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In support of the IGS Multi-GNSS Experiment (MGEX), ESA/ESOC has put in significant effort to derive early phase center corrections for the L-Band transmitter antenna arrays aboard the Inclined Geosynchronous (IGEO) and Medium Earth Orbiting (MEO) BeiDou spacecraft. Almost one and one-half year of BeiDou triple-frequency (B1, B2, B3) measurement data – gathered between February 2013 and May 2014 by 39 ground stations of the MGEX tracking network – was used to derive the satellites’ antenna phase center offsets (PCOs) and variations (PCVs) for the ionosphere-free linear combinations B1-B2 and B1-B3, respectively. Processing was carried out in daily batches using the most recent version (3.8) of ESOC’s multi-GNSS analysis software, the Navigation Package for Earth Observing Satellites (NAPEOS). The parameterization of the PCVs was done in conventional IGS-style, that is, using piece-wise linear functions of the satellite nadir angle with 13 (MEO) and 9 (IGEO), respectively, linear segments (Figure 1). The estimates were found to agree to within 0.1 – 1 millimeter (PCVs) and 1 – 2 decimeter (z-PCOs) with independently-computed values from Wuhan University’s GNSS Research Center (J. Guo).

Figure 1: Satellite- and block-specific IGEO (top) and MEO (bottom) PCV estimates together with error bars representing the formal errors from the variance-covariance matrix. Results for IGEO-4/5 are not shown for reasons of clarity.

Applying the new PCO/PCV corrections to the BeiDou observables shows improved performance relative to the currently recommended standard offset values (x = 0.6 m, y = 0.0 m, z = 1.1 m). Initial comparisons of overlapping orbit solutions suggest that the orbit accuracy (3D-RMS) of the MEO spacecraft is substantially improved by more than a third. The orbital component that benefits most from the advanced phase center modeling is the MEOs along-track component (Figure 2). Similar improvements against the standard offset parameters were reported to us by GFZ (Z. Deng).

Figure 2: Day-to-day orbit overlap differences (RMS) computed for each MEO spacecraft over a five-month processing period. The standard PCO-only solution is shown in red, the advanced PCV-based solution in blue.