Computer Science – Performance
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p71a0456k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P71A-0456
Computer Science
Performance
6900 Radio Science, 6904 Atmospheric Propagation, 6964 Radio Wave Propagation, 6994 Instruments And Techniques
Scientific paper
Atmospheric multipath propagation is a common source of error in radio occultation experiments in dense atmospheres. 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 refractivity with height and result in multivalued frequency with time. Closed-loop radio receivers based on phase-locked loops that are designed to receive single-valued frequency signals will fail to preserve the multivalued nature of these signals. In order to use backprojection methods to mitigate multipath effects in the refractive index profile, it is important that there are no breaks in the collected or backprojected data. Data sets must span a continuous time series over the range of ray asymptotes required for a particular retrieval. The receivers used for radio occultation experiments are therefore expected to reliably and continuously collect data even from signals that have propagated through ducts, turbulence, or other abrupt deviations in atmospheric structure. Both high altitude radiosonde data from Earth's atmosphere as well as model refractive index profiles are used to illustrate instances in which multipath occurs and to characterize the signal structures produced by multipath propagation. A functional representation of atmospheric multipath signal structure is used as the input to a phase locked loop. The phase locked loop simulation is representative of autonomous closed-loop receivers as opposed to those that are manually frequency-steered. The tracking performance of the phase locked loop is evaluated for these dynamic signal structure inputs. Cases where the system fails are examined to determine the extent to which closed-loop systems can be used to observe and accurately characterize such signals.
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