Radio occultation data analysis with analytical ray-tracing

Radio occultation experiments are a sensing technique dedicated to the remote sounding of planetary atmospheres. The technique exploits the frequency shift of a radio signal due to refraction in a planetary atmosphere. The aim is to infer the physical properties of the neutral atmosphere (e.g., pressure and temperature) and ionosphere (e.g., the electron number density). For one-way occultations, the data processing usually relies on Abel transform algorithms when the atmosphere is spherically symmetric. For two-way occultations, such techniques require the introduction of approximate relationships for the bending experienced by the signal to be obtained. In this context, we introduce a new method to process two-way occultations data by spherically symmetric atmospheres using a ray-tracing approach. However, the numerical integration of the geometrical optics equation through the atmosphere requires a significant computational time due to initial pointing issues. For this reason, our novel algorithm exploits a closed-form solution to the equations of geometrical optics (Bourgoin et al., A&A, 624, A41, 2019, https://doi.org/10.1051/0004-6361/201834962) applied to a spherically symmetric atmosphere. Within this approach, the bending is directly provided by the analytical solution and no numerical integration is required. In addition, we develop a procedure enabling us to disentangle the contributions from dispersive and neutral media in the frequency shift. This procedure is validated by comparing our vertical profiles to those obtained using Abel inversion or numerical ray-tracing for Mars and Titan occultation experiments. We show that our algorithm provides similar results to purely numerical ray-tracing algorithms while significantly decreasing the computational time.