Strategy of combination of GPS/VLBI-derived nutation parameters at the normal equation level

Current nutation model adopted by IERS is based on the analytical formulation given by Mathews et al. (1991, 2002) with numerical values of parameters estimated from Very Long Baseline Interferometry (VLBI) observational nutation series. Nevertheless, VLBI observations show discrepancies with the theory of the order 0.2 mas (milliarcsecond) in the time domain (Mathews et al., 2002). In order to improve the model it is thus necessary to improve the parameter determination from observations and as a consequence to refine the observational series themselves. This can be done by mixing different techniques, in particular mixing GPS and VLBI data. In addition, combination at the time series level can suffer from inhomogeneities of the series to be combined, w.r.t. a priori models, reference systems, and parameterizations. The objective of this presentation is to describe the developed strategy of combination of GPS/VLBI-derived nutation parameters and to present the current status of the implementation of this strategy. Our strategy is to work at the level of normal equations. For the purpose of step-by-step validation of our procedure we use one-month-long time series of normal equations produced from VLBI and GPS observations by means of CALC/SOLVE and BERNESE v.5.0 software, respectively. Models and a priori data used to derive the normal equations from both techniques are chosen as close to each other as possible. Earth orientation parameter determination will, in our procedure, benefit from angle and rate observation for a unique estimation.