All available sessions from 1984 up to now have been used.

The solution is updated every three months.

The combination is based on the approach of 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 all three month.

Two combined series are available:

Combined series (dX, dY):
The nutation offsets are dX, dY referred to IAU2006 precession-nutation model excluding free core nutation

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

Combination Procedure

Before accumulating the individual normal equation matrices the matrices are transformed to the mean epoch of all contributions by using the respective earth orientation parameter rates.

Furthermore the individual normal equation matrices have to be transformed to a common set of apriori values for station coordinates and earth orientation parameters.

Weighting factors determined by variance component estimation are applied to the normal equation systems.

The datum-free normal equation matrices are accumulated.

Earth orientataion parameters are estimated by fixing all station components to their ITRF2008 values.

To perform a quality check comparisons are carried out

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 get comparable results, the calculations of bias, WRMS and RMS were carried out only for those sessions that were analysed by every analysis center.