Physics – Optics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p61c0357h&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P61C-0357
Physics
Optics
0350 Pressure, Density, And Temperature, 5409 Atmospheres: Structure And Dynamics, 5494 Instruments And Techniques, 6225 Mars, 6994 Instruments And Techniques
Scientific paper
In planetary radio occultation, Abel inversion is typically used to recover atmospheric refractivity curves from which temperature and pressure can also be derived. But being based in geometrical optics, the Abel inversion is diffraction-limited, yielding profiles with vertical resolutions no better than the diameter of the first Fresnel zone, ~2√ {λ D} where D is the distance between the spacecraft and the observed ray periapse. Consequently, Abel inversion alone cannot resolve sub-Fresnel-scale atmospheric features such as sharp temperature inversions. Further, limb diffraction (as commonly observed in occultation data from Mars Global Surveyor) can aggravate profile reconstruction by masking near-surface atmospheric structure. By itself, Abel inversion cannot uncover near-surface features in the presence of limb diffraction since this is a wave optics phenomenon. This research addresses the resolution limit in Abel inversion by manipulating the angular spectrum of occultation data to reverse propagate the observed fields. In doing so, this technique, called back propagation, effectively reduces the size of the first Fresnel zone. Therefore, when applied to back-propagated data, the Abel inversion can achieve vertical resolutions better than the Fresnel scale. In an occultation simulation mimicking Mars Global Surveyor radio science experiments, back propagation allows Abel inversion to resolve simultaneously both 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 ~4 x 10-6 and a scale height of ~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. Early application of back propagation to real occultation data from Mars Global Surveyor demonstrates similarly promising results for resolving near-surface atmospheric structure at Mars.
Han Cheongho
Tyler L.
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