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Abstract:
Multi-technique combination is predicated upon the fact that different observing systems are capable of contributing to the estimation of shared parameters or parameters whose cross-system relation is well understood. For work oriented towards global reference frames (e.g., ITRF2020, DTRF2020, and JTRF2020), only local (station coordinates) and global (polar motion and length of day) ties are typically employed to combine measurements of different observing systems. Zenith delays and horizontal delay gradients estimated from observations collected at co-location sites are highly correlated and thus offer the opportunity to impose additional constraints during the data combination. In this work, we combine GNSS, VLBI, and SLR normal equation systems employing local, global, and atmospheric ties during CONT14 and CONT17. The modelling and parameterization of the single-technique data analyses have been carried out in Bernese v5.3 for GNSS and SLR, and PORT for VLBI, and are consistent, except for the interval of laser atmospheric refraction coefficient estimates which is dictated by the data availability. We obtain atmospheric ties for co-located stations via ray-tracing through ERA5-derived refractivity fields at three different frequencies (L-Band, X-Band and optical 532 nm). Our contribution discusses the impact of tuning (i) the relative normal equation system weighting to partly account for the fact that correlations between observations have not been considered within the single-system data analysis as well as for the largely different number of observations, and (ii) the uncertainty assigned to the stochastic equivalence constraints, that is, the local and atmospheric ties, on station coordinates, ERP and atmospheric delays.
