Improving Accuracy of Low-Latency Measurements of UT1−UTC with the VLBA

Accurate measurements of the time evolution of Earth’s rotation phase, parameterized as UT1−UTC, are crucial for various applications, such as positioning and navigation. This observable is most precisely measured by geodetic very long baseline interferometry (VLBI). Time series of UT1−UTC measurements from VLBI “Intensive” sessions analyzed at the US Naval Observatory and made with the Maunakea (Mk), Pie Town (Pt), and Hancock (Hn) stations of the Very Long Baseline Array (VLBA) and the KOKEE (Kk) and WETTZELL (Wz) stations coordinated by the International VLBI Service for Geodesy and Astrometry (IVS) are compared to a reference series to assess and identify methods to improve their accuracy and precision. Application of a Nadaraya-Watson estimator weighted by the formal error of each UT1−UTC measurement to the residuals of the Intensive series with respect to a reference series reveals the presence of two sources of systematic inaccuracies. There is a discontinuity in the Mk-Pt series residuals and a periodic signal of varying amplitude in the residuals of all the series. By calculating the sensitivity of the UT1−UTC measurement from the Mk-Pt series to changes in the stations’ positions and examining the station position evolution of GNSS receivers co-located with those stations, the discontinuity is determined to be associated with a 12.4 ± 0.6 mm displacement of the Mk station during the Mw 6.9 Hawai‘i earthquake of May 4th, 2018. The periodic signal is traced to the inaccuracy of the celestial pole orientation model. The IAU 2006/2000A precession-nutation model is insufficiently accurate on its own and the celestial pole offsets (CPOs) determined from diurnal VLBI sessions must also be used in the analysis of Intensive sessions to accurately measure UT1−UTC with low latencies.

These findings highlight the benefit of co-located GNSS and VLBI stations in maintaining the accuracy of UT1−UTC measurements from VLBI Intensive sessions and the need to include the CPOs in the models used in analysis of Intensives. They also support the call to improve the precession and nutation models made in Resolution B2 of the 2021 International Astronomical Union (IAU) General Assembly.