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 OrientationParameters (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 areprocessed to determine the UT1–UTC offset at short latencies. However, also other VLBI session types (mainly 24-hour sessions) are regularly observedand processed, but typically with longer latencies, and those sessions are not regularlycombined yet. As most of these sessions also have the potential to deliver good EOPestimates, their inclusion into the combination would lead to a densification of the VLBIcontribution to combined EOP series. Very recently, the IERS product centers for EOPsemphasized that such a combined IVS contribution would be very valuable to improve theaccuracy 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 theirsuitability for EOP determination. These sessions include, but are not limited to, 24-hoursessions with globally well-distributed networks which were initially scheduled for otherpurposes like the determination of the Terrestrial Reference Frame (TRF) but are alsosensitive to EOPs. Besides the distribution of the stations in a geographical sense, theselection criteria also include investigation of the sensitivity of the observations to theparameters estimated within individual sessions and of correlations between theseparameters. The different characteristics of the various VLBI session types could lead to systematicdifferences in the EOP estimates. We thus investigate the resulting EOPs in view ofsystematics and will set up a procedure to regularly monitor the consistency between thedifferent session types. The combination strategy being developed to enablea flexible combination of pre-selected VLBI session setups for dedicated studies andalso on an operational basis will be based on the current operationalcombination scenario of the BKG/DGFI-TUM IVS Combination Centre.