# Rapid

## Introduction

- Rapid solutions use
**all available 24h sessions**from 1984 to present, and the solution is updated twice per week. - The combination is based on the addition of normal equation matrices, and makes use of datum-free normal equation matrices of VLBI sessions which are provided as SINEX files.
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

- Combined series (dX, dY):

## Combination Procedure

- Matrices are transformed into the
**mean epoch**of all contributions by using the respective earth orientation parameter rates, before accumulating the individual normal equation matrices. - Individual normal equation matrices are then 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 orientation parameters are estimated by fixing all station components to their
**ITRF2008**values. **Comparisons**for the purpose of quality control are carried out:

- internal comparisons of the individual solutions are made with respect to the combined solution,

- external comparisons are carried out 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.