Simulation of VLBI intensive sessions for the estimation of UT1

Abstract

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique capable of estimating all five Earth orientation parameters (EOP) and the International Celestial Reference Frame (ICRF). The measurement principle includes globally distributed radio telescopes observing quasars, thereby deriving the difference in arrival time as the primary observable. In addition to 24 hour sessions, which often involve several VLBI stations, so-called Intensive sessions are observed. These one hour long sessions include only two or three stations and the sole purpose is the determination of Universal Time UT1 with respect to UTC (dUT1) with a short latency, which is required for positioning and navigation purposes. In this study, the scheduling software VieSched++ is used to test the suitability of certain Intensive baselines and networks for the determination of dUT1 in Monte Carlo simulations. The first part of this study deals with possible baselines between existing stations of the VLBI Global Observing System (VGOS) and the Very Long Baseline Array (VLBA). The target is to find the optimal baselines between two and three stations and to assess different session durations in the range from one to four hours. For VGOS, the baseline ISHIOKA to WESTFORD or GGAO12M would deliver the best estimates of dUT1, about 15% better than the actually observed Intensive session between WETTZ13S and KOKEE12M. For VLBA, the baseline MAUNA KEA to HANCOCK would lead to significantly improved values compared to the baseline MAUNA KEA to PIETOWN, which is currently observed.Furthermore, this work assesses baselines in a global artificial network of VLBI antennas, leading to almost 3000 investigated baselines. In general, the optimal baselines show a small angle with the equatorial plane, e.g., between high-to-mid-latitude and low-to-mid-latitude telescopes, while having a baseline length of about 8000 to 11000km. Baselines with a midpoint close to the equatorial plane result in worse dUT1 estimates and are therefore not recommended for VLBI Intensive sessions.