VLBI-GPS Eccentricity Vectors at Medicina's Observatory via GPS Surveys: Reproducibility, Reliability and Quality Assessment of the Results

Local ties are key elements for the computation of combined space geodetic products: the different ITRF realizations determined by merging single technique geodetic reference frames rely on the availability of accurate eccentricities (or intra-site vectors) linking the instrumental reference points (RPs) at co-located observatories. If the RPs cannot be physically materialized and/or directly measured, an indirect approach can be used for their estimation: it is entirely based on a geometrical conditioning applied to observed points' positions of the space geodetic instruments. Intra-site vectors are typically measured by combining terrestrial observations of angles, distances and height differences along with ancillary GPS measurements for assuring a correct alignment of the eccentricity into a global frame (terrestrial local ties). An alternative indirect approach can be used entirely relying on GPS measurements: it may efficiently be applied to VLBI-GPS eccentricities as well as at all co-location sites where ITRF tracking points can be surveyed with GPS technique (GPS-based local ties). In principle such an approach offers advantages with respect to that terrestrial, since it is (i) faster, (ii) semi-automatic and (iii) it provides an immediate eccentricity vector alignment into a global frame. This paper presents the results of the computation of the VLBI-GPS eccentricity at Medicina based on the GPS procedure, with the aim of assessing its quality and repeatability. To this respect, two GPS measurement sessions were performed in 2002 and 2006: the positive results obtained with the first campaign led us to repeat the survey in 2006 in order to better investigate the potential of GPS-based ties. 2002 and 2006 GPS-based ties were compared with terrestrial local ties surveyed at Medicina's observatory in 2001, 2002, 2003. Each of the intra-site vectors was estimated by applying different configurations of the geometrical conditioning with the aim of investigating the effects on the RPs estimations in the terrestrial and in the GPS case. The results demonstrate the efficiency and the precision of the GPS approach for eccentricity vector computation which can be used as an alternative to terrestrial based ties.