Balancing source strength and sky coverage in IVS-INT01 scheduling

The international very-long-baseline interferometry service for Geodesy and Astrometry’s INT01 sessions originally observed small sets of strong quasars (“sources”). But the sources were unevenly distributed, causing bad observation coverage and high UT1 formal errors at times of the year, especially early October. To improve coverage, in 2009, we introduced the maximal source strategy (“MSS”), the strategy of using all geodetic sources that are mutually visible at the two primary INT01 stations. But although the MSS reduced the UT1 formal errors from 32.0 to 15.1 µs in the first half of October in 2 years of testing, the MSS increased them from 10.0 to 12.0 µs in the first half of November; the MSS had introduced weaker sources, which take longer to observe, leading to fewer observations and higher UT1 formal errors. Starting in 2014, we investigated balancing source strength and sky coverage in source sets through their size and through the use of the Goddard Space Flight Center’s Sked program’s “Bestsource” command, which balances these factors. We used the UT1 formal error and two sensitivity metrics to evaluate schedules made from “balanced” source sets of seven sizes and selected 50 as the best size. We tested this “Balanced 50” (BA50) strategy against the MSS strategy in six R&D sessions. The results led to the BA50’s use in operational INT01 scheduling on a trial basis. We report on the selection of the BA50 strategy, the R&Ds that tested it, and its operational use.