Abstract
In the past 30 years, radio occultation experiments were conducted to study the atmospheres of about ten planets and moons in the solar system. Abel inversion, which is the conventional algorithm for retrieving atmospheric profiles in a radio occultation experiment, was applied successfully to obtain extensive information about the vertical structure and dynamics of these atmospheres. However, Abel inversion is derived using geometrical optics and therefore ignores diffraction effects. This suggests that the vertical resolution of the retrieved atmospheric profiles is diffraction limited.

We have investigated the vertical resolution that can be achieved in the presence of small-scale structure that causes diffraction. The results are based on forward simulations of radio-wave propagation through model atmospheres for Earth and Mars. The forward simulations rely on scalar diffraction theory and properly account for diffraction effects. We have found that profiles retrieved through Abel inversion are diffraction limited to about the diameter of the first Fresnel zone, as expected.

To overcome this limitation, we have developed an advanced retrieval algorithm based on diffraction theory. We demonstrate that the method is capable of reconstructing sub-Fresnel-scale structure in the simulated occultations at Earth and Mars. This algorithm also provides a natural means for deciphering multipath propagation that can be a problem in the standard Abel inversion procedure.

Another requirement for the validity of Abel inversion is the absence of atmospheric ducts. We have investigated the effects of ducts on atmospheric profiles retrieved by Abel inversion. We show that the retrieved profiles contain significant errors in and below the duct.