Combining the orbits of the IGS Analysis Centers

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Currently seven Analysis Centers of the International GPS Service for Geodynamics (IGS) are producing daily precise orbits and the corresponding Earth Orientation Parameters (EOP). These individual products are available at several IGS Data Centers (e.g. CDDIS, IGN, SIO, etc.). During 1993 no official IGS orbits were produced, but the routine orbit comparisons by IGS indicated that, after small orientation and scale alignments, the orbit consistency was approaching the 20 cm level (a coordinate RMS), and that some orbit combination should be possible and feasible. An IGS combined orbit could provide a precise and efficient extension of the IERS Terrestrial Reference Frame (ITRF). Another advantage of such a combined orbit would be reliability and precision. Two schemes of orbit combinations are considered here: (a) the first method consists of a weighted averaging process of the earth-fixed satellite positions as produced by the individual Centers; (b) the second method uses the individual IGS orbit files as pseudo-observations in an orbit determination process, where in addition to the initial conditions, different parameter sets may be estimated. Both orbit combination methods have been tested on the January 1993 orbit data sets (GPS weeks 680 and 681) with an impressive agreement at the 5 cm level (coordinate RMS). The quality of the combined orbits is checked by processing a set of continental baselines in two different regions of the globe using different processing softwares. Both types of combined orbits gave similar baseline repeatability of a few ppb in both regions which compared favorably to the best individual orbits in the region.