), On the energy integral formulation of gravitational potential differences from satellite-to-satellite tracking, Celest. Mech. & Dyn. Astr., 121 (4): 415-429

Shang, K., Guo, J., Shum, C.K., Dai, C., Luo, J., (2015), GRACE time-variable gravity field recovery using an improved energy balance approach, Geophys. J. Int. 203, 1773-1786, doi: 10.1093/gji/ggv392.

Akyilmaz, O., Ustun, A., Aydın, C., Arslan, N.,Doganalp, S., Guney, C., Mercan, H., Uygur, S.O., Uz, M., Yağcı, O (2016), High Resolution Gravity Field Determination and Monitoring of Regional Mass Variations using Low-Earth Orbit Satellites, Final Report of Project # 113Y155, pp.86 (in Turkish)