Validation of straight-line signal propagation for radio signal of very inclined cosmic ray air showers

An ongoing challenge for radio-based detectors of high-energy cosmic particles is the accurate description of radio signal propagation in natural non-uniform media. For radio signals originating from extensive air showers, the current state of the art simulations often implicitly assume straight-line signal propagation. This while the refraction due to a non-uniform atmosphere is expected to have an effect on the received signal and associated reconstruction that is currently not completely understood for the most inclined geometries. Here, we present a study regarding the validity of assuming straight-line signal propagation when simulating radio emission associated with very inclined air shower geometries. To this end, the calculation of the electric field based on the end-point formalism used in CoREAS was improved by use of tabulated ray tracing data. We find that including ray curvature effects into the end-point formalism calculation introduces changes of up to a few percent in fluence for frequencies up to 1.2 GHz and zenith angles up to 88{\deg}.