Combining VGOS, legacy S/X and GNSS for the determination of UT1

We perform a combination on the observation level (COL) between VLBI and co-located GNSS in the context of VLBI intensive sessions. Our approach revolves around an estimation procedure which uses 3 h of GNSS data that uniformly encapsulate the 1-h VLBI data, in order to provide consistent troposphere information. We test this approach on both VGOS and Legacy S/X using the VGOS-B and concurrently observed INT1 sessions. The COL strategy is found to increase the precision by 15 % over both session types and leads to an increase of 65 % in the agreement between the sessions when estimating tropospheric gradients every 3 h. A more frequent estimation of the gradients every 1 h, which can be rigorously pursued with the utilization of multi-GNSS, results in a further convergence of the two session types by 30 %. The COL-aided length-of-day (LOD) products also show a 55 % better agreement to external GNSS-derived LOD. In the light of the increasing precision of broadcast GNSS orbits and clocks, this COL strategy can be used to derive rapid UT1-UTC products