Intra-technique VLBI combination by handling software-specific dependencies

The IVS Combination Centre, operated by the Federal Agency for Cartography and Geodesy (BKG, Germany) in close cooperation with the Deutsches Geodätisches Forschungsinstitut (DGFI-TUM, Germany), generates and releases the combined products of the International VLBI Service for Geodesy and Astrometry (IVS). These session-wise solutions are achieved by an intra-technique combination utilizing the individual contributions delivered by multiple IVS Analysis Centres (AC). For the IVS contribution to the ITRF2020, the re-processed sessions containing 24 h VLBI observations have been submitted by eleven different ACs, where a total number of seven different software packages are utilized. As a result, the session-wise SINEX files are delivered, including datum-free normal equations containing the station coordinates, the source positions and full sets of the Earth Orientation Parameters (EOP). As the same software packages are used by various ACs (e.g. CalcSolve by five ACs), the combined solution is in danger of being dominated by these contributions. The estimates of EOP and station coordinates are affected then mainly by the software specific modelling.This work focuses on an optimal weighting strategy to handle dependencies due to identical software packages in the ACs’ contributions. Thereby, software specific sub-combinations are carried out in the first step to employ the strategy. Finally, the ultimate combination consists of the weighted individual contributions of each applied software. In order to assess the quality of the individual components of the combination - especially in comparison to the contributions of the individual ACs – the internal as well as the external comparisons of the estimated EOP are carried out, where the combined solution together with the external time series (e.g., IERS Bulletin A) serve as a reference.