A Priori Gradients in the Analysis of Space Geodetic Observations
We introduce a static a priori gradient model (APG) based on a spherical harmonic expansion up to degree and order nine to describe the azimuthal asymmetry of tropospheric delays. APG is determined from climatology data of the European Centre for Medium-Range Weather Forecasts (ECMWF), and the refined model can be used in the analysis of observations from Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI). Comparisons reveal that gradients estimated in GNSS analysis are mostly smaller than those provided by APG. This difference is also confirmed by station and source coordinate changes if APG is used in GNSS and VLBI analysis.
KeywordsTropospheric delay Tropospheric gradients VLBI GNSS APG
J. Böhm and V. Nafisi are grateful to the Austrian Science Fund (FWF) for supporting this research under project P20902-N10 (GGOS Atmosphere). V. Nafisi would also like to acknowledge Iran Ministry of Science, Research and Technology (MSRT) for funding part of his research at the Vienna University of Technology. L. Urquhart would like to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for providing funding. We like to acknowledge the International VLBI Service for Geodesy and Astrometry (IVS; Schlüter and Behrend 2007) and the International GNSS Service (IGS; Dow et al. 2009) for providing their data and products.
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