Intra-Technique Combination of VLBI Intensives and Rapid Data to Improve the Temporal Regularity and Continuity of the UT1-UTC Series

Abstract The difference UT1-UTC is the most variable quantity among the Earth Orientation Parameters (EOP) with significant unpredictable variation. It can be measured only with the quasi-space-fixed technique VLBI. The IVS organizes two different VLBI observation campaigns: The bi-weekly 24-hour Rapid campaigns and the daily 1-hour Intensive sessions. As a result, two independent UT1-UTC time series are estimated and published as official IVS EOP-S and EOP-I products. These have different strengths and weaknesses in terms of continuity and accuracy, but both are characterized by irregular temporal resolution. We present the current activities of BKG towards a combined processing of VLBI Intensive and Rapid data in one common adjustment. In this way, we unify the strengths of both sessions and generate a UT1-UTC time series characterized by a daily, continuous and temporally regular resolution, e.g., at 12:00 UTC. We achieved a significant improvement in accuracy of 35% lower WRMS values compared to the regular session-wise Intensive-only solution. By using a continuous EOP parameterization, the accuracy is almost at a constant level and less dependent on the irregularity of the VLBI observation period. The processing is based on homogenized, datum-free NEQs which allow a rigorous combination on the normal equation level instead of the observation level. Based on the improved combination method, we intend to set up a new operational VLBI EOP product at BKG. Its characteristics make it suitable as an input for EOP prediction algorithms.