Earth Orientation Parameters from VLBI and GNSS Combined at the Observation Level

Current reference series of the International Earth Rotation and Reference Systems Service (IERS) are produced by a weighted combination of Earth orientation parameters (EOP) time series built up by combination centers of each technique (VLBI, GNSS, Laser ranging, DORIS). In the future, we plan to produce EOP by a direct combination of the four techniques at the observation level. In a first step towards this ambitious combination, we produced EOP time series from the combination of VLBI and GNSS pre-reduced, constraint-free, normal equations with the French DYNAMO geodetic analysis software package developed and maintained by the French GRGS (Groupe de Recherche en Géodésie Spatiale). The normal equations were prepared separately by two analysis centers using a consistent, standard modeling of tropospheric, gravitational, and geophysical effects. The VLBI analysis center of Paris Observatory produced the normal equations relevant to VLBI using the Calc/Solve geodetic VLBI analysis software package while those for GNSS were produced at the French National Space Center (CNES) using the GINS software maintained by the GRGS. Our series cover 2002-2014. The estimation strategy consisted of fixing quasar coordinates to their optimal values given by the latest realization of the International Celestial Reference Frame (ICRF), and most of station coordinates to the ITRF 2014 except for stations undergoing strong nonlinear displacements caused by, e.g., postseismic relaxation. Those station coordinates were estimated as time series. The resulting EOP series were compared to intratechnique combinations and to reference series of the IERS in terms of noise and geophysical signal.