Date: Wednesday, November 18, 1998
Special University Ph.D. Oral Examination
Time: 4:15 - 5:30 pm
Location: Skilling 191
Diffraction, Multipath, and Ducting in Radio Occultation
Dept. of Electrical Engineering,
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.