Quarterly

• All available sessions from 1984 until the present are used, and the solution is updated every three months.

• The combination is based on addition of normal equation matrices, and makes use of datum-free normal equation matrices of VLBI sessions, which are provided as SINEX files.

• The combined earth orientation parameter series will be updated every three months.

• Two combined series are available:

• Combined series (dX, dY):

  The nutation offsets are dX and dY, referring to the IAU2006 precession-nutation model, excluding free core nutation

• Combined series (dPsi, dEps):
  The nutation offsets are dPsi and dEps, referring to the IAU2000A precession-nutation model, excluding free core nutation

Combination Procedure

1. Matrices are transformed to the mean epoch of all contributions by using the respective earth orientation parameter rates, before accumulating the individual normal equation matrices.
2. The individual normal equation matrices are then transformed into a common set of apriori values for station coordinates and earth orientation parameters.
3. Weighting factors determined by variance component estimation are applied to the normal equation systems.
4. The datum-free normal equation matrices are then accumulated.
5. Earth orientation parameters are estimated by fixing all station components to their ITRF2008 values.

6. Finally, two types of comparisons are carried out for the purpose of quality control:
   - internal comparisons of the individual solutions with respect to the combined solution,
   - external comparisons with the earth orientation parameters of the IGS Final Combined Solution (igs00p03.erp), as well as with the IERS C04 (eopc04.62-now) earth orientation parameter series.

   Notice: To ensure comparable results, the calculations of bias, WRMS, and RMS were carried out only for those sessions that were analyzed by every Analysis Center.