All available 24h IVS sessions from 1984 up to now have been used.
The solution is updated half-yearly.
The combination is based on stacking datum-free normal equations from VLBI sessions which are provided by the Analysis Centres in form of SINEX files.
The combined Earth Orientation Parameter series is updated half-yearly.
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 parameter epochs are transformed to the mean epoch of all contributions by using the respective Earth Orientation Parameter rates and regularisation according to the IERS Conventions 2010.
Furthermore, the individual normal equations are be transformed to a common set of a priori 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 systems are accumulated.
Earth Orientation Parameters are estimated, whereby the geodetic datum is realised by fixing all station coordinates to their a priori values.
To perform a quality check, comparisons are carried out, namely
internal comparisons between the individual solutions and the combined solution,
Notice: To get comparable results, the calculations of bias, WRMS and RMS are carried out only for those sessions that were analysed by all Analysis Centres.