Selected Results from Testing the IVS INT01 BA 50 Balanced Scheduling Strategy

The International VLBI Service for Geodesy and Astrometry (IVS) observes one-hour sessions called INT01s that provide quick, preliminary UT1-TAI estimates by observing, generally, with two stations (Kokee Park, Hawaii and Wettzell, Germany). When these sessions began in 2000, the scheduling agency (USNO), used the "STN" strategy, which used a small set of strong sources that were unevenly distributed over the sky. The uneven distribution led to sparse sky coverage at certain times of the year, which led to large UT1 formal errors at those times. In 2009, to improve sky coverage, we proposed and tested a new strategy, the MSS (Maximal Source Strategy), which consisted of observing all geodetic sources that are mutually visible on the Kokee-Wettzell baseline. This strategy was subsequently tested by USNO in the operational INT01 sessions for over two years, with USNO scheduling the MSS and the STN on alternate days. In mid-2016 USNO switched entirely to the MSS. A drawback of the MSS strategy is that it introduces weak sources that, because they take longer to observe, reduce the number of observations and in turn raise the UT1 formal errors at certain times of the year. To address this issue, in 2016 we proposed using an intermediate number of sources (50) and balancing sky coverage and source strength. We call this strategy the Balanced 50 (BA 50) scheduling strategy. We began to test the BA 50 strategy against the MSS strategy in six 2016 and 2017 IVS R&D sessions. USNO plans to begin testing the BA 50 strategy in operational INT01 sessions in the summer of 2018. This poster reports on selected results from testing the BA 50 strategy.