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C. S. Han, G. L. Tyler (Stanford University)
In planetary radio occultation, diffraction is an effect clearly seen in real occultation observations and proves to be a limiting factor in recovered profile resolution. While diffraction is typically associated with sharp, small scale features, data from Mars Global Surveyor (MGS) evidence a diffraction signature at the occultation point similar to a knife edge, but this phenomena occurs even over smooth, flat regions of the Red Planet. When applied to such data, the standard profile retrieval method (i.e., Abel inversion) cannot easily recover atmospheric refractivity profiles near the diffracting limb since the technique is based in geometrical optics. Further, small, sub-Fresnel-scale atmospheric structures such as waves and temperature inversions can diffract as well and compound the challenges to Abel inversion in profile recovery.
This research addresses the diffraction limit in Abel inversion by applying back propagation. By manipulating the angular spectrum of occultation data to reverse propagate the observed fields, a new effective observation distance much less than the actual can be attained thus reducing the Fresnel scale. When applied to such back-propagated data, Abel inversion yields vertical resolution finer than that achievable by applying the inversion alone. In simulation, this technique simultaneously resolves the planet limb to within 10 m as well as a smooth, 40 m refractivity perturbation of magnitude 10-7 superposed over the refractivity profile for a Mars-like atmosphere. The simulated atmosphere has a refractivity of ~4x10-6 and a scale height of approximately 9 km. The diameter of the first Fresnel zone in this simulation is approximately 500 m – greater than ten times the scale of the refractivity feature used in the simulation. This methodology is being further developed for application to real MGS data and early results are pending.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.