Datum realization for the SIRGAS weekly coordinates

M. V. MACKERN; L.SANCHEZ

Buenos Aires

Congreso; IAG 2009 Scientific Assembly; 2009

International Asociation of Geodesy

At present, SIRGAS (Sistema de Referencia Geocéntrico para las Américas) is realized by a network of continuously operating GNSS stations (SIRGAS-CON) with about 190 sites distributed over Latin America and the Caribbean. This network comprises a continental (core) network and three densification sub-networks, which are weekly processed by the SIRGAS Analysis Centres. In the analysis procedure, each sub-network is individually processed by fixing IGS satellite orbits and IGS Earth Orientation Parameters and by giving an a priori sigma of 1 m to all coordinate stations. These loosely constrained solutions are weekly combined in a unified solution with two main products: loosely constrained combinations in SINEX format for later computations (for instance multiyear solutions) and weekly constrained coordinates for practical applications (for instance as reference for differential GNSS surveying). Traditionally, constrained coordinates are obtained after applying the no-net rotation (NNR) and no-net translation (NNT) conditions with respect to the IGS05 values for the IGS05 stations included in the SIRGAS-CON network. However, after comparing SIRGAS weekly coordinates with the IGS weekly solutions for common stations, inconsistencies up to the 1-cm level are found. In order to analyse these discrepancies and to identify possible network deformations due to fixing the geodetic datum, different strategies for the datum realization within the SIRGAS-CON network are evaluated: 1) Fixing coordinates for IGS05 stations, 2) Constraining coordinates for IGS05 stations applying as a priori sigma 1E-05 m, and 3) NNR+NNT with respect to IGS05 stations. These three strategies are numerically evaluated using both the IGS05 values (i.e. IGS05 coordinates + velocities to reduce reference values from reference epoch 2000.0 to observation epoch) and the IGS weekly constrained coordinates (i.e. igsyyPwwww solutions). Linear velocities included in the IGS05 solution do not represent seasonal variations in station positions and reduced coordinates (by means of these velocities) are only approximate. Discrepancies between these approximate coordinates and the actual position of the reference stations (up to 3 cm) deform the network geometry, mainly at the borders. This deformation is minimized by applying IGS weekly coordinates in combination with the NNR+NNT strategy. This paper presents numerical results after applying the described strategies to SIRGAS-CON weekly solutions for the period between October 1, 2006 and December 31, 2008. Obtained conclusions can be generalized for the datum realization of regional reference frames.