Optimal VLBI baseline geometry for UT1-UTC Intensive observations.

One of the main tasks of Very Long Baseline Interferometry (VLBI) is the rapid determination of the highly variable Earth's rotation expressed through the difference between Universal Time UT1 and Coordinated Universal Time UTC (dUT1). For this reason, dedicated one hour, single baseline sessions, called , are observed on a daily basis. Thus far, the optimal geometry of sessions was understood to include a long east-west extension of the baseline to ensure a dUT1 estimation with highest accuracy. In this publication, we prove that long east-west baselines are the best choice only for certain lengths and orientations. In this respect, optimal orientations may even require significant inclination of the baseline with respect to the equatorial plane. The basis of these findings is a simulation study with subsequent investigations in the partial derivatives of the observed group delays with respect to dUT1 . Almost 3000 baselines between artificial stations located on a regular degree grid are investigated to derive a global and generally valid picture about the best length and orientation of baselines. Our results reveal that especially equatorial baselines or baselines with a center close to the equatorial plane are not suited for although they provide a good east-west extension. This is explained by the narrow right ascension band of visible sources and the resulting lack of variety in the partial derivatives. Moreover, it is shown that north-south baselines are also capable of determining dUT1 with reasonable accuracy, given that the baseline orientation is significantly different from the Earth rotation axis. However, great care must be taken to provide accurate polar motion a priori information for these baselines. Finally, we provide a better metric to assess the suitability of baselines based on the effective spread of .