[Previous] | [Session 18] | [Next]
K. Kusza, G. L. Tyler (Stanford University)
Atmospheric multipath propagation is a common source of error in radio occultation experiments in dense atmospheres such as Earth's. If not correctly detected and mapped into the ray asymptote structure, multipath effects produce unrecoverable errors in the inverse problem for the refractive index profile. Raytracing can be used to clearly illustrate multipath caused by interacting signals following closely spaced paths in the atmosphere. Such dynamic signals occur in connection with sharp variations in the refractivity with height profile and result in multivalued frequency with time. Precision radiosonde data [1] exhibit a wide range of these variations at different altitudes, with inversions spread over as much as 2 km, and with changes in refractivity ranging from very small to over 50 N-units.
Multipath propagation effects from these inversions create dynamic signals comprising odd numbers of multiple signals of different frequencies which vary in intensity and phase in a manner unlike both ground multipath and the modulated complex signals that autonomous receivers currently are designed to handle. For example, ground multipath is usually assumed to have intensity of equal or lesser value than the original signal. This is not the case for atmospheric multipath, where closely spaced multipath signals can have rapidly changing intensities that are up to an order of magnitude larger than the original signal. Similar phenomena also occur in association with atmospheric and limb diffraction of occultation signals.
We study several model inversions in the refractivity with height profile using raytracing to experimentally identify the how sensitive the multivalued frequency output is to the altitude spread, sharpness of the inversion, and the change in refractivity of the model. We then consider how the resulting frequency signature would be captured by an autonomous receiver and the extent of the error caused if the multipath signature is not correctly detected and mapped into the ray asymptote structure for each model.
[1] B. Brashers, Precision Radiosonde Data from November 1995, Private Communication, April 1998.
If the author provided an email address or URL for general inquiries,
it is as follows:
Intentionally Blank
Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.