Improving the accuracy of local geodetic ties at colocated sites
The development of the next-generation of VLBI systems, known as VLBI Geodetic Observing System (VGOS), is well underway. Colocation of VGOS stations with instruments from the other space geodetic techniques (i.e., GPS, SLR, and DORIS) is essential for synergistic, robust global reference frame realization. The local ties between reference points of geodetic instruments at colocation sites effectively connect the various techniques together. Precise ties are required for a multi-technique reference frame that is suitable for high-accuracy geophysical applications such as global sea-level change. Unfortunately, local ties remain a significant error towards the realization of global reference frames such as the ITRF. We are investigating approaches that could improve the accuracy of relative positions estimates and ITRF combinations. These approaches use external constraints based on local atmospheric structure at core geodetic sites where multi-techniques are co-located. The challenge is to add information to the geodetic solution based on our knowledge of atmospheric structure without biasing the estimates of the intersite vectors. We will present preliminary results from tests wherein atmospheric structure is used to enhance the strength of geodetic solutions and combinations. In this study, we use data from existing small-scale GPS networks as a proxy for collocation sites instrumented with next-generation geodetic systems.