An investigation on a GPS-based approach to local tie computation

The estimation of eccentricity vectors in co-located sites represents a crucial aspect in order to combine reference frames arising from different space-geodetic techniques; thus their role is pivotal as they deeply influence the intrinsic precision of the combined reference frame and, at the same time, allow a strictly coherent datum realization of combined networks. The way the local ties are estimated through an indirect approach strongly reflects upon the precision of the final multi-technique reference frame and its inner consistency (Ray & Altamimi, 2005). The investigation, here presented, dwells upon the possibility to pursue the local tie estimations through a purely GPS-based survey performed in the ITRF Co-located site of Medicina (Italy). Each local tie is characterized by an intrinsic precision strictly related not only to the way it has been surveyed but also to the applied conditions during the Least Square Estimation in a rigorous statistically-based procedure. Nevertheless, if a terrestrial survey is executed, it should be emphasized that the eccentricity vector components, finally aligned in ITRF, depend on the transformation performed in order to carry out the transition from the local topocentric reference frame, where the tie was classically surveyed, and the final realization of ITRF (Ray & Altamimi, 2005) . Processing and obtaining local eccentricities vectors via GPS-based surveys could allow to overcome the difficulties related to the aforementioned reference frame transition. In fact, since the Global Positioning Systems directly operate in a geocentric reference frame, the passage to ITRF naturally comes neglecting the difficulties related to the gauging of the vertical deflection. The local ties computations are performed through CLEMeNT, an house-made software,which showed satisfactory results in least-squares estimations both for highly precise terrestrial datasets and for simulated and intrinsecally noisy ones ; as far as it is concerned, the accuracies reached in processing noisy datasets have inspired the possibility to perform GPS based local ties by means of a new release of CLEMeNT, modified ad hoc in its structure in order to include a brand new ensemble of constraints which are going to strengthen the definition of the tracking points belonging to the local tie.