Radio Occultation Data Analysis With Analytical Ray‐Tracing

Abstract

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) 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.

An Appraisal of the Progress in Utilizing Radiosondes and Satellites for Monitoring Upper Air Temperature Profiles

“…Based on the bending angle, a profile can be retrieved using statistical algorithms such as Abel inversion [36], ray tracing [108], Monte Carlo simulations [109], optimal estimation [110], Fourier Transform [111], and Chirp Z-Transform [112]. Normally, Abel inversion is used when the occulting atmosphere is assumed to be spherically symmetric.…”

“…Figure 5 illustrates the common RO temperature retrieval techniques used in the literature analyzed in this review from 2000 to 2023. The results show that Abel transform (44.17%) has been the most widely used retrieval algorithm (e.g., [36,108]) despite the need for a priori information. This is because of its efficiency, stability, physical interpretability, and extensive validation in the RO community [98].…”

“…Thus, this therefore warrants more studies on the correction of negative refractivity biases. Several atmospheric retrieval algorithms were reviewed in this study such as Abel inversion [36], ray tracing [108], Monte Carlo simulations [109], optimal estimation [110], Fourier Transform [111], and Chirp Z-Transform [112]. This study has demonstrated that Abel transform is widely preferred despite the need for prior information due to several key advantages such as physical interpretability, and it allows for the retrieval of multiple atmospheric profiles simultaneously.…”

Radio Occultation Measurements of Europa's Ionosphere From Juno's Close Flyby

“…For this reason, we resorted to the use of analytical ray-tracing, which is briefly outlined in the following paragraphs. A complete and thorough description of the methodology can be found in Bourgoin et al (2019) and Caruso et al (2023).…”