ITRS realizations and their impact on VLBI combined EOP and Scale

In this article we investigate the impact of different realizations in framework of the generation of the International Terrestrial Reference Frame (ITRF), Earth Orientation Parameters (EOP), and scale resulting from Very Long Baseline Interferometry (VLBI) intra-technique combination. The first section reviews the input contribution of the International VLBI Service for Geodesy and Astrometry (IVS) to the ITRS realizations. The second section shows the results of using the three different inter-technique combined ITRS realizations as a priori station coordinates in the IVS combination process. We found significant differences in the scale when comparing the results to the respective a priori frames DTRF2014, ITRF2014, and JTRF2014. The scale differences are -0.01 ppb for DTRF2014, -0.59 ppb for ITRF2014, and 0.19 ppb for JTRF2014, with residual WRMS of 0.88 ppb for DTRF2014, 0.95 ppb for ITRF2014, and 0.78 ppb for JTRF2014. In terms of EOP, the differences of using DTRF2014, ITRF2014, and JTRF2014 as a priori values for station coordinates are less distinct. We found differences in particular parameters, i.e. x-pole, y-pole, LOD, and dUT1, using the three different TRS realizations as a priori station coordinates. However, the differences do not emphasize one or another ITRS realization as preferable TRF for VLBI combination. Moreover, we found significant differences in dUT1 when using IERS 08C04 or IERS 14C04 as EOP reference series, which might indicate inconsistencies between these two reference series. Introduction In this chapter we review the IVS contribution to the ITRF2014 and the adjunctive combination process before investigating the impact of the three ITRS realizations on VLBI combined Earth Orientation Parameters (EOP) and scale. The scale parameter is particularly interesting, as VLBI – together with Satellite Laser Ranging (SLR) – are the only space geodetic techniques with reliable access to this parameter. In the framework of the latest realization of the International Terrestrial Reference System (ITRS), each of the three ITRS Combination Centers of the IERS (DGFI-TUM, IGN, JPL) provides an inter-technique combined terrestrial reference frame (TRF) using different station coordinate parameterizations. DGFI-TUM provides the DTRF2014, using a piecewise linear station model with improved geophysical modeling of loading deformation (Seitz et al., 2016). The official ITRF solution, ITRF2014, is provided by IGN, using a piece-wise linear station model and an additional post-seismic deformation model (Altamimi et al., 2016). Finally, JPL provides the JTRF2014, using Kalman filtered weekly estimations of station coordinates with the noise of the time-series triggered by geophysical loading models (Abbondanza et al., 2017). In the following sections the internal VLBI evaluation of the scale parameter is discussed, then we compare the scale parameter between the combined solution resulting from the VLBI-only combination using