Atmospheric delay modelling for ground-based GNSS reflectometry

Abstract

Several studies have demonstrated the utility of global navigation satellite system reflectometry (GNSS-R) for ground-based coastal sea-level altimetry. Recent studies evidenced the presence of atmospheric delays in GNSS-R sea-level retrievals and by-products such as tidal amplitudes. On the one hand, several ad-hoc atmospheric correction formulas have been proposed in the literature. On the other hand, ray-tracing studies applied for GNSS-R show little information about the methods and algorithms involved. This dissertation is based on three articles which establish the theoretical framework of the atmospheric delay experienced in ground-based GNSS-R altimetry. In the first article, we defined the atmospheric interferometric delay in terms of the direct and reflected atmospheric delays as well as the vacuum distance and radio length. Then, we clarified the roles of linear and angular refraction, derived the respective delays and combined them in the total delay. We also introduced for the first time two subcomponents of the atmospheric geometric delay, the geometric-shift and the geometric-excess, unique for reflected signals. The atmospheric altimetry correction necessary for unbiased sea-level retrievals was defined as half the rate-of-change of the atmospheric delay with respect to the sine of satellite elevation angle. We developed a ray-tracing procedure to solve rigorously the three-point boundary value problem involving transmitting satellite, reflecting surface, and receiving antenna. We hence evaluated the atmospheric bias in sea-level retrievals for a range of typical scenarios, showing its dependence on elevation angle and reflector height. In the second article, we demonstrated that rigorous ray-tracing of the bent ray can be simplified by a judicious choice of rectilinear wave propagation model. This facilitates the adaptation by existing GNSS ray-tracing procedures, besides numerical and speed advantages. Further it was emphasized that mapping functions developed for GNSS positioning cannot be reused for GNSS-R purposes without adaptations. In the third article, we developed closed-form expressions of the atmospheric delay and altimetry correction for end-users without access or expertise in ray-tracing. These expressions rely only on direct elevation bending and mean refractivity at the site. Finally, we determined cut-off elevation angle and reflector height, for neglecting atmospheric delays. These limiting conditions are useful in observation planning and error budgeting of the GNSS-R altimetry retrievals.