GOCE Gravity Field Model

Time-Wise Solution - Third generation

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GOCE Input Data:
- Gradients: EGG_NOM_2
- Orbits: SST_PKI (kinematic orbits); SST_PCV (variance information of kinematic orbit 
  positions)
- Attitude: EGG_IAQ_2C
- Data period: 01/11/2009 - 17/04/2011
No a-priori gravity field information applied (neither as reference model, nor for 
constraining the solution)

Processing procedures:
- Gravity from orbits (SST):
- energy integral applied to kinematic orbits, degree/order 100
- orbit variance information included as stochastic model
- Gravity from gradients (SGG):
- parameterization up to degree/order 250
- observations used: Vxx, Vyy, Vzz and Vxz in the Gradiometer Reference Frame (GRF)
- realistic stochastic modelling by applying cascaded digital filters to the observation 
  equations; tuned filters for individual data segments

 Combined solution:
- addition of normal equations (SST D/O 100, SGG D/O 250)
- Constraints: Kaula-regularization applied to (near-)zonal coefficients, and to 
  coefficients of degrees/orders 181 - 250 (contraints towards zero)
- Optimum weighting (SST, SGG, contstraints) based on variance component estimation

Specific features of resulting gravity field:
- Gravity field solution is independent of any other gravity field information
- Constraint towards zero starting from degree/order 181 to improve signal-to-noise ratio
- Related variance-covariance information represents very well the true errors of the 
  coefficients
- Solution can be used for independent comparison and combination on normal equation level 
  with other satellite-only models (e.g. GRACE), terrestrial gravity data, and altimetry
- Since in the low degrees the solution is based solely on kinematic GOCE orbits, it is 
  not competitive with a GRACE model

The processing details are presented in:
Pail R., Goiginger H., Mayrhofer R., Schuh W.-D., Brockmann J.M., Krasbutter I., Hoeck E., 
Fecher T. (2010) Global gravity field model derived from orbit and gradiometry data 
applying the time-wise method. Proceedings of ESA Living Planet Symposium,
28 June 2 - July 2010, Bergen, Norway, ESA SP-686.
Pail R., Bruinsma S., Migliaccio F., Foerste C., Goiginger H., Schuh W.-D, Hoeck E, 
Reguzzoni M., Brockmann J.M, Abrikosov O., Veicherts M., Fecher T., Mayrhofer R., 
Krasbutter I., Sanso F. & Tscherning C.C. (2011): First GOCE gravity field models derived
by three different approaches. Jounal of Geodesy, 81:11, doi: 10.1007/s00190-011-0467-x.