Progress in Understanding Horizontal Resolution of Radio Occultation Measurements

B. Ahmad, G. L. Tyler (Stanford University)

The meaning of high vertical resolution and the ``horizontal'' resolution of radio occultation measurements has been subject to interpretation of the Abel transform relationships between the occultation ray paths and an assumed spherically symmetric atmospheric refractivity field. For the spherically symmetric case the refractivity field is recovered exactly, within the limitations of ray optics. Only brute force methods have been available for analysis of non-spherically symmetric cases. For example, ray tracing is employed to calculate the ray asymptotes for perturbed, approximately symmetric models, followed by application of the Abel inversion to retrieve the apparent refractivity, which then can be compared with the model results. In instances where the bending angle is small a straight ray model leads to the inversion kernel for the Abel transform (Ahmad & Tyler, ``The 2-D Resolution Kernel associated with Retrieval of Ionospheric and Atmospheric Refractivity Profiles by Abelian Inversion of Radio Occultation Phase Data,'' submitted to RADIO SCIENCE, April, 1997). Weighted integrals of the 2-D refractivity field employing the inversion kernel then give the corresponding Abel inversion directly. This approach permits rapid computation of the effects of departures from spherical symmetry. Inspection of the inversion kernel provides insight into the Abel transform inversion procedure. An interesting and useful result is understanding of the artifacts created by atmospheric structures of limited horizontal extent. Transformation of the curved ray case appropriate for thick atmospheres to an equivalent straight ray case yields the kernel function for the curved rays in a spherically symmetric atmosphere. A remaining problem is to estimate the effects of ray path deviation from the background path on the analysis; for realistic cases this effect is believed to be small, however.