Enhanced GNSS Tomography Using Synthetic Ray Augmentation Constrained by Observed Tropospheric Gradients

Abstract

Ground-based GNSS tomography reconstructs three-dimensional tropospheric water-vapor fields from slant observations. However, unconstrained solutions rely heavily on station and satellite geometry. This study introduces a GNSS-only approach that uses precise SP3 orbit products to create pseudo-slant observations in satellite directions not tracked by individual receivers. These directions combine with existing IWV and horizontal-gradient estimates. This completes the ray distribution without adding external atmospheric constraints or independent water-vapor information. The method is tested in Hong Kong and Iceland with GPS-only, GLONASS-only, observed multi-constellation, and SP3-completed setups. GPS-derived gradients were generally in line with the full multi-GNSS solution, but GLONASS-only gradients showed larger differences. Overall, the mapped directions improved or maintained the inversion’s effective rank and numerical stability. They did not systematically degrade the retrieved water-vapor profiles, although the advantages decreased when the additional rays were geometrically redundant.