Application of local ties in the combination on the observation level. A comparison of the established to a novel method using the CONT17 data.

The combination of multiple space-geodetic techniques involves the inclusion of local geometric ties, which are the only direct information that link the reference points of the different space-geodetic instruments. This makes their role important in merging networks of stations of different space-geodetic techniques in one homogenized network with a stable datum definition. The establishment of such a global network is then utilized to derive parameters of interest like station position coordinates or earth orientation parameters (EOP) with enhanced precision. The application of local geometric ties has long been pursued in the form of pseudo-observations that augment the combined multi-technique system of normal equations and weighting has to be applied to them on the basis of some post-fit residual adjustment in order for the combination to be meaningful. We examine this established method on the CONT17 data combining the VLBI with the co-located GPS stations using the combination on the observation level. The c5++ multi-technique space-geodetic software is employed for the data analysis and the local tie discrepancies that occur are presented. We also propose a new method where the local ties are used so that all measurements are projected on the phase center of one of the participating techniques and examine the differences between the two methods regarding EOP and station position repeatabilities.