When calculating radio emission from an air shower, the standard approximation used in all current
air shower radio software is to assume straight-line signal propagation. This approximation is
expected to become less valid for very inclined geometries, but the magnitude of the error caused
by the approximation is yet to be ascertained. Therefore, it is critical to understand the region
of validity for this approximation as it could affect the design of next-generation radio-based
detectors. To investigate the possible error introduced by the approximation, we present results
obtained using a modified version of CoREAS combined with input data from ray tracing to more
correctly describe signal propagation in a non-uniform atmosphere for very inclined geometries
without performing the full ray tracing during simulation. We aim to determine geometries where
the straight-line approximation might introduce significant errors.