The intra-technique combination of normal equations or parameters is a valuable method for quality control and provides the input required for the inter-technique combination, which is the state-of-the-art approach for the determination of terrestrial reference frames, such as the ITRF.

Besides the Korea Astronomy and Space Science Institute (KASI), DGFI, together with the Bundesamt für Kartografie und Geodäsie (BKG), operates the Combination Center of the International VLBI Service for Geodesy and Astrometry (IVS). DGFI provides the software for the combination (DOGS-CS) to BKG and combines and presents the troposphere parameters of IVS:

• the rapid troposphere combination of IVS-R1 and –R4 networks serves as a quality indicator for troposphere and other parameters andt

• the long-term troposphere combination aims at very consistent and homogeneously reprocessed long time-series of precipitable water at core-stations for climatological interpretation.

Research oriented activities in this context are the implementation of advanced intra-technique combination models. For intra-technique combination the same observations are analyzed and provided as results by various Analysis Centers, what is, however, neglected in the case of standard combination methods and thus leading to unrealistic small formal errors of the combined product. For the formal errors the stochastic model of the combination plays the significant role. For the enhancement of the stochastic model, several approaches are under investigation:  variance-covariance-component models and the Operator-Software-Impact (OSI) – model.