Characteristics of different VLBI session types in the view of Earth Orientation Parameter determination

VLBI is the unique space-geodetic technique sensitive to the full set of Earth Orientation Parameters (EOP), i.e., polar motion offsets and rates, UT1–UTC, the length-of-day (LOD) and the nutation offsets. Classically, the Rapid (R1/R4) sessions, scheduled twice a week, are the main contributor to EOP determination as they are processed with highest priority (leading to comparably low latencies). Additionally, Intensive sessions scheduled daily are processed to determine the UT1–UTC offset at short latencies.

However, also other VLBI session types (mainly 24-hour sessions) are regularly observed and processed, but typically with longer latencies, and those sessions are not regularly combined yet. As most of these sessions also have the potential to deliver good EOP estimates, their inclusion into the combination would lead to a densification of the VLBI contribution to combined EOP series. Very recently, the IERS product centers for EOPs emphasized that such a combined IVS contribution would be very valuable to improve the accuracy and reliability of the Earth rotation products according to the user requirements.

This study aims to define a set of criteria to evaluate VLBI sessions with respect to their suitability for EOP determination. These sessions include, but are not limited to, 24-hour sessions with globally well-distributed networks which were initially scheduled for other purposes like the determination of the Terrestrial Reference Frame (TRF) but are also sensitive to EOPs. Besides the distribution of the stations in a geographical sense, the selection criteria also include investigation of the sensitivity of the observations to the parameters estimated within individual sessions and of correlations between these parameters.

The different characteristics of the various VLBI session types could lead to systematic differences in the EOP estimates. We thus will investigate the resulting EOPs in view of systematics and will set up a procedure to regularly monitor the consistency between the different session types. We outline the combination strategy being developed to enable such a flexible combination of pre-selected VLBI session setups for dedicated studies and also on an operational basis, whereby the routines are based on the current operational combination scenario of the BKG/DGFI-TUM IVS Combination Centre.