Comparison of near-field delay models for Earth-orbiting satellites

The classical VLBI delay model is built on the assumption that incoming radiowaves are planar, which holds true for observations of distant celestial radio sources due to their immense distances. However, when applied to VLBI observations of satellites in near-Earth orbit, this assumption becomes inadequate, as the curvature of the wavefronts cannot be neglected. Accurately modeling of these delays is crucial for achieving the precision required in satellite-based VLBI studies. To address this challenge, several near-field delay models have been proposed in the literature. In this study, we focus on four near-field VLBI delay models, comparing their methodologies, and resulting accuracy across a range of orbital configurations. By systematically evaluating these models under varying geometric and dynamic conditions, we aim to identify their strengths, limitations, and potential discrepancies.